Precise measurement of K-shell fluorescence yield in iridium: An improved test of internal-conversion theory

We have measured the total intensity of K x rays relative to 129.4-keV γ rays from decay of the second excited state in ${}^{191}\mathrm{Ir}$. This ($M1+E2$) transition was observed following the β decay of 15.4-d ${}^{191}\mathrm{Os}$. Our measured ratio yields the result ${\alpha }_K{\omega }_K=2.044(11)$. When combined with a recent measurement of the same ratio for the 80.2-keV $M4$ transition from ${}^{193}\mathrm{Ir}{}^m$, this result strongly confirms the need for the K-shell hole to be included in calculations of internal-conversion coefficients ${\alpha }_K$. Since the ${\alpha }_K$ value calculated for the ${}^{191}\mathrm{Ir}$ transition is virtually independent of the hole treatment, our result also yields a model-independent value for the iridium fluorescence yield, ${\omega }_K=0.954(9)$.

Figure 1

Decay scheme for the β decay of ${}^{191}\mathrm{Os}$, including the γ transitions that occur subsequently in its daughter, ${}^{191}\mathrm{Ir}$. The information is taken from Ref. 11. Note that the strong transitions are shown with heavy arrows. The weak 47-keV transition accounts for less than 0.4% of decay from the 129-keV state (including the effects of internal conversion).

Figure 2

A portion of the C1 spectrum from the first series of measurements [part (a)] and of the O2 spectrum from the third series (b) for the energy interval 500–1100 keV. Peaks are identified by their β-decay parent, with only short-lived parents being marked in the top panel, and longer-lived ones in the lower panel. Observed γ-x-ray coincidence sum peaks (s.p.) from ${}^{185}\mathrm{Os}$ decay are also marked: s.p.1 designates the sum with rhenium L x rays, and s.p.2–5 that with rhenium K x rays. The horizontal brackets located near the energy axis indicate regions where peaks in the subtracted background occur, and anomalous fluctuations can thus be expected.

Figure 3

Portion of the spectrum recorded from the C1 source during the second series of measurements; it shows the K x rays and 129.4-keV γ ray from the decay of the 129.4-keV excited state in ${}^{191}\mathrm{Ir}$. Also marked are the K x rays of rhenium following ${}^{185}\mathrm{Os}$ decay.

Figure 4

Decay curves of the impurity-subtracted K x rays (solid circles) and 129.4-keV γ rays (open circles) in ${}^{191}\mathrm{Ir}$. The lines are fitted decay curves with a fixed half-life of 15.5 d for the ${}^{191}\mathrm{Os}$ parent activity. The data are from the osmium-chloride source spectra (numbers 1, 3, and 6 in Table ).