Study of Muon-induced Neutrons in the KIMS Experiment

2005. 4. 22.

The Korean Physical Society J.H. Choi

Underground Laboratory

Yangyang KIMS Lab.

Depth : 700m 2000m(mwe) 2005-04-22 2

2005-04-22

Underground Laboratory

• The high energy muon can pass through an underground in depth.

• Neutrons can be generated by the muon when the muon through out material • The everage energy of muons is 216 GeV at 2500 mwe in IGEX lab.

• The flux of cosmic muon is 4.3x10

-7 cnt/cm 2 /s at 2000 mwe in KIMS lab.

3

Detector

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Detector

• CsI(Tl) crystal is shielded by 30cm mineral oil, 15cm lead, 5cm PE, 10cm Cu • All of other detectors are surrounded by the muon detectors • Neutron detector is located inside PE and outside Cu • CsI crystal detector is in the copper box.

2005-04-22 5 Muon Detector Neutron Detector CsI(Tl) Crystal

PSD of gamma and neutron by neutron detector

Gamma Neutron 2005-04-22 6 PID [MeV/MeV]

500 MeV neutrons

Neutron Problem

IGEX • Neutron signal is identical to the WIMP signal in CsI(Tl) crystal • Neutrons can generated by a variety of process • For example, muon-induced neutrons in the Pb shielding, muon induced neutrons in the rock, neutrons from radioactivities in the rock, neutrons from radioactivities int the air and so forth • Most of the neutrons are easily vetoed because of their low energy compared with moun induced neutrons in the shieldings • We want to avoid neutrons induced by muons in the shieldings 2005-04-22 7

Time Coincidence

• • • • We expected that we can find the coincidence of neutron and muon detector We computed the time difference Δt between muon and neutron detector’s signal Δt = Min(Muon trigger time – Neutron trigger time) Under a micro second, the coincidence is extimated.

coincidence 2005-04-22 8 Δt [sec]

Coincidence Events

All events

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Coincident events with muon

Energy [MeV] 9 60 GeV muon

Coincidence Events

• Events of total energy contain neutron events, gamma events, muon events and so forth.

• The total coincident events’ rate is 12 cnt/day/liter .

• High energy events ( > 3Mev ) in neutron detector are almost from cosmic muon.

• Coincidence events rate in high energy region is 4 cnt/day/liter . – 6x10 -5 cnt/sec • Muon Flux : 4.3x10

-7 cnt/cm 2 /s – 3x10 -5 cnt/sec • Most of the low energy events are gamma events induced by cosmic muon 2005-04-22 10

Energy spectrum in neutron band

Events of neutron energy region

• 2 candidates of muon induced neutrons are found • 0.025 cnt/day/liter • The size of data is too small • The way for shielding(15cm Pb + 5cm PE) is different from simulations of other labs The result of simulations with 35cm Pb shielding in other labs IGEX 0.4 cnt/day/liter EDELWEISS 0.4 cnt/day/liter

Coincident events of neutron energy region

Boulby f(A) 0.7 cnt/day/liter Bergamasco 1 cnt/day/liter Energy [Mev] 2005-04-22 11

Summary

• Total Coincidence events 12 cnt/day/liter • High energy (>3MeV) coincidence events 4 cnt/day/liter • Neutron band coincidence events 0.025 cnt/day/liter • We are studing of the coincident time information of the muon detector and the CsI detector • We are making a study of the simulation with GEANT4 and FLUKA to find a rate of neutrons induced by muons 2005-04-22 12