A betatron-induced $\gamma$, $n$ reaction on ${\mathrm{Zr}}^{90}$ has been found to produce a third branch in the decay of the 4.4-minute state of ${\mathrm{Zr}}^{89}$. The new branch includes a 1.53-Mev gamma ray (8 percent as intense as the 588-kev gamma ray) coincident with positrons of approximately 850-kev maximum energy. These particles (log ft=4.2±0.1) are in competition with the 2.4-Mev positron group (log ft=6.9±0.3) emitted in the decay, to the ground state of ${\mathrm{Y}}^{89}$, of the 4.4-minute ${\mathrm{Zr}}^{89m}$. Anomalous behavior of the comparative half-lives is indicated, since both the 2.4-Mev and the 901-kev, 79-hour positron transitions are thought to be allowed. $\frac{K}{\mathrm{}(L+M)\mathrm{}}$ conversion ratios for the 4.4-minute, 588-kev and the 79 hour, 913-kev gamma radiations are, respectively, 5.4±0.7 and 7±2. A ten-channel scintillation spectrometer, based on a cathode-ray tube, light pipes, and photomultiplier tubes, to effect pulse-height selection, is also described.

• Received 1 April 1953

DOI:https://doi.org/10.1103/PhysRev.91.1203