Total Neutron Cross Section of Lanthanum

Abstract

The total neutron cross section of lanthanum has been measured from 0.1 to 1000 eV with the Argonne fast chopper. In order to explain its variation over the entire range, it is necessary to assume contributions by bound states together with contributions by the measured resonances at positive energies. The data could be fitted by assuming a bound level whose resonance parameters are the resonance energy E 0 = -37.5 eV, the reduced neutron width r n° = 0.115 eV, and the radiation width r 7 = 0.0565 eV. The parameters for the resonances at 0. 734, 2.994, 20.5, 72.3, 618, and 1182 eV have been measured. The state of La 139 +neutron at the neutron energy of 72.3 eV has been identified to be J w =3 + on the basis of the observed 7-ray spectrum. The potential-scattering radius is measured to be 5.2± 1.0 F. Resonances at energies up to about 11 keV have been detected. On the basis of the measured positive-energy resonances and the negative-energy bound level, the thermal capture cross section has been calculated to be 9.6±0.7 b. The resonance-capture integral is 12.1 ±1.0 b. The s-wave neutron strength function So for La 139 is (0.7_ 0 . 3 +1 -0 ) XIO^4.