Electric Properties of the Neutron from Precision Cross Section Measurements

Abstract

The total neutron scattering cross section σ_s contains non vanishing contributions from the electric polarizability α_n and from the n-e⁻ interaction because of their interference terms with coherent nuclear amplitudes. Both contributions can be separated only by an accurate understanding of nuclear amplitudes (for α, and for the n-e⁻ interaction) as well as solid state amplitudes (for the n-e⁻ interaction). Because of the smallness of effects the needed accuracy for the shapes of σ_s is given in orders of 10⁻³ to 10⁻⁴, if we ask for basic electric properties of the neutron. Transmission measurements have been made at the Oak Ridge Electron Linear Accelerator ORELA with low background from the machine and a good n-TOF resolution using the neutron time of flight (n-TOF) technique. To minimize counting errors. detector signal heights (flash-ADC. 8 bit. 100 Mhz) and on line calculated dead time corrections were recorded on tape as a function of n-TOF.

The electric polarizability was determined to be α_n = (1.20 ± 0.15 ± 0.20)10^−-3 fm³. This is the first value different from zero and has been found with radiogenic ²⁰⁸Pb- and highly enriched ²⁰⁸Pb samples for the neutron energy range from 50 eV to 40 keV. α_n is deduced from the linear term in k of the shape of σ_s,corr(k) = 11.508(5) + 0.69(9)k − 448(3)k² + 9500(400)k⁴ as a function of the wave number k in fm⁻¹ where σ_s,corr is σ_s, corrected for Schwinger. n-e⁻ scattering and resonance contributions. The calculation of resonance contributions were made with our new set of nuclear parameters of ²⁰⁸Pb and other Pb isotopes.

To investigate the n-e⁻ interaction, solid state corrections below about 1 eV are of particular interest. As the presently used ones are almost without experimental verifications, work to improve solid state corrections is an important subject of our measurement program. Measured will be liquid Pb, liquid Bi and noble gases in transmission arrangement. The aim is to separate the shape of n-e⁻ scattering coming from the well known atomic form factor. First transmission measurements have been made with solid. powder and liquid samples in the energy range of orders of magnitude around eV.

Supported by the Austrian Fonds zur Förderung der Wissenschaftlichen Forschung, nros 6849, 8489

Managed by the Office of Energy Research, Division of Nuclear Physics, U. S. Department of Energy, under contract DE-AC05- 84OR21400 with Martin Marietta Energy Systems, Inc.