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

In a preliminary study photo-neutrons were observed to be produced by the γ-rays from Na²⁴, Mn⁵⁶, Ga⁷², As⁷⁶, and La¹⁴⁰ when these artificially radioactive isotopes were placed in D₂O or Be; F²⁰, Al²⁸, Cl³⁸, In¹¹⁶, and Sb¹²⁴ produced neutrons only in Be. The first group of isotopes must have γ-rays with energy higher than 2.18 Mev, and the second group must have γ-rays greater than 1.63 Mev but less than 2.18 Mev. No photo-neutrons were observed from Co⁶⁰, Cu⁶⁴, Ag (225 day {}-life), Pr¹⁴², Eu¹⁵¹, Dy¹⁶⁵, Ta¹⁸², Re¹⁸⁸, Ir (19 hours {}-life), and Au¹⁹⁸ in Be; the intense γ-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+D₂O, La+D₂O, Mn+D₂O, Na+D₂O, 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 γ-rays calculated to produce neutrons of these energies are: for Na²⁴, 2.74 Mev; for Mn⁵⁶, 1.81 and 2.7 Mev (a third γ-ray is present); for Ga⁷², 2.50 Mev (at least one other γ-ray is present with an energy between 2.2 and 1.63 Mev); for Sb¹²⁴, 1.67 Mev; and for La¹⁴⁰, 2.49 Mev. The total neutron cross section of carbon is also given for seven of these photo-neutron sources.