Ultra-low background HPGe detector at ChyeongPyung Underground Laboratory
TaeYeon Kimand
KIMS(Korea Invisible Mass Search) Collaboration.
* Contents *
IntroductionMeasurement results
Summary
WIMP Mass (the Lightest Supersymmetry Particle) ~ 10 - 1000 GeVSpeed ~ 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
Requirement of detector
• 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.
Project Review (August 17-18, 2001)
Committee members : S. L. Olsen(chair), M. Fukushima, C. W. Kim, C. S. Lee.
Sample measurements
- Y.D.Kim et al., J. Korean Phys. Soc. 40, 520 (2002).
100 % HPGe Detector
hour
Cou
nt
rate
Total count rate
Raw background spectrum1,386,000 sec = 16.04 days
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
Reductionof
Background
w/o shielding
10 cm lead
Blue line : 10 cm lead + 10 cm copper
Black line : 10 cm lead+ 10 cm copper+ N2 gas flow
234Th, 234mPa, 234U, 230Th, 226Ra
218Po, 214Pb, 214Bi, 214Po,210Pb, 210Bi, 210Po
222Rn (noble gas, 3.82 days)
232Th (1.41x1010 y)238U (4.46x109 y)
206Pb (stable) 208Pb (stable)
228Ra, 228Ac, 228Th, 224Ra
216Po, 212Pb, 212Bi, 212Po, 208Tl
220Rn(noble gas, 55.6 sec)
Decay series of 238U and 232Th
Relative efficiency
232Th : still remains near the detector 238U : Most of them come from outside the copper shielding
All the daughter particles in decay chain are in the secular equilibrium.
w/o N2 gas flow with N2 gas flow
GEANT4 simulation
40K Peak
Attenuation length of 1500 keV photon in copper = > 2.32 cm
10cm lead10cm lead + 10cm copper10cm lead + 10cm copper + N2 gas flow
=> Most of 40K is outside the copper shielding.
Peak count rate:
Red = 1.2 x 10-2/SBlue = 4.9 x 10-4 /SBlack =5.7 x 10-4 /S
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.
NETECOurs
L.A.Currie, Anal. Chem. 40, 586(1968)
T
BsLLD
*66.471.2)(/
Lower Limit of Detection
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.
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