Cosmogenic radionuclides produced in high-purity germanium detectors during fabrication and transport

[Objective] Currently, experiments searching for rare event, such as the interaction of weakly interacting massive particles or the nuclear double beta decay, are important frontier topics in fundamental physics. Due to the considerably low probability of nuclear processes, experiments require ultralow background conditions, particularly for all materials in detection systems. Low-level high-purity germanium (HPGe) gamma-ray spectrometer with an extremely low threshold, high energy resolution, and ultralow radioactive background is critical in material selection for rare event experiments. However, cosmogenic radionuclides contaminate the germanium crystals and other materials of detectors during fabrication, storage, and transport. Effective removal of long-lived radionuclides inside germanium crystals, such as 60Co and 54Mn, within a short period is difficult, which can be considerably problematic for achieving the required sensitivity and seriously affect rare event searches. The direct, experimental information regarding the quantification of cosmogenic activation yields is scarce since analyzing cosmogenic radionuclides requires tracing the entire process from detector material preparation to the implementation of experiments. Most simulation methods were considered to quantify the activation yields of the long-lived radioisotopes that were cosmogenically induced on the ground in germanium crystals and cupreous detector components, such as YIELDX, ACTIVIA, TALYA, and GEANT4.[Methods] Herein, the cosmogenic activation of a newly customized low-level HPGe gamma-ray spectrometer was experimentally investigated. The detector was stored in underground plants except during assembly and manufacture. Immediately after the fabrication, the detector was transported by rail to the China Jinping underground laboratory (CJPL) for background measurement. The cosmogenic activation of materials in the underground can be considered negligible because the flux of cosmic nucleons in the CJPL was suppressed at depths of a few kilometers of water equivalent. Background measurement was performed with nitrogen flushing and multiple shields at different times. The cosmogenic radionuclides in the shielding materials can be ignored because they have been stored in the CJPL for more than six years. The characteristic gama peaks of the cosmogenic radionuclides 57Co,58Co,60Co, and 54Mn were observed in the energy spectra obtained during the experiment. Previous simulations indicated that both germanium crystals and copper bombarded by high-energy cosmic rays would produce these four radionuclides. The detection efficiencies of cosmogenic radionuclides were simulated using the GEANT4 Monte Carlo procedure. By fitting the curves that illustrated the changing characteristic peak counts over time, we obtained the specific activity of cosmogenic radionuclides in germanium crystals and copper.[Results] We assumed that cosmogenic radionuclides were produced during the entire process of fabrication and transport on the ground for approximately one month, and the specific activities of cosmogenic radionuclides inside the detector were calculated via spectral analysis, where the net area of a peak was determined under the assumption of a linear background. In the germanium crystals, the specific activities were obtained as 0.016 mBq/kg for 57Co, 0.046 mBq/kg for 58Co, and 0.012 mBq/kg for 54Mn, while in cupreous detector components, 0.452 mBq/kg for 57Co, 1.245 mBq/kg for 58Co, 0.382 mBq/kg for 54Mn, and 0.389 mBq/kg for 60Co, were obtained.[Conclusions] This research is essential for understanding and analyzing background spectra and creating background models for low-level radioactivity measurements. Moreover, the proposed method for estimating the specific activities of cosmogenic radionuclides provides a reference for assessing the cosmogenic background in rare event detection using HPGe detectors. © 2024 Press of Tsinghua University. All rights reserved.