The reaction ${\mathrm{C}}^{12}(n, n^{\prime }3\alpha )$ has been studied in C-2 emulsions exposed to fifteen discrete neutron energies in the range 12.3-20.1 Mev. Measurements were made of the range and space angles of the three alpha particles for over 2000 events. The observed cross section is 78±15 mb at 12.6 Mev, goes through a broad maximum of 265±47 mb at 16.9 Mev, and is 223±40 mb at 20.1 Mev. It was found that not all the events are observed in the emulsion since (a) one prong may be too short and (b) two prongs which arise from the ground state of ${\mathrm{Be}}^8$ may not be resolvable. A correction is made for these missed stars at four points giving: ${\sigma }_{\mathrm{corr}}=190\mathrm{}\pm 50, 230\pm 50, 316\pm 73, \mathrm{and} 283\pm 59$ mb at the bombarding energies of 12.9, 14.1, 15.5, and 18.8 Mev, respectively. Evidence is found for the excitation of the 9.6-Mev level in ${\mathrm{C}}^{12}$ and the ground state and 3-Mev level in ${\mathrm{Be}}^8$, so that at least some of the events disintegrate via the mode ${\mathrm{C}}^{12}(n, n^{\prime }){\mathrm{C}}^{{12}^{*}}\left(\alpha \right){\mathrm{Be}}^{8^{*}}\left(2\mathrm{}\alpha \right)$. Six events appear to involve the 7.7-Mev level in ${\mathrm{C}}^{12}$. The center-of-mass energy spectrum of the scattered neutrons may be fitted by a four-particle phase space distribution or a Maxwellian distribution. As a result of these measurements, carbon stars in nuclear track plates may be used as a neutron monitor with an accuracy of 15% at 14 Mev and 20% elsewhere in the 12-20 Mev range.

• Received 26 January 1955

DOI:https://doi.org/10.1103/PhysRev.99.1375