Speaker
Description
The procedure followed by the Nuclear Metrology Laboratory (LMN) at the Nuclear and Energy Research Institute (IPEN-CNEN/SP), in São Paulo, Brazil, for the primary standardization of a (243Am + 239Pu) solution is described. 243Am decays almost 100% by alpha transitions (with a very small branch, 10-9 %, of spontaneous fission), to 239Np with a Q value of 5438.8 keV, and a half-life of 7367 years. Most of the decay (86.74 %) populates the excited level of 239Np, with energy of 74.66 keV. The alpha decay is followed by a beta decay from 239Np to 239Pu, with a Q value of 722.5 keV, and a half-life of 2.356 days, populating mainly the excited levels of 228.18 keV (38.6 %) and 277.59 keV (34.8 %) of 239Pu. The measurements were carried out in a 4-pi (PC)-gamma coincidence counting system, composed by a gas-flow proportional counter filled with P-10 mixture and coupled to a 76.2 x 76.2 mm NaI(Tl) crystal. The standardization was accomplished by dropping known aliquots of the solution on Collodion films (20 micro g.cm-2), previously coated with thin gold layers (10 micro g.cm-2) on both sides. The efficiency was varied by placing Collodion films over and under the sources. The counting was performed by an electronic system composed of discriminators, gates and a TAC (Time to Amplitude Converter) module, yielding a time spectrum in a MCA (Multichannel Analyser). In this time spectrum the peaks corresponding to alpha, beta, gamma, alpha-gamma coincidences and beta-gamma coincidences were identified and provided the Nalpha, Nbeta, and Nc alpha+beta counts, to be used in the coincidence equations, for two gamma gates: one at 75 keV, corresponding to alpha decay and another one at (228+278) keV, corresponding to beta decay. The measurements were performed at two plateaus: one for alpha particles at 1.45 kV and another one for alpha+beta particles at 2.05 kV. The extrapolation curves were compared with Monte Carlo calculations, applying the code MCNP6, that considered all aspects of the counting system, including the radioactive source and absorbers details. Code Esquema, used in previous works, was improved and applied in order to calculate the alpha, beta, gamma and coincidence spectra, taking into account the decay scheme information. This provided an independent comparison with the experimental results. Cox-Isham formalism was applied to experimental data and all partial uncertainties were considered, applying the covariance analysis methodology.
