A Comparison of the Gamma-Ray Spectra from 2.8-keV Neutron Capture and Thermal-Neutron Capture in Sodium-23

The gamma-ray spectrum resulting from neutron capture in the 2.8-keV resonance of ²³Na has been measured with the high-resolution annihilation pair spectrometer at the internal-target facility of the CP-5 reactor. The 2.8-keV resonance was populated by using the boron-shielded target technique: A½-in.-thick filter of ¹⁰B surrounding the sodium sample selectively removes low-energy neutrons from the spectrum; the 1/E dependence of the incident neutron flux assures a low intensity of high-energy neutrons. Capture, predominantly in the 2.8-keV resonance, is indicated by a 2- to 3-keV shift in the energies of the primary transitions relative to those observed in thermal-neutron capture. The correlation between the absolute intensities of the resonance transitions and the thermal transitions (measured by others) is computed and discussed in terms of a numerical analysis. (The resonance and thermal intensities are identical within the precision of the measurement.) The results indicate that the resonance total radiation width is 0.24 eV Γγ 0.40 eV.