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.