Contributions from Impurities in Shielding Material
background can come from radioactive impurities in the shielding material
typical lead shielding material contains:
210Pb (46.5 keV -ray)
210Bi (1161 keV -ray)
shielding material may also contain slight amounts of 60Co and 137Cs, which may cause problems for some low-level measurements.
some samples of lead have shown activity levels of 1.5 Bq/g.