Measurement of Scattering Cross Sections for Low-Energy Neutron Resonances

Abstract

The thick-target method for the measurement of the ratio of total to scattering cross section for low-energy neutrons as a function of energy has been extended to include semi-thick targets. A family of calibration curves has been developed by means of which the measured scattered count may be interpreted in terms of ($\frac{{\ensuremath{\sigma}}_{s}}{{\ensuremath{\sigma}}_{t}}$) in the range 0-1.The 5.19-ev and 16.6-ev levels of silver have been investigated by this method. The data agree very well with a Breit-Wigner shape over a wide range centered at the resonance, although slight deviations can be noted at distance $>\ensuremath{\sim}10\ensuremath{\Gamma}$ away from resonance. The scattering measurements yield three independent ratios between various groups of the Breit-Wigner parameters. These ratios may be used together with transmission data to calculate the parameters. The results obtained for silver are for the 5.19-ev level: $\ensuremath{\Gamma}=0.128$ ev, ${\ensuremath{\Gamma}}_{n}=0.0155$ ev, ${\ensuremath{\sigma}}_{t0}=22200$ barns, ${R}_{J=1}=0.66$ ${\mathrm{barn}}^{\mathrm{\textonehalf{}}}$; for the 16.6-ev level: $\ensuremath{\Gamma}=0.110$ ev, ${\ensuremath{\Gamma}}_{n}=0.039$ ev, ${\ensuremath{\sigma}}_{t0}=7300$ barns, ${R}_{J=0}=1.10$ ${\mathrm{barn}}^{\mathrm{\textonehalf{}}}$; ${\ensuremath{\sigma}}_{p}(\mathrm{normal}\mathrm{element})=5.90$ barns.