THE LZ OUTER DETECTORlz.lbl.gov/wp-content/uploads/sites/6/2019/09/UCLA_SShaw_LZOD.pdf · The LZ...

THE LZ OUTER DETECTOR

UCLA Dark Matter 23/02/2018 Sally Shaw

7T active LXe

120 Veto PMTs

17 T gadolinium loaded liquid scintillator (GdLS)

2Tyvek reflector

water

60cmOptical fibres

The LZ Outer Detector S. Shaw UCLA DM 20183

NR

𝝲

𝝲𝝲

𝝲Neutron emitted from PMTs

Gd


Acrylic Tanks

43 bottom tanks

4 side tanks

plug (for when YBe

calibration source not in use) 2 top tanks

Test run with tank frame mock-up - transport tanks down shaft, through mine to Davis cavern

→ ICP-MS radioassay → UV transmission tested → radon exposure & dust monitored → strength & hardness tested

Fabrication almost complete - tanks are in the oven right now!

side tankSide tank panel

Segmented tanks - easier transport and installation underground


Liquid Scintillator

5

Minfang Yeh leading GdLS product at Brookhaven → produced 185T for Daya Bay

Technique - chelation of Gd with TMHA in LAB → result 0.1% Gd by mass

Solvent - Linear Alkylbenzene (LAB)

wavelength (nm)300 350 400 450 500 550 600

0

0.001

0.002

0.003

0.004

0.005

Event.tracks.dWavelength_nm {Event.tracks.iParticleID==0}

Emission spectrum

Wavelength shifter: bis-MSBFluor: PPO


Light Collection

6

Requirement: > 80 phe / MeV

120 Hamamatsu 8-inch PMTs procured, being tested at the

IBS, South Korea

Light collection map for uniform 152Gd 𝝰-decays

Building and testing PMT assemblies and holders

Developing optical calibration system consisting of 35 optical fibre assemblies

Enhancement from tyvek surrounding cryostat

photoelectrons

LZ cryostat

Top tanks

Bottom tanks

Side tanks


3 low background

PMTs

Water buffer

Liquid scintillator

Thoron calibration tube

Reynolds acrylic

Screener lowered into LUX water tank

Tyvek

152Gd &147Sm 𝝰

14C

Other 𝝰

Prototype - LS Screener

7

14C/12C: ~20x10-6 ppt

Sensitive to 𝝰, β, 𝝲:

→ measure whole 238U & 232Th chains

→ 40K, 14C

→ 152Gd, 147Sm low energy 𝝰-particles

→ cavern background (z-scan)


Cavern Backgrounds

8

Initial simulations suggested cavern was dominant background in OD at > 90 Hz,

but Davis cavern 𝝲-flux was uncertain. Background Rate (Hz)

Gd/Sm 𝝰-decays 36 (100 keV)

Internals (238U/232Th/40K/14C)

8-24 (100 keV) ~few (200 keV)

LZ Components 7 (100 keV)

Acrylic / dust <5 (100 keV)

Cavern 𝝲-rays 91 18 (200 keV)

0 500 1000 1500 2000 2500energy (keV)

4−10

3−10

2−10

1−10

1

Cou

nt R

ate

(Hz)

208Tl214Bi

40K

New measurements of 40K, 238U and 232Th concentrations in rock used to simulate

rate in OD - now 18 Hz

OD backgrounds & expected rates

Shielded & unshielded measurements taken with NaI detector in empty water tank

Oct 2017: Cavern flux measurement team


LZ Backgrounds

9

With sim, can assess whether we meet requirements:

Veto >95% of neutrons that scatter in TPC

Veto >70% of 𝝲 -rays that scatter in TPC

Background Expected NR PMTs 0.027

TPC (field rings, PTFE) 0.022

Cryostat 0.018

Muon induced neutrons

0.06

Surface Contamination (ɑ,n)

0.23

Ion Misreconstruction

0.16

Neutrinos 0.51

neutrons backgrounds beaten down by OD

(post cuts, pre 50% acceptance)

No veto Skin+OD vetoes

See talk by J. Dobson on LZ sensitivity

Geant4 simulation - radioactivity placed in detector components

In simulation, veto an event if: energy deposit > 200 keV in

OD, within 500μs of S1 in TPC


Neutron Capture on Gd

10

𝝲

Integration of DICEBOX 𝝲 -cascade code with LZ simulation → 25% increase in vetoing efficiency!

Thermal neutron capture cross sections: 157Gd: 254,000 barns 155Gd: 61,000 barns

Very complex 𝝲-cascade - not

modelled well by Geant4

Capture time in GdLS/LS


Performance

11

0 100 200 300 400 500 600 700 800 900 1000s)µod veto window (

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

inef

ficie

ncy

(%)

OD Threshold

0 keV

100 keV

200 keV

At 200 keV threshold, 500μs window, the OD is only 3.5% inefficient!→96.5 % of neutrons that single scatter within the region of interest in the TPC

(mimicking a WIMP) are vetoed

May be possible to go to 100 keV threshold, reaching < 3% inefficiency

No veto: 12.31 counts/1000 days

OD + Skin veto: 1.24 counts/1000 days

~3.3 T fiducial

5.6 T fiducial

Time between S1 in TPC & signal in OD (μs)


Summary

12

• Construction of LZ veto detector well underway • Tanks almost complete, arriving at SURF in next couple of months • Liquid scintillator production starting July

• Analysis of prototype (LS screener) data ongoing, but already learnt a lot about contaminants and LS handling

• 14C measurement, light yield, quenching • Unexpected backgrounds - 147Sm, 176Lu, 7Be … • Data used to test LZ data analysis framework

• Analysis of NaI measurements of cavern 𝝲-rays also ongoing, but so far suggests 5x lower background rate in OD than first thought

• Performance of OD in simulation so far meeting all goals - vetoes 97.5% of WIMP search background neutrons

• Commission and calibration in 2020 - exciting physics to come!


Thanks!


Backup

14


Non-linearity of scintillator Daya Bay - Phys. Rev. D 95, 072006 (2017)

Pulse shape discrimination in the LS Screener

Proton light response Krosigk et al, Eur. Phys. J. C

External rate in the LS Screener


OD Threshold (keV)0 100 200 300 400 500

Veto

Inef

ficie

ncy

0.02

0.03

0.04

0.05

0.06

0.07

DICEBOX

Geant4-FS

Full Sims, 100 keV Skin Threshold

6 7 8 9 10 11 1210-2

10-1

1

10

102

10

102

103

104

0 1 2 3 4 5 6

10-2

10-1

1

10

1

10

102

103

104

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400

102

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104

¤ 6420

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256

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@60

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780

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8982

998

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911

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4925

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327

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=

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Cou

nt/M

ass/

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×106

Energy (MeV)

182,

199

Sample: Gd2O3 (Gd)Holder: Teflon bagMass: 11 mg (9 mg)Live time: 6,620 sBeam: thermalFlux: 2.3×106 cm-2s-1

Eff. flux: 1×106 cm-2s-1

Vacuum: noCount rate: 3,101 cpsTotal counts: 20,530,263

64:Gd

1186

,118

72

1141

8982

182X

Gd(n,𝝲) spectrum (G. L. Molnar, ed., Handbook of Prompt Gamma Activation Analysis)

Gd neutron capture cross sections

Veto inefficiency (DICEBOX vs default Geant4)

THE LZ OUTER DETECTORlz.lbl.gov/wp-content/uploads/sites/6/2019/09/UCLA_SShaw_LZOD.pdf · The LZ...

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