Absolute activity measurement of electron capture nuclides

The absolute activities of 55Fe and 75Se which are electron capture nuclides was determined. The activity of 55Fe nuclide which emits only characteristic X-rays and Auger electrons was determined by X-ray spectrometry. The X-ray spectrum of 55Fe was measured by Si(Li) detector and the peak area was calculated by summation method. The efficiency of Si(Li) detector was determined by using 51Cr, 54Mn and 57Co standard sources, the measured efficiency was corrected for the attenuation effects of the materials between a source and the detector, and for the probability of X-ray escape from detector crystal. The corrected efficiency values have a value of nearly constant which is independent of X-ray energies. Also the efficiency varepsilon' which means varepsilon cdotωk was used to determine the activity of 55Fe in order to eliminate the uncertainty of fluorescence yield. The activity of 55Fe was 116.72 MBq/g as of January 1, 1990 0 h (UT) and the combined uncertainty was found to be 0.84 MBq(0.72%). However, the activity of 75Se which is isomeric transition nuclide was measured by 4 π(e,x)-r coincidence counting method. The efficiency of the 4 π proportional counter was changed by the variation of low energy threshold level, and the range of the efficiency variation was from 25% to 68%. The result obtained by efficiency extrapolation method was corrected for the delayed transition effect of meta-stable state of 304 keV level. The delayed transition effect was evaluated from the theoretical calculation of the counting probability of the 4π proportional counter and the result was (5.9 pm 0.47)%. The activity of 75Se was 1628.04 kBq/g as of January 1, 1991 0 h(UT) and the combined uncertainty was found to be 7.81 kBq(0.48%). The activity of 75Se was also determined by gamma-ray spectrometry, and the activity was intercompared with the activity which was measured by 4 π(e,x)-r coincidence counting method. The activities obtained from the two methods show a difference of 0.41%, so they are in good agreement with each other in the uncertainty range