Cosmogenic Background Characterization with PROSPECT Xianyi Zhang - PowerPoint PPT Presentation

This work was performed under the auspices of the U.S. Department of Energy by LLNL-PRES-798598 Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Cosmogenic Background Characterization with PROSPECT Xianyi Zhang 12/10/2019 for the PROSPECT collaboration

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 on-surface. • The detector design and event selection strategy enables PROSPECT to measure cosmic background in situ and achieve our physics goal . LLNL-PRES-798598 CPAD Instrumentation Frontier Workshop 2019 2

PROSPECT • Experiment goals: • Probe the ~1 eV sterile neutrino oscillation 7-9 m from the HFIR reactor. • Measure 235 U antineutrino spectrum. Shielding Shield • Remote, and non-intrusive evaluation of reactor wall Electronics activity. • Reactor : HFIR @ ORNL, 99% antineutrino from 235 U. ~7 m Reactor • Detector: core • Optically segmented LS detector; • Detects IBD, 7-9 meter from the HFIR reactor; • 1 m.w.e. overburden. LLNL-PRES-798598 CPAD Instrumentation Frontier Workshop 2019 3

Detector Design • 14x11 optical segmentation with 3D position reconstruction (Topological event selection). Shielding Shield wall Electronics • 6 Li loaded liquid scintillator with Pulse Shape Discrimination (PSD) (for n-capture identification). Reactor core • Multi-layer passive background Water bricks Al tank shielding. 5% borated Acrylic tank polyethylene Segment Plastic lumber supports Lead PMT housings PMT Chassis Optical grid Air caster Specular reflectors NIM A 922 287-309 (2018). PMT JINST 14 (2019) no.04, P04014. SOLIDWORKS Educational Product. For Instructional Use Only. LLNL-PRES-798598 CPAD Instrumentation Frontier Workshop 2019 4

IBD Measurement Strategy • Neutrino induced IBD event signatures in the PROSPECT detector: • Prompt - 𝛄 + ; • Delayed - α and 3 H generated from neutron captured by 6 Li (~10s µs delayed). • PSD of the LS and timing information is used to distinguish β -like and n-like events. IBD Fast neutron n-Li Accidentals Time-PSD selection demonstrated by prototype PROSPECT prototype, 2018 JINST 13 P06023 LLNL-PRES-798598 CPAD Instrumentation Frontier Workshop 2019 5

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. Ambient surface neutron spectrum, and toy model ] 2 incident cosmic neutrons 2 10 /dE [Hz/m neutrons through shielding processed through 1 m.w.e. shielding. muon-induced neutrons 10 Φ d Cosmic fast neutrons dominate flux in detector at × flux E surface. 1 With ~10 m.w.e. overburden, neutrons are 1 − 10 su ffi ciently attenuated that local muon spallation becomes dominant background source. 2 − 10 9 5 3 3 5 − − − 7 1 − − 10 10 10 10 10 10 10 10 neutron energy [MeV] LLNL-PRES-798598 CPAD Instrumentation Frontier Workshop 2019 6

Cosmogenic Neutron Background in the PROSPECT’s IBD Detection Most false IBD events come from one of the two interaction mechanisms: Energy [MeV] • Capture, capture : subsequent 1 event rate [Hz/MeV] event rate [mHz/MeV] preliminary preliminary captures of two thermalized 2 n+H neutrons. The earlier is captured 10 12 C by unintended nucleus and the 1 − 10 neutron capture γ s later captured by 6 Li. 10 n+ 35 Cl • Recoil, capture : gamma-like 2 − 10 events generated from the recoil inelastic recoil γ s accidentals 1 0 2 4 6 8 10 12 of the same or different 0 2 4 6 8 10 12 Energy [MeV] Energy [MeV] neutrons prior to a n-Li capture. 1 event rate [Hz/MeV] Capture, capture Recoil, capture LLNL-PRES-798598 CPAD Instrumentation Frontier Workshop 2019 7

Background Rejection Strategy • Event selection: • Neutron capture identification ( 6 Li Capture + PSD) Shower • 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) IBD-like • Fiducialization (detector as active shielding) • Accidental : prompt and delay off-window event selection. • Background : reactor off measurement Illustration of track and directional reconstruction LLNL-PRES-798598 CPAD Instrumentation Frontier Workshop 2019 8

Background Rejection Strategy • Event selection: • Neutron capture identification ( 6 Li Capture + PSD) rate [mHz/segment] segment z 10 2.15 1.20 0.94 0.86 0.83 0.78 0.79 0.72 0.76 0.82 0.89 1.00 1.18 2.15 2.0 • Prompt event classification (PSD, segmentation) without 1.36 0.34 0.18 0.14 0.15 0.14 0.15 0.15 0.12 0.14 0.14 0.19 0.35 1.31 8 1.00 0.17 0.07 0.03 0.04 0.05 0.05 0.04 0.05 0.04 0.05 0.05 0.17 1.03 PSD, potentially possible with detailed topology 1.5 0.83 0.17 0.03 0.03 0.02 0.04 0.02 0.04 0.04 0.04 0.05 0.04 0.15 0.83 information. 6 0.78 0.12 0.04 0.02 0.02 0.02 0.02 0.03 0.02 0.02 0.03 0.03 0.17 0.78 0.75 0.12 0.05 0.03 0.02 0.01 0.02 0.01 0.03 0.03 0.03 0.02 0.11 0.70 1.0 • Shower veto (high energy muon, recoil, or neutron 4 0.64 0.13 0.02 0.03 0.03 0.02 0.02 0.01 0.03 0.03 0.03 0.04 0.11 0.69 Fiducial volume capture) 0.64 0.11 0.03 0.02 0.03 0.02 0.02 0.02 0.02 0.02 0.02 0.03 0.15 0.64 0.5 2 0.70 0.13 0.04 0.03 0.02 0.02 0.01 0.03 0.01 0.02 0.03 0.04 0.13 0.68 • Prompt-delay distance (position reconstruction) 0.92 0.21 0.10 0.08 0.08 0.06 0.08 0.05 0.05 0.09 0.05 0.10 0.20 0.92 0 1.27 0.69 0.42 0.39 0.35 0.30 0.32 0.32 0.31 0.30 0.36 0.44 0.72 1.34 0.0 0 5 10 • Fiducialization (detector as active shielding) segment x Simulated accidental rate • Accidental : prompt and delay off-window event selection. in each cell. • Background : reactor off measurement. LLNL-PRES-798598 CPAD Instrumentation Frontier Workshop 2019 9

Background Rejection Capability 3 10 Event rate [mHz/MeV] without prompt PSD without fiducialization • Rejecting prompt event neutron recoils via PSD without shower veto without distance 2 10 selection is most critical. with all cuts • Sufficient detector size to fiducialize can increase 10 overall sensitivity. 1 • Shower veto especially effective on capture, capture mechanism. preliminary 1 − 10 0 2 4 6 8 10 12 • Prompt/delay topological cut is straightforward prompt ionization [MeV] Illustration of the impact of each cuts in a detector with few cm position resolution. with PROSPECT reactor o ff background measurement LLNL-PRES-798598 CPAD Instrumentation Frontier Workshop 2019 10

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. The muon simulation with PROSPECT. In the The cosmogenic near surface situation, neutron simulation with muons produce less PROSPECT. correlated background than cosmogenic neutrons. LLNL-PRES-798598 CPAD Instrumentation Frontier Workshop 2019 11

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. RxO ff PRELIMINARY RxO ff RxOn RxOn PRELIMINARY Fast neutron rate vs AP IBD rate vs AP LLNL-PRES-798598 CPAD Instrumentation Frontier Workshop 2019 12

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. 2 2 2 2 Data Counts/200keV 1.8 1.8 1.8 1.8 6.7-7.1 m 7.1-7.5 m 7.5-8.0 m Reactor-on RAA 2500 1.6 1.6 1.6 1.6 Reactor-off 1.4 1.4 1.4 1.4 IBD candidates 1.2 1.2 1.2 1.2 Ratio 2000 1 1 1 1 0.8 0.8 0.8 0.8 1.8 0.6 0.6 0.6 0.6 1500 0.4 0.4 0.4 0.4 0.2 0.2 0.2 0.2 0 2 0 2 0 2 0 2 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 1000 1.8 1.8 1.8 1.8 8.0-8.4 m 8.4-8.8 m 8.8-9.2 m 1.6 1.6 1.6 1.6 1.4 1.4 1.4 1.4 1.2 500 1.2 1.2 1.2 Ratio 1 1 1 1 0.8 0.8 0.8 0.8 1.8 0.6 0.6 0.6 0.6 0 0.4 0.4 0.4 0.4 2 4 6 8 10 12 0.2 0.2 0.2 0.2 Reactor-on and -o ff 0 0 0 3 4 0 5 0 0 0 0 Prompt Energy (MeV) 7 1 1 1 2 2 3 3 4 4 5 5 6 6 7 7 1 1 2 2 3 3 4 4 5 5 6 6 7 7 1 1 2 2 3 3 4 4 5 5 6 6 7 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 Prompt Spectrum(MeV) Prompt Spectrum(MeV) Prompt Spectrum(MeV) Prompt Energy (MeV) Prompt E Prompt Energy (MeV) (MeV) Prompt Energy (MeV) IBD signal rate rec Absolute IBD spectrum Relative IBD spectrum comparison PRL 122 , 251801 (2019) Phys. Rev. Lett. 121 , 251802 LLNL-PRES-798598 CPAD Instrumentation Frontier Workshop 2019 13