Calculation of beta spectra for allowed and unique forbidden transitions

Abstract : In ionizing radiation metrology, the energy spectra of beta decays are often needed, especially when measurements are carried out using Liquid Scintillation Counting. The uncertainties on activity measurements can be reduced with a precise knowledge of the shapes of the beta spectra. The few studies which have been conducted on this subject since the 1970s were mainly focused on allowed and first non-unique forbidden transitions. But it is also necessary to have a good knowledge of spectra for unique and non-unique forbidden transitions of higher orders. Our goal is to develop a computer program for calculating beta spectra, validated by experiments, including evaluated uncertainties on the shapes. The exhaustive evaluation of form factors measured so far shows that there are not enough good data to validate those complex calculations. All these considerations highlight the need for a consistent experimental program in which all the distortion phenomena of beta spectra should be analysed and quantified very carefully. This paper describes the present features of the computer program we have developed, BetaShape, and specifies the future work which should be undertaken to improve it.