Primary standardization of 57Co

This work describes the method developed by the Nuclear Metrology Laboratory in IPEN, São Paulo, Brazil, for the standardization of a 57Co radioactive solution. Cobalt-57 is a radionuclide used for calibrating gamma-ray and X-ray spectrometers, as well as a gamma reference source for dose calibrators used in nuclear medicine services. Two 4πβ–γ coincidence systems were used to perform the standardization, the first used a 4π(PC) counter coupled to a pair of 76 mm×76 mm NaI(Tl) scintillators for detecting gamma-rays, the other one used a HPGe spectrometer for gamma detection. The measurements were performed by selecting a gamma-ray window comprising the (122 keV+136 keV) total absorption energy peaks in the NaI(Tl) and selecting the total absorption peak of 122 keV in the germanium detector. The electronic system used the TAC method developed at LMN for registering the observed events. The methodology recently developed by the LMN for simulating all detection processes in a 4πβ–γ coincidence system, by means of the Monte Carlo technique, was applied and the behavior of extrapolation curve compared to experimental data. The final activity obtained by the Monte Carlo calculation agrees with the experimental results within the experimental uncertainty.