the DarkSide project Andrea Pocar University of Massachusetts, Amherst

Transcription

1 the DarkSide project the DarkSide project Andrea Pocar University of Massachusetts, Amherst Dark Matter Workshop, Aspen Physics Institute, 7-11 February, 2011

2 Outline the DarkSide program and collaboration DarkSide-50: objectives and key design elements two-phase depleted argon, low bg PMTs high efficiency neutron shield/veto integration with Borexino/CTF, muon veto sensitivity goal DarkSide R&D 39 Ar depleted argon procurement and measurement liquid argon light yield backgrounds 10 kg prototype Outlook and near future plans A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/2011 2

3 status of direct x dark matter searches direct detection of non-relativistic dark matter particles in the form of WIMPs is carried out by looking for signals of their elastic scattering on recoiling nuclei current experiments report tonne-day exposure SUSY models suggest that tonne-year exposure might be necessary in order to explore the full parameter space of such theories noble liquids (Xe, Ar, Ne) are used and are being further investigated as targets for large WIMP dark matter detectors A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/2011 3

4 the DarkSide design the DarkSide program combines new technology and design elements in a 2-phase argon TPC to meet the challenges for future dark matter searches with a powerful and scalable strategy i. depleted (low 39 Ar) argon from underground sources ii. high efficiency borated liquid scintillator neutron veto (>99.8% rejection efficiency for radiogenic neutrons) iii. low background photosensors with high QE photocathode iv. large active water shield to reduce external neutron background to tonne-year sensitivity at Gran Sasso depth goal: a background-free measurement via the suppression and best possible identification, discrimination of background events A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/2011 4

5 the DarkSide concept A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/2011 5

6 the DarkSide concept A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/2011 5

7 the DarkSide concept A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/2011 5

8 the DarkSide collaboration IHEP Beijing Augustana College Black Hills State University University of California, Los Angeles Fermilab University of Houston University of Massachusetts at Amherst Princeton University Temple University INFN Laboratori Nazionali del Gran Sasso INFN and Università degli Studi Genova INFN and Università degli Studi Milano INFN and Università degli Studi Napoli INFN and Università degli Studi Perugia JINR Dubna RRC Kurchatov Institute St. Petersburg Nuclear Physics Institute A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/2011 6

9 DarkSide-50 the DarkSide-50 detector is the firstphase implementation of the new technologies i. dual-phase TPC ii. 50 kg (active mass) liquid depleted argon (DAr) iii. designed to operate background-free for 3 years for a sensitivity for WIMP cross sections of ~ cm 2 iv. demonstrator for multi-ton-year background-free sensitivity A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/2011 7

10 why depleted argon? atmospheric argon contains radioactive 39 Ar, produced by cosmic rays via (n,2n) on 40 Ar β emitter, Q = 565 kev, t1/2 = 269 y 39 Ar/Ar ratio = specific activity = 1 Bq/kg atmospheric argon limits the size of an unsegmented two-phase detectors to kg due to the pile-up of 39 Ar events A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/2011 8

11 2-phase TPC gaseous Ar/Xe liquid Ar/Xe transparent inner vessel (optional) photodetectors A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/2011 fiducial volume boundary 9

12 2-phase TPC extraction and acceleration grids multiplication field (~3 kv/cm) field-shaping rings drift field (~1 kv/cm) A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/2011 9

13 primary scintillation photons emitted and detected (S1) 2-phase TPC WIMP scatter deposits energy in FV A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/2011 9

14 2-phase TPC secondary photons emitted by multiplication in gas region (S2) ionized electrons drifted to gas region (~2 mm/μs)) A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/2011 9

15 discrimination x in liquid argon LAr is an excellent scintillator. Pulse shape analysis of primary scintillation allows for very powerful discrimination between nuclear recoil and EM events rejection factors exceed for >50 photoelectrons with 50% nuclear recoil acceptance (Boulay&Hime 2004, Benetti et al (WARP) 2006) Ionization electron drift in argon over long distances is well established. Ionization/scintillation ratio is another strong, semi-independent discrimination mechanism rejection factors ~10 2 (Benetti et al (ICARUS) 2003, Benetti et al (WARP) 2006) Crisp spatial resolution obtained from drifted electrons makes possible the rejection of multiple-site, surface events A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/

16 DarkSide-50 photosensors EM backgrounds in DarkSide are reduced to tonne-year sensitivity by depleted Ar and pulse shape discrimination internal neutrons, mainly from (α,n) reactions in conventional glass PMTs would still interfere with tonne-year sensitivity zero-background operation of a 50 kg detector possible with the new metal-bulb Hamamatsu R11065 PMTs the DarkSide collaboration has procured a set of R11065 PMTs for first installation UCLA has partnered with Hamamatsu to develop the hybrid photomultiplier QuPID detectors A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/

17 QuPIDs fused silica envelope dynode structure substituted by an avalanche photodiode (APD) excellent single p.e. response 30% QE at LAr temperature currently testing first production devices no radioactivity detected in 3 month Ge counting A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/

18 active neutron veto boron-loaded liquid scintillator surrounding the inner detector to detect neutrons which escape the inner detector rely on (n,α) reaction on 10 B α particle is extremely short range allows >99.5% efficiency for radiogenic neutron detection 1 meter-thick shield/veto ~95% efficiency for cosmogenic neutrons Wright,Mosteiro,Loer,Calaprice (2010) heavy use of expertise, technology and facilities developed for Borexino A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/

19 integration in CTF house DarkSide-50 inside the CTF (Borexino test facility) tank in Hall C at Gran Sasso, after appropriate refurbishing and with an upgraded inner vessel (11x10m add CTF tank dimensions) 3m 4m cylindrical stainless steel inner neutron veto vessel instrumented with low-background, high QE PMTs upgrade of current muon veto and electronics integration with Borexino water and scintillator purification plants A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/

20 underground DAr depleted argon can be obtained by centrifugation or thermal diffusion, but it is expensive at the many tonnes scale. 40 Ar is one of the products of 40 K decay, and (n,2n) cosmogenic 39 Ar production in underground Ar is highly suppressed argon is present ~400 ppm in helium extracted from underground natural gas reservoirs; 39 Ar could also originate from (n,p) reactions on 39 K, sites must be individually tested for 39 Ar concentration Ar from the Kinder Morgan Doe Canyon complex (Cortes, CO) was shown to be >25-fold depleted in 39 Ar (Acost-Kane et al 2008) A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/

21 DAr extraction PSA system on site concentrates 400 ppm Ar to 3-4% (balance is He, N2) Princeton extraction plant in Cortes (Kinder Morgan) produced ~45% as of 11/10, at a rate of ~0.7 kg/day repository estimated at ~1000 tonnes of depleted Ar cryogenic distillation column to remove He, N2 being commissioned at Fermilab A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/

22 underground DAr Princeton cryogenic distillation column for depleted argon purification being commissioned at Fermilab A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/

23 DAr counting 39 Ar Counter Design Passive Shielding: 2 OFHC Copper 8 Lead Shielding ( 210 Pb ~65Bq/kg) Active Shielding: 2 Plastic Scintillator Veto, ~2PI S.A Coverage Background in (100,600)keV: ~0.3Bq at sea level Original Setup: Ar mass: LAr ~1kg WLS: TPB PMT R Acrylic Light Guide Lose 50% Light Upgrading Ar mass: LAr ~0.7kg High Crystallize Teflon Cup WLS: TPB/PTP PMT R11065, No Light Guide Kimballton Mine (~1500m.w.e) A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/

24 confirmed >25 depletion Outside View of the Shielding: First Data: Depletion! Filling and Purification System To be Achieved: 1. Higher Light Yield/Collection 2. Better Energy Resolution 3. Reduce Out-gassing Rate 4. Reduce Cosmic Ray Background 5. Thorough Background Study A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/

25 LAr light yield studies 2 kg LAr cell 0.7 kg active mass one Hamamatsu R11065 PMT (QE = 32%) Spectralon reflector cup Light yield: 6-7 p.e./kev A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/

26 10 kg prototype a 10 kg prototype detector is being run at Princeton top: 7 Hamamatsu R PMTs 10 kg active volume inside an acrylic inner vessel with TPB-coated Vikuiti 3M reflective foil bottom: one Hamamatsu R PMT A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/

27 10 kg prototype etched stainless steel grid kapton flex PCB field cage ITO-coated acrylic cathode A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/

28 10 kg prototype A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/

29 background budget A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/

30 background budget screening of materials for radioactivity is key! A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/

31 Summary the DarkSide program uses 2-phase depleted argon TPC detectors, in a design combining new background identification and discrimination techniques, to search for WIMPs with tonne-year, background-free sensitivity the first phase experiment, DarkSide-50, employs 50 kg of active depleted argon and will be housed inside the Borexino/CTF facility at Gran Sasso and have a cm 2 WIMP cross section design sensitivity, with 0.1 tonne-year exposure. DarkSide-50 will also serve as a demonstrator prototype for very large DAr experiments for dark matter searches A. Pocar - Dark Matter Workshop, Aspen Physics Institute - 2/10/

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33 thank you!