Total Neutron Cross Section of Pa²³¹

The total cross section for Pa²³¹ was measured from 0.01 ev to 2 kev, with the use of the Materials Testing Reactor fast chopper with resolutions from 0.040 to 2.0 µsec/meter. The Breit-Wigner resonance parameters have been obtained for the resonances below 11.0 ev. These measurements were made with samples prepared from 0.558 gm of Pa₂O₅. Weighting the level spacings inversely as 2J + 1 gives the average observed level spacings per spin state of 0.72 and 1.2 ev. This is one of the smallest spacings observed in any isotope. The average parameters give a value of 0.63 × 10⁻⁴ for the s-wave neutron strength function . A linear least squares fit to the data between 0.015 and 0.03 ev gives a value of 211 ± 2 barns for the thermal cross section. The resonance absorption integral (for neutrons with energies > 0.1 ev) is 1560 ± 55 barns, with a contribution of approximately 65% from the 0.396 ev resonance.