DarkSide-50: performance and results from the first atmospheric argon run

Transcription

1 DarkSide-50: performance and results from the first atmospheric argon run Yann Guardincerri on behalf of the DarkSide Collaboration August 27th, / 21

2 DarkSide Direct detection search for WIMP dark matter (M χ 100GeV) Based on a two-phase argon time projection chamber (TPC) Design philosophy: having very low background levels, which can be further reduced through active suppression, to achieve a background-free operation 2 / 21

3 DarkSide Collaboration USA Augustana College Black Hills State University University of Chicago University of Hawaii University of Houston University of Massachusetts Princeton University Temple University UC Davis UCLA Virginia Tech FNAL, LANL, LLNL, PNNL, SLAC Italy INFN LNGS Gran Sasso Science Institute INFN and Universita degli Studi Cagliari, Genova, Milano, Napoli, Perugia and Roma 3 Russia Joint Institute for Nuclear Research SINP, Lomonosov Moscow SU NRC Kurchatov Institute St. Petersburg NPI France Universite de Strasbourg APC Universite Paris 7 Diderot Poland - Jagiellonian University Ukraine -Institute for Nuclear Research China - IHEP 3 / 21

4 DarkSide-50 structure In Gran Sasso, Italy, we have: Radon-free clean room Water Tank (Cherenkov muon veto) TPC digitizers and DAQ computers Inner detector TPC (Liquid Argon) Organic Liquid Scintillator (neutron veto) 4 / 21

5 Inner Detector: mechanical layout Cold Amplifiers: allow to operate the PMT at a gain of " PMTs (Top and Bottom) Gas Ar (Elum=4200 V/cm) Liquid Ar (Edrift=200V/cm) 35.6 cm 35.6 cm 5 / 21

6 Detecting WIMPs Cold Amplifiers: allow to operate the PMT at a gain of Ionization signal in gas phase S2 19 3" PMTs (Top and Bottom) Gas Ar (Elum=4200 V/cm) Primary scintillation signal in liquid phase S1 Liquid Ar (Edrift=200V/cm) 35.6 cm 35.6 cm S2 S1 Drift Time 6 / 21

7 Backgrounds Electron Recoils 39 Ar 4 ~10 evt/kg/day γ 2 ~10 evt/kg/day γ 39 Ar γ μ n n Nuclear Recoils μ 2 ~30 evt/m /day Radiogenic n -4 ~6x10 evt/kg/day α α 2 ~10 evt/m /day M χ =100 GeV, σ =10 45 cm 2 WIMP Rate 10 4 ev/kg/day 7 / 21

8 39 Ar Reduction Dominant source of background: 39 Ar 39 Ar radioactivity present in atmospheric argon (1 Bq/kg) 39 Ar is a cosmogenic isotope, and the activity in argon from underground sources can be significantly reduced compared to atmospheric argon Source of underground argon measured to have > 150 times lower rate of 39Ar, compared to atmospheric argon Rate/ (Bq/keV) Atmospheric Argon Underground Argon arxiv: J. Xu, et al Energy/ kev 8 / 21

9 Electron Recoil Discrimination Pulse Shape Discrimination Based on the primary scintillation signal in the liquid phase (S1) Electron and nuclear recoils produce different excitation densities in the argon, leading to different ratios of singlet ( 6ns) and triplet ( 1600ns) inition states Electron Recoil Nuclear Recoil 9 / 21

10 Electron Recoil Discrimination Pulse Shape Discrimination F90: Ratio of detected light in the first 90 ns, compared to the total signal Singlet 6ns and Triplet 1600ns F90: fraction of singlet states Electron Recoils Nuclear Recoils 50 % Acceptance arxiv: Discrimination Power strongly depends on Light Yield 10 / 21

11 Neutron Rejection Liquid Scintillator Veto 4 m diameter sphere containing 50% PC + 50% TMB scintillator Instrumented with PMTs Veto of neutrons coincident in the TPC and provides in situ measurement of the neutron background rate 11 / 21

12 Neutron Rejection Borated Scintillator Veto 50% of TMB Neutron capture results in 1.47 MeV α particle: detected with high efficiency High neutron capture x-section on boron allows for compact veto size Short capture time (2.3 µs) reduces dead time E =1.47MeV α α γ E γ= 0.48 MeV (94%) B* B -12 t~10 sec 7 Li E =0.84MeV Li Veto Efficiency (MC) Radiogenic Neutrons >99% Cosmogenic Neutrons >95% 12 / 21

13 Cosmogenic Neutron Rejection External Water Tank 80 PMTs within Borexino CTF (11m dia. x 10 m high) Acts as a muon detector Cosmogenic neutron veto( 99% efficiency) Provides passive gamma and neutron shielding 13 / 21

14 DS-50 Assembly: Sept - Oct / 21

15 TPC Commissioning: purity Closed loop Ar recirculation ( 30 slpm) Gaseous phase purification using commercial getter Cryogenic charcoal trap to remove Rn contamination How to measure purity? While drifting e can be absorbed by impurities S2 signal is decreased Electron Drift Time [μs] 5 ms 1 month Time Electron drift lifetime > 5 ms, compared to max. drift time of 375 µs 15 / 21

16 TPC Commissioning: calibration TPC currently filled with atmospheric argon (1 Bq/kg) 39 Ar β decay spectrum 83m Kr gas deployed into detector 41.5 kev ee and half-time=1.83 hr 83 Kr Peak ER callibration at null field dominated by 39 Ar 83m Kr decays Fit to the 39 Ar and 83m Kr 39 Ar Beta Decay Light Yield 8 PE/keV ee Measured light yield exceeds previous projections of 6 PE/keVee 16 / 21

17 Neutron Veto Commissioning Neutron veto setup to trigger on events in the liquid Ar TPC Use high energy coincident 60 Co events from cryostat stainless steel to evaluate lightyield in scintillator arb. units 60 Co (1.17 and 1.33 MeV) Lightyield 0.5 PE/keV ee sufficient to detect 50 kevee α from neutron capture charge [PE] Found high rate at low energies due to intrinsic 14 C in (biogenic) TMB Removed TMB from Scintillator (achieved a reduction: 50% 0.16% ) Identified new batch of low- 14 C (underground) TMB 17 / 21

18 S1 pulse shape discrimination Initial Exposure: 280 kg-days of atmospheric argon High rate of 39Ar allows us to study F90 with an exposure that is equivalent to 2.6 yrs with underground argon Single hit events (1 S1 and 1 S2) Drift-time cuts (z-cuts): remove regions near grid and cathode No coincident energy deposition in the neutron veto 18 / 21

19 S1 pulse shape discrimination We have shown that S1 PSD at 200 V/cm (+ z fiducialization from S2) can efficiently suppress the dominant ER background that we expect in 2.6 years of DS-50 UAr run, while maintaining high acceptance for WIMPs. Extracted from Scene arxiv: \% Nuclear Recoil Acceptance Region 70PE 35 kev r 19 / 21

20 DS-50 Projected Sensitivity Assumptions Using F90 NR energy scale & F90 acceptance contour taken from SCENE No S2/S1 rejection Fiducial mass 44 kg (z-cut only) Systematics Estimates of systematics on NR quenching and pulse shape cause a 10% variation at 100 GeV Projected Sensitivities Projected WIMP Search Window 50% 65% 80% 90% 20 / 21

21 DS-50 Current Status Currently acquired 4200 kg days (90 days) of atmospheric argon data with TPC at 200V/cm (and >440 µs windows) TPC+Veto: 2500 kg days (55 days) Data being used to improve understanding of backgrounds, S2 signal, x-y position reconstruction, S1-S2 correlations Neutron veto: High rate of 14C found in TMB During last 2.5 months performed operations to remove TMB and modify the veto daq to run with PC only TMB removed - achieved 300x reduction (50% 0.16%). Will replace with low-14c TMB Source calibration Plan to acquire gamma and neutron data early in the fall 2014 Underground argon Switch to using underground argon foreseen at the beginning of / 21

22 BACK UP SLIDES 22 / 21

23 Argon 40 Ar is mostly produced underground (through decay of 40 K) 39 Ar is cosmogenic, produced by 40 Ar(n,2n) interactions in the atmosphere Argon that has remained underground can therefore have extremely low levels of 39 Ar However, 39 Ar can also be produced underground through 39 K(n, p) interactions, where the neutron originates from (α, n) reactions. 39 Ar/ 40 Ar depends on the local concentration of 238 U and 232 Th 23 / 21

24 SCENE Experimental Scheme Scintillation and Ionization Efficiency of Noble Elements 24 / 21

25 SCENE Results Nuclear Recoil Scintillation Yield and Pulse Shape vs Drift Field 25 / 21