Dark Matter Experiments at Boulby mine The Boulby Dark Matter Collaboration

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

NAIAD - NaI Advanced Detector ZEPLIN - ZonEd Proportional scintillation in LIquid Noble gases

DRIFT - Directional Recoil Identification from TracksMotivation:

• 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

«NAIAD» by John William Waterhouse

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

gammasource

neutronsource

electronrecoils fromgammabackground

nuclearrecoils

Compton calibrationwith gamma source - electron recoils

Data shows one population of scintillation pulses + PMT noise

6-7 keV 7-8 keV

UKDMC, measuredfor DM77

Sakai, IEEE Transactions on Nuclear Science, vol. NS-34, 1987 - 1 cm crystal

DAMA, preprint INFN/AE-00/10, 2000

Energy calibration and energy resolution

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.

1 ton liquid PXE scintillator Veto

Xe Target lined with PTFE reflector

Xe filled Turretscapped by quartz windows

3.7kg liquid Xe

Xe lineCoolant line

PhotomultiplierVacuum pump oninsulation jacket T90 / ns

Boulby underground laboratory - 1100 metres underground in the salt and potash mine

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Fits

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Calibrations with :• Various gamma sources• Am-Be neutron/gamma sourceVarious 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

ZEPLIN I - Liquid XeSinglet/triplet ratio differs for nuclear and electron recoilsRecombination is relevant only for electron recoils (=> t~45ns)Pulse Shape Analysis is applied

Noise cuts (asymmetry cuts, fiducial volume cuts) are applied using projection of normalised amplitude from each PMT onto a plane - S3 cut

Background rejection by Compton veto (liquid scintillator) and S3 cut

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• 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

Spin-independent pb limits for ZEPLIN I, showing other world limits and DAMA signal region

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ZEPLIN I (25 d)ZEPLIN I (est 8m)EdelWeissCDMSIGEXDAMA (ann mod)

ZEPLIN II - Double Phase Xe Detector - under construction (30 kg)

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.

Active volume

Gas phase

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Electro-luminescence

Primary-Scintillation

nuclear recoils: high primary scintillation, low ionisation yield (2nd scintillation)electron recoils: low primary scintillation, high ionisation yield (2nd scintillation)

3 modules

80 kg target

4 sub- units

shielding

ZEPLIN III and ZEPLIN MAX - Double Phase Xe Detector withhigh electric field - at R&D phase

• alpha population gradually moves closer to vertical as the E-field is increased

• Primary vs secondary scintillation for alphas for several values of electric field

The effect of increasing the voltage from 7kV to 12kV

Towards 1 ton Xe detectorZEPLIN MAX

Poster made by V. A. Kudryavtsev, University of Sheffield, UKE-mail: v.kudryavtsev@sheffield.ac.uk, http://www.shef.ac.uk/~phys/people/vitaly/

UKDMC web-site: http:// hepwww.rl.ac.uk/ukdmc/

New Caverns at Boulby50 m

New surface lab

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

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)

Simulation of tracks from different particles in a low pressure gas

Negative ion DRIFT:electron capture by electronegative gas reduces track diffusion (~0.5 mm at 0.5 m drift length)

Electric Field

Scattered WIMP

MWPC Readout

Plane

Cathode

Drift direction

CSCS22

Recoil Atom

DRIFT I - first directional WIMP detector

DRIFT I at Boulby

Large scale DRIFT design

High resolution readout

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b NaI 1996

NAIAD/Xe 2002/3

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DRIFT 2004/5

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Xe-MAX 2006

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ZEPLIN II

DRIFT II and DRIFT III - towards a 100 kg directional detector

Vacuum Vessel Optics Modules

Projected sensitivity of WIMP detectors at Boulby

Preliminary limits from ZEPLIN I

ZEPLIN I: results

Response of a prototype detector to gammas and neutrons

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gammas

recoil discrimination