Beta shapefactor determinations by the cutoff energy yield method

The measurement of spectral deformations due to the forbiddenness of β transitions is commonly resolved by fitting a Kurie plot to experimental data. However, the autoabsorption of the sample and the presence of electromagnetic interferences frequently modify the expected spectral shape, making the determination of the shapefactor function inaccurate in semiconductor and magnetic spectrometers. Although the problem of autoabsorption is not present in liquid-scintillation samples, the sum-coincidence process for pulses and the poor resolution of scintillation spectrometers complicate the deconvolution of the spectra. The goal of this paper is to measure shapefactor functions by making use of observables, such as the maximum point or the cutoff energy yield, which are invariant under resolution changes. As a test of the method, the shapefactor coefficients of the six β-emitters, 36Cl, 204Tl, 210Bi, 89Sr, 90Y and 32P are determined from the analysis of the liquid-scintillation pulse-height spectra. Although the results for 210Bi, 89Sr and 90Y are in good agreement with theory, the measured shapefactors for 36Cl and 204Tl exhibit similar deviations from theory than those referenced in the literature for the Kurie plots.