Jeffrey Kruk

The total neutron cross-section of (‘209)Bi has been measured with high resolution in the vicinity of the prominent resonance at 800 eV. An improved limit on the possible value of the electric polarizability coefficient of the neutron ((alpha)(,n)) has been obtained by using a newly-developed method of analysis of the cross-section data. The total cross-section was derived from a series of measurements of the transmission of neutrons through thin (‘209)Bi targets. The measurements were made using the neutron time-of-flight spectrometer associated with the Yale University Electron Linear Accelerator. The detailed study of neutron cross-sections in the resolved resonance region permitted more precise calculations of the nuclear scattering amplitude than had been possible with optical model analyses at higher energies. The nuclear scattering amplitude was calculated by means of R-matrix theory, and the electromagnetic scattering amplitudes were calculated using the distorted-wave Born approximation.

The shape of the resolution function of the spectrometer was carefully determined over a wide range of energies. A shape fit to the transmission data, after correction for the effects of both Doppler and resolution broadening, established the spin of the resonance in an unambiguous way to be J = 5, and permitted the following improved limit to be set on the value of (alpha)(,n) at the 68% confidence level: -4(.)10(‘-3) fm(‘3) < (alpha)(,n) < 4(.)10(‘-3) fm(‘3). A critique is given of the methods used previously to set limits on this elusive quantity, and the prospects for a future measurement of (alpha)(,n), based on the present method, are discussed.