M,N, andOSubshell Conversion Coefficients inTh228andPu240

The $M$, $N$, and $O$ subshell conversion coefficients of the 57.9-keV, $2^{+}$ → $0^{+}$ transition in ${}_{}{}^{228}{}_{}{}^{}\mathrm{Th}$ and the $M$ and $N$ subshell conversion coefficients of the 42.88-keV, $2^{+}$ → $0^{+}$ transition in ${}_{}{}^{240}{}_{}{}^{}\mathrm{Pu}$ have been measured with an iron-free double-focusing spectrometer. In the case of ${}_{}{}^{228}{}_{}{}^{}\mathrm{Th}$, conversion coefficients obtained for the $M_1$, $M_2$, $M_3$, $M_4+M_5$, $N_2$, $N_3$, $O_2$, and $O_3$ shells are 0.89±0.22, 19.1±1.5, 17.8±1.3, 0.55±0.34, 5.16±0.46, 4.97±0.45, 1.33±0.22, and 1.23±0.20, respectively. For the transition in ${}_{}{}^{240}{}_{}{}^{}\mathrm{Pu}$, the conversion coefficients of the $M_1$, $M_2$, $M_3$, $N_1$, $N_2$, $N_3$ and $N_4+N_5$ are 5.6±2.1, 107.2±7.5, 97.8±7.0, 2.9±0.8, 28.2±2.9, 28.7±2.9, and 2.3±1.0, respectively. Results from the $M$ subshells are (15±8)% to (85±45)% higher than Rose's theoretical values after correcting for screening according to the semiempirical method of Chu and Perlman. The $M$-shell conversion-coefficient ratios for these $E2\mathrm{}$ transitions agree with Rose's unscreened values within our experimental errors. This agreement suggests that the corrections due to screening for the different members in a particular shell are approximately the same.