Dark Matter Experiments at Boulby mine The Boulby Dark Matter Collaboration

1. UKDMC, measured for DM77 Xe Target lined with PTFE reflector recoil discrimination Xe filled Turrets capped by quartz windows DAMA, preprint INFN/AE-00/10, 2000 3 modules gammas 4 sub- units CS2 C recoils Sakai, IEEE Transactions on Nuclear Science, vol. NS-34, 1987 - 1 cm crystal c2 S recoils Electro-luminescence 3.7kg liquid Xe Poisson shielding Gas phase 80 kg target Active volume T90 / ns 10 100 1000 1 ton liquid PXE scintillator Veto 50 m New Caverns at Boulby g Primary-Scintillation Scattered WIMP Recoil Atom Drift direction MWPC Readout Plane Cathode Electric Field Coolant line Xe line gamma source electron recoils from gamma background neutron source Photomultiplier Vacuum pump on insulation jacket nuclear recoils 6-7 keV 7-8 keV Compton calibration with gamma source - electron recoils Data shows one population of scintillation pulses + PMT noise Response of a prototype detector to gammas and neutrons Vacuum Vessel Optics Modules 10-3 NaI 1996 10-4 10-5 WIMP-nucleon cross-section, pb NAIAD/Xe 2002/3 10-6 Xe 2003/4 10-7 DRIFT 2004/5 Xe 2005 10-8 Xe-MAX 2006 10-9 10-10 WIMP mass, GeV Imperial College of Science, Technology and Medicine, London:B. Ahmed, A. Bewick, D. Davidge, J. V. Dawson, A. S. Howard, W. G. Jones, M. K. Joshi, V. Lebedenko, I. Liubarsky, R. LЯscher, T. J. Sumner, J. J. Quenby; Rutherford Appleton Laboratory:G. J. Alner, S. P. Hart, I. Ivaniouchenkov, J. D. Lewin, R. M. Preece, N. J. T. Smith, P.F. Smith;Queen Mary, University of London:J. C. Barton; University of Sheffield:M. J. Carson, T. Gamble, R. Hollingworth,V. A. Kudryavtsev, T. B. Lawson, P. K. Lightfoot, J. E. McMillan, B. Morgan, G. Nicklin, S. M. Paling, J. W. Roberts, M. Robinson, N. J. C. Spooner, D R. Tovey; Occidental College:D. P. Snowden-Ifft, J. Kirkpatrick; Temple University:C. J. Martoff, R. Ayad; UCLA:D. B. Cline, H. Wang, Y. Seo, M. Atac, F. Sergiampietri; LLNL:W. W. Craig; Columbia University: C. J. Hailey, M. Sileo, P. Graham, J. Hong; CERN/ICGF-CNR-Torino/INFN-Padova:P. Picchi, F. Pietropaolo, L. Periale, G. Mannocchi, C. Castagnoli; ITEP, Moscow:D. Akimov, A. Danilov; Texas A&M University: J. T. White, J. Gao; UMSNH, Morelia, Mexico: U. Cotti, M. Reyes, L. Villasenor;CINVESTAV, Mexico City:A. Zepeda Dark Matter Experiments at Boulby mineThe Boulby Dark Matter Collaboration NAIAD - NaIAdvanced Detector ZEPLIN - ZonEd Proportional scintillation in LIquid Noble gases DRIFT - Directional Recoil Identification from Tracks • Motivation: • Two different targets with high and low masses • Sensitive to both spin-independent and spin-dependent WIMP-nucleus interactions • Aimed to confirm or refute annual modulation signal, claimed by DAMA, with similar type detectors (NaI) but different analysis technique • Can be also used as a diagnostic array to study backgrounds and systematic effects for other dark matter experiments at Boulby Boulby underground laboratory - 1100 metres underground in the salt and potash mine «NAIAD» by John William Waterhouse New surface lab ZEPLIN I - Liquid Xe Directionality: • WIMP velocity distribution in the Earth’s frame is strongly peaked in the direction of the solar motion – A WIMP ‘wind’ • A strong signature - sidereal variation of the directions of recoil tracks • Distribution of recoil directions in galactic coordinates is peaked in direction opposite to solar motion Singlet/triplet ratio differs for nuclear and electron recoils Recombination is relevant only for electron recoils (=> t~45ns) Pulse Shape Analysis is applied • Calibrations with : • Various gamma sources • Am-Be neutron/gamma source • Various measurements of pulse shape • Fitted exponential, t • Mean photon arrival time, T90 • Time to reach 70%, T70 • Distributed as a gamma function in 1/X • Signal discrimination: • Pulse Shape Analysis is used to discriminate between nuclear recoils, which can be caused by WIMP interactions, and electron recoils due to gamma background • Light yield determines the discrimination power of the pulse shape analysis • Running unencapsulated crystals requires stability of the light yield • Each crystal is calibrated with gamma and neutron sources • Integrated pulses are fitted to an exponential • Time constant distributions are fitted to the log(Gauss) function (or two log(Gauss) functions in case of two components) + PMT noise • In real data we search for the second (fast) component with known parameters • DRIFT concept -low pressure (40 torr) gas Time Projection Chamber • Ionisation tracks > 1 mm. • Electrons are drifted in an electric field to the x-y readout region. • Drift time measurements provide z-co-ordinates of the tracks. • This allows full 3D reconstruction of events (track length, energy, orientation) Noise cuts (asymmetry cuts, fiducial volume cuts) are applied using projection of normalised amplitude from each PMT onto a plane - S3 cut Negative ion DRIFT: electron capture by electronegative gas reduces track diffusion (~0.5 mm at 0.5 m drift length) Simulation of tracks from different particles in a low pressure gas Background rejection by Compton veto (liquid scintillator) and S3 cut DRIFT I - first directional WIMP detector ZEPLIN I: results • 27 days of live time = 90 kg x days • gamma calibration data from contemporaneous veto events • Gamma function fit to 1/t distribution • Analysis: c2 in high statistics region, Poisson in tail Preliminary limits from ZEPLIN I • Xe detector with field: • Electric field prevents recombination and allows the measurement of the ionisation yield. • For electron recoils the track is less dense and the electric field is more efficient in separating electrons from ions. • Ionisation electron drifts towards a high field region in the gas phase. • Electro-luminescence light from the avalanche process around a multi-wire plane is detected as 2nd scintillation. DRIFT I at Boulby Energy calibration and energy resolution DRIFT II and DRIFT III - towards a 100 kg directional detector ZEPLIN II - Double Phase Xe Detector - under construction (30 kg) nuclear recoils: high primary scintillation, low ionisation yield (2nd scintillation) electron recoils: low primary scintillation, high ionisation yield (2nd scintillation) High resolution readout Large scale DRIFT design ZEPLIN III and ZEPLIN MAX - Double Phase Xe Detector with high electric field - at R&D phase Projected sensitivity of WIMP detectors at Boulby The effect of increasing the voltage from 7kV to 12kV NAIAD results • 6 crystals are currently running at Boulby. Data from 4 crystals have been analysed to set new limits on WIMP-nucleon spin-independent interactions; total exposure = 10.6 kg x years. • Significant improvement over previous limits (1996) has been achieved due to higher light yield and better discrimination. • Extensive studies of NaI(Tl) crystals and their response to various radiations have been performed (energy calibration and resolution, gamma and neutron calibrations etc.). • Pulse shape analysis has been proven to work in NaI detectors and to produce reliable limits. DAMA can use PSA to confirm or refute the positive signal found in annual modulation analysis. • Primary vs secondary scintillation for alphas for several values of electric field Towards 1 ton Xe detector ZEPLIN MAX Poster made by V. A. Kudryavtsev, University of Sheffield, UK E-mail: v.kudryavtsev@sheffield.ac.uk, http://www.shef.ac.uk/~phys/people/vitaly/ UKDMC web-site: http:// hepwww.rl.ac.uk/ukdmc/ • alpha population gradually moves closer to vertical as the E-field is increased ZEPLIN II