Interaction Experiments with Resonance Neutrons

Abstract

A slow neutron collimator of good geometrical proportions with multiple shielding has been constructed especially for use with the cyclotron, and transmission measurements of high precision have been made with neutrons in well-defined regions of the energy spectrum. With the resonance filter method for defining the energy of the neutron beam, cross sections of selected elements were determined for the resonance neutrons of indium (∼0.9 ev energy) and iodine (25-100 ev energy) as well as the cadmium absorption group (thermal energy). Measurements were made by following the induced resonance activities of very thin detectors with an ionization chamber—electrometer system. The experimental work consisted of (1) cross section measurements of selected elements for indium resonance neutrons; (2) investigation of interference effects; (3) measurement of the neutron-proton and neutron-deuteron interactions for indium resonance neutrons and comparison measurements with thermal neutrons; (4) measurement of the neutron-proton interaction for iodine resonance neutrons. The neutron-proton cross section was investigated in some detail to establish definitely the free neutron-proton interaction. The effects of molecular binding should be practically negligible at about 1-ev energy and this is indicated by the fact that the same value of the neutron-proton interaction was obtained for the indium and iodine resonance neutrons; viz., 21×${10}^{-24\mathrm{}}$ ${\mathrm{cm}}^2$. From these measurements it would appear that the free neutron-proton cross section commonly accepted for theoretical calculations; viz., 14×${10}^{-24\mathrm{}}$ ${\mathrm{cm}}^2$ should be revised upward. The comparison measurements with cadmium absorption neutrons agreed well with values obtained by several other investigators using different methods for neutron detection, and the general results show that the method of working inside a collimating enclosure in the immediate vicinity of the cyclotron operates satisfactorily.