Experiments with a Slow Neutron Velocity Spectrometer

Abstract

Neutron bursts were produced by modulation of the arc source of a small cyclotron. Neutrons subsequently slowed down in paraffin were detected by a B${\mathrm{F}}_3$ ionization chamber and linear amplifier. Modulation of an additional amplifier allowed the counting of all detected neutrons as well as those arriving in a specified time interval. Time of flight experiments were carried out at 3.0 meters with "on" times of 50 or 100 μsec. (microseconds) out of a period of 2500 μsec. The modulation of the source and detector was accurate to about 5 μsec. The absorption of thin Cd (0.052 g/${\mathrm{cm}}^2$) shows a resolved resonance absorption with maximum at 0.14 ev. The thick Cd (0.45 g/${\mathrm{cm}}^2$) absorption was also examined and it was found that the absorption edge was not sharp, the transmission being 0.5 at 0.47 ev. Experiments with thick absorbers of Rh and In showed definite evidence of resonance absorption at 1.0 ev for both elements. Experiments were also carried out to study the delayed emission of thermal neutrons from the paraffin surrounding the source. An effective mean life of 170 μsec. was found. The effect of the resolution of the apparatus was examined and found to be small except for the resonances at 1.0 ev. The effect of the delayed emission of thermal neutrons on the time of flight measurements was also examined. From the thin Cd absorption curve T, the total width of the resonance level, was found to be 0.12 ev and from the cross section at resonance $[1\mathrm{}\pm (\frac{1}{2i+1}\left)\right]{\Gamma }_n,$ where ${\Gamma }_n$ is the neutron width and $i$ is the angular momentum of the absorbing nucleus, it was found to be 5×${10}^{-4\mathrm{}}$ ev. In this calculation 13 percent abundance of the effective isotope was assumed.