Transcript Backgrounds measurements and Shielding optimization

Dark Matter Search with CsI crystal
APPI2002 Feb 15/2002
HongJoo Kim, Yonsei University.
For the KIMS Collaboration
Contents
• Introduction
• Characteristics of CsI crystal
• Neutron beam test
• External background
• Internal background
• Prospect of KIMS
• Summary
APPI 2002, HongJoo Kim, Feb 15/2002
Dark Matter ?
* 60 years ago Fritz Zwicky first claim missing mas
in galaxy
*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
APPI 2002, HongJoo Kim, Feb 15/2002
Evidence for the existence of Dark Matter
Rotational curves of galaxies
NGC6503
APPI 2002, HongJoo Kim, Feb 15/2002
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
~
~
~0
~0
~0
χ 1  N11B  N12W  N13 H1  N14 H2
APPI 2002, HongJoo Kim, Feb 15/2002
How to detect WIMP directly ?
Elastic Sacttering of WIMP off a nuclues in the crytsal
~0
~0
 (χ 1 N χ 1 N )
WIMP
10-6 ~ 10-10 pb
Cs
Expected event rate
~ 1/kg/day or less
I
Recoiled nucleus
Energy loss by ionization and lattice vibration
APPI 2002, HongJoo Kim, Feb 15/2002
Direct detection strategies
Nuclear recoil elastic scattering :
exponential spectrum in target material
S  K
B
MT
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
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)
APPI 2002, HongJoo Kim, Feb 15/2002
Annual modulation
WIMP hitting rate depends on season
30km/s
Earth
232km/s
Sun
APPI 2002, HongJoo Kim, Feb 15/2002
Pulse Shape Discrimination Method
Event by event separation (Low temp)
APPI 2002, HongJoo Kim, Feb 15/2002
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
APPI 2002, HongJoo Kim, Feb 15/2002
Status of WIMP Search
CDMS limit 2000
DAMA 1st positive result
based on annual modulatio
KIMS 100kg year
APPI 2002, HongJoo Kim, Feb 15/2002
CsI Crystal
Advantages
High light yield ~50,000 photons/MeV
Pulse shape discrimination
Easy fabrication and handling
CsI(Tl)
Density(g/cm3)
4.53
High mass number
Good spin dependent interaction Decay Time(ns) ~1050
Peak emission(nm) 550
coupling
Hygroscopicity
slight
NaI(Tl)
3.67
~230
415
strong
Disadvantages
Emission spectra does not match with normal bialkali PMT
 effectively reduce light yield
137Cs(t
134Cs(t
1/2 ~30y) ,
1/2 ~2y) may be problematic
APPI 2002, HongJoo Kim, Feb 15/2002
KIMS collaboration
(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)
APPI 2002, HongJoo Kim, Feb 15/2002
CsI(Tl) Detector Unit
CsI(Tl) crystal
PMT
PMT
Amplification & self coincidence
Oscilloscope or digitizer
coincidence
Trigger
Signal
APPI 2002, HongJoo Kim, Feb 15/2002
660 keV g
Typical signals from CsI(Tl)
10 keV g
660 keV a
APPI 2002, HongJoo Kim, Feb 15/2002
Photoelectron yield
Full size crystal (7x7x30) :
~ 4 p.e./keV
Small crystal(3x3x3) :
5~10 p.e./keV
APPI 2002, HongJoo Kim, Feb 15/2002
Neutron Beam Test
KIGAM(Korea Institute of Geology and Material)
3.2 MeV p => 2.4 MeV n
APPI 2002, HongJoo Kim, Feb 15/2002
Quenching factor
QF = Emeasured / Erecoil
APPI 2002, HongJoo Kim, Feb 15/2002
Mean Time Distribution
APPI 2002, HongJoo Kim, Feb 15/2002
Quality Factor
Separation of n/gamma
Ideal case: QF<<1
CsI(Na)
CsI(Tl)
APPI 2002, HongJoo Kim, Feb 15/2002
Background
External Background
 Cosmic ray muons – produce n, g
 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
APPI 2002, HongJoo Kim, Feb 15/2002
Underground Lab. at Cheongpyung
Homyung Mt.(虎鳴山)
Reservoir
350m
Access tunnel(1.4km)
Pukhan River(北漢江)
Laboratory
Power plant
APPI 2002, HongJoo Kim, Feb 15/2002
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
APPI 2002, HongJoo Kim, Feb 15/2002
Gamma Background
100% HPGe installed in CPL
W/o shielding
10cm Pb + 10 cm Cu
(16 days data taking)
222Rn_up(226Ra)/222Rn_down(214PB,214Bi)=0.22+-0.06
APPI 2002, HongJoo Kim, Feb 15/2002
Neutron gamma separation with BC501A
APPI 2002, HongJoo Kim, Feb 15/2002
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
APPI 2002, HongJoo Kim, Feb 15/2002
Sources of internal background
In a crystal currently available
60 cpd
g-rays and
b-rays of
87Rb, 134Cs,
and 137Cs
+
GEANT
137Cs
: ~155mBq/kg, 134Cs : ~35mBq/kg, 87Rb : 3.9 ppb
137Cs
: 13.3mBq/kg, 134Cs : 54.2mBq/kg, 87Rb :~203ppb
238U : 5.5mBq/kg , 232Th : 12.4mBg/kg
Another
crystal
APPI 2002, HongJoo Kim, Feb 15/2002
137
Cs in Pollucite
In CsI : 0.063 Bq/kg
In 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)
APPI 2002, HongJoo Kim, Feb 15/2002
Shielding Design
APPI 2002, HongJoo Kim, Feb 15/2002
Detector Design
CsI(Tl) 9cm x 9cm x
30cm ~ 11 kg
Quartz
PMT (3 inch
RbCs)
5x5 = 25
crystals
~ 275 kg
APPI 2002, HongJoo Kim, Feb 15/2002
Sensitivity of Spin Independent Interaction
After 1 year data taking
with 50 kg CsI(Tl)
assume 2 keV threshold
APPI 2002, HongJoo Kim, Feb 15/2002
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
APPI 2002, HongJoo Kim, Feb 15/2002
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 g-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