1. WIMP Dark Matter Search
Sun Kee Kim
Seoul National University
For the KIMS collaboration

2. KIMS Collaboration
H.J.Ahn, J.M.Choi, H.Y.Yang, M.S.Yang, S.C.Kim, S.K.Kim, T.Y.Kim, H.S.Lee
H.S.Park, I.H.Park, E.I.Won, H.S.Won (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, J.H.Keem (Ihwa 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)

3. Brief History of KIMS
97 Summer : First discussion on WIMP search(cryogenic detector)
97 Fall : Started R&D on CsI(Tl) for WIMP search
98 Summer : First result at ICHEP98
99 Spring : Started background measurement at Cheongphyung
99 Summer : Started measurement of intrinsic
background from crystal, shielding material
99 Fall : Expanded the collaboration
00 Spring : Prototype shielding structure installed
00 Summer : Approval of the proposal for CRI
00 Fall : DMRC established/ KIMS collab. Expanded
01 March : Taiwan, China joined KIMS collab.

4. The phrase "dark matter" means
matter whose existence has been inferred
only through its gravitational effects
Particle data group

5. Evidence for the existence of Dark Matter
Rotational curves of galaxies
Rotation of galaxies in clusters of galaxies …
matter 0.1 ~ 0.3
However,
lum < 0.01
dark matter = matter - lum
NGC6503
K.G.Begeman et. al. Mon. Not. R. Astr. Soc. 249, 523(1991)

6. Dark Matter Candidates
White Dwarfs
Brown Dwarfs
Neutron Stars
Black Holes
Baryonic
Dark Matter
Candidates
Hot Dark Matter
(HDM)
Cold Dark Matter
(CDM)
Light neutrino ~ few tens of eV
Axions ~ eV
WIMP’s (Weakly Interacting Massive Particles)
a) massive neutrino
Dirac ~ excluded by Ge Detector
Majorana > 20 GeV (LEP)
b) SUSY(Super Symmetry) Particles
s-neutrino ~ exluded by LEP
Neutralino > 30 GeV (LEP)
Non-Baryonic

7. 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
In MSSM,

8. How to detect WIMP ?
Elastic Sacttering of WIMP off a nuclues in the crytsal
WIMP
10-6 ~ pb Cs
Expected event rate
~ 1/kg/day or less I
Recoiled nucleus
Energy loss by ionization and lattice vibration

9. Difficulties Needs large amount of sensitive detector material
Cross section is very small : < 10-6 pb
Needs large amount of sensitive detector material
Recoil energy of nucleus is also very small : 10 ~ 100 keV
Qunching effect reduce visible energy further by ~1/10
Detector technique to measure ~ few keV
Background is very large : neutrons, -rays, cosmic rays
Underground experiment
Careful shielding/detector material selection
Additional techniques to reject gamma background

10. WIMP detectors Scintillators : NaI, CsI(Tl), Xe,…
Ionization loss → visible light →PMT
Relatively cheap to acquire large mass
Difficult to reject background
- pulse shape discrimination can reject gamma background
DAMA, UKDMC, …
Low temperature detectors : Si, Ge,…
Phonon excitation → temperature change → SC transition
Good gamma rejection when combined with
ionization loss measurement
Expensive to acquire and operate large mass
CDMS, CRESST,…

11. On going experiments DAMA NaI(Tl) crystal 58 kg
Gran sasso underground lab.
Low threshold ~ 2keV
Low background ~ 1 cpd
Poor gamma background rejection
CDMS
Low temperature detector (20mK)
Si 100 g, Ge 495g
Stanford, 16 m w.e.
Threshold ~ 10 keV
background ~ 60 cpd (2cpd with veto)
Excellent gamma background rejection

12. CDMS and DAMA results are
Status of WIMP Search
CDMS limit 1999
CDMS limit 2000
DAMA limit 1996
DAMA 1st positive result
based on annual modulation
CDMS and DAMA results are
not consistent
Upgrade
DAMA is upgrading to 200 kg
CDMS is moving into Soudan mine

13. WIMP hitting rate depends on season
Annual modulation
WIMP hitting rate depends on season
30km/s
Earth
Sun
232km/s

14. Recoil Energy R : event rate R0: total event rate
E0: most probable incident kinematic energy
r : kinematic factor, 4MDMT/(MD+MT)2

15. What determines the sensitivity
of WIMP search ?
When no signal is observed and background exists,
variance of signal is given by
B : background rate in cpd(counts/keV/kg/day)
M : Mass of the detector in kg,
T : Days of data accumulation in days,
Q : Quality factor
Theory
Experiment
Eth :Threshold energy

16. To improve the sensitivity
Lager detector mass, longer data taking
 easy, but expensive
Lower threshold
 hard
Lower background
 very hard
Smaller Quality factor
 better separation of gamma background
 needs new ideas

17. Sensitivity
Mdet=100,300,600 kg
Eth=1,2,3 keV
Q dependence
B = 1,5,10 cpd

18. CsI(Tl) Crystal Advantage Disadvantages
High light yield ~50,000 photons/MeV
Pulse shape discrimination
Easy fabrication and handling
High mass number
CsI(Tl) NaI(Tl)
Density(g/cm3)
Decay Time(ns) ~ ~230
Peak emmison(nm)
Hygroscopicity slight strong
Disadvantages
Emission spectra does not match with normal bialkali PMT
=> effectively reduce light yield
137Cs(t1/2 ~30y) ,134Cs(t1/2 ~2y) may be problematic

19. Amplification & self coincidence Oscilloscope or digitizer
CsI(Tl) Detector Unit
CsI(Tl) crystal
PMT
PMT
Amplification & self coincidence
Oscilloscope or digitizer
coincidence
Trigger
Signal

20. Typical signals from CsI(Tl)
660 keV g
Typical signals from CsI(Tl)
10 keV g
660 keV a

21. Photoelectron Yield # P.E./keV determines Eth,
also with more P.E., better PSD => improve Q

22. PMT selection RbCs PMT on 3cmx3cmx3cm CsI(Tl) crystal 5.9 keV x-ray
No. P.E./keV= nc/5.9 keV
6 p.e./keV

23. Full size crystal (7x7x30) :
Photoelectron yield
Full size crystal (7x7x30) :
~ 4 p.e./keV
Small crystal(3x3x3) :
5~10 p.e./keV

24. Pulse Shape Discrimination

25. Quality factor  : fraction of signal events passing the cut
 : fraction of  background passing the cut
cut S B
Ideal detector
 ~ 1
~ 0
Q << 1 S B
Separation variable
quality of separation of gamma background from signal

26. Quality factor NaI(Tl) Ideal detector  ~ 1 ~ 0 Q << 1 CsI(Tl)
Measured Energy(keV)

27. Quenching factor Light yield differs by different incident particles
an emprical fomular by Birks
Recoiled nucleus yields smaller amount of light than
recoiled electron by gammas
Quenching factor = Egamma_equivalent/Erecoil
Needs to know Quenching factor to extract Erecoil

28. Neutron Beam Test
KIGAM(지질자원연구원)
3.2 MeV p => 2.4 MeV n

29. QF = Emeasured / Erecoil
Quenching factor n
QF = Emeasured / Erecoil n

30. Background Reduction External Background
Cosmic ray muons – produce n, 
 Underground laboratory
Environmental radio-isotopes – U, Th, K, …
 Heavy shielding (Cu, Pb …)
Internal background
Within crystals – Rb, 137Cs, …
 Material study – chemistry
In other components – Shielding, PMT, cables, …
 Careful selection of materials

31. Underground Lab. at Cheongpyung
Homyung Mt.(虎鳴山)
Reservoir
Access tunnel(1.4km)
350m
Pukhan River(北漢江)
Laboratory
Power plant

35. April 2000

36. April, 2001
April, 2001

37. Prototype Shielding
15cm Pb + 10 cm Cu

38. Background in underground Lab.
Cosmic rays : ~ 10-4 relative to the sea level
Gamma background measured with HPGe detector
~ 10-4 reduction
with 15cm Pb
+ 10cm Cu
* Significant portion
of residual background
is suspected to be
from HPGe itself

39. Neutron background Measured with 0.5 liter BC501A liquid scintillator
~ 1.7x10-5 /cm2/sec
After correcting efficiency and detector volume
~ 5 cpd in CsI detector expected w/o neutron shielding
with ~ 30 cm liquid scintillator active shieling
< 0.05 cpd can be achieved

40. Intrinsic background Radioisotopes in the crystal
- most important background after the proper shielding
137Cs : half life = year (produced by atomic bomb and reactor accidents)
Beta decay to 137Ba meta stable state (Q = keV)
2min life time, emitting keV gamma
Hard to reject => serious background at low energy
134Cs : half life = year( produced by cosmic ray)
Beta deacy to 134Ba (Q= keV) immediate gamma emission
Can be rejected easily => not a serious problem
87Rb : half life = 4.75 x year (27.8% natural abundance)
Beta deacy to 87Sr (Q=282.3 keV) no gamma emission
Hard to reject => potentially a serious problem
=> reduction technique in material is known

41. Background in CsI(Tl) crystals
: Single Crystal Institute (Ukraine)
: Shanghai Institute of Ceramics (China)
: Crismatec (France)

42. Sources of intrinsic background
In the crystals currently available
g-rays and
b-rays of
87Rb, 134Cs,
and 137Cs +
GEANT
87Rb
134Cs
137Cs
Total
<Measured spectrum>
137Cs : ~0.03 Bq/kg, 134Cs : ~0.03 Bq/kg, 87Rb : < 20 ppb

43. Requirement for 1 cpd 87Rb < 0.2 ppb 137Cs < 0.001 Bq/kg

44. When do they enter ? 137Cs in powder 134Cs : 605 keV 137Cs : 662 keV
CsI powder
134Cs : 3~6x10-2 Bq/kg
137Cs : 3~9x10-2 Bq/kg
CsOH/CsNO3
134Cs : 3~4x10-2 Bq/kg
137Cs : 2~3x10-2 Bq/kg
134Cs : 605 keV
137Cs : 662 keV

45. When do they enter ? Pollucite : Upper bound only <6x10-2 Bq/kg
New measurement
< 1x10-2 Bq/kg ?
Water :
3~6x10-2 Bq/liter

46. In Pollucite : < 0.009 Bq/kg
137Cs in Pollucite ?
In CsI : 0.09 Bq/kg
In Pollucite : < Bq/kg

47. Summary of Intrinsic background
134Cs : not a problem - already below the requirement
87Rb : can be reduced
137Cs : does not exist in the pollucite
suspected to be inserted during extraction of Cs powder
measurement of 137Cs contamination in water
- contains 137Cs ~ 0.01 Bq/liter
* a lot of water is used in the process of extracting Cs powder
~ 145 liter/ 1kg Cs (from a company)
R&D with CsI powder companies is ungoing

48. CsI(Na) crystal ?
More light yield than CsI(Tl) due to better matching with PMT
Lower threshold
Maybe better PSD(?)
Hygroscopic behavior may be
a problem
~70% more light yield
CsI(Tl)
CsI(Na)
PSD under investigateion

49. Summary of detector R&D
CsI(Tl) detector
Low energy threshold ~ few keV
( photoelectron yield ~ 4 p.e./keV )
g/a separation with pulse shape analysis
Neutron beam test : quenching factor measured
Background
Cosmic ray reduction at 400 m underground : ~1/10000
Gamma-ray background reduction with shielding : ~1/10000
Intrinsic background : 137Cs exists in CsI powder
=> not in the pollucite (?)
=> need to understand Cs extraction procedure

50. Pilot Experiment Purpose : To test the concept of the experiment
and to find possible problems for the main experiment
One 7cm x 7 cm x 30 cm crystal
+ 12 surrounding veto crystals + extended prototype shielding
will start this month !

51. Experiment Setup (base line design)
CsI(Tl) 9cm x 9cm x 30cm ~ 11 kg
PMT (3 inch RbCs)
5x5 = 25 crystals ~ 275 kg

52. Experimental Setup (base line design)
220 cm
140 cm
140 cm
Total weight ~ 35 t

53. WIMP search Prospect After 1 year data taking with 100 kg CsI(Tl)
* 3x3 crystals ~ 100 kg
assume 2 keV threshold

54. Summary & Prospect
Underground Lab. at CheongPhyung is being established
Environmental background : small enough
Comfortable place for long term experiment
CsI(Tl) crystal R&D has been carried out
Low energy can be measured with good resolution
Pulse shape discrimination of -rays : promising
Intrinsic background : alsmost understood
Prospect
Pilot run with ~ 7kg CsI(Tl) will start soon
Started engineering design of the detector setup
~100 kg CsI(Tl) crystal → plan to start to data taking this year
1 year data taking would confirm or reject DAMA result

55. Astroparticle physics session Electroweak session Neutrino session
Physics In Collision 2001 
June 28-June 30, 2001  
The conference will review and update key topics in elementary particle
physics, with the aim of encouraging informal discussions experimental results
and their implications.
New Phenomena session
QCD session
Heavy flavor session
Astroparticle physics session
Electroweak session
Neutrino session