Publications

Dual-phase liquid-xenon time projection chambers (LXe TPCs) deploying a few tonnes of liquid are presently leading the search for WIMP dark matter. Scaling these detectors to 10-fold larger fiducial masses, while improving their sensitivity to low-mass WIMPs presents difficult challenges in detector design. Several groups are considering a departure from current schemes, towards either single-phase liquid-only TPCs, or dual-phase detectors where the electroluminescence region consists of patterned electrodes. Here, we discuss the possible use of Thick Gaseous Electron Multipliers (THGEMs) coated with a VUV photocathode and immersed in LXe as a building block in such designs. We focus on the transfer efficiencies of ionization electrons and photoelectrons emitted from the photocathode through the electrode holes and show experimentally that efficiencies approaching 100% can be achieved with realistic voltage settings. The observed voltage dependence of the transfer efficiencies is consistent with electron transport simulations once diffusion and charging-up effects are included.

The nature of dark matter and properties of neutrinos are among the most pressing issues in contemporary particle physics. The dual-phase xenon time-projection chamber is the leading technology to cover the available parameter space for weakly interacting massive particles, while featuring extensive sensitivity to many alternative dark matter candidates. These detectors can also study neutrinos through neutrinoless double-beta decay and through a variety of astrophysical sources. A next-generation xenon-based detector will therefore be a true multi-purpose observatory to significantly advance particle physics, nuclear physics, astrophysics, solar physics, and cosmology. This review article presents the science cases for such a detector.

Cryogenic versions of Resistive WELL (RWELL) and Resistive Plate WELL (RPWELL) detectors have been developed, aimed at stable avalanche multiplication of ionization electrons in the vapor phase of LAr (dual-phase TPC). In the RWELL, a thin resistive DLC layer deposited on top of an insulator is inserted in between the electron multiplier (THGEM) and the readout anode; in the RPWELL, a resistive ferrite plate is directly coupled to the THGEM. Radiation-induced ionization electrons in the liquid are extracted into the gaseous phase. They drift into the THGEM's holes where they undergo charge multiplication. Embedding resistive materials into the multiplier proved to enhance operation stability due to the mitigation of electrical discharges thus allowing operation at higher charge gain compared to standard THGEM (a.k.a. LEM) multipliers. We present the detector concepts and report on the main preliminary results.

We detail the sensitivity of the proposed liquid xenon DARWIN observatory to solar neutrinos via elastic electron scattering. We find that DARWIN will have the potential to measure the fluxes of five solar neutrino components: pp, ⁷Be, ¹³N, ¹⁵O and pep. The precision of the ¹³N, ¹⁵O and pep components is hindered by the double-beta decay of ¹³⁶Xe and, thus, would benefit from a depleted target. A high-statistics observation of pp neutrinos would allow us to infer the values of the electroweak mixing angle, sin ²θ_w, and the electron-type neutrino survival probability, P_ee, in the electron recoil energy region from a few keV up to 200 keV for the first time, with relative precision of 5% and 4%, respectively, with 10 live years of data and a 30 tonne fiducial volume. An observation of pp and ⁷Be neutrinos would constrain the neutrino-inferred solar luminosity down to 0.2%. A combination of all flux measurements would distinguish between the high- (GS98) and low-metallicity (AGS09) solar models with 2.12.5σ significance, independent of external measurements from other experiments or a measurement of ⁸B neutrinos through coherent elastic neutrino-nucleus scattering in DARWIN. Finally, we demonstrate that with a depleted target DARWIN may be sensitive to the neutrino capture process of ¹³¹Xe.

The bubble-assisted Liquid Hole Multiplier (LHM) is a novel concept for the combined detection of ionization electrons and scintillation photons in noble-liquid time projection chambers. It consists of a perforated electrode immersed in the noble liquid, with heating wires generating a stable vapor bubble underneath. Radiation-induced ionization electrons in the liquid drift into the electrode's holes and cross the liquid-vapor interface into the bubble where they induce electroluminescence (EL). The top surface of the electrode is optionally coated with a CsI photocathode; radiation-induced UV-scintillation photons extract photoelectrons that induce EL in a similar way. EL-photons recorded with an array of photo-sensors, e.g. SiPMs, provide event localization. We present the basic principles of the LHM concept and summarize the results obtained in LXe and LAr.

We present for the first time, discharge-free operation at cryogenic conditions of a Resistive-Plate WELL (RPWELL) detector. It is a single-sided Thick Gaseous Electron Multiplier (THGEM) coupled to a readout anode via a plate of high bulk resistivity. The results of single- and double-stage RPWELL detectors operated in stable conditions in Ne/5%CH₄ at 163 K are summarized. The RPWELL comprised a ferric-based (Fe³⁺) ceramic composite ("Fe-ceramic") as the resistive plate, of volume resistivity similar to 10¹¹ Ω·cm at this temperature. Gains of similar to 10⁴ and similar to 10⁵ were reached with the single-stage RPWELL, with 6 keV X-rays and single UV-photons, respectively. The double-stage detector, a THGEM followed by the RPWELL, reached gains similar to 10⁵ and similar to 10⁶ with X-rays and single UV-photons, respectively. The results were obtained with and without a CsI photocathode on the first multiplying element. Potential applications at these cryogenic conditions are discussed.

Charging-up processes affecting gain stability in Thick Gas Electron Multipliers (THGEM) were studied with a dedicated simulation toolkit. Integrated with Garfield++, it provides an effective platform for systematic phenomenological studies of charging-up processes in MPGD detectors. We describe the simulation tool and the fine-tuning of the step-size required for the algorithm convergence, in relation to physical parameters. Simulation results of gain stability over time in THGEM detectors are presented, exploring the role of electrode-thickness and applied voltage on its evolution. The results show that the total amount of irradiated charge through electrode's hole needed for reaching gain stabilization is in the range of tens to hundreds of pC, depending on the detector geometry and operational voltage. These results are in agreement with experimental observations presented previously.

The XENON1T experiment at the Laboratori Nazionali del Gran Sasso (LNGS) is the first WIMP dark matter detector operating with a liquid xenon target mass above the ton-scale. Out of its 3.2 t liquid xenon inventory, 2.0 t constitute the active target of the dual-phase time projection chamber. The scintillation and ionization signals from particle interactions are detected with low-background photomultipliers. This article describes the XENON1T instrument and its subsystems as well as strategies to achieve an unprecedented low background level. First results on the detector response and the performance of the subsystems are also presented.

A study of the localization properties of a single-element Resistive Plate WELL (RP-WELL) detector is presented. The detector comprises of a single-sided THick Gaseous Electron Multiplier (THGEM) coupled to a segmented readout anode through a doped silicate-glass plate of 10¹⁰ Ω·cm bulk resistivity. Operated in ambient\nech gas, the detector has been investigated with 150 GeV muons at CERN-SPS. Signals induced through the resistive plate on anode readout strips were recorded with APV25/SRS electronics. The experimental results are compared with that of Monte Carlo simulations. The effects of various physics phenomena on the position resolution are discussed. The measured position resolution in the present configuration is 0.28 mm RMS - compatible with the holes-pattern of the multiplier. Possible ways for improving the detector position resolution are suggested.

A thin single-element THGEM-based, Resistive-Plate WELL (RPWELL) detector was operated with 150 GeV/c muon and pion beams in Ne/(5%CH₄), Ar/(5%CH₄) and Ar/(7%CO₂); signals were recorded with 1 cm² square pads and SRS/APV25 electronics. Detection efficiency values greater than 98% were reached in all the gas mixtures, at average pad multiplicity of 1.2. The use of the 10⁹ Ω cm resistive plate resulted in a completely discharge-free operation also in intense pion beams. The efficiency remained essentially constant at 9899% up to fluxes of ∼10⁴ Hz/cm², dropping by a few % when approaching 10⁵ Hz/cm². These results pave the way towards cost-effective, robust, efficient, large-scale detectors for a variety of applications in future particle, astroparticle and applied fields. A potential target application is digital hadron calorimetry.

The phenomenon of avalanche-gain variations over time, particularly in Micro Pattern Gaseous Detectors (MPGD) incorporating insulator materials, have been generally attributed to electric-field modifications resulting from "charging-up" effects of the insulator. A robust methodology for characterization of gain-transients in such detectors is presented. It comprises three guidelines: detector initialization, long gain-stabilization monitoring and imposing transients by applying abrupt changes in operation conditions. Using THWELL and RPWELL detectors, we validated the proposed methodology by assessing a charging-up/charging-down model describing the governing processes of gain stabilization. The results provide a deeper insight into these processes, reflected by different transients upon abrupt variations of detector gain or the irradiation rate. This methodology provides a handle for future investigations of the involved physics phenomena in MPGD detectors comprising insulating components.

We perform a low-mass dark matter search using an exposure of 30 kg×yr with the XENON100 detector. By dropping the requirement of a scintillation signal and using only the ionization signal to determine the interaction energy, we lowered the energy threshold for detection to 0.7 keV for nuclear recoils. No dark matter detection can be claimed because a complete background model cannot be constructed without a primary scintillation signal. Instead, we compute an upper limit on the WIMP-nucleon scattering cross section under the assumption that every event passing our selection criteria could be a signal event. Using an energy interval from 0.7 keV to 9.1 keV, we derive a limit on the spin-independent WIMP-nucleon cross section that excludes WIMPs with a mass of 6 GeV/c2 above 1.4×10-41 cm2 at 90% confidence level.

The XENON1T experiment is currently in the commissioning phase at the Laboratori Nazionali del Gran Sasso, Italy. In this article we study the experiment's expected sensitivity to the spin-independent WIMP-nucleon interaction cross section, based on Monte Carlo predictions of the electronic and nuclear recoil backgrounds.The total electronic recoil background in 1 tonne fiducial volume and (1, 12) keV electronic recoil equivalent energy region, before applying any selection to discriminate between electronic and nuclear recoils, is (1.80 ± 0.15) centerdot 10−4 (kgcenterdotdaycenterdotkeV)−1, mainly due to the decay of 222Rn daughters inside the xenon target. The nuclear recoil background in the corresponding nuclear recoil equivalent energy region (4, 50) keV, is composed of (0.6 ± 0.1) (tcenterdoty)−1 from radiogenic neutrons, (1.8 ± 0.3) centerdot 10−2 (tcenterdoty)−1 from coherent scattering of neutrinos, and less than 0.01 (tcenterdoty)−1 from muon-induced neutrons. The sensitivity of XENON1T is calculated with the Profile Likelihood Ratio method, after converting the deposited energy of electronic and nuclear recoils into the scintillation and ionization signals seen in the detector. We take into account the systematic uncertainties on the photon and electron emission model, and on the estimation of the backgrounds, treated as nuisance parameters. The main contribution comes from the relative scintillation efficiency Script Leff, which affects both the signal from WIMPs and the nuclear recoil backgrounds. After a 2 y measurement in 1 t fiducial volume, the sensitivity reaches a minimum cross section of 1.6 centerdot 10−47 cm2 at mχ = 50 GeV/c2.

In-beam evaluation of a fully-equipped medium-size 30 × 30 cm² Resistive Plate WELL (RPWELL) detector is presented. It consists here of a single element gas-avalanche multiplier with Semitron ESD225 resistive plate, 1 cm² readout pads and APV25/SRS electronics. Similarly to previous results with small detector prototypes, stable operation at high detection efficiency (> 98%) and low average pad multiplicity (~ 1.2) were recorded with 150 GeV muon and high-rate pion beams, in Ne/(5%CH₄), Ar/(5%CH₄) and Ar/(7%CO₂). This is an important step towards the realization of robust detectors suitable for applications requiring large-area coverage; among them Digital Hadron Calorimetry.

We discuss recent advances in the development of cryogenic gaseous photomultipliers (GPM), for possible use in dark matter and other rare-event searches using noble-liquid targets. We present results from a 10 cm diameter GPM coupled to a dual-phase liquid xenon (LXe) TPC, demonstrating - for the first time - the feasibility of recording both primary (''S1'') and secondary (''S2'') scintillation signals. The detector comprised a triple Thick Gas Electron Multiplier (THGEM) structure with cesium iodide photocathode on the first element; it was shown to operate stably at 180 K with gains above 10⁵, providing high single-photon detection efficiency even in the presence of large α particle-induced S2 signals comprising thousands of photoelectrons. S1 scintillation signals were recorded with a time resolution of 1.2 ns (RMS). The energy resolution (σ/E) for S2 electroluminescence of 5.5 MeV α particles was ∼ 9%, which is comparable to that obtained in the XENON100 TPC with PMTs. The results are discussed within the context of potential GPM deployment in future multi-ton noble-liquid detectors.

In this work we discuss the mechanism behind the large electroluminescence signals observed at relatively low electric fields in the holes of a Thick Gas Electron Multiplier (THGEM) electrode immersed in liquid xenon. We present strong evidence that the scintillation light is generated in xenon bubbles trapped below the THGEM holes. The process is shown to be remarkably stable over months of operation, providing - under specific thermodynamic conditions - energy resolution similar to that of present dual-phase liquid xenon experiments. The observed mechanism may serve as the basis for the development of Liquid Hole Multipliers (LHMs), capable of producing local charge-induced electroluminescence signals in large-volume single-phase noble-liquid detectors for dark matter and neutrino physics experiments.

Two-phase Cryogenic Avalanche Detectors (CRADs) with THGEM multipliers, optically read out with Geiger-mode APDs (GAPDs), were proposed as potential technique for charge recording in rare-event experiments. In this work we report on the degradation of the GAPD performance at cryogenic temperatures revealed in the course of the study of two-phase CRAD in Ar, with combined THGEM/GAPD-matrix multiplier; the GAPDs recorded secondary scintillation photons from the THGEM holes in the Near Infrared. The degradation effect, namely the loss of the GAPD pulse amplitude, depended on the incident X-ray photon flux. The critical counting rate of photoelectrons produced at the 4.4 mm² GAPD, degrading its performance at 87 K, was estimated as ∼ 10⁴ s^-1. This effect was shown to result from the considerable increase of the pixel quenching resistor of this CPTA-made GAPD type. Though not affecting low-rate rare-event experiments, the observed effect may impose some limitations on the performance of CRADs with GAPD-based optical readout at higher-rate applications.

Novel high-efficiency fast-neutron detectors were suggested for fan-beam tomography applications. They combine multi-layer polymer converters in gas medium, coupled to thick gaseous electron multipliers (THGEM). Neutron-induced scattering on the converter's hydrogen nuclei results in gas ionization by the escaping recoil-protons between two successive converters. The electrons drift under the action of a homogeneous electric field, parallel to the converter-foil surfaces, towards a position-sensitive THGEM multiplying element. In this work we discuss the results of a systematic study of the electron transport inside a narrow gap between successive converter foils, which affects the performance of the detector, both in terms of detection efficiency and localization properties. The efficiency of transporting ionization electrons was measured along a 0.6 mm wide gas gap in 6 and 10 mm wide polymer converters. Computer simulations provided conceptual understanding of the observations. For drift lengths of 6 mm, electrons were efficiently transported along the narrow gas gap with minimal diffusion-induced losses; an average collection efficiency of 95% was achieved for ionization electrons induced by few keV photoelectrons. The 10 mm height converter yielded considerably lower efficiency due to electrical and mechanical flaws of the converter foils. The results indicate that detection efficiencies of ∼ 7% can be expected for 2.5 MeV neutrons with 300-foils converters, of 6 mm height, 0.4 mm thick foils and 0.6 mm gas gap.

The second Jπ=2⁺ state of C12, predicted over 50 years ago as an excitation of the Hoyle state, has been unambiguously identified using the C12(γ,α₀)Be8 reaction. The alpha particles produced by the photodisintegration of C12 were detected using an optical time projection chamber. Data were collected at beam energies between 9.1 and 10.7 MeV using the intense nearly monoenergetic gamma-ray beams at the HIγS facility. The measured angular distributions determine the cross section and the E1-E2 relative phases as a function of energy leading to an unambiguous identification of the second 2⁺ state in C12 at 10.03(11) MeV, with a total width of 800(130) keV and a ground state gamma-decay width of 60(10) meV; B(E2:22+→01+)=0.73(13)e2 fm4 [or 0.45(8) W.u.]. The Hoyle state and its rotational 2⁺ state that are more extended than the ground state of C12 presents a challenge and constraints for models attempting to reveal the nature of three alpha-particle states in C12. Specifically, it challenges the ab initio lattice effective field theory calculations that predict similar rms radii for the ground state and the Hoyle state.

The Thick Gaseous Electron Multiplier (THGEM) is a simple and robust electrode suitable for large area detectors. In this work the results of extensive comparative studies of the physical properties of different THGEM-based structures are reviewed. The focus is on newly suggested THGEM-like WELL configurations as well as on recently developed characterization methods. The WELL structures are single-sided THGEM electrodes directly coupled to different anode readout electrodes; they differ by the coupling concept of the bottom THGEM electrode to the metallic readout pads. The results are compared to that of traditional double-sided THGEM electrodes followed by induction gaps - in some cases with moderate additional multiplication within the gap. We compare the different configurations in terms of gain, avalanche extension, discharge-rate and magnitude as well as rate capabilities over a broad dynamic range - exploiting a method that mimics highly ionizing particles in the laboratory. We report on recent studies of avalanche distribution in THGEM holes using optical readout.

We present the results of first studies of the Resistive Plate WELL (RPWELL): a single-faced THGEM coupled to a copper anode via a resistive layer of high bulk resistivity. We explored various materials of different bulk resistivity (10⁹ - 10¹² Ωcm) and thickness (0.4 - 4 mm). Our most successful prototype, with a 0.6 mm resistive plate of ∼ 10⁹ Ωcm, achieved gains of up to 10⁵ with 8 keV x-ray in Ne/5%CH₄; a minor 30% gain drop occurred with a rate increase from 10 to 10⁴ Hz/mm2. The detector displayed a full "discharge-free" operation - even when exposed to high primary ionization events. We present the RPWELL detector concept and compare its performance to that of other previously explored THGEM configurations - in terms of gain, its curves, dependence on rate, and the response to high ionization. The robust Resistive Plate WELL concept is a step forward in the Micro-Pattern Gas-Detector family, with numerous potential applications.

Two-phase Cryogenic Avalanche Detectors (CRADs) with GEM and THGEM multipliers have become an emerging potential technique for charge recording in rare-event experiments. In this work we present the performance of two-phase CRADs operated in Ar and Ar+N₂. Detectors with sensitive area of 10 × 10 cm², reaching a litre-scale active volume, yielded gains of the order of 1000 with a double-THGEM multiplier. Higher gains, of about 5000, have been attained in two-phase Ar CRADs with a hybrid triple-stage multiplier, comprising of a double-THGEM followed by a GEM. The performance of two-phase CRADs in Ar doped with N₂ (0.1-0.6%) yielded faster signals and similar gains compared to the operation in two-phase Ar. The applicability to rare-event experiments is discussed.

Gaseous PhotoMultipliers (GPM) are a very promising alternative of vacuum PMTs especially for large-size noble-liquid detectors in the field of Functional Nuclear Medical Imaging and Direct Dark Matter Detection. We present recent characterization results of a Hybrid-GPM made of three Micropattern Gaseous Structures; a Thick Gaseous Electron Multiplier (THGEM), a Parallel Ionization Multiplier (PIM) and a MICROMesh GAseous Structure (MICROMEGAS), operating in Ne/CF ₄ (90:10). Gain values close to 10 ⁷ were recorded in this mixture, with 5.9 keV x-rays at 1100 mbar, both at room temperature and at that of liquid xenon (T=171 K). The results are discussed in term of scintillation detection. While the present multiplier was investigated without photocathode, complementary results of photoextraction from CsI UV-photocathodes are presented in Ne/CH ₄ (95:5) and CH ₄ in cryogenic conditions.

Preliminary results of a photon detector combining a Micromegas-like multiplier coated with a UV-sensitive CsI photocathode are described. The multiplier is made in a CMOS compatible InGrid technology, which allows to postprocess it directly on the surface of an imaging IC. This method is aimed at building light-sensitive imaging detectors where all elements are monolithically integrated. We show that the CsI photocathode deposited in the InGrid mesh does not alter the device performance. Maximum gains of ∼6000 were reached in a single-grid element operated in Ar/CH₄, with a 2% ion backflow fraction returning to the photocathode.

A new concept for avalanche ion back-flow (IBF) reduction in cascaded gaseous detectors is presented. The Thick-COBRA is a Thick-GEM (THGEM) with a patterned electrode, aiming at reducing secondary effects by trapping avalanche ions. The patterned electrode faces the electron drift region, trapping ions flowing back from the cascaded multiplier and those produced within the Thick-COBRA. Total IBF values of about 5% were obtained, about six times below that reached with a THGEM in a triple-THGEM configuration, keeping full single-electron detection efficiency. Experimental and simulation studies of IBF and electron collection/transmission efficiencies are presented; the suitability for application of Thick-COBRA in photon detectors for RICH is discussed.

Characteristic properties of a Geiger Mode APD (G-APD) in a THGEM-based cryogenic two-phase Ar avalanche detector were studied in view of potential applications in rare-event experiments. G-APD signal amplitude and noise characteristics at cryogenic temperatures turned out to be superior to those at room temperature. The effective detection of avalanche scintillations from THGEM-multiplier holes in two-phase Ar has been demonstrated using a G-APD without wavelength shifter. At an avalanche gain of 60, the avalanche scintillation yield measured by the G-APD was as high as 0.9 photoelectrons per avalanche electron, extrapolated to 4π acceptance.

The CERES experiment has measured inclusive photon production in S-Au collisions of 200 GeV/nucleon at the CERN SPS. No evidence for direct emission of photons was found. For the kinematic region 2.1

We describe the concept and properties of a time-resolved integrative optical neutron (TRION) detector, a novel high spatial resolution neutron imaging system in the energy range of 110 MeV, for fast-neutron resonance radiography (FNRR), with multiple-energy TOF-spectrometry capability. Two generations of TRION detectors have already demonstrated their suitability for detecting small quantities of thin-sheet explosives. TRION holds promise for fully automatic detection and identification of standard and improvised explosives concealed in luggage and cargo, by determining the density distribution of light elements such as C, N and O.

We report on the construction, tests, calibrations and commissioning of an Optical Readout Time Projection Chamber (O-TPC) detector operating with a CO₂(80%) + N₂(20%) gas mixture at 100 and 150 Torr. It was designed to measure the cross sections of several key nuclear reactions involved in stellar evolution. In particular, a study of the rate of formation of oxygen and carbon during the process of helium burning will be performed by exposing the chamber gas to intense nearly mono-energetic gamma-ray beams at the High Intensity Gamma Source (HIγS) facility. The O-TPC has a sensitive target-drift volume of 30x30x21 cm³. Ionization electrons drift towards a double parallel-grid avalanche multiplier, yielding charge multiplication and light emission. Avalanche-induced photons from N₂ emission are collected, intensified and recorded with a Charge Coupled Device (CCD) camera, providing two-dimensional track images. The event's time projection (third coordinate) and the deposited energy are recorded by photomultipliers and by the TPC charge-signal, respectively. A dedicated VME-based data acquisition system and associated data analysis tools were developed to record and analyze these data. The O-TPC has been tested and calibrated with 3.183 MeV alpha-particles emitted by a ¹⁴⁸Gd source placed within its volume with a measured energy resolution of 3.0%. Tracks of alpha and ¹²C particles from the dissociation of ¹⁶O and of three alpha-particles from the dissociation of ¹²C have been measured during initial in-beam test experiments performed at the HIγS facility at Duke University. The full detection system and its performance are described and the results of the preliminary in-beam test experiments are reported.

Recent progress is described in gaseous photon detectors combining bialkali photocathodes and cascaded patterned gas-avalanche electron multipliers. Efficient avalanche-ion blocking permitted the development and feasibility demonstration of high-gain operation in continuous mode, of gaseous photomultipliers sensitive to single photons in the visible spectral range.

A novel concept of optical signal recording in cryogenic two-phase avalanche detectors, with Geiger-mode Avalanche Photodiodes (G-APD) measuring avalanche-scintillation photons in a thick Gas Electron Multiplier (THGEM), has been studied in view of its potential applications in rare-event experiments. The effective detection of avalanche scintillations in THGEM holes has been demonstrated in two-phase Ar with a bare G-APD without wavelength shifter, i.e. insensitive to VUV emission of Ar. At gas-avalanche gain of 400 and under ±70° viewing-angle, the G-APD yielded 640 photoelectrons (pe) per 60 keV X-ray converted in liquid Ar; this corresponds to 0.7 pe per initial (prior to multiplication) electron. The avalanche-scintillation light yield measured by the G-APD was about 0.7 pe per avalanche electron, extrapolated to 4π acceptance. The avalanche scintillations observed occurred presumably in the near infrared (NIR) where G-APDs may have high sensitivity. The measured scintillation yield is similar to that observed by others in the VUV. Other related topics discussed in this work are the G-APD's single-pixel and quenching resistor characteristics at cryogenic temperatures.

The article deals with the detection efficiency of UV-photon detectors consisting of Thick Gas Electron Multipliers (THGEM) coated with CsI photocathode, operated in atmospheric Ne/CH₄ and Ne/CF₄ mixtures. We report on the photoelectron extraction efficiency from the photocathode into these gas mixtures, and on the photoelectron collection efficiency into the THGEM holes. Full collection efficiency was reached in all gases investigated, in some cases at relatively low multiplication. High total detector gains for UV photons, in excess of 10⁵, were reached at relatively low operation voltages with a single THGEM element. We discuss the photon detection efficiency in the context of possible application to RICH.

The present work deals with the analysis of prostatic-zinc-concentration images. The goal is to evaluate potential clinically relevant information that can be extracted from such images. In the absence of experimental images, synthetic ones are produced from clinically measured zinc-concentration distributions in certified benign and cancerous tissue samples, classified by the lesion grade. We describe the method for producing the images and model the effect of counting statistics noise. We present in detail the image analysis, which is based on a combination of standard image processing and segmentation tools, optimized for this particular application. The information on lowest zinc value obtained from the image analysis is translated to clinical data such as tumour presence, location, size and grade. Their confidence is evaluated with the help of standard statistical tools such as receiver operating characteristic analysis. The present work predicts a potential for detecting small prostate-cancer lesions, of grade (4+3) and above, with very good specificity and sensitivity. The present analysis further provides data on the pixel size and image counting statistics requested from the trans-rectal probe that will record in vivo prostatic-zinc maps in patients.

We briefly review the concept and properties of the THick Gaseous Electron Multiplier (THGEM); it is a robust, high-gain gaseous electron multiplier, manufactured economically by standard printed-circuit drilling and etching technology. Its operation and structure resemble that of gaseous electron multiplier's (GEM's) but with 5-20-fold expanded dimensions. The millimeter-scale hole-size results in good electron transport and in large avalanche-multiplication factors, e.g. reaching 10⁷ in double-THGEM cascaded single-photoelectron detectors. The multiplier's material, parameters and shape can be application-tailored; it can operate practically in any counting gas, including noble gases, over a pressure range spanning from 1 mbar to several bars; its operation at cryogenic (LAr) conditions was recently demonstrated. The high gain, sub-millimeter spatial resolution, high counting-rate capability, good timing properties and the possibility of industrial production capability of large-area robust detectors, pave ways towards a broad spectrum of potential applications; some are discussed here in brief.

We shortly describe recent progress in photon detectors combining bi-alkali photocathodes and cascaded patterned gas-avalanche electron multipliers. It permitted the development and the first feasibility demonstration of high-gain gaseous photomultipliers sensitive in the visible spectral range, operated in continuous-mode with single-photon sensitivity.

Efforts are being made in direct Dark Matter search experiments to detect the primary ionisation in the liquid by extracting the electrons to the gas phase and use the secondary ionization produced in the micropattern electron multipliers for signal amplification in noble-liquid dual-phase TPCs. We have studied the secondary scintillation yield of a single Gas Electron Multiplier (GEM) and of a Micro-Hole & Strip Plate (MHSP) for xenon at room temperature. Values for secondary scintillation yield between 5.0 × 10³ and 1.3 × 10³ photons per primary electron were obtained for the GEM and between 7.2 × 10⁴ and 1.8 × 10³ photons per primary electron for the MHSP, as the pressure increased from 1.0 to 2.5 bar in the GEM-setup and from 1.0 to 3.3 bar in the MHSP-setup, respectively. These values can be more than one order of magnitude higher than what has been obtained in the uniform-field scintillation gaps of the XENON and ZEPLIN-III experiments. Although in the present setups the amount of secondary scintillation obtained is sufficient in view of the use of PMTs, if a different type of readout is considered, such as large area avalanche photodiodes, it will be important to increase the amount of secondary scintillation. The attained results demonstrate the clear advantage of reading the secondary scintillation instead of the charge produced in the electron avalanches of micropattern electron multipliers, in low-background and low-rate experiments, as is the case in direct Dark Matter search.

The properties of the THick Gas Electron Multiplier (THGEM) operated with soft X-rays in Kr at room temperature, are presented. Charge gains ranging from 10⁴ to 3 × 10² and from 4 × 10⁴ to 10³ were reached with single- and double-THGEM detectors in a pressure range of 0.5-3.0 bar, respectively, for operation voltages in the range of 800-2600 V. The gain was limited by photon- and ion-feedback effects, and by electron field-emission from the Cu-electrodes at THGEM operation voltages above 2000 V. Energy resolutions in the range of 21-34% FWHM were measured with 5.9 keV X-rays. Other characteristics, including pulse-shape and electric fields in THGEM structures, are discussed.

A novel concept for ion blocking in gas-avalanche detectors was developed, comprising cascaded micro-hole electron multipliers with patterned electrodes for ion defocusing. This leads to ion blocking at the 10^{- 4} level, in DC mode, in operation conditions adequate for TPCs and for gaseous photomultipliers. The concept was validated in a cascaded visible-sensitive gas-avalanche photomultiplier operating at atmospheric pressure of Ar / CH₄ (95/5) with a bi-alkali photocathode. While in previous works high gain, in excess of 10⁵, was reached only in a pulse-gated cascaded-GEM gaseous photomultiplier, the present device yielded, for the first time, similar gain in DC mode. We describe shortly the physical processes involved in the charge transport within gaseous photomultipliers and the ion blocking method. We present results of ion back-flow fraction and of electron multiplication in cascaded patterned-electrode gaseous photomultiplier with K-Cs-Sb, Na-K-Sb and Cs-Sb visible-sensitive photocathodes, operated in DC mode.

A tracking ion counting nanodosimeter was employed to acquire spatial ionization patterns produced by charged particles in propane gas at 1.3 mbar. Data were taken at the James M. Slater MD Proton Treatment and Research Center with 250 MeV, 17 MeV, 5 MeV and 1.5 MeV proton beams, 4.8 MeV alpha particles and electrons from a Sr-90/Y-90 source. For each particle type, measurable quantities used for track structure reconstruction included the number of ionizations and their location within a wall-less, cylindrical sensitive volume measured with a resolution of about 5 tissue-equivalent nanometers, and primary particle coordinates. Measured ionization frequency distributions as a function of distance from particle track were compared with results of a dedicated Monte Carlo track structure code.

We review latest progress in gaseous photomultipliers (GPMs) combining solid photocathodes and various types of novel electron multipliers. Cascaded gaseous electron multipliers (GEMs) coated with CsI photocathodes can efficiently replace UV-sensitive wire chambers for single-photon recording in Cherenkov and other detectors. Other hole-multipliers with patterned electrodes (Micro-Hole and Strip Plates) and improved ion-blocking properties are discussed; these permit reducing considerably photon- and ion-induced secondary effects. Photon detectors with other electron-multiplier techniques are briefly described, among them GPMs are based on Micromegas, capillary plates, Thick-GEMs and resistive Thick-GEMs. The two latter techniques, robust and economically produced, are particularly suited for large-area GPM applications, e.g. in RICH. Cascaded hole-multipliers with very high ion-blocking performance permitted the development and the first demonstration of visible-sensitive GPMs operated in continuous mode, with bialkali photocathodes and single-photon sensitivity. Recent progress is described in GPMs operating at cryogenic temperatures for rare-event noble-liquid detectors and medical imaging.

A newly developed experimental system for measurements of electron emission induced by low energy ions at below low ion flux (10⁵ ions s ^-1 cm^-2) avoiding the fast emission degradation caused by high ion fluxes (usually applied 10¹⁰-10¹³ ions s ^-1 cm^-2) has been established. The method, based on an event-by-event recording of the emitted cluster of electrons for each impinging ion event, overcomes the difficulty of measurement of reliable ion-induced secondary electron emission (IIEE) yield, associated with fast degradation of the electron yield. This method provides not only the average number (γ) of IIEE under a given ionic impact, but actually higher moments of their distribution, which contains more information on the physical processes involved in the IIEE phenomenon. The secondary electron emission coefficients showed a dependence on the kinetic energy of incident argon ions and protons ranging from 1 keV to 10 keV on hydrogenated diamond film surfaces.

We present the results of our recent studies of a Thick Gaseous Electron Multiplier (THGEM)-based detector, operated in Ar, Xe and Ar:Xe (95:5) at various gas pressures. Avalanche-multiplication properties and energy resolution were investigated with soft x-rays for different detector configurations and parameters. Gains above 10⁴ were reached in a double-THGEM detector, at atmospheric pressure, in all gases, in almost all the tested conditions; in Ar:Xe (95:5) similar gains were reached at pressures up to 2 bar. The energy resolution dependence on the gas, pressure, hole geometry and electric fields was studied in detail, yielding in some configurations values below 20% FWHM with 5.9 keV x-rays.

The operation of the recently introduced Photon Assisted Cascaded Electron Multiplier (PACEM) in CF₄ is investigated. The PACEM uses the VUV scintillation produced in the electron avalanches of the first multiplier of the cascade to transfer the signal to the subsequent ones. The VUV scintillation induces the emission of a large number of photoelectrons from a CsI photocathode placed on the top-surface of the second multiplier. The photoelectrons are further multiplied in the subsequent stages of the cascade, resulting in efficient signal amplification. A mesh, set at a fixed voltage, is placed between the first and the second multipliers to block the charge transfer between them, thus suppressing all the ion backflow (IBF) heading to the first cascade element. The PACEM electrically isolates the first multiplier of the cascade and only the ions produced in the electron avalanches of the first element may flow back into the drift region. Operating in CF₄, absolute IBFs as low as ∼1 ion per primary electron are achieved for drift fields of 0.1 kV/cm, while for fields of 0.5 kV/cm the absolute IBF is ∼10 ions/primary electron. This corresponds to an IBF of 10^-4 at gains of 10⁴, for TPC operating conditions, and IBFs of ∼10^-5 at gains of 10⁶ for GPM operating conditions.

We present the Photon-Assisted Cascaded Electron Multiplier (PACEM) as a potential alternative for ion back-flow suppression in gaseous cascade electron multipliers. Using a Micro Hole and Strip Plate-Gas Electron Multiplier (MHSP-GEM) configuration, the number of ions flowing back to the scintillation region is about 1.5 ions per primary electron at an optical gain of 6.5 and a drift field of 0.1 kV/cm, and about 10 ions per primary electron at an optical gain of 10 and a drift field of 0.5 kV/cm. These allow reaching ion back-flow values close to 10^-4 and 10^-5 at typical operation conditions of TPCs and GPMs, respectively.

The performance of a new VUV photosensor, based on an micro-hole and strip plate (MHSP) electron multiplier with a CsI photocathode deposited on its top electrode, is described. This photosensor presents gains above 10⁴ when operating in an Ar-5%Xe gas mixture at 1 atm. Although the gain of a single MHSP is not sufficient for efficient detection of single photons, this simple UV photosensor may be useful for the detection of higher light levels, such as primary- and secondary scintillation in noble gases.

We report the performance of the Micro-Hole-and-Strip-Plate (MHSP) operated in xenon, under high-flux irradiation of a 30-kV X-ray Cu-tube. Stable operation in pulse-counting mode for gas gains above 10⁴ was measured up to counting rates of 5×10⁵ Hz/mm². A gain variation of less than 5% was observed for gains of 10⁴, while the energy resolution for 8-keV X-rays degrades from ∼16% to ∼18%. No visible degradation was observed on the MHSP irradiated area.

A newly developed experimental system enables measurements of IIEE at a very low ion flux (10⁵ ions/s cm²) avoiding the fast emission degradation caused by high ion fluxes (usually applied 10¹⁰-10¹³ ions/s cm²). The method overcomes the difficulty of measurement of reliable ion-induced secondary electron emission (IIEE) yield, associated with fast degradation of the IIEE yield. We report on the investigation of the IIEE from hydrogenated undoped and B-doped diamond films as a function of (i) moderate heating in vacuum prior to the measurements, (ii) H⁺ and Ar⁺ energy in the range of 1-10 KeV, and (iii) film thickness and microstructure. An IIEE yield (γ) enhancement was typically detected when the films were heated to 300 °C in vacuum. In the B-doped diamond film heated to 300 °C, γ rose nearly linearly from ∼ 20 to ∼ 100 electrons/ion, for 1 to 10 KeV Ar⁺ ions, respectively. The values of γ obtained with H⁺ showed a more moderate, nonlinear increase from ∼ 8 electrons/ion at 1 KeV up to ∼ 90 electrons/ion at 10 KeV. In heated undoped diamond films of different thicknesses the measured values of γ were similar for all the studied films and somewhat lower than in B-doped film: from ∼ 10-16 to 60-70 electrons per 1 to10 KeV Ar⁺, respectively, and from 14-26 to 50-60 electrons per 1 to10 KeV H⁺, respectively. The experimental results were interpreted using TRIM calculations.

A new imaging method that combines high-efficiency fast-neutron detection with sub-ns time resolution is presented. This is achieved by exploiting the high neutron detection efficiency of a thick scintillator and the fast timing capability and flexibility of light-pulse detection with a dedicated image intensifier. The neutron converter is a plastic scintillator slab or, alternatively, a scintillating fibre screen. The scintillator is optically coupled to a pulse counting image intensifier which measures the 2-dimensional position coordinates and the Time-Of-Flight (TOF) of each detected neutron with an intrinsic time resolution of less than 1 ns. Large-area imaging devices with high count rate capability can be obtained by lateral segmentation of the optical readout channels.

We present our studies of prostatic Zn concentration measurements, carried out in the light of a novel prostate cancer (CAP) diagnosis method proposed by us. The method is based on in vivo prostatic Zn mapping by XRF trans-rectal probe. We report on the extensive clinical studies, intended to assess the validity of the novel proposed diagnostic method. Zn content was measured in vitro in needle-biopsy samples from several hundreds of patients, and was correlated with histological findings and other patient parameters. For this purpose, a technique of absolute Zn content determination in ∼1 mm³ fresh tissue samples by XRF was developed. The experimental details and the main clinical-evaluation results are presented. We further outline the suggested design of the XRF trans-rectal probe for an efficient in vivo detection and mapping of the Zn fluorescence radiation from the prostate through the rectal wall. Laboratory phantom studies, a preliminary design concept and its expected performance are also reported.

Nanodosimetric spectra, measured in a well-defined ionisation sensitive volume of an ion-counting gaseous nanodosemeter, may have a valuable predictive value of radiation damage to DNA. In such devices, the distributions of radiation-induced ions are measured after their drift in gas. The sensitive-volume size, corresponding to a DNA segment length, can be tuned by selecting an appropriate time window for ion counting; the method's accuracy depends on the velocity distribution of the drifting ions. The results of ion-drift measurements in an ion-counting nanodosemeter were used for the precise calculation of its sensitive volume length. Monte Carlo simulations of nanodosimetric spectra, performed with the obtained data, are in good agreement with experimental data. The method's limitations, arising from the spread of drift velocities, are discussed.

An ion-counting nanodosemeter (ND) yielding the distribution of radiation-induced ions in a low-pressure gas within a millimetric, wall-less sensitive volume (SV) was equipped with a silicon microstrip telescope that tracks the primary particles, allowing correlation of nanodosimetric data with particle position relative to the SV. The performance of this tracking ND was tested with a broad 250 MeV proton beam at Loma Linda University Medical Center. The high-resolution tracking capability made it possible to map the ion registration efficiency distribution within the SV, for which only calculated data were available before. It was shown that tracking information combined with nanodosimetric data can map the ionisation pattern of track segments within 150 nm-equivalent long SVs with a longitudinal resolution of ∼5 tissue-equivalent nanometers. Data acquired in this work were compared with results of Monte Carlo track structure simulations. The good agreement between 'tracking nanodosimetry' data acquired with the new system and simulated data supports the application of ion-counting nanodosimetry in experimental track-structure studies.

We present a Photon-Assisted Cascaded Electron Multipliers (PACEM) which has a potential for ion back-flow blocking in gaseous radiation detectors: the avalanche from a first multiplication stage propagates to the successive one via its photons, which in turn induce photoelectron emission from a photocathode deposited on the second multiplier stage; the multiplication process may further continue via electron-avalanche propagation. The photonmediated stage allows, by a proper choice of geometry and fields, complete blocking of the ion back-flow into the first element; thus, only ions from the latter will flow back to the drift region. The PACEM concept was validated in a double-MHSP (Micro-Hole & Strip Plate) cascaded multiplier operated in xenon, where the intermediate scintillation stage provided optical gain of ∼ 60. The double-MHSP detector had a total gain above 10⁴ and energy resolution of 18% FWHM for 5.9 keV x-rays.

We report on the performance of a Micro-Hole & Strip Plate (MHSP) electron multiplier operating in pure Xe, Kr, Ar and Ne at the pressure range of 1 to 6 bar. The maximal gains at 1 bar Xe and Kr are 5×10⁴ and 10⁵, respectively; they drop by about one order of magnitude at 2 bar and by almost another order of magnitude at 5-6 bar; they reach gains of 500 and 4000 at 5 bar in Xe and Kr, respectively. In Ar, the gain varies very little with pressure, being 3 - 9 ×10³; in Ne the maximum attainable gain, about 105, is pressure independent above 2 bar. The results are compared with that of single- and triple-GEM multipliers operated in similar conditions. Potential applications are in hard X-ray imaging and in cryogenic radiation detectors.

We present a review on the research carried out with the Micro-Hole & Strip plate (MHSP) gaseous electron multiplier. In this new device charge multiplication occurs in two stages: radiation-induced electrons are multiplied in small holes and the resulting avalanche is further multiplied on anode strips patterned on the multiplier's bottom electrode. This structure provides fast multiplication process, has a high total gain even in noble gas mixtures and provides good localization properties. Detectors based on this principle have shown gains above 10⁴ and energy resolution of about 14% for 5.9-keV X-rays; large gains were obtained at high gas pressures as well, allowing for efficient detection of higher-energy X-rays. The properties of UV-photon detectors comprising an MHSP coupled to semitransparent CsI photocathodes or with reflective ones deposited on the multiplier's top surface were investigated. The operation mechanism of these detectors and their most significant results are presented.

We report on the properties of thick GEM-like (THGEM) electron multipliers made of 0.4 mm thick double-sided Cu-clad G-10 plates, perforated with a dense hexagonal array of 0.3 mm diameter drilled holes. Photon detectors comprising THGEMs coupled to semi-transparent CsI photocathodes or reflective ones deposited on the THGEM surface were studied with Ar/CO2 (70:30), Ar/CH4 (95:5), CH4 and CF4. Gains of ∼105 or exceeding 106 were reached with single- or double-THGEM, respectively; the signals have 5-10 ns rise times. The electric field configurations at the THGEM electrodes result in an efficient extraction of photoelectrons and their focusing into the holes; this occurs already at rather low gains, below 100. These detectors, with single-photon sensitivity and with expected sub-millimeter localization, can operate at MHz/mm2 rates. We discuss their prospects for large-area UV-photon imaging for RICH.

A new mathematical method of measuring electron emission induced by low energy ions from solids is described and used to calculate secondary electron emission according to the recorded pulse-height spectra of ions and ultraviolet (UV) photons. Using the UV single secondary electron spectra, we predict the shape of many secondary electron distributions under consideration of detection efficiency of MCP detector. These calculated distributions allow us to characterize the secondary electrons yield, and to give a secondary electron distribution for measured data. It seems rather feasible to determine secondary electron yield emitted by low energy ions at very low ion fluxes.

One of our two methods for fast-neutron imaging with spectrometric capability is presented here. It is a neutron-counting technique based on a hydrogenous neutron converter coupled to Gaseous Electron Multipliers (GEM). The principles of the detection techniques and the optimization of the converter, electron amplification and the readout are described. Evaluation of the properties is derived from a experiment in a pulsed neutron beam of spectral distribution between 2 and 10 MeV.

In this work we report on absolute quantum photoyield (QPY) measurements in the 140-220 nm (8.85-5.63 eV) photon energy range of nanometer thick diamond films. The QPY value at a photon wavelength of 140 nm increases with film deposition time from 2.5% (carbonized silicon substrate) to 14% for a nominal film thickness of 70 nm, followed by a decrease to 12% and stabilization for continuous films of thicknesses above ∼150-200 nm. Prior to QPY measurements the film's surface was conditioned by exposure to microwave hydrogen plasma, to induce negative electron affinity. The surface properties, phase composition and microstructure of the films were examined by electron spectroscopic and microscopic methods. Different factors are considered to explain the enhancement of QPY of the nano-metric thick films.

We present results from our recent investigations on detectors comprising cascaded gas electron multipliers (GEMs) and cascaded GEMs with microhole and strip plate (MHSP) multiplier as a final amplification stage. We discuss the factors governing the operation of these fast radiation-imaging detectors, which have single-electron sensitivity. The issue of ion-backflow and ion-induced secondary effects is discussed in some detail, presenting ways for its suppression. Applications are presented in the fields of photon imaging in the ultraviolet-to-visible spectral range as well as X-ray and neutron imaging.

We present the performance of a Micro-Hole and Strip Plate (MHSP) electron multiplier in argon-xenon mixtures, at pressures of 1-7 bar. This microstructure can operate at high pressures without significant reduction of the maximum achievable gain. Absolute gains of 1-4 × 10³ were reached in Ar/50mbar Xe over this pressure range; the maximum gain is imposed by the discharge limit, dropping at higher pressures. Energy resolutions between 14% and 16% were reached for 6 keV X-rays; they do not degrade significantly with increasing pressure. Better performances are expected by improved manufacturing of the MHSP.

The microhole and strip plate (MHSP) gas electron multiplier was studied as a stand-alone device or in combination with a cascade of gas electron multipliers (GEMs), for X-ray and UV-photon detection. An MHSP operating in Ar/5%Xe yielded gains above 10⁴ and energy resolutions of about 14% FWHM for 5.9-keV X-rays. Gains as high as 10⁷ were reached in a 3-GEM/MHSP gaseous photomultiplier operating in an Ar/5%CH₄; the ion back-flow ratio to the top of the first GEM could be reduced to ∼2%. Two-dimensional (2-D) imaging was performed using signals induced by avalanche ions on a wedge and strip (W&S) readout cathode, placed at close proximity to the anode strips; position resolution of 200-250 μm FWHM for 5.9-keV X-rays was measured.

We report on avalanche-ion back-flow measurements in the novel Micro-Hole and Strip-Plate (MHSP) multiplier and in gaseous photomultipliers comprising Gas Electron Multipliers (GEMs) followed by an MHSP. In a 3-GEMs/MHSP photomultiplier with reflective photocathode, avalanche-ion back-flow fraction of ∼7% and ∼2% were recorded for respective effective gains of 10 ⁷ and 10⁶, in Ar/CH₄ (95/5) at 760Torr. This is about one order of magnitude reduction in ion back-flow compared to the best values measured in 4-GEMs photomultiplier at the same gain. We describe the mode of operation of the MHSP and explain its ion back-flow reduction features.

This paper describes a phantom-based feasibility study for a potential in vivo determination of zinc in prostate, which could bring about improved diagnosis of prostate cancer. An x-ray fluorescence topographic technique was developed, which will permit determination of the Zn content in the prostate through the rectum, namely behind a 2-3 mm thick layer of the rectal wall. The topographic approach, together with a reconstruction method developed here, minimizes the interference of Zn from non-prostatic tissue. The phantom studies show that it will be possible to determine Zn in a prostatic compartment behind a few mm thick layer of tissue using a specially designed transrectal probe. Such a probe is currently under development in our laboratories.

We present results from our recent investigations on detectors comprising cascaded gas electron multipliers (GEM) and cascaded GEMs with micro-hole and strip (MHSP) electrodes as a final amplification stage. We discuss the factors governing the operation of these fast radiation-imaging detectors, which have single-charge sensitivity. The issue of ion-backflow and ion-induced secondary effects is discussed in some detail, presenting ways for its suppression. Applications are presented in the fields of photon imaging in the UV-to-visible spectral range as well as x-ray and neutron imaging.

Purpose: In cancer affected prostate cells lose the ability to concentrate zinc, resulting in a substantial decrease in Zn in the prostate. We investigated the possibility of using prostatic zinc combined with prostate specific antigen (PSA) as a novel tool for the reliable diagnosis of prostate cancer. Materials and Methods: Using the x-ray fluorescence method the Zn concentration was determined in vitro in prostate samples extracted by surgery from 28 patients. Clinical records included age, serum PSA, sextant prostate needle biopsy, previous medical therapy, surgical procedure and histological findings. Results: A new relationship was found between Zn in prostate tissue and PSA in blood, which allows improved separation between prostate cancer and benign prostate hyperplasia, and might have a significant impact on the reliable diagnosis of prostate cancer. Conclusions: Zn concentration is not uniform even in the same anatomical region of the prostate, so that a number of measurements at various locations are required for a diagnostic procedure. The most interesting finding in this study is the relationship between Zn concentration and PSA. A combination of these parameters represents a significant improvement on the diagnostic value of each of them separately and provides a powerful tool for more accurate diagnosis. Although the method may be applied in vitro on biopsy samples, our study underlines the importance of developing a facility for in vivo Zn determination in the prostate.

We summarize our recent advances in gaseous photomultipliers (GPMTs) for the UV and visible spectral range. They combine photocathodes and advanced multi-stage Gas Electron Multipliers (GEMs). Principles of operation and properties are discussed, with emphasis on time and localization resolutions, ion-feedback suppression, novel electron multipliers and sealed photon detectors for visible light.

We report on the performance of a new gaseous electron multiplier: the Micro-Hole & Strip Plate (MHSP). It consists of two independent charge-amplification stages in a single, double-sided micro-structured electrode, deposited on a thin insulating substrate. Charge gains in excess of 10³ were obtained in a MHSP operated with soft X-rays in Ar/CO₂ (70/30). We present the results of a systematic study of the MHSP properties and those of a double-stage GEM+MHSP multiplier. Applications to gaseous photomultipliers are discussed.

We discuss a new thermal-neutron imaging detector that combines a neutron converter coupled to a position-sensitive gaseous photodetector. The neutron converter consists of a thin (0.5mm) layer of boron-rich liquid scintillator. It is viewed by an atmospheric-pressure, gas-avalanche photomultiplier with a bialkali photocathode. Scintillation-induced photoelectrons are multiplied by a cascade of Gas Electron Multipliers (GEM). The multi-GEM supplies the pulse-height, time and position information for each converted neutron. Such a fast, large-area detector can operate at high radiation flux.

The transport of low-energy (

The properties of a 3-GEM (Gas Electron Multiplier) element photomultiplier, with a semitransparent CsI photocathode and CF₄ gas filling, are presented. Compared to other gas mixtures, such as CH₄, Ar/CH₄, Ar/N₂ and He/Ar/N₂, CF₄ has superior performance: the highest gain, approaching 10⁷, the fastest, 8 ns wide signal and the lowest photoelectron backscattering; the latter allows to reach photocathode quantum efficiency values approaching that in vacuum. The time resolution of the multi-GEM photomultiplier for single photoelectrons was measured to be 2 ns. These properties are of high relevance for applications in Cherenkov detectors and in tracking devices.

We present the performance of a sealed gaseous photomultiplier consisting of a cascade of 3 or 4 Gas Electron Multiplier (GEM) elements coupled to a semitransparent CsI photocathode, in Ar/CH₄ (95/5). A few-month stability study of the photocathode in a sealed mode is presented. Increasing the number of GEMs in cascade substantially reduces the ageing of the detector under strong irradiation. The ion feedback to the photocathode has probably a minor effect on the ageing rate.

We report on the improvements in the position sensitive readout of a xenon-filled gas scintillation proportional counter. Using an imaging optic for UV-light in the region of 170 nm, the position resolution could be improved by more than 30%. In addition, we have obtained first encouraging results for the use of the recently developed gas electron multiplier together with a CsI-photocathode as a large area UV-detector system.

We report on the efficient operation of a CsI-coated GEM photon detector. We describe its operation mode and the dependence of the single electron detection efficiency on the electric fields. Conditions for obtaining full efficiency of photoelectron extraction and their focusing into the GEM apertures, in 1 atmCH₄, are presented. The quantum efficiency of the CsI-coated GEM is 35% at 150 nm.

The effect of moderate heating (200-300°C) in vacuum on the photoemission from air-exposed hydrogen-terminated chemical vapor deposited diamond films was studied in the photon spectral range of 140-210 nm (8.9-5.9 eV). A three- to fivefold enhancement was observed, stable in high vacuum and in some high purity gases, but unstable in air. The surfaces were also examined by x-ray induced photoelectron spectroscopy and ultraviolet induced photoelectron spectroscopy before and after the heating process and upon exposure to air and to oxygen. The results provide good evidence that the strong dipole originating from H₂O molecules absorbed on the diamond surface is responsible for the observed effect. A simple model is presented for quantitative estimation of the effect.

Ultrananocrystalline diamond (UNCD) films 0.1-2.4 μm thick were conformally deposited on sharp single Si microtip emitters, using microwave CH₄-Ar plasma-enhanced chemical vapor deposition in combination with a dielectrophoretic seeding process. Field-emission studies exhibited stable, extremely high (60-100 μA/tip) emission current, with little variation in threshold fields as a function of film thickness or Si tip radius. The electron emission properties of high aspect ratio Si microtips, coated with diamond using the hot filament chemical vapor deposition (HFCVD) process were found to be very different from those of the UNCD-coated tips. For the HFCVD process, there is a strong dependence of the emission threshold on both the diamond coating thickness and Si tip radius. Quantum photoyield measurements of the UNCD films revealed that these films have an enhanced density of states within the bulk diamond band gap that is correlated with a reduction in the threshold field for electron emission. In addition, scanning tunneling microscopy studies indicate that the emission sites from UNCD films are related to minima or inflection points in the surface topography, and not to surface asperities. These data, in conjunction with tight binding pseudopotential calculations, indicate that grain boundaries play a critical role in the electron emission properties of UNCD films, such that these boundaries: (a) provide a conducting path from the substrate to the diamond-vacuum interface, (b) produce a geometric enhancement in the local electric field via internal structures, rather than surface topography, and (c) produce an enhancement in the local density of states within the bulk diamond band gap.

Modern gas avalanche detectors are instruments of choice for detecting single charges deposited in gas or emitted from thin solid radiation converters. We discuss principal factors governing the operation of gas avalanche photomultipliers, combining solid photocathodes with advanced micro-pattern gaseous multipliers and summarize the properties of UV photocathodes and film-protected photocathodes for the visible spectral range. We review recent progress and applications of single-charge counting detectors and discuss in some detail their application to nanodosimetry and its relevance to studies of radiation damage to DNA.

We report on the photoemission properties of 300 angstroms thick transmissive- and 5000 angstroms thick reflective UV-sensitive CsBr photocathodes. Following post-evaporation heat treatment at 70 °C the absolute quantum efficiency is 35% at 150 nm, with a red boundary cut-off at about 195 nm. Extensive aging studies of CsBr and CsI photocathodes, under high photon flux and under ion bombardment in gas avalanche multiplication mode, were carried out for the first time without exposure to air. The results are compared with the previously published data on CsI aging and the methodology of the aging tests is discussed in details.

We present the results of our further investigation of the Gas Electron Multiplier (GEM) photomultiplier consisting of a solid CsI photocathode coupled to a cascade of GEMs. It operates with mixtures of pure noble gases or with noble gases having small additives of nitrogen or methane. The GEM photomultiplier (GPM) reaches multiplication factors of 10⁵-10⁶, allowing for detection of single photons. We discuss the limitations of the GPM, imposed by ion feedback and charging-up of the GEM elements, and some phenomena related to the avalanche development in such devices.

The single-ion detection efficiency of two types of electron multipliers is provided for incident energies reaching 10 keV. Absolute efficiencies above 90% were measured for a discrete dynode electron multiplier (DDEM), in response to Ar⁺ ions and protons, above an energy of 5 keV. The efficiency to detect protons is consistently higher than that of Ar⁺ ions, due to a higher secondary electron emission yield. For a channel electron multiplier we have measured a similar maximal detection efficiency for Ar⁺ ions; though strongly varying across the detector surface.

We report on our latest results of K-Cs-Sb visible photocathodes, coated with thin CsI and CsBr protective films, for applications within gas avalanche photomultipliers. Data on the sensitivity to exposure to oxygen and moisture is presented, as well as on the stability under gas multiplication and aging under gas avalanche. There are good indications that the protected photocathodes can withstand long-term multiplication mode, with low-level impurity gases. Possible applications are discussed.

The absolute quantum photoyield (QPY) of polycrystalline diamond films in the range of 140-210 nm is reported, for undoped and B-doped films, as a function of deposition conditions and postgrowth surface treatment. B-doping, as well as geometrical structure, crystalline size and quality of the deposited films did not affect the photoemission properties, whereas exposure to microwave (MW) hydrogen plasma significantly improved the QPY to more than 12% at 140 nm, compared to 4-6% measured for untreated films deposited by hot filament chemical vapor deposition (HFCVD) at temperatures of ≥900 °C. Undoped diamond films deposited by the MW plasma chemical vapor deposition (MWCVD) at the same temperature showed a QPY of ~12% at 140 nm without additional hydrogen plasma treatment. A reduction in deposition temperature in the HF reactor resulted in an increase of QPY up to 11%. We have observed a decrease of the QPY in time, down to QPY values of 5-6%, for samples exposed to ambient air. The decrease occurred on time scales of hours to weeks, for non-hydrogenated and postgrowth hydrogenated films. The high QPY values were regenerated by repeating the hydrogenation process. The degradation of hydrogen-terminated films was found to be related to the adsorption of small amounts of oxygen, as detected by Auger electron spectroscopy. The observed oxygen adsorption at room temperature is in contrast to that in previous studies, claiming the stability of hydrogen-terminated diamond surfaces.

We report on the properties of the Gaseous Electron Multiplier (GEM) operated at 10-40 Torr isobutane and methane. We found stable operation at gains of a few thousand, fast response and effective photon-feedback reduction. The transmission of single electrons through the GEM apertures was studied. Ion-induced feedback, from a wire chamber following the GEM, was found to limit the total two-stage multiplication at high GEM gains. A stable double-GEM operation with reduced ion-feedback was demonstrated. Some applications are discussed.

We report about a nuclear track imaging system which is designed to study in detail the ionization topology of charged particle tracks in a low-pressure gas. The detection method is based on a time projection chamber (TPC) filled with low-pressure triethylamine (TEA). Ionization electrons produced by energetic charged particles are three-dimensionally imaged by recording light from electron avalanches with an intensified CCD system. The detector permits to investigate the spatial ionization distributions of particle tracks in gas, of equivalent length and resolution in tissue of 4 μm and 40 nm (RMS), respectively. We explain the relevance of this technique for dosimetry, describe the experimental method and the basic operation parameters. First results of the chamber response to protons and alpha particles are presented.

We describe a technique for protecting alkali-antimonide visible light photocathodes against deterioration by exposure to impurities, during handling or storage in poor vacuum or gas. The photocathodes are coated with a ∼ 1 μm vacuum-deposited hexatriacontane film, which can be subsequently removed by low-temperature sublimation. We show that Cs₃Sb coated photocathodes can be exposed for several minutes to considerable amounts of oxygen, without deterioration. Their initial photoemission properties are almost fully recovered after film removal.

We report on the measurements of the electron drift velocity and longitudinal diffusion in n-pentane gas mixtures and on the effect of the drift velocity on the timing properties of Thin Gap Chambers. Gas mixtures of n-pentane-CO₂, n-pentane-CO₂-CH₄, and n-pentane-CO₂-CF₄ were investigated. An increase of the drift velocity has been observed with mixtures containing CF₄, while still maintaining the very high, saturated gain, typical for these chambers. The overall improvement of TGC timing properties is such that 99% detection efficiency can be reached within a 20 ns gate. A simulation of the chamber timing properties using the measured drift velocities, reproduces well the measured data and can be used to predict chamber performance as a function of geometry and gas mixture.

The optical and photoemission properties of hydrogen-free DLC films prepared under conditions which result in different types of carbon bonding (previously determined from electron energy loss spectroscopy) are reported. The films were prepared using a mass selected ion beam deposition system covering the C⁺ energy range 10 eV-2 keV, mostly on Si held at room temperature. Previous measurements have shown that the sp³ content of these films varied from 0 to > 80%. The optical constants (n, k, absorption coefficient (α)) of these films were measured by ellipsometry. The energy gaps were derived from Tauc plots (E_g) and from α=10⁴ cm^-1 values (E₀₄). The energy gaps were found to vary with the sp³ content from E

In order to operate gaseous photomultipliers in the visible range it was suggested to protect sensitive photocathodes against contact to air and counting gases by their coating with a thin solid dielectric film. We present data on coating of cesium-antimony photocathodes with alkali-halide (NaI, CsI, CsF, NaF), oxide (SiO) and organic (hexatriacontane, calciumstearate) films. The photoelectron transmission through these films and their protection capability have been studied in detail. Cesium-antinomy photocathodes are shown to withstand exposure to considerable doses of oxygen and dry air when coated with NaI films. This opens ways to their operation in gas media.

High accuracy imaging of thermal neutrons has been demonstrated with detectors having solid converters coated with secondary electron emitters and coupled to gaseous electron multipliers. The present work concentrates on study of the neutron detection efficiency of detectors equipped with composite CsI coated Li-6 neutron converters. Monte Carlo simulations of the neutron conversion and the charged particle and electron emission processes permit to optimise the converter parameters. The experimental study of the detection efficiency is described in detail and the results are compared with the theoretical predictions.

An approach, which permits to overcome certain fundamental limitations iu spatial resolution in nanodosimetry is been developed. It is based on counting the number of radiation induced single ions, extracted from a wall-less sensitive region, filled with gas. The extracted ions are acceleratecl in vacuum into an electron multiplier.Such development requires the knowledge of the ion diffusion within the dectector. As the gases of interest for nanodosimetry have multi-atomic molecules, the transport picture is rather complicated and involves a variety of mechanisms, resulting in series of ion species produced in the track, and their subsequent transformations during the diffusion process. The diffusion parameters, which should actually take into account all these mechanisms, are not available for most of the gases of interest.A special setup for measuring ion transport parameters has been built, in which first measurements for propane were performed. Calculations, based on the obtained data, show that in an ion counting nanodosimeter a closed wall-less sensitive volume of about 0.2 nm to 1nm across can be formed. Due to the absence of electron avalanche (like in electron counting devices), the choice of working gas is not restricted. This permits the use of various gases forsimulation of the chemical composition of living cell fine subsystems.

We report on photoemission properties of visible K-Cs-Sb reflective photocathodes coated with thin CsBr protective films. We present the absolute quantum efficiency and its stability under prolonged contact with oxygen. It is found that K-Cs-Sb photocathodes coated with 280 Å thick CsBr films have 5% quantum efficiency at 312 nm and can withstand an exposure to 150 Torr of oxygen for half an hour. We discuss the possible role of good lattice constants matching between the photocathodes and the protecting film.

Microdot avalanche chambers (MDOT) equipped with thin semitransparent Cr photocathodes, were characterized with UV photons at low gas pressure. Gains superior to 104 were reached with gas multiplication at the dots. In a mode where preamplification in the gas volume precedes the additional dot multiplication, gains superior to 106 were measured at 30-60 torr of propane. The fast amplification mechanism results in narrow high amplitude pulses with 2-3 ns rise time, visible with no further electronic amplification means. We present here our preliminary results and briefly discuss potential applications.

The absolute photoyields of chemical vapor deposited (CVD) diamond and amorphous hydrogen-free diamondlike carbon (DLC) films, in the range of 140-300 nm, are reported. CVD diamond films exhibit a large photoyield, of a few percent in the range 140-180 nm. DLC films have a 20-50 times lower yield. Post growth hydrogenation is found to substantially increase the photoyield of CVD diamond films. We discuss the applicability of these films as UV photocathodes coupled to electron multipliers based on gaseous charge multiplication.

The photoemission from solid surfaces into gas is important in view of the application of solid photocathodes in fast-RICH devices. The photoemission from CsI into gas has been investigated in He-, Ar-and CH₄-based gas mixtures as a function of the electric field at the photocathode surface. Measurements were made both in laboratory, with a UV source, and in a beam with a RICH detector. The results are interpreted in terms of current models of electron transport in gas. The electron collection efficiency, below gas ionization threshold, is reduced by backscattering. This phenomenon is particularly important in He-based gas mixtures. In CH₄ and Ar/CH₄ mixtures the backscattering is very low. At high electric field, under charge multiplication, a full collection efficiency, similar to that in vacuum, is obtained in all gases investigated. We discuss the parameters governing the choice of the gas mixture in this kind of photon detectors.

We present the first results of the application of a novel digital X-ray imaging detector, based on secondary electron emission from a solid converter, to high-rate crystallographic studies. Results from diffraction and small-angle scattering experiments from several crystallized proteins, performed at the European Synchrotron Radiation Facility, ESRF, Grenoble, are presented and compared with a phosphor-based imaging system. Future developments of this detector system are proposed.

Radiation-induced low-energy secondary electrons multiplied in gas by a series of wire electrodes provide an efficient means for fast and accurate radiation localization under very high flux. We discuss here the properties of secondary electron emission (SEE) X-ray detectors, operated at atmospheric pressure helium-based mixtures.

A new method is presented for deriving the Fano factor, F, and the mean energy per ion pair, W_i, in counting gases. It is based on the technique of individual counting of single ionization electrons induced in low-pressure gas samples by soft x-ray photons. A correlation of the experimental data with a detailed simulation of the electron deposition and counting process permits the extraction of the Fano factor and the mean energy per ion pair values. We present data of F and W_i for C₂H₆ and Ar/C₂H₆ over the energy range of 100-1500 eV. The energy dependence of these parameters reflects the atomic level structure of the gases. We discuss in detail the accuracy of this technique and its advantages and limitations. Ways are proposed for improving the technique and for broadening the energy range.

We report here on the UV-induced photoemission from CsI photocathodes coated with LiF or NaF films. A considerable enhancement of the quantum efficiency was found after contact with water vapour. We interpret this effect as a decrease of the electron affinity of LiF and NaF, induced by adsorption of polarized H₂O molecules.

A new class of radiation imaging detectors is described. They combine thin solid converters and gaseous electron multipliers. CsI or CsI-coated films provide an efficient means for UV, X-ray and thermal neutron conversion. The latter is followed by the emission of single photoelectrons or multiple secondary electrons in the eV range. The surface conversion and the electron multiplication close to the radiation impact location guarantee an accurate localization, fast response and low occupancy, regardless of the angle of incidence. A low gas pressure permits the operation at very intense radiation flux, with a low sensitivity to ionizing background. The article briefly reviews the new technique and its potential applications.

A novel, fast, hadron-blind imaging TRD is proposed. Traditonal radiators are followed by novel, particle-blind photon imaging detectors, combining thin Csl photon convertors and fast gaseous electron multipliers. The expected performance of the TRD, based on multiple radiator/detector elements, is simulated, using experimental and theoretical data of the photon detector characteristics. Such TRDs can provide 100 GeV electron detection efficiency above 90% with hadronic background rejection factors of 10² - 10⁴. The nanosecond timing, very low occupancy and the high rate capability make these devices very competitive tools for ultrarelativistic electron identification under critical conditions expected in future particle physics experiments.

We discuss the idea of a vacuum or gaseous photomultiplier with a CsI-coated wire photocathode. It benefits from an electric field enhanced quantum efficiency. We present a method for protecting air-sensitive photocathodes with thin dielectric film coating. Results of CsI coated with thin LiF and MgF2 films are shown. The protection of other UV and visible photocathodes is discussed.

We report here on the results obtained by the CERN RD26 collaboration on the production and characterization of large area photocathodes, susceptible to equip fast UV-photon imaging devices. Such detectors are planned for some Ring Imaging Cherenkov (RICH) detector projects, in particular HADES at SIS Darmstadt, BABAR at the SLAC asymmetric B-factory, and ALICE at the LHC (CERN).

A detector has been devised able to measure with high resolution the primary ionisation yield in tissue-equivalent gas volumes of a few nanometres equivalent length. The sensitive ionisation volume is a wall-less millimetric region defined by a properly shaped electric field. Free electrons created by the radiation inside the sensitive volume are collected into an electron multiplier, capable of efficiently counting single electrons at low gas pressure. The single-electron detection system consists of a long drift column attached to a multistep proportional counter. The electron cloud created by the radiation inside the sensitive volume, diffuses along the drift column. Single electrons, successively arriving at the multiplier are amplified, giving rise to a pulse trail from which the original number of ionisation electrons is counted. The experimental set-up, the electron counting principle, and first data are presented and discussed.

We report on measurements of low-mass electron pairs in 450 GeV p-Be, p-Au, and 200 GeV/nucleon S-Au collisions at central rapidities. For the proton induced interactions, the low-mass spectra are, within the systematic errors, satisfactorily explained by electron pairs from hadron decays, whereas in the S-Au system an enhancement over the hadronic contributions by a factor of 5.0±0.7(stat)±2.0(syst) in the invariant mass range 0.2

We revise the quantum efficiency of NaI and CuI reflective photocathodes. We have shown that the photoyield from NaI and CuI can be considerably enhanced by a post-evaporation heat treatment, similarly to that observed before for the CsI photocathode. We discuss possible reasons for the heat enhancement.

The potential of a thermal neutron imaging system based on a composite neutron convertor foil combined with a low-pressure, multistep avalanche chamber is demonstrated. Neutron-induced charged particles from a primary convertor element induce multiple low-energy electrons escaping from a second thin film of high electron-emissive material. We investigated the performance of detectors with Gd- and Li-based primary convertors coated with CsI as a secondary electron emitter. It is shown, that the detector can be operated with high stability at a sufficiently high gain to detect all escaping particles. A localisation resolution of 0.4 mm (fwhm) was obtained. With Li-based convertors a very low γ-ray sensitivity was established. The good imaging performance, free of parallax errors in divergent neutron beams, fast time resolution, low occupation time and high count rate capability, make this instrument an excellent tool for time-resolved neutron scattering experiments and for neutron radiography and tomography.

We performed a systematic investigation of the quantum efficiency of some solid reflective photocathodes in the spectral range 140-240 nm. The measurements were made without gaseous amplification in vacuum and in methane. No significant difference was found among CsI photocathodes prepared by vacuum deposition at different institutes, either from powders or from crystals of different origins, and measured either in vacuum or in methane. Amorphous silicon photocathodes were prepared by the plasma enhanced chemical vapor deposition technique. We present the results for several doping conditions of amorphous silicon and for p-n junctions. Some organometallic photocathodes, containing iron or other transition metals (cerium), were evaporated and measured. Among them decamethylferrocene exhibits the highest quantum efficiency in the range 190-240 nm.

The quantum efficiency (QE) of four photomultipliers of the R1460 series of Hamamatsu, used in the last years as a reference in our UV-photocathode studies, was determined with the help of a calibrated vacuum UV-photodiode. Large discrepancies were found with respect to the QE curves supplied for each tube by the manufacturer. The correction curves of the photomultipliers are presented and should be used to normalize our previously published QE data of various photoemissive materials. Corrected data of CsI photocathodes are presented here.

We investigated the photoyield from CsI, p and n doped and undoped amorphous silicon and some organometallic (decamethyl-ferrocene,1,1,dimethylferrocene and ruthenocene) reflective photocathodes, in the spectral range 140-220 nm. The measurements were made in a current collection mode, in vacuum and in methane. Powder and crystal forms of CsI were used for evaporation of bulk and porous photocathodes. The radiation resistance of CsI was measured at doses reaching 4500 Gy. The effect on the quantum efficiency of various n and p doping levels of amorphous silicon was measured.

New detectors for fast, real-time, high resolution X-ray imaging at high photon fluxes are described. A thin solid photoconverter is coupled to a multistage gaseous electron multiplier operating at low pressure. The readout electronics connected to the wire electrodes of the chamber provides two-dimensional localization of single registered photons. Prototype detectors were tested with Csl, Ag and Ta converters in the photon energy range of 8-60 keV. Radiographic digital images are presented compared to that of X-ray films. It is shown that compared to the film technique the secondary electron emission detector provides radiographic images of an equivalent contrast at an order of magnitude lower exposure. This novel type of detector is suited for static and dynamic in-line quality control on industrial production lines and for medical imaging.

A model of electron transport in alkali halides, below 10 eV, is described. It is based on theoretically calculated microscopic cross sections of electron interactions with lattice phonons. Both acoustic and optical scatterings are taken into account, the former being also treated as a quasielastic process that randomizes the electron motion. Monte Carlo calculations based on the model simulate the UV-induced photoelectron emission from CsI. The calculated quantum efficiency and energy spectra are in good agreement with experimental data, in the photon energy range of 6.3-8.6 eV. The probability for an electron to escape from CsI, NaCl, and KCl is provided as a function of its energy and creation depth. A comparison is made between our approach and other phenomenological models.

A two-stage multiplication mechanism has been observed in microstrip gas chambers (MSGC) operated in a pressure range of 10-50 Torr of isobutane. The resulting high gain (> 10⁴) of single electrons photo-produced on a CsI photocathode is attributed to a preamplification in the gas gap followed by anode strip multiplication. The large and fast rise of the induced pulse in this mode leads to an efficient single electron detection at relatively low charge gain. The reduced positive ion feedback preserves radiation convertors coupled to such electron multipliers from sputtering damage. MSGCs operated in this mode are expected to have a subnanosecond time resolution and very high rate capability. Some potential applications are briefly discussed.

Radiobiological data reveal that radiation action on the living cell is at the nanometre level. In this paper a new approach is proposed for the measurement of the radial radiation induced ionisation distribution across the particle track at this level. It is based on coupling a parallel-plate ionisation chamber to a cylindrical proportional electron multiplier. A detector of this type, filled with propane at pressures of 227 and 454 Pa, was used for the measurement of the radial ionisation distribution induced by a particles. The displacement of the detector with respect to the α particle beam provided a tissue-equivalent ionisation distribution at the nanometre level. The data obtained with this novel technique are presented and the secondary effects due to radiation induced electron emission from the detector walls are discussed. Further developments are also discussed.

We present the result of a systematic investigation of the quantum efficiency of reflective CsI photocathodes in the spectral range 140-220 nm. Powder and crystal forms of CsI were used for evaporation of bulk and porous photocathodes. The measurements were made without gaseous amplification in vacuum and in methane. The results of radiation resistance measurements at doses reaching 4500 Gy are also reported.

Ionization electrons deposited by soft X-rays in a low pressure (10 Torr) gas medium are efficiently counted by a multistage electron multiplier, providing an accurate measurement of the X-ray photon energy. Energy resolutions of 56-28% FWHM were measured for X-rays of 110-676 eV, recording electrical induced charges or visible photons emitted during the avalanche process. It is demonstrated that a combined analysis of the number of electrons and the electron trail length of an event, provides a powerful and competitive way of resolving ultra-soft X-rays. We present the experimental technique, discuss the advantages and limitations of the Primary Electron Counter, and suggest ways to improve its performances.

The results of a systematic investigation of the properties of CsI and CsI-TMAE photocathodes, in the spectral range of 150-200 nm, are presented. The measurements were carried out in high vacuum and in CH₄ atmosphere. Effects of the photocathode regeneration and the enhancement of the quantum efficiency due to temperature and TMAE adsorption are observed. The stability of the TMAE effect was investigated and is discussed. A comparison is given with different results of other groups.

We present a new X-ray imaging method with a potential application to ultrarelativistic particle identification by Transition Radiation detection. It is based on the conversion of the X-ray photons in a thin layer of CsI and the amplification of secondary emitted electrons in a low-pressure multistep avalanche electron multiplier. The obvious advantages of the solid X-ray convertor are the parallax-free imaging and the ultrafast response. The detector has an X-ray localization resolution accuracy better than 0.2 mm (fwhm), a subnanosecond time reponse and a reduced sensitivity to minimum ionizing particles as compared to Xe-filled detectors. We present the experimental results on the detector performance with X-rays of 6-60 keV and with relativistic electrons. They are accompanied by mathematical modelling and Monte Carlo simulations of Transition Radiation Detectors based on this novel technique.

We propose new techniques of X-ray spectroscopy and imaging, based on the use of low-pressure multistep gaseous electron multipliers. Ultrasoft X-rays are detected by counting single-electron clusters induced in the gas. X-ray induced UV-photons in gas scintillation chambers are read out with wire chambers coupled to CsI photocathodes. X-rays converted in foil-electrodes are imaged by fast multistep avalanche electron multipliers. We discuss the advantages of the various techniques and present experimental results and Monte Carlo simulations.

We report on a search for supermassive nuclei in nature with masses up to 10⁷ amu. Such exotic nuclei might consist, for example, of stable strange matter, which comprises a mixture of up, down, and strange quarks, or of relic particles from the early Universe. The experiments are based on Rutherford backscattering of heavy ions, preferably²³⁸U, from various target samples. The measured parameters of a detected particle are its time-of-flight, scattering angle, and specific ionization. From this information the mass of the target nucleus can be inferred. Upper limits for the abundance of strange supermassive nuclei with masses A-4·10² to 10⁷ amu relative to the number of nucleons were found to be in the range 10^-11 to 10^-15. For the narrower mass range A -10³ to 10⁴ amu the limit is 2· 10^-17.

UV-photons from an Xe-filled gas scintillation counter are detected with a CsI photocathode coupled to a double stage, low-pressure wire chamber. The high quantum efficiency, (9 pct), of the UV-detector yields a high detection efficiency of both primary and secondary scintillation photons. An energy resolution of 4.1 pct (FWHM) was recorded with 60 keV X-rays, inducing secondary scintillation in 5 bar of Xe; the wire chamber operated at 20 Torr of CH4. The two-dimensional planar localization of UV-photons, combined with the drift time measurement of primary electrons, provides a three-dimensional, multihit, imaging capability of X-ray photon interactions, with a resolution of 2-3 mm (FWHM). The stability of the CsI photocathode under different operation conditions and its sensitivity to exposure to air are discussed.

UV-photons are detected by a low-pressure photosensitive multistep gaseous detector. Photoelectrons are multiplied in two charge amplification stages. A third, light amplification stage operating in a scintillation mode, provides light yields > 5×10⁷ visible photons per single photoelectron avalanche, in ArC₂H₆-TMAE gas mixtures. We present results on absolute photon yields in various TMAE gas mixtures, at low gas pressures and at low charge gains. We describe the operation mechanism and some basic properties of the gated 3-stage detectors, such as stability of operation at high background rates and localization resolution, particularly at large TMAE concentration and high temperature operation conditions. Further applications are discussed.

A simple method for the determination of the total width of the 9.17 MeV level in ¹⁴N is described. The method is based on the use of a resonant detector which contains nitrogen in its active volume. With the help of the resonant detector the ratio of Γ_γ0/Γ_T was found to be 0.052 ± 0.004. This result together with the data from a conventional resonant absorption experiment yields for the total width of the level a value of 122±8 eV.

We describe a new method of Primary ionization Cluster Counting (PCC), using a low-pressure multistep counter coupled to a conversion volume. Due to the small density of the primary clusters in the low-pressure gas, and to the high gain of the multistep amplification structure, the clusters are efficiently counted. Taking advantage of the enhanced relativistic rise at low gas pressures, and the reduced interference from δ-electrons, the PCC can be efficiently applied to dE/dx relativistic charged particle identification. We present here preliminary results obtained with relativistic electrons and a UV laser. For minimum ionizing electrons we counted about 2 primary clusters/cm, at 20 Torr of isobutane. The distribution is poissonian, with σ ~ n0.5. The PCC is shown to be an efficient tool for the study of the primary ionization process in gas media.

We have made a quantitative systematic study of the light emitted through the electron avalanche process in low-pressure gases. The mixtures under investigation were argon/(CH₄ or C₂H₆)/(TMAE or TEA). For a given avalanche charge gain, we measured comparable light yields for mixtures containing TEA and TMAE; typical values are of 0.1-1 photons per electrons in the avalanche. We found a linear increase of the light intensity with the amplification gap width. Light production by electrons drifting in these gas mixtures without charge multiplication was also demonstrated.

According to a number of suggestions, stable strange matter could exist in the form of supermassive nuclei (or 'strange nuggets')^1,2. In contrast to ordinary nuclei, which contain only 'up' and 'down' quarks, a piece of strange matter should comprise a mixture of 'up', 'down' and 'strange' quarks in roughly equal proportions. Small amounts of strange matter could have survived from the early stages of the Universe¹. Alternatively, strange matter might reach the Earth as a flux of strange nuggets produced in collisions of neutron stars³. Limits to the cosmic flux of strange nuggets with masses in the range from 10^-4 to 250 g have been obtained in a search for light produced by the nuggets in the upper atmosphere⁴. Here we report the results of a search for supermassive nuclei by using Rutherford backscattering of heavy ions. The method is sensitive to a broad range of masses extending to those that exceed the projectile mass by several orders of magnitude. Upper limits for the abundance of strange nuggets with masses A ≈ 4 × 10² to 10⁷ AMU relative to the number of nucleons were found to be in the range 10^-10 to 10^-14.

The application of low-pressure two-step gas detectors to the ring-imaging Cherenkov (RICH) technique is demonstrated. A RICH prototype with a UV-photon detector of 20×20 cm², filled with 53 mbar of C₂H₆ + TMAE at 30°C, was exposed to 3 GeV celectrons. Photoelectrons were localized with a FADC system with a spatial resolution of 2 mm rms. The ring reconstruction technique is presented. The advantages of using this method in high background experiments, such as in relativistic heavy ion collisions, are discussed.

Transverse-energy distributions have been measured in the pseudorapidity region -0.1

A novel two-dimensional readout scheme for gas detectors is presented that uses small metal pads with 2.54-mm pitch as an anode. The pads are read out via 128-channel VLSI low-noise preamplifier/multiplexer chips. These chips are mounted on 2.8-cm*2.8-cm modules, which are directly plugged onto the detector backplane, delay-chained with jumpers, and read out sequentially. The readout has been successfully tested with a low-pressure, two-step, TMAE-filled, ultraviolet-sensitive ring imaging Cerenkov counter. A single-electron efficiency of >90% was observed at moderate chamber gains (

We have developed a low-pressure multistep, TMAE- filled, UV sensitive detector for Cerenkov ring imaging (RICH) for ultrarelativistic heavy ion experiments (HELIOS-CERN). In the present work we describe the detector structure and its basic properties and we present here the experimental setup and the detector performance in tests with high energy electrons using three modes of readout: 3 wire coordinates and flash ADC's Pad readout - optical recording of the avalanches. Single photons can be localized with an accuracy of σ⋍a 2 mm, mainly due to single photoelectron diffusion and to chromatic dispersion in the radiator gas. The experimental No values, obtained by the three methods, are close to those expected, taking into account the various losses due to the test conditions.

A position sensitive Multistep Avalanche Chamber (MSC), operated with low pressure isobutane (5-20 Torr) was coupled to a solid photocathode. It is shown that gaseous amplification factors above 10⁸ are obtained, thus enabling the efficient detection of single photoelectrons. Imaging of single photoelectrons was performed using the delay-line technique. Position resolution of the order of 200 μm (fwhm) is obtained with a photoelectron detection efficiency close to 100%. Further applications are discussed.

MWPCs operated at gas pressures around 1 Torr are shown to have excellent timing and position response for heavily ionizing particles. The avalanche growth process at such conditions is of a double-step nature and is characterized by fast collection times of electrons and positive ions. A substantial spatial distribution of the avalanche (1-2 mm fwhm) leads to an excellent position resolution even when interpolating between neighbouring wires. Very thin (100-200 φg/cm² of polypropylene), fast timing and position-sensitive detectors are providing time resolutions of the order of 100-130 ps (fwhm), and position resolutions, with delay-line read-out, of the order of 100 φm (fwhm). Various detectors of this type, with sizes reaching 80 cm², operating at counting rates up to 10⁵ c/s·mm², are used as routine tools for timing and imaging of heavily ionizing particles.

The ions liberated in a high-pressure gas or in some liquids can be collected, by electric fields, on the surface of insulators and can be accurately localized. In a simulation of this method at atmospheric pressure, we applied it to α particles, with the additional amplification from a parallel grid gap. By directly measuring the static electric charges collected on mylar foils, we observe tracks of 1 mm fwhm and charge densities as low as 10⁴ electrons/mm². The combination of multistep gated avalanche chambers with this read-out method should permit high-accuracy measurements of minimum ionizing particles. The limits of the method and some conditions for detection by liquid toners are discussed.