Measurements of extremely low radioactivity levels in stainless steel for GERDA

The radioisotope concentrations in stainless steel for the cryostat of the GERDA experiment at the Laboratori Nazionali del Gran Sasso (LNGS) have been measured. A total of 13 different stainless steel batches have been screened by low-level γγ-ray spectrometry with germanium detectors located at two underground laboratories. The article reports the discovery of commonly available stainless steel with very low concentrations of primordial and cosmogenic radionuclides. The concentrations are in the range of 1 mBq/kg and below. Only the manmade 60Co-isotope is present in all samples with a higher concentration of around 19 mBq/kg. Further, deviations from secular equilibrium in the natural decay chains of U and Th have been observed in some cases.