Sodium-iodide with Active Background REjection. Irene Bolognino

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1 Direct search of Dark Matter through the SABRE experiment SABRE Sodium-iodide with Active Background REjection Irene Bolognino Università degli studi di Milano, INFN 53 rd Rencontres de Moriond, La Thuile, 20 th March 2018

2 The Dark Matter Annual Modulation: a Model Independent Signature Standard halo model hypothesis: spherical halo of cold, dark matter (WIMP particles) permeating the galaxy local energy density ρ WIMP 0.3 GeV/cm 3 Maxwell speed distribution WIMP Rate dr de R (t) =S 0 (E R )+S m (E R )cos (t t 0 ) Rare and low energy events: expected WIMP-nucleons s cm 2 very low expected rate < 1 count/day/kg expected recoil energy is kev for a WIMP of mass GeV Annual modulation having 1 year as a period due to the Earth motion around the Sun Maximum and minimum expected on June 2 nd and on 2nd December Annual modulation is a model independent signature of Dark Matter interaction! 20/03/18 Irene Bolognino 2

3 Motivation for the SABRE experiment European Physical Journal C 73, 2648 (2013) DAMA/NaI and DAMA/LIBRA observed modulation The DAMA/LIBRA experiment has observed a modulation for 14 years: located at Laboratori Nazionali del Gran Sasso, Italy total mass: 250 kg of NaI observed ~0.01 cpd/kg/kev modulation in the 2-6 kev energy range 9.3 s significance If interpreted as WIMPs undergoing a spinindependent nuclear scattering: Na nuclei M WIMP 10 GeV, σ cm 2 I nuclei M WIMP 80 GeV, σ cm 2 This claim is still unverified inasmuch as the other experiments are based on different target materials A model independent experiment is therefore required having: The same target material Lower background 20/03/18 Irene Bolognino 3

4 S A BR The SABRE (Sodium-iodide with Active Background REjection) Principles E Model Independent Test 1 High pure crystals 20/03/18 2 Active background rejection 3 Low Energy Threshold 4 Double location Irene Bolognino 1. Development of low background NaI(Tl) crystals Ultra-high pure NaI powder Ultra-clean crystal growth method 2. Active Veto and Passive Shielding Highly efficient background rejection 3. Low energy threshold High Quantum Efficiency PMTs directly coupled to the crystal 4. Two identical detectors in northern and southern hemispheres Reduction of seasonal effects with opposite phase Different rock composition and radiopurity Independent radon, temperature, pressure control systems and power supply 4

5 Ultra pure NaI Crystals Collaboration with industrial partners (now Sigma Aldrich) led to NaI crystal with unmatched purity: nat K content 9 ppb Independent impurity assessment of powder and crystal with ICP-MS, ICP-OES, and AMS Record sensitivity achieved Crystal growth Minimize crystal contamination during growth procedure Test growth of small crystals indifferent crucibles Careful material selection Precision cleaning Successfully grown 2kg crystal with breakthrough purity SABRE DAMA nat K 9 ppb 13 ppb 87 Rb < 0.1 ppb < 0.35 ppb 238 U chain 0.6 ppt ppt Full size crystal (5.5 kg) - under production Growth method validation for big crystals. Larger crucible will improve the surface to volume ratio and the expected backgrounds 232 Th chain 0.5 ppt ppt 20/03/18 Irene Bolognino 5

6 Active Background Rejection External and Intrinsic backgrounds (radioactive and cosmic-induced processes) will be rejected by 2 tons of active liquid scintillator veto veto threshold:100 kev 10 PMTs 8 Hamamatsu R5912 Additional passive shielding made of lead, water and polyethylene modulation region 40 K DAMA/LIBRA spectrum 40 K veto efficiency 84% Most dangerous long-lived background in ROI: 40 K decays by e - capture (BR 11%) à excited state of 40 Ar emitting a 1461 kev gamma à Auger e - or X-ray with a total energy of 3.2 kev 22 Na emits with a BR of 10% a 0.8 kev X-ray and a 1275 kev gamma 20/03/18 Irene Bolognino 6

7 Low Energy Threshold DAMA/LIBRA modulation region: 2-6 kev SABRE goal is to investigate also in the region 1-2 kev à it is essential to lower the energy threshold High light yield: the light yield depends on crystals but also on PMTs Quantum Efficiency and how light is collected Low dark noise: Less dark rate means better Signal to Noise ratio Selected (Hamamatsu 3" R ) PMTs with High QE: > 35%, low radioactivity. This allows direct coupling with the crystal, improving the light collection efficiency The original ceramic feedthrough plate is replaced with a synthetic one with significantly lower background The usage of super bialkali photocathodes to minimize noise is under investigation à quantum efficiency improvement up to about 40% and a dark noise reduction from 5 to 10 times 20/03/18 Irene Bolognino 7

8 SABRE Proof-of-Principle (PoP) Hosted in Laboratori Nazionali del Gran Sasso (LNGS), Italy 3600 m w.e. Well known background Located in Hall C Goals Test active veto performance o 40 K and 22 Na Fully characterize the intrinsic and cosmogenic backgrounds o 87 Rb, 238 U chain, 232 Th chain, 210 Pb, 3 H Layout 1 NaI(Tl) crystal (~5.5 kg). Crystal coupled directly with 2 PMTs in a highly radiopure copper enclosure Active veto: o Cylindrical vessel ( x h) = (1.40 m x 1.50 m) o Agreement to use liquid scintillator from Borexino (mass 2ton) o 10 PMTs External shielding: combination of water, PE and lead. The inner space of the shielding is sealed and purged with nitrogen to preventbackground fromradon 20/03/18 Irene Bolognino 8

9 Copper Enclosure & Crystal Insertion System Copper Enclosure Essential to insulate and protect crystal Continuous purge with nitrogen to suppress radon and moisture Highly radiopure copper enclosure U, Th <1µBq/kg Tested at LNGS with a mock-up crystal Crystal Insertion System High-purity copper tube to insert crystal enclosure Continuous purge with nitrogen to suppress radon Allows the insertion of calibration sources 20/03/18 Irene Bolognino 9

10 SABRE Proof-of-Principle (PoP) - State of Art Passive Shielding Installation of shielding at LNGS Hall C has started Polyethylene Lead DAQ & Trigger Trigger firmware implemented and tested with liquid scintillator majority trigger mode DAQ software finished and tested Veto Vessel Currently in Hall B at LNGS Inner parts coated with Lumirror All veto PMTs installed and tested Data taking start in summer /03/18 Irene Bolognino 10

11 SABRE in the South Hosted in Stawell Underground Physics Laboratory (SUPL), Australia 250 km from Melbourne 3000 m w.e. similar to LNGS Background similar to LNGS Expected to be completed in 2019 SABRE Layout 50 kg of ultra low-background NaI(Tl) crystals Liquid scintillator veto Lead shielding Vessel tests in /03/18 Irene Bolognino 11

12 SABRE PoP Monte Carlo Simulations Background is fully simulated through Monte Carlo GEANT4 based code with detailed PoP geometry Veto threshold 100 kev Contamination values from: o SABRE material screening on 2 kg crystal plus a contribution of 30 µbq/kg of 210 Pb o material screening from other experiments Next step full-scale experiment 20/03/18 Irene Bolognino 12

13 Total Background Contributions on PoP Rate [cpd/kg/kev] SABRE PoP - Preliminary Tot Background, veto off Tot Background, veto on Veto CIS Crystal Crystal PMTs Crystal (Cosmogenic) Enclosure 2 10 *Background contributions from crystal cosmogenic activation (after180 days) E [kev] Expected background in ROI (2-6 kev): 0.2 cpd/kg/kev ee Total veto rejection of internal background: 3.5 factor Crystal is the mainsource of background 20/03/18 Irene Bolognino 13

14 SABRE Expected Sensitivity Dark Matter standard halo model (Rev. Mod. Phys. 85, 1561, 2013) Region of interest: 2-6 kev Crystal mass ~50 kg 3 years measuring time Background taken from PoP Monte Carlo simulations Quenching factors (arxiv, v1, 2015) o Q Na range from 0.13 to 0.21 depending on the recoil energy o Q I = years x 50 kg exposure are sufficient to confirm or exclude the DAMA modulation signal 20/03/18 Irene Bolognino 14

15 SABRE Collaboration Australian National University Swinburne University University of Adelaide University of Melbourne LNGS & GSSI INFN Rome University of Milano & INFN LLNL PNNL Princeton University 20/03/18 Irene Bolognino 15

16 Conclusions DAMA/LIBRA claim is still unverified à A model independent check requires a new NaI(Tl) experiment SABRE can perform an independent high sensitivity verification of the DAMA/LIBRA modulation through: o ultra low-background crystal o low energy threshold o active liquid scintillator veto o twinexperiments locatedinboth hemispheres Monte Carlo simulations performed taking into account the PoP geometry led to the following outcomes: o expected background in ROI (2-6 kev): 0.2 cpd/kg/kev ee o active veto rejection factor 3.5 Proof of Principle phase is in progress at LNGS and expected to start data taking in summer 2018 SABRE implementation in the south hemisphere is in progress and it is expected to start data taking in 2019 (SUPL laboratory ready by the beginning of 2019) Full scale experiment (50 kg) can confirm/reject DAMA/LIBRA results with 3 years of data 20/03/18 Irene Bolognino 16

17 Back Up 20/03/18 Irene Bolognino 17

18 Interpretation of DAMA/LIBRA results Interaction on Na nuclei M wimp ~ 10 GeV σ ~ cm 2 In the simplest interpretation of SI WIMP-nucleus interaction there are two allowed regions with very similar χ 2 Howeverthere areseveral assumptions here: ü Astrophysics: DM halo ü Dark matter candidate: WIMP ü Nature of interaction: elastic and Spin Independent ü Target of Interaction: nuclei Interaction on I nuclei M wimp ~ 80 GeV σ ~ cm 2 20/03/18 Irene Bolognino 18

19 Background measurements summary LNGS Stawell Neutron Flux (n/s/cm 2 ) 4 x x 10-6 Gamma-rayflux <3MeV (γ/s/cm 2 ) 0.73 ~1 Rock Hall 238 U (ppm) Rock Hall 232 Th (ppm) Concrete 238 U (ppm) 1.05 Concrete 232 Th (ppm) 0.66 Radon Bq/m 3 (12 day accumulation) ~50 <1.86 (wall) <2.18 (floor) 3.84 (wall) 3.49 (floor) 408±40 (free air) 36±5 (comp. air) Muon Flux (μ/h/m 2 ) ~1.2 to be measured 20/03/18 Irene Bolognino 19

20 K measurement Target 40 K electron capture (3 kev auger e MeV g) in the crystal and other processes with large energy deposits in the scintillator Coincidences Cystal+Scintillator allow to study other intrinsic BKGs that give a energy release in the scintillator Largest bkg contribution from 22 Na mostly below threshold of 2 kev 20/03/18 Irene Bolognino 20

21 Cosmogenics 20/03/18 Irene Bolognino 21

22 Crystal Intrinsic 20/03/18 Irene Bolognino 22

23 S A BR A (Upper Dam) E world wide effort DM-ICEKorea 17kg Middleland operating since Power2011 Co. Yangyang Pumped Storage Power Plant Y2L Seoul 5 ON OF THE EXPERIMENT 3. DESCRIPTION OF THE EXPERIMENT: scale conclusion of previous studies erent prototypes b by the h U University i i PRESENT off STATUS Canfranc. CHARACTERIZATION OF BACKGROUNDS FOR AN ULTRAPURE NaI(Tl) CRYSTAL: ANAIS PROTOTYPE IV of 40 cm neutron shielding A PRELIMINAR DARK MATTER RUN: ANAIS-0 Active vetos tudy d In the best background conditions: already running but only 9.6 kg ion (Power Plant) 20 cm lead Dark Matter Searches get was me of PVC box 2 mm Cd!7 20/03/18 12 crysta RENO 2.5 year d Backgrou Source ca 1 year of PSD anal Backgrou Rick Gaitskell, Brown University, LUX / DOE _DM2014_Gaitskell_Review_v05 28, 2014 Total- February backgrounds Matter Search) (Lower Dam) KIMS7 (Dark Data AMoRE (Double Beta Decay Experiment) kevee region is measured to be 7.9 ± 0.4 counts/day/kev/kg." Dominated by assembly housing and PMT - no ice contribution 10 cm Roman lead Data wit MC Minimum depth : 700 m / Access to the lab by car (~2km) Vibration isolator!6 6 Rick Gaitskell, Brown University, LUX / DOE Irene Bolognino _DM2014_Gaitskell_Review_v05 - February 28,