A lithium ion source has been developed which can be used in a standard 2-Mev van de Graaff generator, producing a focussed beam of 1-2 microamperes of magnetically separated ${\mathrm{Li}}^7$. Above 1.4 Mev the beam produced disintegration particles from lithium targets, and above 1.8 Mev disintegrations from beryllium were observed. The reaction ${\mathrm{Li}}^7({\mathrm{Li}}^7, p){\mathrm{B}}^{13}$ was studied by observing the protons at 90° from the lithium beam. The $Q$ value is 5.97±0.05 Mev and the $M-A$ of ${\mathrm{B}}^{13}$ is 20.39±0.05 Mev, corresponding to a physical atomic weight of 13.02190±0.00005. The cross section for the ejection of ${\mathrm{B}}^{13}$ protons into unit solid angle at 90° rises monotonically and exponentially from 1.2 to 22 microbarns between 1.4 and 2.0-Mev bombarding energy.

Three groups of protons from the reaction ${\mathrm{Li}}^6({\mathrm{Li}}^7, p){\mathrm{B}}^{12}$ were identified, and the two of lesser energy corresponded well with the known excited states of ${\mathrm{B}}^{12}$ at 0.950 and 1.67 Mev. The intensities of the proton groups at 2 Mev were ground: first excited: second excited = 1:1.1:0.8.

Two groups of protons were identified from the reaction ${\mathrm{Be}}^9({\mathrm{Li}}^7, p){\mathrm{C}}^{15}$. The more energetic of these established the $Q$ value of the reaction as 9.05±0.05 Mev, and corresponds to an $M-A$ of ${\mathrm{C}}^{15}$ of 14.35±0.05 Mev, or a physical atomic weight of 15.01541±0.00005. The proton group of lesser energy is 2.1 times as intense as the ground state group at 2-Mev bombarding energy and at 90° from the beam; it corresponds to an excited state of the ${\mathrm{C}}^{15}$ nucleus lying 0.70±0.05 Mev above the ground state.

• Received 8 August 1956

DOI:https://doi.org/10.1103/PhysRev.105.204