Double Decay Processes inIn114m

A search has been made for double internal conversion in competition with the 192-keV $E4\mathrm{}$ transition of ${\mathrm{In}}^{114m}$, by measuring the coincidence intensity of equal-energy electrons emitted into separate sections of an orange beta-ray spectrometer. No evidence for double $\mathrm{KK}$, $\mathrm{KL}$, or $\mathrm{LL}$ electron ejection was found. The upper limit obtained for the intensity of double $K$-electron ejection relative to single $K$ conversion may be expressed approximately as: $\frac{\mathrm{KK}}{K}<1.3\mathrm{}\times {10}^{-4\mathrm{}}$. This limit on $\mathrm{KK}$ ejection is appreciably lower than those determined for $\mathrm{KL}$ and $\mathrm{LL}$, since the measurements of the latter were restricted to large angular separations between the two electrons, to avoid effects of the scattering in the source of the intense single conversion electrons of the isomeric transition. Interpretation of experimental limits on double electron ejection in terms of double internal conversion requires the knowledge of the contributions of a variety of additional scattering processes. Quantitative estimates of these processes are not yet available for ${\mathrm{In}}^{114m}$, but qualitative estimates indicate that their intensities may be comparable with the present experimental upper limits. If the contribution of these scattering processes is taken to be zero, and the present $\mathrm{KK}$ limit is interpreted directly in terms of the double-conversion formalism of Eichler, an approximate equivalent limit on double gamma-ray emission is obtained: $\frac{\gamma \gamma }{\gamma }<3\mathrm{}\times {10}^{-4\mathrm{}}$. This is to be compared with the direct limit on double gamma-ray emission set by Grabowski, Gustafsson, and Bäckström: $\frac{\gamma \gamma }{\gamma }<3\mathrm{}\times {10}^{-5\mathrm{}}$. Both of these upper limits are significantly less than the dimensional estimate for an allowed double $E2\mathrm{}$ cascade proceeding through nearby 3+ states in ${\mathrm{In}}^{114}$. Such states, composed principally of $d$-neutron configurations, are expected on the basis of systematics, but have not yet been identified. It is pointed out that the present situation is analogous to the well-known presence of inhibited particle $E2\mathrm{}$ transitions in nearby Sn, Cd, and Sb isotopes, which have received an explanation in terms of pairing-correlation effects. It is emphasized that a similar explanation is available here, both for the inhibition of the double gamma-ray process, and the inhibition of the competing $E4\mathrm{}$ transition. These suggestions are illustrated by calculations based on the extended shell model of Kisslinger and Sorensen, as simply applied to odd-odd ${\mathrm{In}}^{114}$. The present results are then interpretable as further evidence of the effects of pairing correlations in the tin region.