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Muon Detection and Veto

Haoqi Lu

Institute of High Energy physics

The joint DC-RENO-DYB workshop

Oct. 16-18,2016

Outline

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• Muon system

• Muon energy and flux

• Muon detector performance

• Muon detection efficiency

• Muon veto

• Summary

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Experiment layout

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Muon system

• The ADs are immersed in pool with thickness of water at least 2.5 meters, to shield backgrounds from neutrons and gamma’s from lab walls .

• The water pools are equipped with PMTs as a water Cerenkov detector. The water pools are covered with RPC detectors.

• Water Cerenkov detector and RPC system with efficiency>99.5% and error<0.25%.

Background due to rock radioactivity vs thickness of water. Neutron background vs. thickness of water.

2.5 m of water

• Multiple AD modules at each site to check uncorr. syst. err. – Far: 4 modules，near: 2 modules

• Multiple muon detectors to reduce veto eff. uncertainties – Water Cherenkov： 2 layers – RPC： 4 layers at the top + telescopes

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• Outer layer of water veto (sides and bottom) is 1m, inner layer >1.5m. Water extends 2.5m above ADs

• 288 8” PMTs per near hall

• 384 8” PMTs in Far Hall

• Two detectors

• Inner water shield(IWS)

• Outer water shield(OWS)

Far Site Near Site

Water Cherenkov detector

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RPC detector

RPC chamber design module effi. vs. HV • 4-layer RPC above pool

• 54 modules in near halls

• 81 modules in Far Hall

• 2 Telescope RPC modules in each hall for precise muon track measurement.

Tele-RPC

RPC modules

Tele-RPC 4 layer RPCs in one module,

dimension 2 m x 2m.

Muon energy and flux in experiment hall

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3D image of the DayaBay

• In order to get the muon energy and flux

underground:

• Generate a statistical sample of sea-level muon events according to a modified standard Gaisser’s formula;

• Calculate the slant depth of muons passing through the mountain using an interpolation method based on the digitized data of the mountain;

• Get the underground muon sample and underground muon flux with MUSIC.

The modified Gaisser formula

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Using the modified angle θ and energy E, the formula is consistent with experimental results in the low energy range.

• The muon flux at sea level usually can be described by the standard Gaisser’s formula. The muon flux of deep underground experiments and at polar angles smaller than 70 degrees is well-described by the standard formula.

• In low energy region, the formula can’t give a good description. For DayaBay, the mountain is shallow(~100m), so a modified Gaisser formula was developed for Daya Bay.

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3. Simulation Results

Muon flux and energy at Daya Bay underground halls

Detector trigger threshold

• RPC system: – 3 of 4 layer of a module to trigger one event

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• Water pool trigger threshold

• Offline threshold:

• Muon events:

• Pool muon:IWS or OWS PMT Multiplicity>12

• RPC: 3 of 4 layer of a module

• AD muon: visible energy in AD>20MeV

• AD shower muon: visible energy in AD>2.5GeV

Muon detector performance

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Water Cherenkov PMT gains (single photoelectron charge) vs. time.

Water Cherenkov PMT dark noise rate vs time, from periodic triggers.

NIM, A 773(2015)8–20

PMT gain PMT dark noise rate

The jumps in the EH1 gains in early May were caused by temperature changes in the electronics crates.

Detector trigger rate

EH1

EH1

EH2

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Multiplicity trigger rate vs time. Rate of muons (depositing more than 20 MeV in an AD) in the ADs versus time.

Water pool Antineutrino Detector(AD)

Muon events:

• The trigger events will be treated as a tagged muon.

• IWS muon: PMT Multiplicity>12

• OWS muon: PMT Multiplicity>12

• AD muon: visible energy in AD>20MeV

EH3

EH3

EH3

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Muon flux in experiment hall

Measured underground muon flux(Hz/m^2)

• Flux could be measured by 4 independent detectors : AD, IWS, OWS, RPC.

• The data are consist with simulation result.

NIM, A 773(2015)8–20

Muon angular distributions measurement by RPC

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• 2 Telescope-RPC(Tele-RPC) modules about 2 meter above bottom array RPC in each hall;

• Muon position resolution of RPC tracking system ~0.1m;

• The combined telescope-RPC modules and and array RPC system can measure the muon tracks and angular distribution.

Tele-RPC

RPC modules

Tele-RPC

• Tele-RPC and RPC reconstructed muon tracks in EH1. The MC and data points are normalized by their respective total counts.

• This is a good validation about the muon flux MC.

Muon

PMT Multiplicity per muon

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The peak below 20 PMTs, most prominent for EH3 OWS, is mainly due to PMT and electronics noise.

Mean PMT multiplicity per muon vs. time. Distributions of PMT Multiplicity-per-muon.

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τ value vs time in each halls.

Resistivity of the pool water in each hall vs. time.

• This measurement is made where water from the pool re-enters the polishing loop. The water delivered to the pool is typically14–16 MΩ-cm.

• The jump in EH1 on May 4 was due to sensor maintenance.

• Longer decay times correspond to clearer water

The decay time τ from the PMT hit time

Water property

Muon detection efficiency • For muon efficiency study, we use the going through AD muons to measure

IWS and OWS detection efficiency. – Directly measurement from data;

– Monitoring the detector performance change;

– Do MC and data comparison.

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IWS efficiency for AD-tagged muons vs time.

The small inefficiency is due to events identified as muons in an AD,but which are actually a neutrons created by muons in the rock.

• Neutrons can propagate undetected through the water into an AD, and deposit sufficient energy to be tagged as a muon;

• MC indicate that the IWS efficiency for AD-tagged muons should be 100%;

• MC show a very small fraction of muons showering in the rock,where an neutron deposits energy in an AD but without trigger IWS/OWS;

• MC is is consistent with the observed IWS inefficiency.

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OWS efficiency for AD-tagged muons vs time.

• OWS efficiency by AD-tagged muons is expected to a little lower than the true OWS efficiency.

– Some muons that deposit energy in the ADs but stop AD/IWS(stop muons) without traversing the OWS.

– EH3 efficiency is higher than EH1 and EH2. Stop muons ratio is lower than EH1/EH2 because of higher mean muon energy.

– These are validated by MC.

Fast neutron background

• Muon detection:

– The muons going through IWS have very high detect efficiency because of the long track length in water;

– The undetected events is corner clipper events with very short track length in water and far from central detector.

• Fast neutron background

– The stop muon induced fast neutron background is neliglible;

– The undetected corner clipper muon events is very small;

– The main contribution is from muons going through the rock(boundary muons) induced fast neutron background.

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Muon veto for background reduction

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• The cosmic muon induced background in IBD candidate

• Fast neutron background

• Proton recoil: prompt signal,neutron capture: delayed signal

• Veto time should be rather long in comparison with∼28 µs neutron capture time in the GdLS target.

• Double neutron

• Prompt signal: neutron capture on GdLS, delay signal: neutron in LS region and slowly diffused into the GdLS and capture in GdLS

• Veto time should be seveal times longer than ~200 µs( neutron capture time in Liquid Scintillator).

• Li9/He8 background

• β-decay + neutron

• A longer veto need because of 178.3 ms half-life of 9Li

• Need balance the background reduction and live time loss.

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• Muon veto in IBD selection(Gd capture):

• Muon Veto: 0.6 ms after a Pool muon (reject fast neutron), 1 ms after an AD muon (reject double neutron), 1 s after an AD shower muon (reject 9Li/8He)

• 0.7 MeV < Ep < 12.0 MeV

• 6.0 MeV < Ed < 12.0 MeV

• 1 μs < Δtp-d < 200 μs

• Multiplicity cut

• Muon events:

• Pool muon:IWS or OWS PMT Multiplicity>12

• AD muon: visible energy in AD>20MeV

• AD shower muon: visible energy in AD>2.5GeV

• Background level after veto:

• Fast neutron: B/S ~0.1%;

• Li9/He8: B/S~0.4%;

• Double neutron: ~negligible;

• live-times loss:

• 2% of the far and 14%–18% of the near hall

Chinese Phys. C37 (2013) 011001

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Summary

• The Daya Bay use multiple detectors for muon tagging. The veto detectors have very high efficiency for muon tagging.

• The fast neutron background in IBD sample is mainly from the boundary muons contribution( muons going through the rock around the detectors).

• We use different veto time windows for different background reduction.

• Background level after veto:

• Fast neutron: B/S ~0.1%;

• Li9/He8: B/S~0.4%;

• Double neutron: ~negligible;

• Live-times loss: 2% of the far and 14%–18% of the near hall.

Thanks!

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