appi 2002, hongjoo kim, feb 15/2002 dark matter search with csi crystal appi2002 feb 15/2002 hongjoo...

APPI 2002, HongJoo Kim, Feb 15/2002

Dark Matter Search with CsI crystal

APPI2002 Feb 15/2002

HongJoo Kim, Yonsei University.

• Introduction• Characteristics of CsI crystal• Neutron beam test• External background• Internal background • Prospect of KIMS• Summary

Contents

For the KIMS Collaboration


*The phrase "dark matter" means matter whose existence has been inferred only through its gravitational effects ( Particle data group)

*It is believed that 90% of universe is filled with unknown matter and about 30% could be from dark matter

Dark Matter ?

* 60 years ago Fritz Zwicky first claim missing mass in galaxy


Evidence for the existence of Dark Matter

Rotational curves of galaxies

NGC6503


Weakly Interacting Massive Particle (WIMP)

Certain classes of SUSY model predict Neutralino as LSP :

Stable, weak interaction scale annihilation cross section gives proper relic density for dark matter

Excellent CDM candidate (proposed by M.Goodman, E.Witten PRD 31, 1985)

Neutralino : super-partner of neutral gauge bosons and higgs partciles

0214

01131211

01

~~~~χ~

HNHNWNBN


Elastic Sacttering of WIMP off a nuclues in the crytsal

How to detect WIMP directly ?

Energy loss by ionization and lattice vibration

WIMP

Cs

I

Recoiled nucleus

)χ~

χ~

( 01

01 NN

10-6 ~ 10-10 pb

Expected event rate~ 1/kg/day or less


Direct detection strategies

We need • Large mass and long time measurement• Low background • Low threshold and further background rejection method ( K factor)• Annual modulation• Nuclear recoil discrimination (PSD)

MT

BKS

Nuclear recoil elastic scattering : exponential spectrum in target material

B : background rate in cpd(counts/keV/kg/day) M : Mass of the detector in kg, T : Days of data accumulation in days,K : Quality factor Lower value gives better sensitivity


Annual modulation

WIMP hitting rate depends on season

232km/s

30km/s

Sun

Earth


Pulse Shape Discrimination Method

Event by event separation (Low temp)


DAMA experiment

DAMA NaI(Tl) crystal ~100 kg Gran sasso underground lab. Low threshold ~ 2keV Low background ~ 1 cpd Annual modulation method Found a evidence of the annual modulation


Status of WIMP Search

DAMA 1st positive resultbased on annual modulation

CDMS limit 2000

KIMS 100kg year


CsI CrystalCsI Crystal

CsI(Tl) NaI(Tl)Density(g/cm3) 4.53 3.67Decay Time(ns) ~1050 ~230Peak emission(nm) 550 415Hygroscopicity slight strong

Advantages High light yield ~50,000 photons/MeV Pulse shape discrimination Easy fabrication and handling High mass number Good spin dependent interaction coupling

Disadvantages Emission spectra does not match with normal bialkali PMT effectively reduce light yield 137Cs(1/2 ~30y) ,134Cs(1/2 ~2y) may be problematic


KIMS collaborationKIMS collaboration(Korean Invisible Mass Search)(Korean Invisible Mass Search)

H.J.Ahn, J.M.Choi, R.K.Jain,S.C.Kim, S.K.Kim, T.Y.Kim, H.S.Lee,S.E.Lee,H.Park, I.H.Park, E.I.Won, H.S.Won,H.Y.Yang,M.S.Yang (Seoul National Univ.,

Korea)

W.K.Kang, Y.D.Kim (Sejong Univ., Korea)

M.J.Hwang, H.J.Kim, J.H.Lee, Y.J.Kwon (Yonsei Univ., Korea)

I.S.Han, E.K.Lee (Ehwa Womans Univ., Korea )

I.S.Cho, D.H.Choi, S.H.Noh, I.T.Yu (SeongKyunKwan Univ., Korea)

S.Y.Choi (Chonbuk National Univ., Korea)

P.Ko (KAIST, Korea)

M.H.Lee, E.S.Seo (Univ. Maryland, USA)

H.B.Li, C.H.Tang, M.Z.Wang (National Taiwan Univ., Taiwan)

W.P.Lai, H.T. Wong (Academia Cinica, Taiwan)

J.Li, Y.Liu, Q.Yue (Inst. Of High Energy Physics, China)

B.Xin, Z.Y.Zhou (Inst. Of Atomic Energy, China)

J.J.Zhu(Tsing Hua University, China)


CsI(Tl) Detector Unit

CsI(Tl) crystalPMT PMT

Amplification & self coincidence

coincidence

Oscilloscope or digitizer

Trigger

Signal


Typical signals from CsI(Tl)

660 keV

660 keV 10 keV


Photoelectron yield

Full size crystal (7x7x30) :~ 4 p.e./keV

Small crystal(3x3x3) :5~10 p.e./keV


Neutron Beam Test

KIGAM(Korea Institute of Geology and Material)3.2 MeV p => 2.4 MeV n


Quenching factor

QF = Emeasured / Erecoil


Mean Time Distribution


Quality Factor

CsI(Na)

CsI(Tl)

Separation of n/gammaIdeal case: QF<<1


BackgroundBackground External Background

Cosmic ray muons – produce n, Underground laboratory

Environmental radio-isotopes – U, Th, K, … Heavy shielding with low background(Cu, Pb )

Neutron background -> Most serious problem with PSD method (n and WIMP scattering is the same)

Heavy shielding (Polyethylene), Active shielding with BC501A (n background measurement)

Radon background Pure nitrogen gas flowing Internal background

Within crystals – 87Rb, 137Cs, 134Cs … Material study – chemistry, low background powder selection


Power plant

Homyung Mt.( 虎鳴山 )

Reservoir

Laboratory

Access tunnel(1.4km)350mPukhan River( 北漢江 )

Underground Lab. at Cheongpyung


Phase II ( 2000,4 - 2001,4 )

Used for the null background and the sample measurements with HPGe detector.

Size :External volume = 75x75x85 cm3 Internal volume = 10x15x23 cm3


W/o shielding

10cm Pb + 10 cm Cu(16 days data taking)

100% HPGe installed in CPL

Gamma Background

222Rn_up(226Ra)/222Rn_down(214PB,214Bi)=0.22+-0.06


Neutron gamma separation with BC501A


Neutron background at underground

Measured with 0.5 liter BC501A liquid scintillator after unfolding ~ 4x10-5 /cm2/sec ~ 5 cpd in CsI detector expected w/o neutron shielding

GEANT4 simulation with measured neutron flux with 30cm Polyethylene + 20 cm liquid scintillator active shieling < 0.05 cpd can be achieved


Sources of internal background

137Cs : ~155mBq/kg, 134Cs : ~35mBq/kg, 87Rb : 3.9 ppb

In a crystal currently available

rays andrays of 87Rb, 134Cs, and 137Cs

+ GEANT

60 cpd

137Cs : 13.3mBq/kg, 134Cs : 54.2mBq/kg, 87Rb :~203ppb238U : 5.5mBq/kg , 232Th : 12.4mBg/kgAnother

crystal


137137Cs in PolluciteCs in Pollucite

In CsI : 0.063 Bq/kgIn Pollucite : < 0.008 Bq/kg

Suspected to be inserted during extraction of Cs powder measurement of 137Cs contamination in water 137Cs ~ 0.01Bq/liter

* a lot of water is used in the process of extracting Cs powder ~ 145 liter/ 1kg Cs (from a company)


Shielding Design


Detector Design

CsI(Tl) 9cm x 9cm x 30cm ~ 11 kg

PMT (3 inch RbCs)

5x5 = 25 crystals ~ 275 kg

Quartz


Sensitivity of Spin Independent Interaction

After 1 year data takingwith 50 kg CsI(Tl)

assume 2 keV threshold


Spin Dependent WIMP search Prospect

•Spin dependent interaction sensitivity : If WIMP interact only with spin of nucleus• Spin factor of Cs is bigger than I•After 1 year data taking with 100 kg CsI(Tl) : Sensitivity is better than DAMA’s limit


Summary & Prospect

Underground Lab. at CheongPyung is being established Environmental backgrounds : Understood. Small enough after the shielding

Extensive R&D on CsI(Tl) crystal has been carried out Low energy can be measured with good resolution Pulse shape discrimination of -rays : promising Intrinsic background : almost understood

Shielding design/Simulation/Electronics/DAQ Design almost at final stage

Prospect Started engineering design of the detector

~100 kg CsI(Tl) crystal 1 year data taking would confirm or reject DAMA result

appi 2002, hongjoo kim, feb 15/2002 dark matter search with csi crystal appi2002 feb 15/2002 hongjoo...

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