The SABRE Proof of Principle Simone Copello on behalf of the SABRE - - PowerPoint PPT Presentation

The SABRE Proof of Principle

Simone Copello on behalf of the SABRE collaboration

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TAUP 2019 - 8/14 September, Toyama, Japan

*Gran Sasso Science Institute, L’Aquila, Italy

Simone Copello - TAUP 2019, September 9th - Toyama, Japan

Outline

• Experimental goal and strategies

○ Annual modulation signature in DAMA ○ SABRE key features

• SABRE Proof of Principle (PoP)

○ PoP and Full Scale Experiment ○ PoP status

• NaI-33 crystal

○ Alpha rate

• Conclusions

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Simone Copello - TAUP 2019, September 9th - Toyama, Japan

Annual modulation signature

Single-site nuclear recols are considered for DM direct

• detection. Annual modulation of event rate is a powerful

signature caused by the combination of Earth and Sun velocities within the dark matter halo. Expected event rate is 10-1 to 10-6 events/day/kg

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Ingredients for the annual modulation:

• Standard halo model:

spherical halo surrounding the galaxy, with a local mass density of ~0.3 GeV/c2/cm3

• WIMP velocity (with respect

to Earth): [220 + 15 cos ω(t-t0)] km/s

A signal has been observed by the DAMA/LIBRA experiment at LNGS, Italy.

DAMA/LIBRA–phase2 Exposure: 1.13 ton × year (6 years) Sensitive mass: about 250 kg of radio-pure NaI(Tl) crystals Statistical significance: 9.5 σ in (1 - 6) keV, 12.9 σ in (2 - 6) keV*

* Including DAMA/NaI and DAMA/LIBRA–phase1 data

From arXiv:1805.10486

Simone Copello - TAUP 2019, September 9th - Toyama, Japan

Sodium iodide with Active Background REjection

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SABRE aims to detect the annual modulation signal by using NaI(Tl) crystals, in

• rder to have a direct (model independent) confirmation/confutation of DAMA results.

4 key features: 1. Active background rejection: active veto of liquid scintillator 2. Low energy threshold: High QE Hamamatsu PMTs, directly coupled to the crystals 3. Double location: both in Northern and Southern hemispheres 4. High purity crystals: High purity powder and clean crystal growth method

More detail about the SABRE project can be found the paper “The SABRE project and the SABRE Proof-of-Principle” https://link.springer.com/article/10.1140/epjc/s10052-019-6860-y (arXiv:1806.09340)

Simone Copello - TAUP 2019, September 9th - Toyama, Japan

SABRE collaboration

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~50 physicists from three countries

U.S.A

• Princeton University
• Lawrence Livermore

National Laboratory (LLNL)

• Pacific Northwest

National Laboratory (PNNL) Italy

• Laboratori Nazionali del Gran

Sasso (LNGS)

• University of Milano and INFN
• University of Roma Sapienza

and INFN

• Gran Sasso Science Institute

Australia

• Australian Nuclear Science

and Technology Organization

• Australian National University
• Swinburne University of

Technology

• University of Adelaide
• University of Melbourne

Simone Copello - TAUP 2019, September 9th - Toyama, Japan

• 1. Active veto

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Intrinsic and environmental backgrounds are reduced by means

• f material selection, underground labs and shielding.

40K represents a consistent fraction of the background in the

ROI but can be tagged, as well as 22Na. Other intrinsic background sources, that cannot be tagged by the veto, are 87Rb, 232Th (chain), 238U (chain) and 3H

40K background with active veto:

Rejection efficiency ~85%

Picture from paper on the SABRE PoP Monte Carlo simulations: https://www.sciencedirect.com/science/article/ pii/S0927650518301804?via%3Dihub

Simone Copello - TAUP 2019, September 9th - Toyama, Japan

• 2. Low energy threshold

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Modulation amplitude, in general, is larger at smaller energies.

Light production: High crystal light yield Light collection: Hamatsu R11065 3” PMTs

• 2 PMT per crystal: coincidence trigger
• PMTs directly coupled to the crystal: no light guides
• High quantum efficiency (~35%)

Simone Copello - TAUP 2019, September 9th - Toyama, Japan

• 3. Double location

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Seasonal effects have opposite phases in the opposite hemispheres.

SABRE North at Laboratori Nazionali del Gran Sasso (LNGS), Italy SABRE South at Stawell Underground Physics Laboratory (SUPL), Australia

Simone Copello - TAUP 2019, September 9th - Toyama, Japan

Example of a good growth of a 2 kg crystal obtained in

• 2015. 40K content was 9 ppb from ICP-MS (the same

content of the Astro-Grade powder)

• 4. High purity crystals

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Residual impurities in the crystal are predominant source of background, once reduced the environmental radioactivity, for DM search. Effort of Princeton University with some Industrial Partners:

• Production of Ultra-high purity powder: Astrograde powder (PU &

Sigma Aldrich – now Merck)

• Powder handling and treatment methods
• Crucible selection, cleaning and treatment
• Crystal growth using vertical Bridgman technique in a sealed ampoule

(PU & RMD)

• Crystal cutting, polishing and handling
• Development of high sensitivity ICP-MS measurements

2kg test crystal

Simone Copello - TAUP 2019, September 9th - Toyama, Japan

Layout:

• 1 NaI(Tl) crystal module per time (“dry” insertion)
• Crystal mounted inside a Cu enclosure and directly

coupled to 2 PMTs Hamamatsu ULB R11065-20 3” PMTs, High QE

• Active veto: 2 t PC+PPO (3g/l) scintillator (from

Borexino exp) read by 10 Hamamatsu R5912-100 PMTs

• External passive shielding (lead, polyethylene and

water) purged with gas nitrogen

• Material selection (using ICP-MS and HPGe)

SABRE Phase I - Proof of Principle

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Setup ready at LNGS

Goals:

• Fully characterise SABRE NaI(Tl) intrinsic and cosmogenic

backgrounds

• Test few (≈ 3) crystals
• Test active veto performance

veto

Simone Copello - TAUP 2019, September 9th - Toyama, Japan

SABRE Phase II: Full Scale Experiment

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Twin detectors SABRE North at LNGS and SABRE South at SUPL

Goals:

• Lowest background NaI experiment for Dark Matter search
• Confirm/Reject annual modulation with amplitude observed by

DAMA/LIBRA with 3 years of data Layout:

• > 50 kg of NaI(Tl) crystal, active veto, external shielding, double

location

• Final design and schedule depends on the outcome of PoP

DAMA/LIBRA FACTS

• NaI(Tl) crystal detector

mass 250 kg

• natK

≈13 ppb

• Rb

< 0.35 ppb

• Th

≈ 0.5 – 7.5 ppt

• U

≈ 0.7 – 10 ppt

• 210Pb

10-30 µBq/kg

• LY

6-10 pe/keV

• Threshold

1 keV

• Rate in the ROI (2-6 keV) 1 cpd/keV/kg (single hit)

SABRE GOALS

• natK

≤ 10 ppb

• Rb

≤ 0.1 ppb

• U/Th

< 1ppt

• low 3H and 210Pb contamination
• LY

> 10 pe/keV

• Rate in the ROI (2-6 keV)

< 1 cpd/keV/kg

Simone Copello - TAUP 2019, September 9th - Toyama, Japan

Proof of Principle status

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The veto vessel, cleaned and internally covered with Lumirror reflector, is in its final position. All the veto PMT have been tested.

The copper tube will host the crystal enclosure. This solution avoids contact of liquid scintillator with air: safe and practical way to mount/change crystal in PoP set-up.

Simone Copello - TAUP 2019, September 9th - Toyama, Japan

Proof of Principle status

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Crystal Insertion System

A frame is mounted on a steel plate above the vessel. A motorized pulley connected to an alignment system, guides the enclosure into the copper tube. The enclosure is connected to a flange via a steel bar.

Enclosure mock-up

Simone Copello - TAUP 2019, September 9th - Toyama, Japan

Proof of Principle status

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Shielding

• Water tanks ( ≥ 80 cm top and side)
• High density Polyethylene (10 cm top and bottom, ≥ 40 cm side)
• Lead basement (15 cm)

Simone Copello - TAUP 2019, September 9th - Toyama, Japan

Proof of Principle status

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All the needed infrastructures are completed:

• Fluid Handling
• Safety plant
• Slow control system
• Power (normal + UPS power)
• Network
• Control room (DAQ, High voltage...)

PoP is ready to be filled with liquid scintillator but we still waiting for the approval from the laboratory.

Simone Copello - TAUP 2019, September 9th - Toyama, Japan

Passive shielding setup at LNGS

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Small underground shielding used for preliminary crystal measurements, it can host two crystal enclosures. Same DAQ and reconstruction software of PoP. Enclosed into a Radon box. 5/10 cm of copper + ≥ 15 cm of lead. NaI-33 data have been collected here (next slides)

Simone Copello - TAUP 2019, September 9th - Toyama, Japan

NaI-33 crystal

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Underground at LNGS

• Astrograde powder by Sigma Aldrich and crucible prepared at PU.
• Ready October 2018 - Assembled in mid-May 2019.
• Potassium measurement via ICP-MS: ~ 4 ppb
• Mass ~ 3.5 kg after cut and polishing
• Arrived at LNGS on August 6, 2019 (by boat to reduce cosmogenic

activation).

K content: fit from three samples taken near tip, tail and far-end tail measured by ICP-MS (at Seastar).

Final crystal portion

Simone Copello - TAUP 2019, September 9th - Toyama, Japan

NaI-33: LY and resolution

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Currently the spectrum is still dominated by cosmogenic isotopes decays.

241Am source used to measure energy resolution and light yield (LY) on the 59.5 keV line:

• Peak at (12.17 ± 0.01) nVs
• σ = (0.62 ± 0.01) nVs

Light Yield 11 phe/keV FWHM/E 12.3%

NOTE: all the plots and numbers reported in this slide must be considered preliminary results

For comparison…

• DAMA/LIBRA

phase-2: 15.8% @59.5 keV

• ANAIS-112:

11.2% @59.5 keV

• COSINE-100:

11.8% @59.5 keV

Preliminary

Simone Copello - TAUP 2019, September 9th - Toyama, Japan

NaI-33: Alpha rate

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Alpha rate: 0.4 mBq/kg

Higher than DAMA but lower than other competitors,

~ 0.36 mBq/kg is due to 210Po (see next slide)

Pulse charge-weighted mean-time used to distinguish alpha events from beta/gamma or muon events.

NOTE: all the plots and numbers reported in this slide must be considered preliminary results

The spectrum is still dominated by cosmogenic isotopes decays but alpha decays can be easily distinguished.

α β, γ

noise

𝝂

Preliminary Preliminary

Good pulse shape discrimination

noise

Simone Copello - TAUP 2019, September 9th - Toyama, Japan

NaI-33: 238U and 232Th chains

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NOTE: all the plots and numbers reported in this slide must be considered preliminary results

226Ra (238U chain) activity evaluated by means of Bi-Po events (easily taggable with efficiency ~1):

• beta decay followed by an alpha T1/2 = 164.3 μs

(0.057 ± 0.015) evt/h Activity(226Ra) = (4.7 ± 1.2) μBq/kg

228Ra (232Th chain) activity evaluated by means of

• BiPo events: beta decay followed by an alpha with T1/2 = 299 ns (easily taggable with efficiency ~1)
• and the alphas sequence 224Ra - 220Rn - 216Po in the 232Th decay chain (T1/2 = 55.6 s and T1/2 = 0.145 s,

respectively)

(0.011 ± 0.006) evt/h Activity(228Ra) = (0.9 ± 0.5) μBq/kg

BiPo event from 232Th chain

QFα = 0.64-0.66

This analysis is described in Ambra Mariani’s

poster, ID 344 (tomorrow poster session)

Assuming secular equilibrium*:

0.4 ppt 238U

Assuming secular equilibrium*:

0.2 ppt 232Th

* Secular equilibrium assumption is likely wrong, e.g. because of 222Rn contamination for 238U

Simone Copello - TAUP 2019, September 9th - Toyama, Japan

Conclusions

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SABRE Proof of Principle

○ Designed to test few crystals to validate the growth technique and the veto rejection capability ○ Ready to be filled with liquid scintillator at Laboratori Nazionali del Gran Sasso

NaI-33 crystal (currently in measurement at LNGS) is a promising detector

○

40K content ~ 4 ppb (ICP-MS), more than 3 times lower than DAMA

○ Good light yield 11 phe/keV (preliminary) ○ Good energy resolution 12% @ 59.5 keV (preliminary) ○ Current alpha rate is ~ 0.4 mBq/kg (preliminary), possibly mainly due to 210Po.

• Thanks for the attention -

Backup

Simone Copello - TAUP 2019, September 9th - Toyama, Japan

238U and 232Th chains

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Simone Copello - TAUP 2019, September 9th - Toyama, Japan

NaI-33: 2 keV event

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NOTE: all the plots and numbers reported in this slide must be considered preliminary results

Time [ns]

Samples [ADC counts]

An example of a 2 keV event in NaI-33 Ch 0 Ch 1 Sum

single or couples of photoelectrons