Speaker
Description
Rare-event search experiments such as AMoRE (Advanced Mo-based Rare process Experiment) require ultra-low background levels and therefore operate in underground laboratories. In such environments, where ventilation is limited, ²²²Rn progeny accumulate on detector surfaces and contribute to the experimental backgrounds. The AMoRE-II, the phase-2 of the AMoRE program, is located underground at Yemilab (²²²Rn ~205 Bq/m³), with various metal components — Cu, Pb, SS, and Fe — stored prior to deployment. Past cases of elevated surface α activity on Pb components exposed to Rn-rich environments, and the subsequent removal of ²¹⁰Po contamination via HNO₃ cleaning, indicate that quantitative characterization of Rn-induced surface contamination and the validation of acid cleaning protocols for metal components are required as inputs to the AMoRE-II background budget.
To this end, we apply a three-step protocol to Cu 4×4 cm² samples prepared identically to the AMoRE-II cans: (i) surface sanding, (ii) ²²²Rn exposure, and (iii) standard acid cleaning. Surface α activity is measured after each step using a 12 cm² Si α counter with a background of 0.05 count/hr/cm². The ²²²Rn exposure condition is set using the Jacobi deposition model to be equivalent to 49 days of exposure in the Yemilab environment, reproducing the actual ²¹⁰Po accumulation behavior on an accelerated timescale.
Sanding reduced the surface α activity to approximately one-third of its initial level. Over ~100 days of follow-up measurements after Rn exposure, we observed a time-dependent increase in the 5.3 MeV peak rate, attributed to ²¹⁰Po ingrowth from ²¹⁰Pb implanted on the surface. In this poster, we report the surface accumulation rate and saturation surface activity of ²¹⁰Po extracted from the ingrowth fit, together with the post-cleaning saturation surface activity, the ²¹⁰Po removal efficiency, and the surface contamination depth profile.
These results provide direct inputs to the AMoRE-II background simulation and serve to validate the acid cleaning protocol for metal components. Extension of the same protocol to Pb, SS, and Fe samples is planned as a follow-up study.