Photo-Neutron Sources and the Energy of the Photo-Neutrons

In a preliminary study photo-neutrons were observed to be produced by the $\gamma$-rays from ${\mathrm{Na}}^{24}$, ${\mathrm{Mn}}^{56}$, ${\mathrm{Ga}}^{72}$, ${\mathrm{As}}^{76}$, and ${\mathrm{La}}^{140}$ when these artificially radioactive isotopes were placed in ${\mathrm{D}}_2$O or Be; ${\mathrm{F}}^{20}$, ${\mathrm{Al}}^{28}$, ${\mathrm{Cl}}^{38}$, ${\mathrm{In}}^{116}$, and ${\mathrm{Sb}}^{124}$ produced neutrons only in Be. The first group of isotopes must have $\gamma$-rays with energy higher than 2.18 Mev, and the second group must have $\gamma$-rays greater than 1.63 Mev but less than 2.18 Mev. No photo-neutrons were observed from ${\mathrm{Co}}^{60}$, ${\mathrm{Cu}}^{64}$, Ag (225 day ½-life), ${\mathrm{Pr}}^{142}$, ${\mathrm{Eu}}^{151}$, ${\mathrm{Dy}}^{165}$, ${\mathrm{Ta}}^{182}$, ${\mathrm{Re}}^{188}$, Ir (19 hours ½-life), and ${\mathrm{Au}}^{198}$ in Be; the intense $\gamma$-rays from these isotopes must have energies less than 1.63 Mev. In a second experiment the energy of the photo-neutrons from nine of these sources was determined by measuring the hydrogen scattering cross section of the emitted neutrons. Seven, of the sources seem to emit homogeneous groups of neutrons; they are: Sb+Be, Ga+${\mathrm{D}}_2$O, La+${\mathrm{D}}_2$O, Mn+${\mathrm{D}}_2$O, Na+${\mathrm{D}}_2$O, La+Be, and Na+Be with neutron energies 0.024, 0.13, 0.13, 0.22, 0.22, 0.62, and 0.83 Mev, respectively, for the particular sized sources studied. Mn+Be emits three groups of neutrons of different energies, but more than 80 percent of the neutrons seem to be in a group at 0.14 Mev. The energy of the $\gamma$-rays calculated to produce neutrons of these energies are: for ${\mathrm{Na}}^{24}$, 2.74 Mev; for ${\mathrm{Mn}}^{56}$, 1.81 and 2.7 Mev (a third $\gamma$-ray is present); for ${\mathrm{Ga}}^{72}$, 2.50 Mev (at least one other $\gamma$-ray is present with an energy between 2.2 and 1.63 Mev); for ${\mathrm{Sb}}^{124}$, 1.67 Mev; and for ${\mathrm{La}}^{140}$, 2.49 Mev. The total neutron cross section of carbon is also given for seven of these photo-neutron sources.