Corrections for β-Absorption in the Source Mount in 4π β-Counting

In 4π β-counting an important source of error is the absorption of radiation in the mounting film on which the source is deposited; for this reason very thin mounting films (20 -30 μg/cm2) are usually employed. A method to correct for β-absorption in the source mount is proposed here, and is shown to be valid also for thick mounts (of the order of 1 μg/cm2). The method consists in making a series of counts of the same source sandwiched between foils of increasing thickness and then fitting the data obtained with the expression I/I0 = [ e-μ-μt∫∞μt (e-x/x) * dx] which is derived on the supposition that β-rays are absorbed exponentially with an absorption coefficient μ, and where I0 represents the counting with zero thickness and I the counting with thickness t. In this expression the coefficient μ is varied until a best fit is obtained. When this is done, the data from the calculations give directly the corrections for various thicknesses. The method has been tested with aluminium foils 1.34 mg/cm2 thick and with various nuclides of different β-end-point energies; satisfactory fits have been obtained even for β-end-point energies as low as 0.156 MeV of 14C and up to aluminium thicknesses for which a reduction of a factor of 40 in the original counting rate was reached. In this work the data obtained are presented together with a plot of the μ-values derived against β-end- point energies of the various nuclides studied. (author)