HPGe Array

High Purity Germanium detector (HPGe) system

High Purity Germanium (HPGe) detectors as a semiconductor detector are used to analyze activities of radioactive isotopes in materials by measuring gamma rays from the isotopes. In Center for Underground Physics (CUP), there are two single p-type coaxial detectors made of one HPGe crystal with 100% relative efficiency each, Canberra Coaxial 1 & 2(CC1 & CC2), and an array detector with fourteen HPGe crystals with 70% relative efficiency each at Y2L.

(Left) Figure 1 CC1 HPGe detector, (Center) Figure 2 CC2 HPGe detector, (Right) Figure 3 Array HPGe detector

CC1 & CC2 single HPGe detector

The YangYang Underground Laboratory is placed in YangYang pumped storage power Plant and there are lots of background gamma-ray signals coming from the rocks surrounding the experiment place. To reduce the background signals, we installed lead and copper shielding systems in reliable geometries.

CC1 installed in 2010 at A6 laboratory is shielded from outside to inside by 10 cm-thick Pb layer for top and bottom, 15 cm-thick Pb layer for sides and 10 cm-thick Cu layer for all the innermost sides. In 2015, 5 cm-thick ancient lead was installed near the HPGe detector to reduce β-decays and bremsstrahlung backgrounds of 210Pb which has 22.2 y half-life. The background count rate of the CC1 is 0.0079 Hz in an energy range of 50 ~ 4000 keV.

Likewise, CC2 installed in 2016, is shielded from inside to out by a layer of 10 cm thick copper, followed by 10 cm thick lead produced by the J. L. Goslar company, and a generically-sourced lead with 10 cm thickness. The background count rate of the CC2 is 0.0073 Hz in an energy range of 50 ~ 3200 keV.

Both detectors have sensitivities to 228Th and 226Rn of about 1 mBq/kg for samples on a scale of about one kg and counting time of roughly two weeks. A similar condition produces 40K sensitivity of about 5 mBq/kg. They are rough estimates and depend greatly on a number of factors that vary from sample to sample.

Figure 4. CC1 detector with shielding system (left), background spectrum (right)
Figure 5 CC2 detector with shielding system(left), background spectrum (right)

Array HPGe detector system

In early 2017, the CUP installed an array of fourteen 70% HPGe detectors composed of two arrays facing each other and each array has seven p-type coaxial HPGe detectors. The device is being used to study rare decays (i.e. decay of 180mTa) and to reach new sensitivity levels for background assays. By screening several candidate materials for the components using the single HPGe (CC1 mostly) detectors and studying backgrounds in different configurations using simulation tools, the CUP has developed it in collaboration with CANBERRA. The array detector is shielded, from inside to out, by a layer of 10 cm thick copper, followed by 10 cm thick lead produced by the J. L. Goslar company, and further 20 cm thick generically-sourced lead. The shielding system has 2 doors on two sides with a motor system to open for a replacement of samples. To prevent other particles, especially radon, enter through the door gaps, Vikuiti sheets were sealed and dry nitrogen gas from boil-off of liquid nitrogen is supplied continuously. The background count rate of the Array is 0.062 Hz in an energy range of 50 ~ 4000 keV for all fourteen detectors. Compared with the CC1 for the same relative efficiency, the background count rate of the array detector is reduced by 25%, therefore, the array detector is suitable to measure more sensitive samples.

Figure 6 Array detector in a shielding system
Figure 7 A schematic of the Array detector in a cross-section view

Reference

  • E. Sala et al., “Development of an underground low background instrument for high sensitivity measurements,” J. Phys.: Conf. Ser (2016) 062050.
  • E. Lee et al.,”Measurements of detector material samples with two HPGe detectors at the YangYang Underground Lab.,” PoS(ICHEP2018)809
  • G. Kim et al., “DAQ optimization, signal processing and simulations for an ultra low background HPGe detectors Array,” LRT 2017 Poster
  • S. Park et al., “An enriched Mo-100 powder measurement by a HPGe array detector,” PoS(ICHEP2018)783
  • M. H. Lee et al., “An ultra-low radioactivity measurement HPGe facility at the Center for Underground Physics,” PoS(ICHEP2018)363