1. Ultra-low background HPGe detector at ChyeongPyung Underground Laboratory TaeYeon Kim and KIMS(Korea Invisible Mass Search) Collaboration. * Contents * Introduction Measurement results Summary

2. WIMP Mass ( the Lightest Supersymmetry Particle ) ~ 10 - 1000 GeV Speed ~ 270 km/s Density ~ 0.3 GeV/cm 3 Cross section is very small : less than 10 -41 cm 2 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

3. 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.

4. Sample measurements - Y.D.Kim et al., J. Korean Phys. Soc. 40, 520 (2002).

5. 100 % HPGe Detector

6. hour Count rate Total count rate Raw background spectrum 1,386,000 sec = 16.04 days

7. Energy (keV) IsotopeEmission Probabilty(%) Count rate ( day -1 ) 352 214 Pb35.17.29 ± 2.68 609 214 Pb44.612.8 ± 2.3 1120 214 Bi14.79.79 ± 2.37 1764 214 Bi15.19.85 ± 1.75 238 212 Pb43.559.2 ± 3.9 583 208 Tl30.630.2 ± 2.8 727 212 Bi6.645.7 ± 1.4 2614 208 Tl35.937.8 ± 1.5 911 228 Ac26.613.6 ± 2.4 969 228 Ac16.214.6 ± 3.4 1461 40 K10.649.2 ± 2.4 Reduction of Background w/o shielding 10 cm lead Blue line : 10 cm lead + 10 cm copper Black line : 10 cm lead + 10 cm copper + N 2 gas flow

8. 234 Th, 234m Pa, 234 U, 230 Th, 226 Ra 218 Po, 214 Pb, 214 Bi, 214 Po, 210 Pb, 210 Bi, 210 Po 222 Rn (noble gas, 3.82 days) 232 Th (1.41x10 10 y) 238 U (4.46x10 9 y) 206 Pb (stable) 208 Pb (stable) 228 Ra, 228 Ac, 228 Th, 224 Ra 216 Po, 212 Pb, 212 Bi, 212 Po, 208 Tl 220 Rn (noble gas, 55.6 sec) Decay series of 238 U and 232 Th

9. 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 N 2 gas flowwith N 2 gas flow

10. GEANT4 simulation

11. 40 K 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 => Most of 40 K is outside the copper shielding. Peak count rate: Red = 1.2 x 10 -2 /S Blue = 4.9 x 10 -4 /S Black =5.7 x 10 -4 /S

12. HPGe detector system of NETEC(Nuclear Environment TECnology institute)

13. Comparison with the 100% HPGe of NETEC NETEC detector : Cosmic-ray veto. Compton suppressor. located at sea level. NETEC Ours

14. L.A.Currie, Anal. Chem. 40, 586(1968) 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

15. 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.