Ultra-low background HPGe detector
at ChyeongPyung Underground Laboratory
TaeYeon Kim
and
KIMS(Korea Invisible Mass Search) Collaboration.
* Contents *
Introduction
Measurement results
Summary
Requirement of detector
WIMP Mass (the Lightest Supersymmetry Particle) ~ 10 - 1000 GeV
Speed ~ 270 km/s
Density ~ 0.3 GeV/cm3
Cross section is very small : less than 10-41 cm2
Recoil energy of nucleus is also very small :
less than100 keV
Background(gamma-rays, neutrons, X-rays,
cosmic rays) is very large
• Detector has large target mass and long-term stability
• Detector can measure tiny energy ~ few keV
• Detector have to be made of pure material
• Detector have to be located underground
Project Review (August 17-18, 2001)
Committee members : S. L. Olsen(chair),
C. W. Kim,
M. Fukushima,
C. S. Lee.
• First (real) data-taking in August 2002.
<- An ambitious schedule, but it should be maintained.
• Continue testing and trying to improve the PSD quality factor.
<- The PSD power of the production crystal should be confirmed.
• Reducing impurities is an iterative and time consuming process.
<- It is recommended to pursue the process of improving the
crystal performance and building the rest of the experiment
simultaneously.
Sample measurements
- Y.D.Kim et al., J. Korean Phys. Soc. 40, 520 (2002).
100 % HPGe Detector
Total count rate
hour
1,386,000 sec = 16.04 days
Raw background spectrum
Reduction
of
Background
Blue line :
10 cm lead
+ 10 cm copper
w/o shielding
10 cm lead
Black line :
10 cm lead
+ 10 cm copper
+ N2 gas flow
Energy
(keV)
Isotope
Emission
Probabilty(%)
Count rate
( day-1)
352
214Pb
35.1
7.29 ± 2.68
609
214Pb
44.6
12.8 ± 2.3
1120
214Bi
14.7
9.79 ± 2.37
1764
214Bi
15.1
9.85 ± 1.75
238
212Pb
43.5
59.2 ± 3.9
583
208Tl
30.6
30.2 ± 2.8
727
212Bi
6.64
5.7 ± 1.4
2614
208Tl
35.9
37.8 ± 1.5
911
228Ac
26.6
13.6 ± 2.4
969
228Ac
16.2
14.6 ± 3.4
1461
40K
10.6
49.2 ± 2.4
Decay series of
238U
238U
and 232Th
(4.46x109 y)
234Th, 234mPa, 234U,
230Th, 226Ra
232Th
(1.41x1010 y)
228Ra, 228Ac, 228Th, 224Ra
222Rn
220Rn
(noble gas, 3.82 days)
(noble gas, 55.6 sec)
218Po, 214Pb, 214Bi, 214Po,
216Po, 212Pb, 212Bi,
210Pb, 210Bi, 210Po
212Po, 208Tl
206Pb
(stable)
208Pb
(stable)
Relative efficiency
w/o N2 gas flow
with N2 gas flow
All the daughter particles in decay chain  232Th : still remains near the detector
are in the secular equilibrium.
 238U : Most of them come from
outside the copper shielding
GEANT4 simulation
40K
Peak
Attenuation length of 1500 keV photon in copper
= > 2.32 cm
10cm lead
10cm lead + 10cm copper
10cm lead + 10cm copper + N2 gas flow
Peak count rate:
Red = 1.2 x 10-2/S
Blue = 4.9 x 10-4 /S
Black =5.7 x 10-4 /S
=> Most of
40K
is outside the copper shielding.
HPGe detector system of NETEC(Nuclear Environment TECnology institute)
Comparison with the 100% HPGe of NETEC
NETEC detector : Cosmic-ray veto.
Compton suppressor.
located at sea level.
NETEC
Ours
Lower Limit of Detection
L.A.Currie, Anal. Chem. 40, 586(1968)
LLD (/ s ) 
2.71  4.66 * B
T
When T = 350,000 sec,
23 % : LLD = 1.9 x 10-4 /s
100 % : LLD =1.1 x 10-4 /s
23 % HPGe
100 % HPGe
Summary
• A HPGe detector for low-level counting was installed
at CheongPyung underground laboratory and
the background level was reduced to less than 10
cpd/keV/kg.
• A feasible study if our detector could be used to
find rare process events is going on.