David Vartsky

New approaches to spectroscopy and imaging of ultrasoft-to-hard X-rays

Abstract 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.

Investigation of CsI and CsI-TMAE VUV-photocathodes in vacuum and gas media

Abstract 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 CH 4 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.

First observation of liquid-xenon proportional electroluminescence in THGEM holes

Radiation-induced proportional-electroluminescence UV signals, emitted from the holes of a Thick Gas Electron Multiplier (THGEM) electrode immersed in liquid xenon, were recorded with a PMT for the first time. Significant photon yields were observed with gamma photons and alpha particles using a 0.4 mm thick electrode with 0.3 mm diameter holes; at 2 kV across the THGEM the photon yield was estimated to be ~ 600 UV photons/electron over 4π. This may pave the way towards the realization of novel single-phase noble-liquid radiation detectors incorporating liquid hole-multipliers (LHM); their concept is presented.

Liquid Hole Multipliers: bubble-assisted electroluminescence in liquid xenon

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.

First results of a large-area cryogenic gaseous photomultiplier coupled to a dual-phase liquid xenon TPC

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 10^5, providing high single-photon detection efficiency even in the presence of large alpha 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 ({sigma}/E) for S2 electroluminescence of 5.5 MeV alpha 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.

A comprehensive simulation study of a Liquid-Xe detector for contraband detection

Recently, a new detector concept, for combined imaging and spectroscopy of fast-neutrons and gamma was presented. It encompasses a liquid-xenon (LXe) converter-scintillator coupled to a UV-sensitive gaseous Thick Gas Electron Multiplier (THGEM)-based imaging photomultiplier (GPM). In this work we present and discuss the results of a systematic computer-simulation study aiming at optimizing the type and performance of LXe converter. We have evaluated the detector spectral response, detection efficiency and spatial resolution for gamma-rays and neutrons in the energy range of 2-15 MeV for 50 mm thick converters consisting of plain LXe volume and LXe-filled capillaries, of Teflon, Polyethylene or hydrogen-containing Teflon (Tefzel). Neutron detection efficiencies for plain LXe, Teflon-capillaries and Tefzel-capillaries converters were about 20% over the entire energy range. In polyethylene capillaries converters the neutron detection efficiency was about 10% at 2 MeV and increased up to about 20% at 14 MeV. Detection efficiencies of gammas in Teflon, Tefzel and polyethylene converters were ~35%. The plain-LXe converter provided the highest gamma-ray detection efficiency, of ~40-50% for 2-15 MeV energy range. Optimization of LXe-filled Tefzel capillary dimensions resulted in spatial resolution of ~1.5mm (FWHM) for neutrons and up to 3.5 mm (FWHM) for gamma-rays. Simulations of radiographic images of various materials using two discrete energy gamma-rays (4.4 MeV and 15.1 MeV) and neutrons in broad energy range (2-10 MeV) were performed in order to evaluate the potential of elemental discrimination.

Direct observation of bubble-assisted electroluminescence in liquid xenon

Bubble formation in liquid xenon underneath a Thick Gaseous Electron Multiplier (THGEM) electrode immersed in liquid xenon was observed with a CCD camera. With voltage across the THGEM, the appearance of bubbles was correlated with that of electroluminescence signals induced by ionization electrons from alpha-particle tracks. This confirms recent indirect evidence that the observed photons are due to electroluminescence within a xenon vapor layer trapped under the electrode. The bubbles seem to emerge spontaneously due to heat flow from 300K into the liquid, or in a controlled manner, by locally boiling the liquid with resistive wires. Controlled bubble formation resulted in energy resolution of {sigma}/E~7.5% for ~6,000 ionization electrons. The phenomenon could pave ways towards the conception of large-volume local dual-phase noble-liquid TPCs.

First demonstration of VUV-photon detection in liquid xenon with THGEM and GEM-based Liquid Hole Multipliers

Abstract The bubble-assisted Liquid Hole-Multiplier (LHM) is a recently-introduced detection concept for noble-liquid time projection chambers. In this “local dual-phase” detection element, a gas bubble is supported underneath a perforated electrode (e.g., Thick Gas Electron Multiplier – THGEM, or Gas Electron Multiplier – GEM). Electrons drifting through the holes induce large electroluminescence signals as they pass into the bubble. In this work we report on recent results of THGEM and GEM electrodes coated with cesium iodide and immersed in liquid xenon, allowing – for the first time – the detection of primary VUV scintillation photons in addition to ionization electrons.

Cryogenic gaseous photomultipliers and liquid hole- multipliers: advances in THGEM-based sensors for future noble-liquid TPCs

Dual-phase noble-liquid TPCs are presently the most sensitive instruments for direct dark matter detection. Scaling up existing ton-scale designs to the multi-ton regime may prove to be technologically challenging. This includes both large-area coverage with affordable high-QE UV-photon detectors, and maintaining high precision in measuring the charge and light signals of rare events with keV-scale energy depositions. We present our recent advances in two complementary approaches to these problems: large-area cryogenic gaseous photomultipliers (GPM) for UV-photon detection, and liquid-hole multipliers (LHM) that provide electroluminescence light in response to ionization electrons and primary scintillation photons, using perforated electrodes immersed within the noble liquid. Results from a 10 cm diameter GPM coupled to a dual-phase liquid- xenon TPC demonstrate the feasibility of recording - for the first time - both primary (S1) and secondary (S2) scintillation signals, over a very broad dynamic range. The detector, comprising a triple-THGEM structure with CsI on the first element, has been operating stably at 180 K with gains larger than 105; it provided high single-photon detection efficiency - in the presence of massive alpha-particle induced S2 signals; S1 scintillation signals were recorded with time resolutions of 1.2 ns (RMS). Results with the LHM operated in liquid xenon yielded large photon gains, with a pulse-height resolution of 11% (RMS) for alpha-particle induced S2 signals. The detector response was stable over several months. The response of the S2 signals to rapid changes in pressure lead to the conclusion that the underlying mechanism for S2 light is electroluminescence in xenon bubbles trapped below the immersed THGEM electrode. Both studies have the potential of paving the way towards new designs of dual- and single-phase noble-liquid TPCs that could simplify the conception of future multi-ton detectors of dark matter and other rare events.

Method for detection of explosives based on nuclear resonance absorption of gamma rays in 14 N

The physical principles of the nuclear resonance absorption method and its application to explosives detection are described. In this method, the object to be tested is scanned by a beam of 9.17 MeV gamma rays, which undergoes resonant attenuation whenever the beam encounters regions of nitrogen concentration. This resonant component of attenuation is detected using an array of gamma ray detectors containing a nitrogen-rich medium. From the reconstructed spatial distribution of nitrogen density obtained in multiview scanning, the presence of an explosive can be determined.