Cosmogenic Background Characterization with PROSPECT Xianyi Zhang - - PowerPoint PPT Presentation

Xianyi Zhang 12/10/2019 for the PROSPECT collaboration

Cosmogenic Background Characterization with PROSPECT

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

LLNL-PRES-798598

CPAD Instrumentation Frontier Workshop 2019 LLNL-PRES-798598

Introduction

• Cosmogenic neutron background is one of the challenges in rare event detection

experiments, especially for on surface experiments.

• Many experiments rely on simulation of cosmogenic or local neutron

backgrounds.

• The PROSPECT detector is deployed to measure reactor antineutrino spectrum
• n-surface.
• The detector design and event selection strategy enables PROSPECT to measure

cosmic background in situ and achieve our physics goal.

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CPAD Instrumentation Frontier Workshop 2019 LLNL-PRES-798598

PROSPECT

• Experiment goals:
• Probe the ~1 eV sterile neutrino oscillation 7-9 m from

the HFIR reactor.

• Measure 235U antineutrino spectrum.
• Remote, and non-intrusive evaluation of reactor

activity.

• Reactor: HFIR @ ORNL, 99% antineutrino from 235U.
• Detector:
• Optically segmented LS detector;
• Detects IBD, 7-9 meter from the HFIR reactor;
• 1 m.w.e. overburden.

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Reactor core Shield wall Shielding

Electronics

~7 m

CPAD Instrumentation Frontier Workshop 2019 LLNL-PRES-798598

Detector Design

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Water bricks 5% borated polyethylene Plastic lumber Lead Chassis Air caster Al tank Acrylic tank Segment supports PMT housings Optical grid

Reactor core Shield wall Shielding

Electronics

• 14x11 optical segmentation with 3D position

reconstruction (Topological event selection).

• 6Li loaded liquid scintillator with Pulse Shape

Discrimination (PSD) (for n-capture identification).

• Multi-layer passive background

shielding.

PMT PMT Specular reflectors

NIM A 922 287-309 (2018). JINST 14 (2019) no.04, P04014.

CPAD Instrumentation Frontier Workshop 2019 LLNL-PRES-798598

IBD Measurement Strategy

• Neutrino induced IBD event signatures in the PROSPECT detector:
• Prompt - 𝛄+;
• Delayed - α and 3H generated from neutron captured by 6Li (~10s µs delayed).
• PSD of the LS and timing information is used to distinguish β-like and n-like events.

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IBD Fast neutron Accidentals n-Li

PROSPECT prototype, 2018 JINST 13 P06023 Time-PSD selection demonstrated by prototype

CPAD Instrumentation Frontier Workshop 2019 LLNL-PRES-798598

Background Characterization

• Cosmogenic fast neutron within the IBD selection window (10s µs scale) is the

primary source of the IBD-mimic events near the earth surface.

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neutron energy [MeV]

9 −

10

7 −

10

5 −

10

3 −

10

1 −

10 10

3

10

5

10 ]

2

/dE [Hz/m Φ d × flux E

2 −

10

1 −

10 1 10

2

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incident cosmic neutrons neutrons through shielding muon-induced neutrons

Ambient surface neutron spectrum, and toy model processed through 1 m.w.e. shielding. Cosmic fast neutrons dominate flux in detector at surface. With ~10 m.w.e. overburden, neutrons are sufficiently attenuated that local muon spallation becomes dominant background source.

CPAD Instrumentation Frontier Workshop 2019 LLNL-PRES-798598

Cosmogenic Neutron Background in the PROSPECT’s IBD Detection

• Capture, capture: subsequent

captures of two thermalized

• neutrons. The earlier is captured

by unintended nucleus and the later captured by 6Li.

• Recoil, capture: gamma-like

events generated from the recoil

• f the same or different

neutrons prior to a n-Li capture.

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2 4 6 8 10 12 Energy [MeV] 1 10

2

10 event rate [mHz/MeV]

preliminary

neutron capture γs n+H n+35Cl accidentals

1 event rate [Hz/MeV] Energy [MeV] 2 4 6 8 10 12 Energy [MeV]

2 −

10

1 −

10 1 event rate [Hz/MeV]

preliminary

inelastic recoil γs

12C

Most false IBD events come from one of the two interaction mechanisms:

Capture, capture Recoil, capture

CPAD Instrumentation Frontier Workshop 2019 LLNL-PRES-798598

Background Rejection Strategy

• Event selection:
• Neutron capture identification (6Li Capture + PSD)
• Prompt event identification (PSD, segmentation) without

PSD, potentially possible with detailed topology information.

• Shower veto (high energy muon, recoil, or neutron

capture)

• Prompt-delay distance (position reconstruction)
• Fiducialization (detector as active shielding)
• Accidental: prompt and delay off-window event selection.
• Background: reactor off measurement

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Illustration of track and directional reconstruction Shower IBD-like

CPAD Instrumentation Frontier Workshop 2019 LLNL-PRES-798598

Background Rejection Strategy

• Event selection:
• Neutron capture identification (6Li Capture + PSD)
• Prompt event classification (PSD, segmentation) without

PSD, potentially possible with detailed topology information.

• Shower veto (high energy muon, recoil, or neutron

capture)

• Prompt-delay distance (position reconstruction)
• Fiducialization (detector as active shielding)
• Accidental: prompt and delay off-window event selection.
• Background: reactor off measurement.

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rate [mHz/segment] 0.0 0.5 1.0 1.5 2.0

segment x 5 10 segment z 2 4 6 8 10

Fiducial volume

Simulated accidental rate in each cell.

CPAD Instrumentation Frontier Workshop 2019 LLNL-PRES-798598

Background Rejection Capability

• Rejecting prompt event neutron recoils via PSD

selection is most critical.

• Sufficient detector size to fiducialize can increase
• verall sensitivity.
• Shower veto especially effective on capture,

capture mechanism.

• Prompt/delay topological cut is straightforward

in a detector with few cm position resolution.

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2 4 6 8 10 12 prompt ionization [MeV]

1 −

10 1 10

2

10

3

10 Event rate [mHz/MeV]

without prompt PSD without fiducialization without shower veto without distance with all cuts

preliminary

Illustration of the impact of each cuts with PROSPECT reactor off background measurement

CPAD Instrumentation Frontier Workshop 2019 LLNL-PRES-798598

Detector Response to Cosmic Backgrounds

• PROSPECT’s Geant4 simulation is able to predict the patterns of background rejection.
• On the earth’s surface, muons produce much less correlated background than

cosmogenic neutrons.

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The cosmogenic neutron simulation with PROSPECT. The muon simulation with PROSPECT. In the near surface situation, muons produce less correlated background than cosmogenic neutrons.

CPAD Instrumentation Frontier Workshop 2019 LLNL-PRES-798598

Background Normalization

• Correlations between atmospheric pressure (AP) and background rates were studied among

various backgrounds.

• IBD-like background is subtracted upon veto induced dead time and the variation from the

atmospheric pressure.

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PRELIMINARY PRELIMINARY Fast neutron rate vs AP IBD rate vs AP

RxOff RxOn RxOff RxOn

CPAD Instrumentation Frontier Workshop 2019 LLNL-PRES-798598

IBD After Background Subtraction

• With proper background measurements and subtraction, PROSPECT achieved

precise measurement of spectra along the 7-9 m baseline, on earth surface.

• S:B ~ 1.7:1.

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PRL 122, 251801 (2019)

• Phys. Rev. Lett. 121 , 251802

1 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

1.8 Ratio

1 2 3 4 5 6 7 Prompt Energy (MeV) 1 2 3 4 5 6 7 Prompt Energy (MeV) 1 2 3 4 5 6 7 Prompt Energy (MeV)

1.8 Ratio

1 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

3 4 5 (MeV)

rec

Prompt E

6.7-7.1 m Data RAA

8.0-8.4 m

7.1-7.5 m

8.4-8.8 m

7.5-8.0 m

8.8-9.2 m

Prompt Energy (MeV)

2 4 6 8 10 12

Counts/200keV

500 1000 1500 2000 2500

Reactor-on Reactor-off IBD candidates

Reactor-on and -off IBD signal rate Relative IBD spectrum comparison Absolute IBD spectrum

CPAD Instrumentation Frontier Workshop 2019 LLNL-PRES-798598

Contribution of PROSPECT in Neutron Background Measurement

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• PROSPECT can be used to demonstrate the cosmic neutron modeling with its data-

MC comparison.

• PROSPECT-style detector (with PSD + 6Li + segmented scintillator) can also be

utilized to measure neutrons that hard to be modeled:

• Making in situ measurement of near surface cosmogenic neutron. (i.e. SBN)
• Measuring beam induced neutron background. (i.e. decay at rest boosted neutrino

measurement, coherent neutrino scattering)

• Underground neutron background measurements. (i.e. DUNE, LZ)

CPAD Instrumentation Frontier Workshop 2019 LLNL-PRES-798598

Summary

• Neutron background is one of the outstanding challenges among on-surface and

beam induced neutrino experiments.

• PROSPECT is able to measure reactor antineutrino spectrum with background

measurement in situ.

• PROSPECT’s measurement of the near surface cosmogenic neutron can benefit

the community by benchmarking cosmic ray models.

• PROSPECT-style detector can also be applied in other P5 experiments to measure

cosmogenic and beam generated neutron backgrounds.

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Thank you!

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