The energy threshold for the production of neutrons by bombardment of targets of Li, Be, B, and C with high energy protons has been measured with a boron trifluoride ionization chamber, surrounded with paraffin, as a neutron detector. The energy of the protons was calibrated against the 0.862-Mev $\mathrm{F}(p,\gamma )$ resonance. With $\mathrm{H}_2^{}{}_{}{}^{+}$ and $\mathrm{H}_3^{}{}_{}{}^{+}$, this calibration was extended to 1.724 Mev and 2.586 Mev. The thresholds are: ${\mathrm{Li}}^7$, 1.86 Mev; ${\mathrm{Be}}^9$, 2.03 Mev; ${\mathrm{B}}^{11}$, 2.97 Mev and ${\mathrm{C}}^{13}$, 3.20 Mev. From these values can be calculated the mass differences ${\mathrm{Be}}^7$-${\mathrm{Li}}^7$=0.87 Mev, ${\mathrm{B}}^9$-${\mathrm{Be}}^9$=1.08 Mev, ${\mathrm{C}}^{11}$-${\mathrm{B}}^{11}$=1.97 Mev and ${\mathrm{N}}^{13}$-${\mathrm{C}}^{13}$=2.22 Mev. ${\mathrm{B}}^9$ is shown to be certainly unstable with respect to disintegration into ${\mathrm{Be}}^8$+${\mathrm{H}}^1$. The ${\mathrm{N}}^{13}$-${\mathrm{C}}^{13}$ difference agrees closely with the maximum positron energy from ${\mathrm{N}}^{13}$, and thereby confirms the voltage scale.

• Received 30 August 1940

DOI:https://doi.org/10.1103/PhysRev.58.1035