Neutron Monitoring Detector in KIMS Jungwon Kwak Seoul National University 2003 October 25 th KPS meeting

2003 Oct 25thKPS meeting2 CSI NMD KIMS experiment Ordinary matter which we know is only few % of Universe Dark Energy and Dark matter is main component of Universe Strong Candidate of Dark Matter is WIMP (Weakly interacting Massive Particle) In 1997, WIMP search started in Korea by KIMS collaboration At CheongPhyung Lab(CPL), pilot experiment started in 1999 Spring In 2003, YangYang Lab(Y2L), KIMS main detector built First Dark matter search experimental run in Korea started in 2003 Summer. Korean Invisible Mass Search group

2003 Oct 25thKPS meeting3 New laboratory, Y2L 양양 양수 발전소 청평 양수 발전소 100 Km GaPyeung Lab. ChungPyeung Lab. YangYang Lab.

2003 Oct 25thKPS meeting4 Comparison between Y2L & CPL Laboratory Y2L CPL Depth Minimum 700 m ( times of Cosmic Muon ) Access tunnel 2 Km Minimum 350 m ( times of Cosmic Muon ) Access tunnel 1.4 Km Temperature 20 ~ 25 o C 11 ~ 16 o C Humidity 50 %more than 90 % Rock contents 238 U less than 0.5 ppm 232 Th 5.6 +/- 2.6 ppm K 2 O 4.1 % 238 U 4.8 +/- 1.8 ppm 232 Th 6.0 +/- 2.2 ppm K 2 O 4.2 % Size74 m 2 x 5 m55.25 m 2 x 2.5 m Networking & Phone Permitted and installedNot permitted

2003 Oct 25thKPS meeting5 CSI Pb shield (15cm) PE shield (5cm) Copper shield (10cm) Mineral Oil shield (30cm) NMD MUD Diagram of KIMS detector NMD Neutron Monitor Detector CSI CsI(Tl) Crystal Detector MUD Muon Detector - LS 5% Active Detector Passive shield Copper : 10 cm PE : 5 cm Lead : 15 cm Mineral Oil : 30 cm

2003 Oct 25thKPS meeting6 CSI MUD Neutron background in Dark matter experiment  To understand neutron background well is important in dark matter search experiment because neutron signals mimic the WIMP signals.  Neutron background source:  cosmic ray induced neutron  spontaneous fission of U 238  (α,n) reaction of U 238 & Th 232 in the rock  Neutron flux inside shield is very small, so we need a large and sensitive detector and a long term measurement.

2003 Oct 25thKPS meeting7 CSI MUD NMD Neutron Monitor Detector BC501A Liquid Scintillator 20 liter ( 16 cm diameter x 100 cm ) 10 liter ( 16 cm diameter x 50 cm ) 2 x 5” PMT times of Neutron rate inside 30cm of Mineral Oil inside Pb 5 cm shield inside main shield

2003 Oct 25thKPS meeting8 LED test of 5“ PMT  Single photoelectron distribution (200 times preamp)  2.6 ADC counts/ p.e. & 0.77 KeV /ADC counts ( Co 60 ) 0.5 p.e./ KeV Attenuation length Fitting result : 330 e - X /42cm e - X /25cm

2003 Oct 25thKPS meeting9 Digital Oscilloscope DAQ a.Setting of Digital Oscilloscope -1GHz sampling and 8 bit digitization of 1V window size b.Trigger -Coincidence of two PMT signals - Threshold 30mV -27 Hz trigger rate and 15 Hz DAQ rate c.t 0 Determination from normalized signal d. Use DCC method -PSD using the ratio of partial and full signal Pedestal Full Partial t0t0 50ns

2003 Oct 25thKPS meeting10 Analysis cuts Relative ratio vs Energy (MeV) Relative ratio = (PMT1-PMT2)/(PMT1+PMT2) abs(Relative ratio) < 0.5 => 7.8 liter of BC501A volume Part/Full signal ratio vs Energy (MeV) Partial signal – remove first 50ns region from full signal Energy > 0.3 MeV Part/ full ratio > 0.26 Neutron region Gamma region

2003 Oct 25thKPS meeting11 Decay Chain of U 238 & Th 232 Family in 238 U IsotopeLifetimeDecayQ-value (MeV) 238 U 234 Th 234 Pa * 234 Pa 4.47x10 9 y 24.1 d 1.18 m 6.7 h Alpha Beta Beta (IT) Beta (0.08) U 2.45x10 5 yAlpha Th 8.0x10 4 yAlpha Ra 222 Rn 218 Po 214 Pb 214 Bi 214 Po 1600 y 3.83 d 3.05 m 26,8 m 19.7 m  s Alpha Beta Alpha Pb 210 Bi 210 Po y 5.01 d d Beta Alpha Family in 232 Th IsotopeLifetimeDecayQ-value (MeV) 232 Th x yAlpha Ra 228 Ac 5.75 y 6.15 h Beta Th 224 Ra 220 Rn 216 Po 212 Pb 212 Bi 212 Po 208 Tl y 3.66 d 55.6 s s h m  s m Alpha Beta Alpha(64%) Beta(36%) Alpha Beta

2003 Oct 25thKPS meeting12 Fitting function = lifetime (3.8 days for Rn 222 ) + constant Constant flux : 27 counts / day /liter Data taken just after windows are sealed Time variation

2003 Oct 25thKPS meeting13 Energy Spectra of Neutron region Total 5112 counts Use 45 days data taken 2 month after Window seal No time variation 25.5 counts /liter /day

2003 Oct 25thKPS meeting14 Neutron Flux Summary Counts/day/liter Lead shieldMain Shield CPL liter BC501A CamacADC 11bit liter BC501A CamacADC 11bit Y2L liter BC501A CamacADC 11&10 bit Rn-contamination liter BC501A Digital Oscilloscope Counts in Neutron Detector is maximum number of Neutron counts Counts = Neutron + Rn + internal background (U 238 & Th 232 ) More R&D in progress DAQ Upgrade – Reduce DAQ Dead time Liquid Scintillator purification – Reduce Internal Background Time Co-relation Study for U and Th chain components