Following the suggestion that ${\mathrm{H}}^5$ might be stable against nucleon emission, a search was made for delayed neutrons resulting from the reaction ${\mathrm{Li}}^7(\gamma , 2p){\mathrm{H}}^5$. The targets were bombarded with a 340-MeV bremsstrahlung beam and the delayed neutrons detected in a shielded paraffin-moderated array of B${\mathrm{F}}_3$ counters. Counts were scaled in 14 delay channels of variable width and variable initial delay. Separated isotopes of ${\mathrm{Li}}^7$ and ${\mathrm{Li}}^6$ were used as targets. As ${\mathrm{H}}^5$ cannot be produced from the bombardment of ${\mathrm{Li}}^6$, counts observed during the bombardment of ${\mathrm{Li}}^6$ were used to measure the background. After background subtraction, the data fall uniformly about zero and show no apparent lifetime. Assuming a 10-msec half-life, an upper limit for the activation cross section is ${\sigma }_A\le 3\times {10}^{-32\mathrm{}}$ ${\mathrm{cm}}^2$. A comparison of this with the yield expected from the extrapolation of the measured yields of the reactions ${\mathrm{B}}^{11}(\gamma , 2p){\mathrm{Li}}^9$ and ${\mathrm{F}}^{19}(\gamma , 2p){\mathrm{N}}^{17}$ indicates that the upper limit for the activation cross section is of the order of 1% of the expected yield. Hence it appears unlikely that ${\mathrm{H}}^5$ is stable against particle emission.

• Received 12 July 1962

DOI:https://doi.org/10.1103/PhysRev.128.1788