Capture of Slow Neutrons

TLDR

Current theories of large slow‑neutron cross sections conflict with the frequent lack of strong scattering in good absorbers and the presence of resonance bands. The study proposes that slow‑neutron anomalies arise from transitions to virtual nuclear excitation states involving an excited particle of the original nucleus. The authors model the process as transitions to virtual excitation states, with γ‑ray emission causing radiation damping that broadens resonances, reduces scattering, and the interaction occurs mainly via the s‑wave component. The model predicts that absorption decreases with higher resonance energy, yielding cross sections up to 10⁻¹⁹ cm² at 50 V and 0.5×10⁻²⁰ cm² at thermal energies, and the high probability of low‑energy nuclear levels, along with temperature and filtered‑radiation effects, supports the existence of resonance bands.

Abstract

Current theories of the large cross sections of slow neutrons are contradicted by frequent absence of strong scattering in good absorbers as well as the existence of resonance bands. These facts can be accounted for by supposing that in addition to the usual effect there exist transitions to virtual excitation states of the nucleus in which not only the captured neutron but, in addition to this, one of the particles of the original nucleus is in an excited state. Radiation damping due to the emission of $\ensuremath{\gamma}$-rays broadens the resonance and reduces scattering in comparison with absorption by a large factor. Interaction with the nucleus is most probable through the $s$ part of the incident wave. The higher the resonance region, the smaller will be the absorption. For a resonance region at 50 volts the cross section at resonance may be as high as ${10}^{\ensuremath{-}19}$ ${\mathrm{cm}}^{2}$ and 0.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}20}$ ${\mathrm{cm}}^{2}$ at thermal energy. The estimated probability of having a nuclear level in the low energy region is sufficiently high to make the explanation reasonable. Temperature effects and absorption of filtered radiation point to the existence of bands which fit in with the present theory.