Self-absorption correction in standardisation of 204Tl

In the frame of the CCRI(II) (Consultative Committee for Ionizing Radiation at the BIPM) working group on 204Tl, dedicated to investigate the problem and explain the reasons for the discrepancy between methods in standardisation of 204Tl, IRMM was assigned the task of studying the influence of self-absorption in solid sources of 204Tl. This was investigated by measuring the activity of quantitative solid sources of different carrier content, prepared by employing a special source drying technique, developed at IRMM. The activity of the solid sources was determined using a 4π pressurised proportional counter. The self-absorption was calculated using the activity determined from liquid scintillation spectrometry, unaffected by self-absorption, as the reference value. After correction for foil absorption and non-detected X-rays, the self-absorption showed a linear relation to the logarithm of the dry mass of the source. For a typical source, the self-absorption correction for 204Tl was found to be as high as 8%. This article describes a way to minimise and correct for self-absorption in solid sources of 204Tl or nuclides with similar decay properties.