Search

We have developed and successfully tested a low-pressure Time Projection Chamber (TPC) with Gas Electron Multiplier (GEM) readout for Accelerator Mass Spectrometry (AMS). AMS facility in Novosibirsk has a problem of separating isobar ions of different chemical elements that have the same atomic mass. The typical example is radioactive isotopes 10Be and 10B that are used to date geological objects at a time scale of ten million years. To solve this problem, a new ion identification technique, namely that based on measuring both ion track ranges and ion energies in low-pressure TPCs with GEM readout, has been developed. This technique is proposed to be applied in AMS for dating geological objects, namely for geochronology of Cenozoic era. In this work, we developed a new larger version of the TPC with a dedicated thin silicon nitride window for an efficient passage of ions. The TPC characteristics were studied in isobutane at low pressures using alpha particle sources. In addition, the use of GEM instead of THGEM has been shown to substantially improve the energy resolution at a nominal pressure (50 torr). Using these results and SRIM code simulations, it is shown that isobaric boron and beryllium ions can be effectively separated at AMS, providing efficient dating on a scale of ten million years. This technique will be applied in the AMS facility in Novosibirsk in the near future.

Large liquid argon detectors offer one of the best avenues for the detection of galactic weakly interacting massive particles (WIMPs) via their scattering on atomic nuclei. The liquid argon target allows exquisite discrimination between nuclear and electron recoil signals via pulse-shape discrimination of the scintillation signals. Atmospheric argon (AAr), however, has a naturally occurring radioactive isotope, $^{39}$Ar, a $\beta$ emitter of cosmogenic origin. For large detectors, the atmospheric $^{39}$Ar activity poses pile-up concerns. The use of argon extracted from underground wells, deprived of $^{39}$Ar, is key to the physics potential of these experiments. The DarkSide-20k dark matter search experiment will operate a dual-phase time projection chamber with 50 tonnes of radio-pure underground argon (UAr), that was shown to be depleted of $^{39}$Ar with respect to AAr by a factor larger than 1400. Assessing the $^{39}$Ar content of the UAr during extraction is crucial for the success of DarkSide-20k, as well as for future experiments of the Global Argon Dark Matter Collaboration (GADMC). This will be carried out by the DArT in ArDM experiment, a small chamber made with extremely radio-pure materials that will be placed at the centre of the ArDM detector, in the Canfranc Underground Laboratory (LSC) in Spain. The ArDM LAr volume acts as an active veto for background radioactivity, mostly $\gamma$-rays from the ArDM detector materials and the surrounding rock. This article describes the DArT in ArDM project, including the chamber design and construction, and reviews the background required to achieve the expected performance of the detector., Comment: 13 pages, 8 figures. Corresponding author: E. S\'anchez Garc\'ia

In liquid argon TPCs for dark matter search and neutrino detection experiments, primary scintillation light is used as a prompt signal of particle scattering, being intensively produced in the vacuum ultraviolet (VUV) due to excimer emission mechanism. On the other hand, there were indications on the production of visible-light emission in liquid argon, albeit at a much lower intensity, the origin of which is still not clear. The closely related issue is visible-light emission in liquid argon doped with methane, the interest in which is due to the possible use in neutron veto detectors for those experiments. In this work we study in detail the properties of such light emission in pure liquid argon and its mixtures with methane. In particular, the absolute photon yield of visible-light emission in pure liquid argon was measured to be about 200 and 90 photon/MeV for X-rays and alpha particles respectively. In liquid argon doped with methane the photon yield dropped down significantly, by about an order of magnitude at a methane molar content varying from 0.01 to 1%, and then almost did not change when further increasing the methane content up to 10%., 22 pages, 15 figures, 3 tables

A two-phase argon detector is being developed in our laboratory for dark matter search and low-energy neutrino experiments. A 109Cd γ-ray source was used to calibrate the energy scale of the detector. In this paper, a detailed emission spectrum of the 109Cd source was measured using a YAP:Ce based scintillation detector and high-purity germanium detector. It was shown that the 109Cd source equipped with a tungsten substrate and a copper filter can provide a complete set of γ-ray lines in the range from 8 to 90 keV for energy calibration of two-phase detectors. These measurements allowed us to successfully quantify the shape of the amplitude spectrum observed when the two-phase detector was irradiated by the 109Cd source.

Electroluminescence and electron avalanching are the physical effects used in two-phase argon and xenon detectors for dark mater search and neutrino detection, to amplify the primary ionization signal directly in cryogenic noble-gas media. We review the concepts of such light and charge signal amplification, including a combination thereof, both in the gas and in the liquid phase. Puzzling aspects of the physics of electroluminescence and electron avalanching in two-phase detectors are explained and detection techniques based on these effects are described., 30 pages, 33 figures

Proportional electroluminescence (EL) in noble gases is used in two-phase detectors for dark matter search to record ionization signals in the gas phase induced by particle scattering in the liquid phase (S2 signals). In this work, the EL pulse-shapes in a two-phase argon detector have for the first time been studied systematically in a wide range of reduced electric field, varying from 3 to 9 Td. The pulse-shapes were studied at different readout configurations and spectral ranges: using cryogenic PMTs and SiPMs, with and without a wavelength shifter (WLS), in the VUV and visible range. We observed the fast component and two unusual slow components, with time constants of about 5 $\mu$s and 40 $\mu$s. The unusual characteristic property of slow components was that their contribution and time constants increased with electric field., Comment: 18 pages, 14 figures, 1 equation. Proceedings paper of the INSTR20 conference

International audience; We report on the cryogenic characterization of Red Green Blue - High Density (RGB-HD) SiPMs developed at Fondazione Bruno Kessler (FBK) as part of the DarkSide program of dark matter searches with liquid argon time projection chambers. A cryogenic setup was used to operate the SiPMs at varying temperatures and a custom data acquisition system and analysis software were used to precisely characterize the primary dark noise, the correlated noise, and the gain of the devices. We demonstrate that FBK RGB-HD SiPMs with low quenching resistance (RGB-HD-LRq) can be operated from 40 K to 300 K with gains in the range 10(5) to 10(6) and noise rates at a level of around 1 Hz/mm(2).

A two-phase Cryogenic Avalanche Detector (CRAD) with electroluminescence (EL) gap, operated in argon doped with a minor (49$\pm$7 ppm) admixture of nitrogen, has been studied. The EL gap was optically read out using cryogenic PMTs located on the perimeter of the gap. We present the results of the measurements of the N$_2$ content, detector sensitivity to X-ray-induced signals, EL gap yield and electron lifetime in the liquid. The detector sensitivity, at a drift field in liquid Ar of 0.6 kV/cm, was measured to be 9 and 16 photoelectrons recorded at the PMTs per keV of deposited energy at 23 and 88 keV respectively. Such two-phase detectors, with enhanced sensitivity to the S2 (ionization-induced) signal, are relevant in the field of argon detectors for dark matter search and low energy neutrino detection., Comment: 4 pages, 5 figures. Presented at Vienna Conference of Instrumentation (2016). To be published in Nucl. Instr. Meth. A

We further study the effect of neutral bremsstrahlung (NBrS) in two-phase argon electroluminescence (EL), revealed recently in [1]. The absolute EL yield due to NBrS effect, in the visible and NIR range, was remeasured in pure gaseous argon in the two-phase mode, using a two-phase detector with EL gap read out directly by cryogenic PMTs and SiPMs. Possible applications of the NBrS effect in detection science are discussed, including those in two-phase dark matter detectors., 4 pages, 5 figures, presented at VCI2019 conference, to be published in NIM A

One of the main features of two-phase detectors with electroluminescence (EL) gap being developed in our laboratory for dark matter search is the extensive use of THGEMs (Thick Gas Electron Multipliers). In various versions of the detector, the THGEMs are used as electrodes in the gas and liquid phases to form the drift, electron emission and EL regions, as well as for avalanche amplification of a signal in the gas phase. In this work the simulations of the electric field and electron transport through such THGEM electrodes were performed. In the liquid phase, these simulations allowed to determine the optimal parameters, such as the hole diameter of THGEM and applied voltage across it, that can provide effective transmission of the electrons from the drift region to that of the EL gap. In the gas phase, the effect of the THGHEM voltage on the electric field uniformity in the EL gap was studied., Comment: 7 pages, 11 figures, 1 table

Neutron veto detector based on liquid scintillator containing hydrogen atoms is an integral part of any underground experiment for dark matter search. So far, a flammable mixture of liquid hydrocarbons was used as a liquid scintillator in such detectors. A safe alternative might be a liquid scintillator based on liquid argon doped with methane. In this work, we have studied the primary scintillations in pure liquid argon and its mixtures with methane, the CH4 content varying from 100 ppm to 5%. The primary scintillations have for the first time been observed in liquid argon doped with methane, in the visible and near infrared range, and their relative light yields have been measured as a function of the CH4 content., 10 pages, 6 figures. Presented at Instrumentation for Colliding Beam Physics Conference (INSTR20)

A new technique for ion identification in Accelerator Mass Spectrometry (AMS) has been proposed based on measuring the ion track ranges using a low-pressure TPC. As a proof of principle, a low-pressure TPC with charge readout using a THGEM multiplier was developed. The tracks of alpha particles from various radioactive sources were successfully recorded in the TPC. The track ranges were measured with a rather high accuracy, reaching 2%. Using these results and SRIM code simulation, it is shown that the isobaric boron and beryllium ions can be effectively separated at the 10 sigma level. This technique is expected to be applied in the AMS facility in Novosibirsk for dating geological objects, in particular for geochronology of Cenozoic Era.

A new technique for ion identification in Accelerator Mass Spectrometry (AMS) has been proposed by measuring the ion track ranges using a low-pressure TPC. As a proof of principle, a low-pressure TPC with charge readout using a THGEM multiplier was developed. The tracks of alpha particles from various radioactive sources were successfully recorded in the TPC. The track ranges were measured with a high accuracy, reaching the 2% resolution level. Using these results and the SRIM code simulation, it is shown that the isobaric boron and beryllium ions can be effectively separated at ten sigma level. It is expected that this technique will be applied in the AMS facility in Novosibirsk for dating geological objects, in particular for the geochronology of Cenozoic Era., 8 pages, 13 figures, 2 tables. Short paper version will be published in Nucl. Instr. Meth. A

A measurement of ionization yields in noble-gas liquids is relevant to the energy calibration of nuclear recoil detectors for dark matter search and coherent neutrino-nucleus scattering experiments. In this work we further study the ionization yield of nuclear recoils in liquid Ar, using a two-phase detector with an electroluminescence gap and DD neutron generator. The ionization yields of nuclear recoils in liquid Ar were measured at 233 keV and electric fields of 0.56 and 0.62 kV/cm; their values amounted to 5.9 $\pm$ 0.8 and 7.4 $\pm$ 1 e$^-$/keV, respectively. The characteristic dependences of the ionization yield on energy and electric field were determined, while comparing the results obtained to those at lower energies and higher fields., 10 pages, 8 figures, 1 table. Submitted to the Proceedings of the Instrumentation for Colliding Beam Physics (INSTR17) Conference. To be published in JINST

We report the results of a characterization study of several types of cryogenic photomultipliers manufactured by Hamamatsu Photonics and intended for operation in liquid Ar conditions, namely: compact 2-inch R6041-506MOD tubes, 3-inch R11065-10 and R11065-MOD tubes for operation in liquid Ar and 3-inch R11410-20 tubes originally designed for operation in liquid Xe. These types of PMT are proposed for installation into the future two-phase cryogenic avalanche detector that is developed in the Laboratory of Cosmology and Particle Physics of the Novosibirsk State University jointly with the Budker Institute of Nuclear Physics. Eight R11065 PMTs and seven R11410-20 tubes were tested and all demonstrated excellent performance in liquid Ar in terms of gain and relative single electron efficiency. All 3-inch PMTs showed a maximal gain in liquid Ar above 5x10^6 and relative single electron efficiency higher than 95%. Compact R6041-506MOD tubes have dynode system different from that of the 3-inch photomultipliers and thus their single electron energy resolution and relative efficiency is much worse than that of 3-inch tubes. From 21 2-inch PMTs only 12 tubes were selected with acceptable, i.e. higher than 75%, relative single electron efficiency and the maximal gain higher than 5x10^6. However, these PMTs are very attractive because they are the only compact type of tubes that can operate in liquid Ar., Submitted to the Proceedings of the Instrumentation for Colliding Beam Physics INSTR17 Conference, to be published in JINST

The study of proportional electroluminescence in two-phase argon is relevant in the field of noble-gas liquid detectors for dark matter search and low-energy neutrino experiments. In this work, we continued to study proportional electroluminescence (EL) in two-phase argon doped with a minor (9 ppm) admixture of nitrogen, in the VUV, UV and visible spectral ranges. We confirmed the effect of enhancement of the EL yield, as well as the presence of non-VUV component in addition to that of VUV, in proportional electroluminescence in two-phase Ar. On the other hand, the contribution of non-VUV component determined here within the model of N2 emission in the UV, turned out to be insufficient to explain the EL yield enhancement effect. Accordingly, the problem of proportional electroluminescence in two-phase Ar remains unresolved., Comment: 12 pages, 7 figures, 1 table. Presented at Instrumentation for Colliding Beam Physics Conference (INSTR17). To be published in JINST

We describe extensive tests of Double Gas Electron Multiplier (GEM) and Triple GEM detectors, including large size prototypes for the COMPASS experiment, exposed to high intensity muon, proton and pion beams at the Paul Scherrer Institute and at CERN. The measurements aim at detecting problems possibly appearing under these harsh operating conditions, the main concern being the occurrence of discharges induced by beam particles. Results for the dependence of the probability for induced discharges on the experimental environment are presented and discussed. Implications for the application of GEM detectors in experiments at high luminosity colliders are illustrated.

We study the operation of the Gas Electron Multiplier (GEM) in pure Ar, in comparison to that in Ar–CO2 mixture. In pure Ar, high GEM gains, of above 700 and 3000 for single and double GEM structures, respectively, have been obtained. It is observed that the GEM effective gain and its charging-up are strongly affected by electric field values above and below the GEM. Applications to the development of non-ageing gas photomultiplier are discussed.

The present work describes the development of a new detection technology for medical imaging using the method of Single-Photon Emitted Computed Tomography (SPECT) combined with X-rays Computed Tomography (CT). Our ultimate goal is to advance disease detection by imaging quantitatively molecular agents and localizing their distribution in tissues and organs by SPECT simultaneously with the anatomical CT image. To assess the imaging characteristics of the imaging agents developed, we tested and advanced a detection technique first developed for high-energy physics: Gas Electron Multipliers (GEMs).

Remarkable features of gas electron multipliers (GEMs) make them attractive for numerous applications in high-energy physics, nuclear physics, astrophysics, and the medical-imaging field. This review presents the results of research and development of GEM-based radiation detectors. The operating principles of GEMs and their main characteristics, including the physics of multistage GEMs and their operation in pure noble gases, are expounded. GEM-based detectors of ionizing radiations are considered. In particular, the data on tracking detectors are briefly described, while more detailed data, including the data on two-phase avalanche detectors, are presented for detectors operated at high pressures and cryogenic temperatures. A special Section is devoted to the development of GEM-based gas photodetectors.

Two-phase Cryogenic Avalanche Detectors (CRADs) with combined THGEM/GAPD multiplier have become an emerging potential technique for dark matter search and coherent neutrino-nucleus scattering experiments. In such a multiplier the THGEM hole avalanches are optically recorded in the Near Infrared (NIR) using a matrix of Geiger-mode APDs (GAPDs). To select the proper sensor, the performances of six GAPD types manufactured by different companies, namely by Hamamatsu (MPPCs), CPTA (MRS APDs) and SensL (SiPMs), have been comparatively studied at cryogenic temperatures when operated in two-phase CRADs in Ar at 87 K. While the GAPDs with ceramic packages failed to operate properly at cryogenic temperatures, those with plastic packages, namely MPPC S10931-100P and MRS APD 149-35, showed satisfactory performances at 87 K. In addition, MPPC S10931-100P turned out to be superior in terms of the higher detection efficiency, lower nose rate, lower pixel quenching resistor and better characteristics reproducibility., 11 pages, 8 figures. Submitted to JINST

The main purpose of this work is to provide reference data on X-ray ionization yields and energy spectra in liquid Ar to the studies in the field of Cryogenic Avalanche Detectors (CRADs) for rare-event and other experiments, based on liquid Ar detectors. We present the results of two related researches. First, the X-ray recombination coefficients in the energy range of 10-1000 keV and ionization yields at different electric fields, between 0.6 and 2.3 kV/cm, are determined in liquid Ar based on the results of a dedicated experiment. Second, the energy spectra of pulsed X-rays in liquid Ar in the energy range of 15-40 keV, obtained in given experiments including that with the two-phase CRAD, are interpreted and compared to those calculated using a computer program, to correctly determine the absorbed X-ray energy. The X-ray recombination coefficients and ionization yields have for the first time been presented for liquid Ar in systematic way., Comment: 13 pages, 9 figures, 3 tables. Submitted to NIM. 2nd version: substantially modified. New Fig. 7: summary of ionization yields in liquid Ar

We further study the performance of cryogenic two-phase avalanche detectors, based on Gas Electron Multipliers (GEMs) and operated in an electron-avalanching mode in Kr. Stable operation of the triple-GEM in saturated vapor, above the liquid, has been demonstrated for several hours. The signals induced by 0.5 MeV γ-rays were recorded from the two-phase detector in coincidences with a BGO counter, aiming at potential applications in Positron Emission Tomography. Charging-up effects were observed at high anode current densities in GEM-based two-phase detectors.

We study the feedback of positive ions in triple and quadruple Gas Electron Multiplier (GEM) detectors. The effects of GEM hole diameter, detector gain, applied voltages, number of GEMs and other parameters on ion feedback are investigated in detail. In particular, it was found that the ion feedback is independent of the gas mixture and the pressure. In the optimized multi-GEM structure, the ion feedback current can be suppressed down to 0.5% of the anode current, at a drift field of 0.1 kV/cm and gain of 10^4. A simple model of ion feedback in multi-GEM structures is suggested. The results obtained are relevant to the performance of time projection chambers and gas photomultipliers., 9 pages, 11 figures. Submitted to Nucl. Instr. and Meth. A

The results of testing of several photomultiplier tubes with microchannel plates (MCP PMT) and phototriodes developed in the Budker Institute of Nuclear Physics in 1997–2002 are presented. The study includes single anode MCP PMTs and several types of multianode tubes. The results on counting rate capabilities, dependence of the tube gain on external magnetic field and cross-talks between individual channels are presented. The design of a sealed gas photomultiplier is described.

We show that physics of multi-GEM structures is rather complex, regarding the number of phenomena affecting detector performance. The high-pressure operation in noble gases and the ion feedback are considered in more detail. It is proposed that the dominant avalanche mechanism in He and Ne, at high pressures, is the associative ionization. Ion feedback effects related to the dependence on gas, pressure and gain and to possible avalanche extension outside the GEM holes are discussed., Comment: Presented at the 8th International Conference on Instrumentation for Colliding Beam Physics, Novosibirsk, Febuary 28 - March 6, 2002. To be published in Nucl. Instr. and Meth. A

An upgrade of a special system to tag scattered electrons from two-photon processes, based on triple-GEM detectors, is described. The system is intended for the experiments at the VEPP-4M storage ring. Tests of the detector prototype with two gas mixtures confirm the feasibility of the chosen upgrade layout.

The properties of a 3-GEM (Gas Electron Multiplier) element photomultiplier, with a semitransparent CsI photocathode and CF 4 gas filling, are presented. Compared to other gas mixtures, such as CH 4 , Ar/CH 4 , Ar/N 2 and He/Ar/N 2 , CF 4 has superior performance: the highest gain, approaching 10 7 , 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/CH 4 (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.

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 10000 per second. 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., 11 pages, 9 figures. Submitted to JINST

Characteristic properties of two-phase avalanche detectors are further studied. Such detectors are relevant in the field of coherent neutrino-nucleus scattering, dark matter search and positron emission tomography (PET). The detectors investigated comprised a liquid Ar, Ar+N2 or Xe layer followed by a multi-GEM multiplier with CsI photocathode operated in saturated vapor. It was found that doping of two-phase Ar with N2 removed the slow electron emission component, existing in two-phase Ar, and enhanced the fast component. The electron emission efficiency was close to 100% both in two-phase Ar and Ar+N2 even at lower electric fields. The detection of 511 keV gamma-ray induced signals was demonstrated in two-phase Ar (scintillation signals) and two-phase Xe (ionization signals) avalanche detectors, in connection with PET applications.

The current design of the Micro Strip Gas Chamber Tracker of the Compact Muon Solenoid experiment (CMS) at LHC is presented. An extensive study of the performance of the final layout MSGC detectors is being carried out in dedicated tests that reproduce LHC-like conditions. Results from these tests are reported, showing the reliability of the MSGC detectors optimized for tracking in the CMS environment.

Gas avalanche detectors, combining solid photocathodes with fast electron multipliers, provide an attractive solution for photon localization over very large sensitive areas and under high illumination flux. They offer single-photon sensitivity and the possibility of operation under very intense magnetic fields. We discuss the principal factors governing the operation of gas avalanche photomultipliers. We summarize the recent progress made in alkali-halide and CVD-diamond UV-photocathodes, capable of operation under gas multiplication, and novel thin-film protected alkali-antimonide photocathodes, providing, for the first time, the possibility of operating gas photomultipliers in the visible range. Electron multipliers, adequate for these photon detectors, are proposed and some applications are briefly discussed.

We present the results of detailed investigations of the Gas Electron Multiplier (GEM)-based photomultiplier, consisting of a solid CsI photocathode coupled to a cascade of GEM elements. The detector is "lled with non-ageing mixtures based on noble gases: Ar, Ne, Ar#Ne, Ar#Xe, Ar#CH 4 and Ar#N 2 . Very high gas gains, reaching 106, and rather fast anode pulses, of a width of 10 ns, were observed in some mixtures. Various phenomena and physical processes, found to a!ect the device operation, are discussed here: additional gain due to secondary scintillation; mixtures with enhanced ionization e$ciency; improvement of pulse-height resolution due to avalanche con"nement in the GEM holes; avalanche extension outside the GEM holes; gain limitation due to ion feedback and charging-up of GEM electrodes; photoelectron backscattering. ( 2000 Elsevier Science B.V. All rights reserved.

We present the results of our further investigation of the Gas Electron Mutiplier (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. ( 2000 Elsevier Science B.V. All rights reserved.

We report on the results of coating K–Cs–Sb visible photocathodes, with thin CsBr and CsI protective films, in view of their possible combination with gas electron multipliers. Absolute quantum efficiency, photoelectron transport through the coating film, the effect of exposure to oxygen and to air and the photocathode behavior under intense photon flux are presented. The coated K–Cs–Sb photocathodes have typically maximum quantum efficiency values of 5–6% at 312 nm. They can withstand exposure to 150 Torr of oxygen for more than half an hour. Appropriate electron multipliers and possible applications are briefly discussed.

We have studied the operation of the Gas Electron Multiplier (GEM) in gas mixtures Xe–CO 2 , Ar–CO 2 and CH 4 at different pressures varying from 0.1 to 5 atm. In Ar- and Xe-based mixtures, the maximum GEM gain considerably decreases with pressure, from a few hundreds at 1 atm to below 10 at 5 atm. Combined gain of GEM and the Micro-Strip Gas Chamber (MSGC) can exceed values of 10000 at 1 atm and 100 at 5 atm. High GEM gains, of above 1000, were obtained in CH 4 at low pressures. We have observed the effect of the avalanche confinement in GEM micro-holes, resulting in violation of the pressure scaling and in the possibility of GEM operation in pure noble gases.

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 μ m vacuum-deposited hexatriacontane film, which can be subsequently removed by low-temperature sublimation . We show that Cs 3 Sb 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.

An experimental setup and procedures for the preparation of high-quality reflective and semitransparent alkali-antimonide photocathodes in laboratory conditions is described. Absolute quantum efficiency curves for KCsSb and Cs 3 Sb photocathodes are presented in the spectral range of 250–550 nm; their average quantum efficiency values at 312 nm are 32% and 15%, respectively. The photocathodes are stable in vacuum and in high-purity methane over a few days period. The system also permits photocathode coating with protective films.

A new semiconducting coating material for microstrip gas chambers (MSGCs) has been studied: AlN. The conductivity of AlN films at room temperature varies from 10−10 to 10 −12 Ω −1 cm −1 depending on the preparation conditions; it increases with temperature with an activation energy of 0.8–0.9 eV. The electron-induced secondary electron emission yield from AlN has been measured: it is about 5 at the maximum. The influence of the substrate nature and that of the film’s preparation conditions on the composition of AlN films are investigated using X-ray photoelectron spectroscopy .

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 A 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.

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, calcium stearate) 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.

Wire-chamber gaseous electron multipliers coupled to CsI photocathodes, provide interesting means for UV-photon imaging over a very large area. We report on the measurement of the CsI resistivity, important for the understanding of rate-dependent effects in CsI-based detectors. Photon counting at high rates is also closely connected to long-term photocathode stability. We present results on ageing of CsI photocathodes under high photon flux, with one atmosphere CH4 and under a detector gain of 105, for several substrate materials. We compare the ageing results with that of CsI photocathodes coated with thin NaF protective films.

The multi-channel optical readout of a THGEM multiplier coupled to a matrix of 3x3 Geiger-mode APDs (GAPDs) was demonstrated in a two-phase Cryogenic Avalanche Detector (CRAD) in Ar. The GAPDs recorded THGEM-hole avalanches in the Near Infrared (NIR). At an avalanche charge gain of 160, the yield of the combined THGEM/GAPD-matrix multiplier amounted at ~80 photoelectrons per 20 keV X-ray absorbed in the liquid phase. A spatial resolution of 2.5 mm (FWHM) has been measured for the impinging X-rays. This technique has potential applications in coherent neutrino-nucleus scattering and dark matter search experiments., Comment: 4 pages, 6 figures. Presented at Vienna Conference of Instrumentation (Feb 15-20, 2013, Vienna, Austria). Submitted to the Proceedings

The scintillation properties of CF4 are presented in comparison with those of Xe and CH4. Alpha-particle induced photon emission was measured with vacuum phototubes and with a CsI-based gaseous photomultiplier. The latter method provides an absolute sensitivity of such devices to particle-induced UV-photon background in CF4 and CH4 gaseous Cherenkov radiators. Integrated CF4 scintillation yields over the range of 150–220 nm are, on the average, 315±95 to 242±60 photons/MeV, in the respective pressure range of 0.063 to 0.75 atm, compared to CH4 which emits 0.06±0.01 photons/MeV at 1 atm. The total photon yield, integrated over the full emission spectrum of CF4 (150–500 nm), is of the order of 1200 photons/MeV × 4 π. The primary scintillation photon yield of CF4 is about 16(±5)% of that of Xe. No proportional secondary scintillation was observed in CF4. The avalanche-induced photon yield was measured to be of the order of 0.3 photons per electron. The implications of this considerable photon emission, are discussed.

A methodology to measure Near Infrared (NIR) scintillations in gaseous and liquid Ar, using Geiger-mode APDs (GAPDs) sensitive in the NIR and pulsed X-ray irradiation, is described. This study has been triggered by the development of Cryogenic Avalanche Detectors (CRADs) with optical readout in the NIR using combined THGEM/GAPD multiplier, which may come to be in demand in coherent neutrino-nucleus scattering and dark matter search experiments. A new approach to measure the NIR scintillation yield at cryogenic temperatures has been developed, namely using GAPDs in single photoelectron counting mode with time resolution. The time structure of NIR scintillations and their light yield were measured both for primary scintillations and that of secondary at moderate electric fields (electroluminescence), in gaseous and liquid Ar., Comment: 17 pages, 15 figures. Submitted to JINST

We present here a comprehensive study of the light yield of primary and secondary scintillations produced in gaseous and liquid Ar in the near infrared (NIR) and visible region, at cryogenic temperatures. The measurements were performed using Geiger-mode avalanche photodiodes (GAPDs) and pulsed X-ray irradiation. The primary scintillation yield of the fast emission component in gaseous Ar was found to be independent of temperature in the range of 87-160 K; it amounted to 17000+/-3000 photon/MeV in the NIR in the range of 690-1000 nm. In liquid Ar at 87 K, the primary scintillation yield of the fast component was considerably reduced, amounting to 510+/-90 photon/MeV, in the range of 400-1000 nm. Proportional NIR scintillations (electroluminescence) in gaseous Ar were also observed; their amplification parameter at 160 K was measured to be 13 photons per drifting electron per kV. No proportional scintillations were observed in liquid Ar up to the electric fields of 30 kV/cm. The applications of NIR scintillations in dark matter search and coherent neutrino-nucleus scattering experiments and in ion beam radiotherapy are considered., Comment: 20 pages, 11 figures. Submitted to JINST

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 10x10 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., Comment: 22 pages, 13 figures. Submitted to JINST