Absolute activity measurement of electron capture nuclides

Abstract

The absolute activities of $^{55}Fe$ and $^{75}Se$ which are electron capture nuclides was determined. The activity of $^{55}Fe$ nuclide which emits only characteristic X-rays and Auger electrons was determined by X-ray spectrometry. The X-ray spectrum of $^{55}Fe$ 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 $^{51}Cr$, $^{54}Mn$ and $^{57}Co$ 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 $\varepsilon\cdot\omega$k was used to determine the activity of $^{55}Fe$ in order to eliminate the uncertainty of fluorescence yield. The activity of $^{55}Fe$ 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 $^{75}Se$ which is isomeric transition nuclide was measured by $4 \pi$(e,x)-r coincidence counting method. The efficiency of the $4 \pi$ 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 4$\pi$ proportional counter and the result was $(5.9 \pm 0.47)\%$. The activity of $^{75}Se$ 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 $^{75}Se$ was also determined by gamma-ray spectrometry, and the activity was intercompared with the activity which was measured by $4 \pi$(e,x)-...