S- and p-wave neutron spectroscopy: Part IV. Experimental methods in the kev region

Abstract

A new high-resolution collimation-detection system has been developed for neutron total cross-section measurements in the kev region. The instrument has a variable opening between conical polyethylene collimating surfaces and admits neutrons emitted from the Li( p, n) Be reaction at about 160° with respect to the direction of the protons. The neutrons are detected by two separate banks of boron trifluoride counters. Neutron energy spread is relatively constant over a wide range of neutron energies. The resolution is apparently limited at present by nonuniformity of the thin Li targets. The optimum resolution obtainable with a reasonable counting rate has been calculated. Measurements have been made of the yield at 160° of the small extraneous background scattered into the detectors by the target backing and other sources. The total cross section for Bi 209 has been measured from 10 to 136 kev; prominent s-wave resonances occur at 12, 15, 32, 44.5, 68, 80, 84, 94, 101, 112, 116, and 134 kev in addition to five other less prominent levels and three known resonances below 10 kev. A resolution correction was applied to the first four resonances and the peak cross sections corrected in this manner are close to the theoretical values. The average level spacing is 10 ± 3 kev per s-wave channel (spin state). An estimate has been made of the neutron widths Γ n by a new method (see previous paper). The s-wave strength function based on more than twenty resonances is (0.57 ± 0.17) × 10 −4.