Zb. Alfassi et A. Tsechanski, Efficiency calibration of Ge detectors in wide energy range with natural thorium and Cs-137, J RAD NUCL, 246(2), 2000, pp. 309-312
For the efficiency calibration of Ge detectors the Th-232 natural radionucl
ide chain is suggested with a standard source of Cs-137 which due to its lo
ng half-life requires a change of the calibrated standard only in about 30
to 50 years.