Decay of 6.3-MinBr78

Abstract

The nuclide 6.3-min ${\mathrm{Br}}^{78}$ has been made by the reactions ($\gamma , n$), ($n, 2n$), ($p, n$), ($d, 2n$),and ($\alpha , n$), and its decay properties have been investigated by positron decay-curve analysis, by application of a standard chemical isomer-separation procedure, by searching for conversion electrons, and by studying the gamma-ray and positron-gamma-coincidence spectra. The early portion of the positron decay curve exhibited a single 6.25±0.2-min component, and no active daughter of this species was chemically separable from active C${\mathrm{Br}}_4$. Conversion electrons were not found, and soft gamma rays were shown to be absent. There are 12-kev x rays, of intensity about 0.05 relative to the positrons and therefore presumed to be $K$ x rays of Se resulting from electron capture. These results show that 6.3-min ${\mathrm{Br}}^{78}$ has no daughter isomer and probably no >10-sec parent isomer. There is a 615-kev gamma ray, in coincidence with positrons, and of intensity 0.139 that of all positrons, representing decay of ${\mathrm{Br}}^{78}$ to ${\mathrm{Se}}^{78}$ in the 615-kev 2+ state. However, no evidence for decay to ${\mathrm{Se}}^{78}$ in the 1.32-Mev state could be found. From these data it was deduced that 6.3-min ${\mathrm{Br}}^{78}$ decays 81% by positron emission and 6% by electron capture to ground-state ${\mathrm{Se}}^{78}$, 11% by positron emission and 2% by electron capture to ${\mathrm{Se}}^{78}$ in the 615-kev state, and ${\mathrm{Se}}^{78}$ in the 1.32-Mev state. The disintegrations of ${\mathrm{Br}}^{78}$ to ${\mathrm{Se}}^{78}$ in the ground state and 615-kev state have $log\mathrm{ft}$ values of 4.8 and 5.2, respectively, indicating that the spin of ground-state ${\mathrm{Br}}^{78}$ is 1, with even parity. The absence of isomerism is discussed in terms of the locations of expected energy levels, with reference to the known locations of these levels in ${\mathrm{Br}}^{80}$.