A measurement of the differential cross section for elastic scattering of 14 MeV neutrons in niobium.

Abstract

The differential elastic scattering cross section of niobium was measured for 16 angles in the range of $16\sp\circ$-$160\sp\circ$ at incident neutron energies between 14.75 MeV (at $16\sp\circ )$ and 13.48 MeV (at $160\sp\circ )$ using the time-of-flight technique. The measurements were performed in a low background environment using a ring geometry. Nanosecond pulses of D-T neutrons with average intensities of 4-5 $\times$ $10\sp7$ n/s/sr were produced by a 150 kV Cockroft-Walton linear accelerator. Scattered neutrons were detected by a heavily shielded, high resolution, large volume liquid scintillator (NE 213). The operational characteristics of the various components of the time-of-flight system were determined in a series of independent tests and preliminary time-of-flight measurements with direct source neutrons and neutrons scattered by a carbon ring sample. The relative efficiency of the detector as a function of energy was measured using a time-of-flight technique with a $\rm\sp{252}Cf$ source, while absolute values were provided by Monte Carlo calculations. The neutron pulse width and current were monitored by a secondary time-of-flight system and a $\rm BF\sb3$ long counter. Aluminum activation disks were used to determine the integrated neutron flux. The activity of the disks was measured with a standard gamma spectroscopy system based on a high purity germanium detector. The differential cross section was calculated in the laboratory and center-of-mass systems from the area of the elastic scattering peak in the time-of-flight spectra after the necessary corrections (background, multiple scattering etc.) were applied. The overall uncertainty in the final results was in the range of 7-10% with some higher values observed around the minima of the angular distribution. The measured differential cross sections were compared with previous experimental data and calculated values in three major databases (ENDF-VI/B, JEF-2, and JENDL-3). A generally good agreement was found between the present measurements, the previous experimental data and the values in the ENDF-VI/B database, while the values from the JEF-2 and JENDL-3 databases appear to overestimate the differential cross section for large scattering angles.