N.J.C. Spooner

Large underground, liquid based detectors for astro-particle physics in Europe: Scientific case and prospects

This document reports on a series of experimental and theoretical studies conducted to assess the astro-particle physics potential of three future large scale particle detectors proposed in Europe as next generation underground observatories. The proposed apparatuses employ three different and, to some extent, complementary detection techniques: GLACIER (liquid argon TPC), LENA (liquid scintillator) and MEMPHYS (water Cherenkov), based on the use of large mass of liquids as active detection media. The results of these studies are presented along with a critical discussion of the performance attainable by the three proposed approaches coupled to existing or planned underground laboratories, in relation to open and outstanding physics issues such as the search for matter instability, the detection of astrophysical neutrinos and geo-neutrinos and to the possible use of these detectors in future high intensity neutrino beams.

The ZEPLIN-III dark matter detector: Instrument design, manufacture and commissioning

We present details of the technical design, manufacture and testing of the ZEPLIN-III dark matter experiment. ZEPLIN-III is a two-phase xenon detector which measures both the scintillation light and the ionisation charge generated in the liquid by interacting particles and radiation. The instrument design is driven by both the physics requirements and by the technology requirements surrounding the use of liquid xenon. These include considerations of key performance parameters, such as the efficiency of scintillation light collection, restrictions placed on the use of materials to control the inherent radioactivity levels, attainment of high vacuum levels and chemical contamination control. The successful solution has involved a number of novel design and manufacturing features which will be of specific use to future generations of direct dark matter search experiments as they struggle with similar and progressively more demanding requirements.

Limits on WIMP cross-sections from the NAIAD experiment at the Boulby Underground Laboratory

The NAIAD experiment (NaI Advanced Detector) for WIMP dark matter searches at the Boulby Underground Laboratory (North Yorkshire, UK) ran from 2000 until 2003. A total of 44.9 kg x years of data collected with 2 encapsulated and 4 unencapsulated NaI(Tl) crystals with high light yield were included in the analysis. We present final results of this analysis carried out using pulse shape discrimination. No signal associated with nuclear recoils from WIMP interactions was observed in any run with any crystal. This allowed us to set upper limits on the WIMP-nucleon spin-independent and WIMP-proton spin-dependent cross-sections. The NAIAD experiment has so far imposed the most stringent constraints on the spin-dependent WIMP-proton cross-section.

Neutron background in large-scale xenon detectors for dark matter searches

Abstract Simulations of the neutron background for future large-scale particle dark matter detectors are presented. Neutrons were generated in rock and detector elements via spontaneous fission and (α,n) reactions, and by cosmic-ray muons. The simulation techniques and results are discussed in the context of the expected sensitivity of a generic liquid xenon dark matter detector. Methods of neutron background suppression are investigated. A sensitivity of 10 −9 –10 −10 pb to WIMP-nucleon interactions can be achieved by a tonne-scale detector.

Optical readout tracking detector concept using secondary scintillation from liquid argon generated by a thick gas electron multiplier

For the first time secondary scintillation, generated within the holes of a thick gas electron multiplier (THGEM) immersed in liquid argon, has been observed and measured using a silicon photomultiplier device (SiPM). 250 electron-ion pairs, generated in liquid argon via the interaction of a 5.9 keV Fe-55 gamma source, were drifted under the influence of a 2.5 kV/cm field towards a 1.5 mm thickness THGEM, the local field sufficiently high to generate secondary scintillation light within the liquid as the charge traversed the central region of the THGEM hole. The resulting VUV light was incident on an immersed SiPM device coated in the waveshifter tetraphenyl butadiene (TPB), the emission spectrum peaked at 460 nm in the high quantum efficiency region of the device. For a SiPM over-voltage of 1 V, a THGEM voltage of 9.91 kV, and a drift field of 2.5 kV/cm, a total of 62±20 photoelectrons were produced at the SiPM device per Fe-55 event, corresponding to an estimated gain of 150±66 photoelectrons per drifted electron.

NaI dark matter limits and the NAIAD array – a detector with improved sensitivity to WIMPs using unencapsulated NaI

Re-analysis of published data from the UKDMC NaI Tl dark matter experiment is presented using latest spin factors and comparison is made with the sensitivity predicted for NAIAD, a 100 kg NaI detector concept based on unencapsulated . NaI Tl . We present experimental results and Monte Carlo simulations for NAIAD and show that a factor of 1.5-2 improvement in energy threshold is achievable over conventional NaI dark matter detectors with consequent ; 50% improvement in nuclear recoil discrimination at 10 keV. An overall improvement in sensitivity to spin dependent WIMP interactions of factor 50, based on 100 kg = yrs of data, is predicted relative to previous UKDMC limits. q 2000 Published by Elsevier Science B.V. All rights reserved.

Development of wavelength shifter coated reflectors for the ArDM argon dark matter detector

To optimise the design of the light readout in the ArDM 1-ton liquid argon dark matter detector, a range of reflector and WLS coating combinations were investigated in several small setups, where argon scintillation light was generated by radioactive sources in gas at normal temperature and pressure and shifted into the blue region by tetraphenyl butadiene (TPB). Various thicknesses of TPB were deposited by spraying and vacuum evaporation onto specular 3MTM-foil and diffuse Tetratex® (TTX) substrates. Light yields of each reflector and TPB coating combination were compared. Reflection coefficients of TPB coated reflectors were independently measured using a spectroradiometer in a wavelength range between 200 and 650 nm. WLS coating on the PMT window was also studied. These measurements were used to define the parameters of the light reflectors of the ArDM experiment. Fifteen large 120 × 25 cm2 TTX sheets were coated and assembled in the detector. Measurements in argon gas are reported providing good evidence of fulfilling the light collection requirements of the experiment.

Characterisation of a silicon photomultiplier device for applications in liquid argon based neutrino physics and dark matter searches

The performance of a silicon photomultiplier has been assessed at low temperature in order to evaluate its suitability as a scintillation readout device in liquid argon particle physics detectors. The gain, measured as 2.1 × 106 for a constant over-voltage of 4V was measured between 25°C and -196°C and found to be invariant with temperature, the corresponding single photoelectron dark count rate reducing from 1MHz to 40Hz respectively. Following multiple thermal cycles no deterioration in the device performance was observed. The photon detection efficiency (PDE) was assessed as a function of photon wavelength and temperature. For an over-voltage of 4V, the PDE, found again to be invariant with temperature, was measured as 25% for 460nm photons and 11% for 680nm photons. Device saturation due to high photon flux rate, observed both at room temperature and -196°C, was again found to be independent of temperature. Although the output signal remained proportional to the input signal so long as the saturation limit was not exceeded, the photoelectron pulse resolution and decay time increased slightly at −196°C.

A Search for the Dark Matter Annual Modulation in South Pole Ice

Astrophysical observations and cosmological data have led to the conclusion that nearly one quarter of the Universe consists of dark matter. Under certain assumptions, an observable signature of dark matter is the annual modulation of the rate of dark matter-nucleon interactions taking place in an Earth-bound experiment. To search for this e ect, we introduce the concept for a new dark matter experiment using NaI scintillation detectors deployed deep in the South Pole ice. This experiment complements dark matter search e orts in the Northern Hemisphere and will investigate the observed annual modulation in the DAMA/LIBRA and DAMA/NaI experiments. The unique location will permit the study of background e ects correlated with seasonal variations and the surrounding environment. This paper describes the experimental concept and explores the sensitivity of a 250 kg NaI experiment at the South Pole.

Measurements of muon flux at 1070 m vertical depth in the Boulby underground laboratory

Abstract Measurements of cosmic-ray muon rates and energy deposition spectra in a 1 t liquid scintillator detector at 1070 m vertical depth in the Boulby underground laboratory are discussed. In addition, the simulations used to model the detector are described. The results of the simulations are compared to the experimental data and conclusions given. The muon flux in the laboratory is found to be (4.09±0.15)×10 −8 cm −2 s −1 .