RESONANT SCATTERING OF SLOW NEUTRONS

Abstract

A method is described for measurement, as a function of neutron energy, of the scattering cross section for slow neutrons of materials having high absorption. The necessary corrections are discussed. The results are normalized to a vanadium scattering cross section of 5.07 barns inferred from the measured total cross section, [Formula: see text]. Measurements are presented for the low energy neutron resonances in cadmium, samarium, gadolinium, dysprosium, europium, rhodium, and indium. The single level Breit-Wigner formula is accurately obeyed for cadmium and samarium but not for gadolinium in which two resonances contribute. The scattering results indicate that the absorption cross sections of cadmium and samarium at resonance are higher than the values previously accepted. This has been confirmed by total cross section measurements on cadmium for which the Breit-Wigner parameters are E0 = 0.180 (±.003) ev., Γ = 0.113 ev., σa0 = 7750 barns, σP = 4.7 barns, aR = 7 × 10−3 cm. The spin of the compound nucleus is unity. For Sm149 the spin of the compound nucleus (0.096 ev. resonance) is [Formula: see text]. The scattering cross section of europium for neutrons of energy 0.03 ev. to 0.15 ev. is 8 barns, almost independent of the energy. The scattering cross section of dysprosium is 100 barns at 0.03 ev. and decreases slowly with increasing energy, thus confirming the existence of a level at negative neutron energy. The spin of the compound nucleus for the 1.26 ev. resonance in Rh103 is found to be unity and that for the 1.45 ev. resonance in In115 is found to be four.