научная статья по теме DARK MATTER SEARCHES BY THE BOULBY COLLABORATION AND LIQUID XENON PROTOTYPE DEVELOPMENT Физика

Текст научной статьи на тему «DARK MATTER SEARCHES BY THE BOULBY COLLABORATION AND LIQUID XENON PROTOTYPE DEVELOPMENT»

HREPHAH 0H3HKA, 2004, moM 67, № 11, c. 2053-2061

= DOUBLE-BETA DECAY AND RARE PROCESSES -

DARK MATTER SEARCHES BY THE BOULBY COLLABORATION AND LIQUID XENON PROTOTYPE DEVELOPMENT

© 2004 A. S. Howard* (on behalf of the Boulby Dark Matter Collaboration1))

High Energy Physics, Blackett Laboratory, Imperial College London, United Kingdom

Received April 16, 2004

The current status of direct dark matter searches by the Boulby Dark Matter Collaboration is presented with the latest result from the ZePLiN I liquid xenon detector. An upper limit in the interaction cross section per nucleon of x 10~6 pb for a WIMP mass of 100 GeV is found. Details of ZePLiNs II and III — two future liquid xenon dark matter are presented. Extensive two-phase liquid-gas xenon prototype work has been undertaken and results of characterization studies are presented. The detector response to internal alpha and external gamma and neutron sources is shown. The potential discrimination power of the two-phase technique is displayed. Finally, prospects for the future dark matter search program is discussed.

1. INTRODUCTION

The evidence for Dark Matter has recently been given a boost from the results of the WMAP Cosmic Microwave Background Survey [1]. The results of the fitted power spectrum indicate that —23% of the Universe is made up of nonbaryonic "dark" matter, whereas the luminous baryonic matter constitutes only —4%. The most theoretically favored candidate for nonbaryonic Dark Matter is the Lightest Super-symmetric Party (LSP) which should only interact weakly. The current most probable mass for the LSP is 100 GeV due to SUSY model constraints from existing parameter measurements. In this paper the current status of direct Weakly Interacting Massive Particle (WIMP) dark matter searches is presented together with prospects for future experiments and results from prototype detectors.

The Boulby Dark Matter Collaboration (formerly UKDMC) has been searching for nonbaryonic matter since the early 1990's. The collaboration comprises 9 institutes from 5 countries. Recently a new large (150 x 4 m) experimental facility has been constructed in the Boulby mine in North Yorkshire, England. The mine is a 1.1 km deep working salt mine that offers an excellent environment for searching for dark matter. The depth and lack of local radiation is a particular benefit and reduces significantly background contributions from neutrons, muons and other cosmic or radioactive sources.

1)Boulby Dark Matter Collaboration: Imperial, RAL, Sheffield;

UCLA, Texas A&M; Pisa; ITEP; Coimbra; Edinburgh.

* E-mail: a.s.howard@imperial.ac.uk

The Boulby Dark Matter Collaboration is using liquid xenon as a target material for current (ZePLiN I) and future (ZePLiNs II, III) dark matter detectors. Xenon offers a close match between the expected 100-GeV most probable LSP and the xenon nucleus (121 GeV). Liquid xenon is an attractive choice of target material for a number of reasons, including:

1) The nuclear mass very close to favored LSP mass (-100 GeV),

2) Can be produced in large quantities relatively cheaply due to its cryogenic nature,

3) Can be purified to very high levels (measured electron lifetimes >1 ms),

4) It is very low background (85Kr is the only major radioactive contaminant),

5) Pulse-Shape Discrimination via scintillation,

6) Two-Phase Discrimination via ionization and scintillation channels,

7) The decay time of the scintillation has two major components: recombination and deexcitation,

8) Ionization is also produced.

The primary signal source in xenon is scintillation in the vacuum ultraviolet at 175 nm. Initially an electromagnetic interaction produces either excitation or ionization. The excited state can then form a dimer and deexcite via either a longer-lived triplet or shorter singlet state. A schematic for the scintillation mechanism is shown in Fig. 1.

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HOWARD

Xe*

+Xe

175

Xe*

Triplet H

27 nsJl

Singlet 3 ns

Xe+

+Xe

Xe2

+ e-

(recombination)

Xe** + Xe

175 nm

2 Xe 2 Xe

Fig. 1. The scintillation mechanism in liquid xenon.

PMT

LXe

Veto

Fig. 2. A cut-away schematic of the ZePLiN I detector.

Two-Phase Xenon

In addition to the primary scintillation, interactions in xenon can also produce free ionization. Without an applied electric field this leads to characteristic recombination which allows the distinction between nuclear and electron recoils. Upon application of an electric field, the ionization can drift through the liquid phase and up towards the gas. With a sufficiently high electric field, the electrons can cross the liquidgas interface and then drift through the gas at high velocity [2]. These electrons can then produce further photons via electroluminescence [3]. The multitude of subsequent photons produced leads to an "amplification" of the ionization signal and the possibility of single electron detection. Due to the different energy-loss densities, the probability for recombination between densely ionizing massive nuclear recoils and higher velocity electron recoils is vastly different. This then gives the possibility of excellent discrimination by merely measuring the scintillation and ioniza-tion signals. A WIMP interaction should generate a greater proportion of primary scintillation compared to ionization, whilst a background gamma interaction should produce approximately equal quantities of scintillation and ionization. Xenon can thus be used as a two-phase dark matter detector (so-called due to the utilization of both the liquid and gas phases for the detection of scintillation and ionization).

2. THE ZePLiN I DARK MATTER DETECTOR

The ZePLiN I (ZonEd Proportional scintillation LIquid Noble Gas) liquid xenon detector has

been underground since January 2001. The detector comprises 3.1 kg of liquid xenon and utilizes three photomultipliers to detect the scintillation emission. The use of multiple PMT's allows the definition of an internal fiducial volume and also removes PMT glass radioactivity via the implementation of stand-off "spigots". A cut-away schematic is shown in Fig. 2. The target vessel is constructed out of oxygen free copper to reduce radioactivity and impurity levels. Surrounding the vessel is a PXE-based organic liquid scintillator veto to remove high-energy gammas giving low-energy Compton scatters in the target. Liquid xenon scintillation has previously been shown to give good discrimination between background gammas and signal-like neutron-induced nuclear recoils [3—6]. With the ZePLiN I detector a neutron calibration was carried with an Am—Be source at the surface prior to underground installation. The resultant time constant distribution (fitted to the exponential-like scintillation decay time) is shown in Fig. 3 for gamma Compton interactions (top graph) and for the Am—Be source (lower graph). A clear population of faster time constant nuclear recoils can be seen on the left of the graph. So far, 220 kg d of data have been acquired with the ZePLiN I detector and the resulting preliminary WIMP cross-section limit is shown in Fig. 4, the minimum of which is 10_6 pb for an optimum WIMP mass of ~ 100 GeV.

This result is preliminary and requires further understanding in terms of threshold, energy resolution homogeneity, fiducial definition and discrimination power, particularly at low energy.

Time constant

Fig. 3. The time constant measured in the ZePLiN I detector for Compton gamma interactions (top) and Am— Be elastic and inelastic nuclear recoils (bottom).

2.1. ZePLiN II

A two-phase xenon detector, ZePLiN II, is currently under construction and due for deployment at the end of 2004. The detector comprises 30 kg of liquid xenon operating in a low-electric-field "two-phase" liquid-gas mode. The choice of field provides enhanced positive gamma detection whilst still allowing limited pulse-shape discrimination between WIMP-induced nuclear recoils and background gamma events. A design schematic of the detector is shown in Fig. 5. The vessel is cast in brass to ease construction and reduce radioactivity. Internally, the use of PTFE and field shaping grids provides close to a 100% active volume. The electric field is insufficient (~100 V/cm) to allow nuclear recoil ionization to separate, thus a WIMP recoil signal does not contain secondary scintillation. Therefore, any insensitive regions will result in reduced performance.

Examples of PMT signals from a 1-kg ZePLiN II prototype are shown in Fig. 6. In the case of the alpha event (left-hand trace), clear suppression of the ionization and hence secondary scintillation can be seen, especially compared to a gamma interaction (right-hand trace). A two-phase plot of secondary-vs.-primary scintillation can be seen in Fig. 7 for signals induced by an Am—Be neutron source. A

Cross section, cm2 (normalised to nucleón)

Fig. 4. The preliminary WIMP cross section/baryonic nucleon after 220 kg d of data from the ZePLiN I detector

(2), in comparison with CDMS [7] (/), EDELWEISS [8]

(3) and DAMA [9] (closed region).

clear population with zero secondary ionization can be seen close to the ^-axis. In addition, the reduced electric field should allow second-order pulse-shape discrimination between gammas and nuclear recoils.

2.2. ZePLiN III In parallel to the development of ZePLiN II, a high electric field two-phase liquid xenon detector, ZePLiN III, is also being constructed and is due for installation underground by the end of 2004. The detector comprises 50 kg of xenon giving an active volume of ^7 kg, fiducially defined as a high electric field (10 kV/cm), high extraction (~99%) electroluminescent region. The electric field will, in particular, give extreme sensitivity to the ionization produced (single electron), whilst the internal PMT array provides position sensitivity, and the large xenon volume removes background (passive shielding through periphery volumes). A design drawing of the detector is shown in Fig. 8. Thirty-one 2

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