Investigation of radioactive radon daughters removal methods from copper surface

BackgroundCopper is one of materials often used in rare decay experiments due to its low intrinsic radioactivity. During the installation and assembly of detectors, copper parts may be exposed to air-born radon, therefore the decay daughters of radon (210Pb, 210Bi and 210Po) will be deposited on copper surfaces, and their α, β and γ decays will result in the increase of the experimental radioactive background.PurposeThis study aims to suppress this background by carefully cleaning the radioisotopes on copper surface.MethodsFirst of all, eight pickling recipes were used to test the efficacy of 210Po removal from copper surfaces. Copper test samples were prepared by exposing to high concentration radon gas environment. Then, counting rates of deposited 210Po on copper surfaces were measured by an alpha detector before and after cleaning for the calculation of 210Po removal efficiency. The relationship between removal efficiencies and concentration of pickling cocktails were investigated and pickling recipe with the highest removal efficiency was obtained. Finally, with the highest removal efficiency pickling cocktail, the radioactivity of 232Th/238U on copper surface was measured by an inductively coupled plasma mass spectrometry (ICP-MS) to estimate the radioactive contamination level in passivation layer.ResultsMixing 5% C6H8O7 and 8% H2O2 as pickling solution can remove 99% of 210Po, and 232Th/238U concentration in passivation layer is less than ng·g-1 level.ConclusionsThis best pickling recipe found by this study can be used in rare decay experiments to remove radon daughters (210Po) from copper surfaces to suppress the background induced by radon daughters.