Beta-Ray Spectra of Arsenic, Rubidium, and Krypton

Cloud-chamber measurements are described in which the $\beta$-rays emitted by the radioactive nuclei ${\mathrm{As}}^{76}$ ($T=27\mathrm{}$ hours), ${\mathrm{Kr}}^{88,\ge 90}$ ($T=3\mathrm{}$ hours), and the latter's decay product, ${\mathrm{Rb}}^{88,\ge 90}$ ($T=18\mathrm{}$ minutes), were investigated. The krypton and rubidium activities were separated from uranium fission products. The observed upper energy limit of the disintegration electrons from ${\mathrm{As}}^{76}$ is 2.71 Mev. The complexity of the spectrum is apparent from the observed momentum distribution. A Fermi plot yields an extrapolated end point of 2.78 Mev. A K-U plot analyzes into components with extrapolated end points of 0.97 Mev and 3.32 Mev. The decay of ${\mathrm{As}}^{76}$ was followed and found to have a half-life of 26.8 hours. The $\beta$-rays from the active rubidium have an observed upper energy limit at 5.1 Mev. Although a Fermi plot analyzes into two components with extrapolated end points at 2.5 Mev and 5.0 Mev, a K-U plot indicates a single group with extrapolated end point at 5.8 Mev. The possible instability of the rubidium decay product, a strontium isotope, is discussed. A measurement of the half-life of the rubidium activity yields $T=17.5\mathrm{}$ minutes. The measurement and K-U analysis of the complex spectrum due to krypton and rubidium disintegrations yields an extrapolated upper energy limit of 2.5 Mev for the $\beta$-rays emitted by ${\mathrm{Kr}}^{88,\ge 90}$. A plot of the Sargent points indicates that all of the $\beta$-transitions involved in the above measurements are of the "forbidden" type.