DarkSide-50 Results from first Argon run Davide DAngelo Universit - PowerPoint PPT Presentation

DarkSide-50 Results from first Argon run Davide D’Angelo Università degli Studi di Milano e I.N.F .N. 
 for the DarkSide collaboration CoEPP-CAASTRO Workshop 
 28-30 September 2014 
 Melbourne, Australia

DarkSide Keywords ➡ Direct detection of dark matter ➡ Wimp-nucleus scattering in liquid Argon ➡ Dual-phase Time Projection Chambers (TPC) ➡ Multi-stage approach ➡ At Laboratori Nazionali del Gran Sasso (LNGS) in central Italy: rock coverage ~3500m w.e. B a ➡ Very low intrinsic background levels c k g o r p o e u ➡ Electron recoil discrimination r n a d t i - o f n r e ➡ Neutron active suppression e 2 CoEPP-CAASTRO Workshop 2014 - DS50: Results from first Argon run D. D’Angelo for the DarkSide coll.

Why Liquid Argon (LAr) ? LAr advantages LAr disadvatanges • Bright scintillator -> low energy • Cosmogenic radioactive 39 Ar: threshold: Atmospheric argon (AAr) 1 Bq/kg ~40 photons/keVee Underground argon (UAr) has low ~8-10 pe/keVee possible 39 Ar but: • AAr is cheap, UAr is not. • Powerful PSD in scintillation signal • Scintillation light at 128 nm. separates background from ER from • Need wavelength shifters. WIMP induced NR. • Special PMTs developed: • Moderate cryogenic requirements • low radioactivity • Good ionization detector for TPC • working at LAr temperature • Well defined fiducial volume is � possible. • S2/S1 helpful for discrimination. • Easily scalable to large masses. • Liquids and gasses can be radio-pure. Internal background reduced by online purification. 
 3 CoEPP-CAASTRO Workshop 2014 - DS50: Results from first Argon run D. D’Angelo for the DarkSide coll.

DarkSide Multi-stage Program DarkSide-50 DarkSide DarkSide-10 
 First physics detector Future multi-ton Prototype detector recently commissioned detector 
 Astropart.Phys. 49 (2013) 44-51 ~10 -45 cm 2 @100GeV ~10 -47 cm 2 @100GeV 4 CoEPP-CAASTRO Workshop 2014 - DS50: Results from first Argon run D. D’Angelo for the DarkSide coll.

χ χ Detecting WIMPs 19x2 3” Photomultiplier Tubes � (Top & Bottom) � ~20% photocathode coverage � ~60% of end plate surface Total mass: 145kg S2 Active mass: 49.4kg Fiducial mass: 44.9kg M a x d r i f t t i m e ~ 3 7 5 u s Gas Ar (E lum ~ 4200 V/cm) 35.6cm Liquid Ar (E drift ~ 200 V/cm) S1 S1 S2 Drift Time 5 CoEPP-CAASTRO Workshop 2014 - DS50: Results from first Argon run D. D’Angelo for the DarkSide coll.

DarkSide 50 Housed in CTF Borexino Counting Test Facility Radon-free clean room Instrumented water tank Organic liquid scintillator Inner detector TPC Designed to hold a 5t TPC within the same Veto system 6 CoEPP-CAASTRO Workshop 2014 - DS50: Results from first Argon run D. D’Angelo for the DarkSide coll.

Liquid Scintillator Veto • 4 m diameter sphere containing 1:1 PC + TMB scintillator • Instrumented with 110 8” PMTs 1. Coincident veto of neutrons in the TPC � 2. in situ measurement of the neutron background rate 7 CoEPP-CAASTRO Workshop 2014 - DS50: Results from first Argon run D. D’Angelo for the DarkSide coll.

Borated Liquid Scintillator • High neutron capture cross section on boron allows for compact veto size • Short capture time (2.3 μ s) reduces dead time loss • Capture results in 1.47 MeV α particle, quenched to ~50 keV : it must be detected with high e ffi ciency! Veto Efficiency (MC) Radiogenic Neutrons > 99% Cosmogenic Neutrons > 95% Nuclear Instruments and Methods A 644, 18 (2011) 8 CoEPP-CAASTRO Workshop 2014 - DS50: Results from first Argon run D. D’Angelo for the DarkSide coll.

External Water tank • Ultra-pure Water Cherenkov detector 
 (11m dia. x 10 m high) • 80 8” PMTs from Borexino’s CTF • Acts as a muon and cosmogenic veto 
 (~ 99% e ffi ciency) • Provides passive gamma and neutron shielding 9 CoEPP-CAASTRO Workshop 2014 - DS50: Results from first Argon run D. D’Angelo for the DarkSide coll.

DS-50 Assembly Sept - Oct 2013 10 CoEPP-CAASTRO Workshop 2014 - DS50: Results from first Argon run D. D’Angelo for the DarkSide coll.

PMTs with cold-amplifiers • 3” PMTs • Hamamatsu R11065 series • The “/20” have good background levels but show problems at nominal gain at LAr temperature • Require low PMT Gain 
 ~ 4 x 10 5 • Custom cold amplifiers: Noise ~3 μ V on 200 MHz 11 CoEPP-CAASTRO Workshop 2014 - DS50: Results from first Argon run D. D’Angelo for the DarkSide coll.

DS-50 Status All 3 detectors are filled TPC and currently operating μ Liquid Scintillator Liquid Scintillator LAr Water Cherenkov Water Tank 12 CoEPP-CAASTRO Workshop 2014 - DS50: Results from first Argon run D. D’Angelo for the DarkSide coll.

Ar purification: electron lifetime 7000" 6000" Electron Drift Lifetime [us] 5 ms 5000" 4000" Life%me'[us]' 3000" 2000" 1000" 0" 10/10/13"0:00" 10/17/13"0:00" 10/24/13"0:00" 10/31/13"0:00" 11/7/13"0:00" 11/14/13"0:00" 11/21/13"0:00" 11/28/13"0:00" 1 month Time Electron lifetime > 5 ms >> max. drift time ~ 375 us 13 CoEPP-CAASTRO Workshop 2014 - DS50: Results from first Argon run D. D’Angelo for the DarkSide coll.

39 Ar • ~1 Bq/kg in atmospheric argon: • primary background for argon-based detectors! • β emitter with Q β =565 keV and T 1/2 =269 years • Cosmogenic via 40 Ar(n,2n) 39 Ar: • in argon from underground sources it can be significantly reduced Energy Spectra Rate/(Bq/keV) -2 10 Identified source of underground argon Atmospheric Ar Underground Ar in Colorado measured to have -3 10 <6.5mBq/kg i.e. > 150 times lower rate compared to -4 10 atmospheric argon 
 -5 10 Plant (including cryogenic distillation at FNAL) produces ~0.5 kg/d -6 10 0 200 400 600 800 1000 Energy/keV 14 CoEPP-CAASTRO Workshop 2014 - DS50: Results from first Argon run D. D’Angelo for the DarkSide coll.

Pulse Shape Discrimination Electron and nuclear recoils produce di ff erent excitation densities in the argon, leading to di ff erent ratios of singlet and triplet excitation states Recoil Excitation Ionization τ singlet ~ 7 ns Ar + Ar * Electrons τ triplet ~ 1600 ns Ar 2+ Ar 2* S2 Ar ** Triplet Singlet Recombination S1 15 CoEPP-CAASTRO Workshop 2014 - DS50: Results from first Argon run D. D’Angelo for the DarkSide coll.

Pulse Shape Discrimination 90 ns ∫ dt f ( t ) # 0.3 ER F 0 90 = = $ ∞ 0.7 NR ∫ dt f ( t ) % Electron Recoil 0 Nuclear Recoil 16 CoEPP-CAASTRO Workshop 2014 - DS50: Results from first Argon run D. D’Angelo for the DarkSide coll.

ER Light Yield 83m Kr spike in the recirculation system τ 1/2 ~1.8h 
 (possibly concentrated near the cathode where S1 light collection is higher) ³ � Ar spectrum 565 keV LY null ~ 8 pe/keV from 39 Ar (energy independent within 3%) quenching factor from 83m Kr and used to scale the LY null 
 LY 200 ~7.2 pe/keV at 200 V/cm 17 CoEPP-CAASTRO Workshop 2014 - DS50: Results from first Argon run D. D’Angelo for the DarkSide coll.

Nuclear Recoil From SCENE (SCintillation E ffi ciency of Noble Elements): 1. nuclear recoil quenching 2. the F90 distribution by processing SCENE data with DS-50 code and extrapolated to DS-50 detector along with the systematics. 0.32 Kr(0V/cm) f90 Median 0.75 f90 0.3 0.28 0.7 83m 0.26 0.65 S1 Relative to 0.24 0.6 0.22 0 V/cm 0 V/cm 0.2 100 V/cm 0.55 200 V/cm 200 V/cm 0.18 300 V/cm 300 V/cm 0.5 0.16 1000 V/cm 1000 V/cm 0.14 0.45 10 20 30 40 50 60 10 20 30 40 50 60 Recoil Energy [keV] Recoil energy [keV] arXiv:1406.4825 18 CoEPP-CAASTRO Workshop 2014 - DS50: Results from first Argon run D. D’Angelo for the DarkSide coll.

Neutron Veto Commissioning Use high energy coincident 60 Co events scintillator VETO LY of about 0.5 PE/keV ee , ements. 60 Co from cryostat stainless steel to evaluate 0.51±0.07 PE/keV ee Light Yield in scintillator. 
 Norm. Counts/bin (1.17 and 1.33 MeV) Confirmed by 14 C and 208 Tl fits. Light yield ~0.5 PE/keV su ffi cient to detect ~ 50 keV ee charge [PE] • Found high rate of intrinsic 14 C in (biogenic) TMB: ~10 -13 g/g • TMB temporarily removed via distillation: currently running in pure PC- mode • Identified new batch of low- 14 C (underground) TMB (<10 -15 g/g) to be used in November 2014 19 CoEPP-CAASTRO Workshop 2014 - DS50: Results from first Argon run D. D’Angelo for the DarkSide coll.

Initial Exposure (280 kg-days) 1 F90 total_s1_corr_f90_after_lsv_cuts_hist total_s1_corr_f90_after_lsv_cuts_hist Entries Entries 2.119474e+07 2.119474e+07 0.9 1400 Mean x Mean x 346.6 346.6 Mean y Mean y 0.2939 0.2939 0.8 RMS x RMS x 154.6 154.6 1200 RMS y RMS y 0.03621 0.03621 0.7 1000 0.6 800 0.5 0.4 600 0.3 400 0.2 200 0.1 0 0 100 200 300 400 500 600 S1 [PE] 20 CoEPP-CAASTRO Workshop 2014 - DS50: Results from first Argon run D. D’Angelo for the DarkSide coll.