kps @ iwha womans university 2005/04/22 hyunsu lee & jungwon kwak current limit of wimp search...

KPS @ Iwha Womans University 2005/04/22 Hyunsu Lee & Jungwon KwakHyunsu Lee & Jungwon Kwak

Current Limit of WIMP search with CsI(Tl) crystal in KIMS

이현수 *, 곽정원 , 김상열 , 김선기 , 김승천 , 명성숙 , 방형찬 , 양혜영 , 이명재 , 이주희 , 이직 , 최정훈 ( 서울대 )

김영덕 , 이정일 ( 세종대 ) 김 홍주 , 소중호 , 양성철 ( 경북대 )

권 영준 , 황 명진 ( 연세대 ) 박일흥 , 한인식 ( 이화여대 )

서은숙 , 이무현 ( Univ. of Maryland) J. J. Zhu, D. He, Q. Yue (Tsinghua University)

J. Li (IHEP)


KIMS experiment

Purpose : Search Dark matter ( Weakly Interacting Massive Particle )

1. Find unknown components of our Universe

- 73% Dark Energy(???) + 23% Dark matter(?) + 4% Known matter(…)

2. Probe various theories of Super Symmetry Particle

Required

1. Developed low background CsI(Tl)detector

- Successful reduction of the internal background of CsI(Tl) crystal

2. Deep underground experimental hall to avoid cosmic ray and induced backgrounds

3. Constructed a shield composed of pure materials and a muon detector with full

coverage of solid angle.


Underground experimental hall in Yangyang

JumBong Mt. 1424m

750 m

Entrance 250 m

Experimental hall 50m

Depth Minimum 700 m

Temperature 20 ~ 25 oC

Humidity 35 ~ 60 %

Rock contents 238U less than 0.5 ppm 232Th 5.6 2.6 ppm 40K 270 5% ppm

Muon flux 4.3 10-7 /cm2/s ( 1 10-2 /cm2/s )

Neutron flux 8 10-7 /cm2/s ( 1.4 10-3 /cm2/s )

222Rn in air 1 pCi/liter ( 4 pCi/liter )

Gneiss (2 Gyr )

HPGe detector measurement


WIMP search experiments

Gaitskell’s summary talk IDM2004


Best available Crystal at MarketBest available Crystal at Market

Powder SelectionPowder Selection

Cs137 Reduction Using Pure waterCs137 Reduction Using Pure water

Rb87 Reduction by Re-crystallizationRb87 Reduction by Re-crystallization

CsI Crystal detector

CsI(Tl) Crystal 8x8x30 cm3 (8.7 kg)

3” PMT (9269QA) Quartz window, RbCs photo cathode 4~5 Photo-electron/keV DAQ 500MHz Home Made FADC 5 photo-electron within 2μsec trigger condition total 32μsec window

Data taking with 3 crystals – 24 kg - 5.5cpd background level

Produced 600 kg of More purified powder

- use 18 M pure water - More re-crystallization - Expected Less than 2cpd background

level - Current shield can house up to 250kg

crystal

Typical Fe55 Typical Fe55 SignalSignal


CsI Data Taking

Data for WIMP search 430 kg days for 8x8x30 cm3 crystal of 14 cpd background level 2004 spring 237 kg days for 8x8x23 cm3 crystal of 6 cpd background level 2004 summer 679 kg days for 24 kg CsI(Tl) crystal of 6 cpd background level (~ now)

MC data using Geant4 simulation

Calibration data• Neutron data from Neutron calibration facility Reference distribution for recoil events• 57Co ( 122.06 keV ) and 55Fe (5.9 keV) Compton data Check Surface events, Single photon calibration • 137Cs ( 661.657 keV ) Compton data Reference distribution for events


Dark matter density at the solar system

D = 0.3 GeV c-2 cm -3

Use annual average parameters

V0 = 220 km s-1, VE = 232 km s-1, VEsc = 650 km s-1

Differential rate per unit detector mass

,

Limit curve Calculation


i

ii

A

tAt

Pulse Shape Discrimination (Mean time method)


Limit curve with 237 kg days data ( 2004.10. )

Background level

Recoil energy threshold > 20 keV

Used measured quanching factor of Recoil energy


Analysis with 679 kg days data

CsI(Tl) Crystal detectors • 0406 crystal - 6.7 kg • 0501A and 0501B crystals - 8.7 x 2 kg- Total 24 kg Crystal ( 5 ~ 6 cpd internal background )

log(Mean Time) spectrum depend on # of P.E

Calibration Gamma reference & B.G. use Charge sum of Am241 peak

(59.54keV) Neutron reference

- Cluster charge was normalized with cluster charge of B.G.

Analysis cuts - Optimized with Gamma reference data- To avoid electrical noises and abnormal events


0501B Crystal – w/o Quartz light guide ~ 4.7 p.e./keV

2-3keV2-3keV 3-4keV3-4keV 4-5keV4-5keV



PreliminaryPreliminary


0501A Crystal – with Quartz light guide ~ 3.7 p.e./keV






Module dependency

0406040605010501AA05010501BB

Data # of p.e. Size of Crystal Quartz wave guide

0406 237 kg days

4.9 8x8x23 cm3 6.7 kg

W

0501A 263 kg days

3.7 8x8x30 cm3 8.7 kg

W

0501B 179 kg days

4.7 8x8x30 cm3 8.7 kg

W/O

SB

S B

cut

2)(

)1(

K


Limit curve using All data

Weighted sum

more # of p.e., better PSD

Larger size, less surface effects



Future plan

600 kg of 2cpd background level powder was produced

- Small size crystal growing has done

Improve # of p.e. - Light shielding, reflector - Quartz window

Need more study - Size dependency - Energy calibration - Surface effect


kps @ iwha womans university 2005/04/22 hyunsu lee & jungwon kwak current limit of wimp search...

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