Radioactive Kr Isotopes

A radioactive isotope of 1.1-hour half-life has been produced in krypton by alpha-particle bombardment of ${\mathrm{Se}}^{74}$, enriched electromagnetically from 0.9 percent to 14.1 percent. Assignment of the isotope is made to ${\mathrm{Kr}}^{77}$. Aluminum absorption measurements indicate a positron end point of 1.7 Mev. In addition to annihilation radiation, gamma-rays and $K$-capture have been observed. The ratio of $K$-capture to positron emission from the ${\mathrm{Se}}^{74}(\alpha ,n)$ reaction is computed as 2.6. The krypton 1.42-day isotope has been produced by an $\alpha ,n$ reaction on electromagnetically enriched ${\mathrm{Se}}^{76}$. The isotope is located as ${\mathrm{Kr}}^{79}$ and its half-life confirmed. A positron end point of 1.0 Mev is determined by aluminum absorption measurements. In addition to annihilation radiation, gamma-rays and $K$-capture have been observed. The ratio of $K$-capture to positron emission from the ${\mathrm{Se}}^{76}(\alpha ,n)$ reaction is computed to be 50. The cross-section ratio for formation of ${\mathrm{Kr}}^{77}$ compared to ${\mathrm{Kr}}^{79}$ by alpha-particle bombardment of selenium is computed as 1.4. The 4.6-hour ${\mathrm{Kr}}^{85}$ isotope has been produced by a $\mathrm{Se}(\alpha ,n)$ reaction.