Efficiency Calibration of Ge Detectors in Wide Energy Range with Natural Thorium And

Introduction We would like to present the use of another natural radionuclide chain, that of 232Th. It is suggested to be used as a relative energy calibrator, while absolute calibration will be done by a standard source of 137Cs which, due to its long half-life, requires a change of the calibrated standard only in about 30 to 50 years. Most Ge detector/MCA set-ups are calibrated for energy and efficiency by the use of commercially available radioactive point sources. Several national standard laboratories and commercial firms offer calibrated sources of mixed radionuclides. A favorite mixture1 consists of 109Cd, 57Co, 139Ce, 203Hg, 113Sn, 85Sr, 137Cs, 60Co and 88Y, which covers the range from 88 to 1836 keV. The disadvantage of this mixture lies in the short half-lives of several nuclides, so that the source is only useful over a period of a few months. COURSEY et al.2 suggested the use of 125Sb +154Eu+155Eu which covers the energy range from 27 to 1596 keV, with halflives beyond 2.5 years. This mixture suffers from the disadvantage of many cascade transitions which require correction for coincidence summing. In addition, while 2.5 years is longer than the half-life of the previous mixture, it is still too short, requiring replacement of the standard in less than 5 years. The advantages of Th