Reduction of Coincident Photomultiplier Noise Relevant to Astroparticle Physics Experiment

In low background and low threshold particle astrophysics experiments using observation of Cherenkov or scintillation light it is common to use pairs or arrays of photomultipliers operated in coincidence. In such circumstances, for instance in dark matter and neutrino experiments, unexpected PMT noise events have been observed, probably arising from generation of light from one PMT being detected by one or more other PMTs. We describe here experimental investigation of such coincident noise events and development of new techniques to remove them using novel pulse shape discrimination procedures. When applied to data from a low background NaI detector with facing PMTs the new procedures are found to improve noise rejection by a factor of 20 over conventional techniques, with significantly reduced loss of signal events.Comment: Submitted to NIM

Cryogenic Gaseous Photomultiplier for position reconstruction of liquid argon scintillation light

Presented here are first tests of a Gaseous Photomultiplier based on a cascade of Thick GEM structures intended for gamma-ray position reconstruction in liquid argon. The detector has a MgF2 window, transparent to VUV light, and a CsI photocathode deposited on the first THGEM . A gain of 8⋅ 105 per photoelectron and ~ 100% photoelectron collection efficiency are measured at stable operation settings. The excellent position resolution capabilities of the detector (better than 100 μm) at 100 kHz readout rate, is demonstrated at room temperature. Structural integrity tests of the detector and seals are successfully performed at cryogenic temperatures by immersing the detector in liquid Nitrogen, laying a good foundation for future operation tests in noble liquids

Low energy neutron propagation in MCNPX and GEANT4

Simulations of neutron background from rock for underground experiments are presented. Neutron propagation through two types of rock, lead and hydrocarbon material is discussed. The results show a reasonably good agreement between GEANT4, MCNPX and GEANT3 in transporting low-energy neutrons.Comment: 9 Figure

Muon-induced neutron production and detection with GEANT4 and FLUKA

We report on a comparison study of the Monte Carlo packages GEANT4 and FLUKA for simulating neutron production by muons penetrating deep underground. GEANT4 is found to generate fewer neutrons at muon energies above ~100 GeV, by at most a factor of 2 in some materials, which we attribute mainly to lower neutron production in hadronic cascades. As a practical case study, the muon-induced neutron background expected in a 250 kg liquid-xenon WIMP dark matter detector was calculated and good agreement was found for the recoil event rates. The detailed model of neutron elastic scattering in GEANT4 was also shown to influence the nuclear recoil spectrum observed in the target, which is presently a shortcoming of FLUKA. We conclude that both packages are suited for this type of simulation, although further improvements are desirable in both cases.Comment: (23 pages, 14 figures) To appear in Nucl. Instrum. Meth. A v2: Changes to format only; v3: Corrected typo in front matter; v4: Looked up additional experimental data for comparison with simulation

Demonstration of ThGEM-multiwire hybrid charge readout for directional dark matter searches

Sensitivities of current directional dark matter search detectors using gas time projection chambers are now constrained by target mass. A ton-scale gas TPC detector will require large charge readout areas. We present a first demonstration of a novel ThGEM-Multiwire hybrid charge readout technology which combines the robust nature and high gas gain of Thick Gaseous Electron Multipliers with lower capacitive noise of a one-plane multiwire charge readout in SF6 target gas. Measurements performed with this hybrid detector show an ion drift velocity of 139 ± 12 ms−1 in a reduced drift field E/N of 93 Td (10−17 V cm2) at a gas gain of 2470±160 in 20 Torr of pure SF6 target gas

First measurement of low intensity fast neutron background from rock at the Boulby Underground Laboratory

A technique to measure low intensity fast neutron flux has been developed. The design, calibrations, procedure for data analysis and interpretation of the results are discussed in detail. The technique has been applied to measure the neutron background from rock at the Boulby Underground Laboratory, a site used for dark matter and other experiments, requiring shielding from cosmic ray muons. The experiment was performed using a liquid scintillation detector. A 6.1 litre volume stainless steel cell was filled with an in-house made liquid scintillator loaded with Gd to enhance neutron capture. A two-pulse signature (proton recoils followed by gammas from neutron capture) was used to identify the neutron events from much larger gamma background from PMTs. Suppression of gammas from the rock was achieved by surrounding the detector with high-purity lead and copper. Calibrations of the detector were performed with various gamma and neutron sources. Special care was taken to eliminate PMT afterpulses and correlated background events from the delayed coincidences of two pulses in the Bi-Po decay chain. A four month run revealed a neutron-induced event rate of 1.84 +- 0.65 (stat.) events/day. Monte Carlo simulations based on the GEANT4 toolkit were carried out to estimate the efficiency of the detector and the energy spectra of the expected proton recoils. From comparison of the measured rate with Monte Carlo simulations the flux of fast neutrons from rock was estimated as (1.72 +- 0.61 (stat.) +- 0.38 (syst.))*10^(-6) cm^(-2) s^(-1) above 0.5 MeV.Comment: 37 pages, 24 figures, to be published in Astroparticle Physic

CsI(Tl) for WIMP dark matter searches

We report a study of CsI(Tl) scintillator to assess its applicability in experiments to search for dark matter particles. Measurements of the mean scintillation pulse shapes due to nuclear and electron recoils have been performed. We find that, as with NaI(Tl), pulse shape analysis can be used to discriminate between electron and nuclear recoils down to 4 keV. However, the discrimination factor is typically (10-15)% better than in NaI(Tl) above 4 keV. The quenching factor for caesium and iodine recoils was measured and found to increase from 11% to ~17% with decreasing recoil energy from 60 to 12 keV. Based on these results, the potential sensitivity of CsI(Tl) to dark matter particles in the form of neutralinos was calculated. We find an improvement over NaI(Tl) for the spin independent WIMP-nucleon interactions up to a factor of 5 assuming comparable electron background levels in the two scintillators.Comment: 16 pages, 8 figures, to be published in Nucl. Instrum. and Meth. in Phys. Res.

Measurements of Scintillation Efficiency and Pulse-Shape for Low Energy Recoils in Liquid Xenon

Results of observations of low energy nuclear and electron recoil events in liquid xenon scintillator detectors are given. The relative scintillation efficiency for nuclear recoils is 0.22 +/- 0.01 in the recoil energy range 40 keV - 70 keV. Under the assumption of a single dominant decay component to the scintillation pulse-shape the log-normal mean parameter T0 of the maximum likelihood estimator of the decay time constant for 6 keV < Eee < 30 keV nuclear recoil events is equal to 21.0 ns +/- 0.5 ns. It is observed that for electron recoils T0 rises slowly with energy, having a value ~ 30 ns at Eee ~ 15 keV. Electron and nuclear recoil pulse-shapes are found to be well fitted by single exponential functions although some evidence is found for a double exponential form for the nuclear recoil pulse-shape.Comment: 11 pages, including 5 encapsulated postscript figure

Charge amplification in sub-atmospheric CF4:He mixtures for directional dark matter searches

Low pressure gaseous Time Projection Chambers (TPCs) are a viable technology for directional Dark Matter (DM) searches and have the potential for exploring the parameter space below the neutrino fog [1,2]. Gases like CF4 are advantageous because they contain flourine which is predicted to have heightened elastic scattering rates with a possible Weakly Interacting Massive Particle (WIMP) DM candidate [3,4,5]. The low pressure of CF4 must be maintained, ideally lower than 100 Torr, in order to elongate potential Nuclear Recoil (NR) tracks which allows for improved directional sensitivity and NR/Electron Recoil (ER) discrimination [6]. Recent evidence suggests that He can be added to heavier gases, like CF4, without significantly affecting the length of 12C and 19F recoils due to its lower mass. Such addition of He has the advantage of improving sensitivity to lower mass WIMPs [1]. Simulations can not reliably predict operational stability in these low pressure gas mixtures and thus must be demonstrated experimentally. In this paper we investigate how the addition of He to low pressure CF4 affects the gas gain and energy resolution achieved with a single Thick Gaseous Electron Multiplier (ThGEM)

Energy calibration of large underwater detectors using stopping muons

We propose to use stopping cosmic-ray muons in the energy calibration of planned and deployed large underwater detectors. The method is based on the proportionality between the incident muon energy and the length of the muon path before it stops. Simultaneous measurements of the muon path and the amplitude of the signal from the photomultiplier tubes allow a relation between the energy deposited in the sensitive volume of the detector and the observed signal to be derived, and also provide a test of detector simulations. We describe the proposed method and present the results of simulations