Mass, Half-Life, and Excited States ofB8

The mass of ${\mathrm{B}}^8$ has been determined by measuring the threshold energy of the reaction ${\mathrm{Li}}^6({\mathrm{He}}^3, n){\mathrm{B}}^8$. Observations were made on both the neutrons from the reaction and the positrons from the decay of ${\mathrm{B}}^8$. The threshold energy is 2.9661±0.0017 Mev, giving a mass of 8.027157±0.000008 amu for ${\mathrm{B}}^8$. The half-life of ${\mathrm{B}}^8$ was measured to be 0.78±0.01 sec. Evidence was found for possible excited states in ${\mathrm{B}}^8$ at 0.6±0.1 and 0.80±0.05 Mev with respective widths of 0.2±0.1 and 0.07±0.04 Mev. Assuming that the decay proceeds exclusively to the 2.9-Mev state of ${\mathrm{Be}}^8$, the $log\mathrm{ft}$ value is 5.7.