Precision cross section measurements of neutron-induced non-elastic gamma production reactions at 14 MeV

Abstract

We present a technique for high-precision absolute measurements of gamma-ray production cross sections (n,xg) induced by 14 MeV neutrons. The technique is based on the Associated Particle Imaging (API) method, which tags individual neutrons emitted from a deuterium-tritium source with their associated alpha-particles, enabling coincidence-based suppression of lower-energy neutrons and room background signals, while also providing neutron flux measurements with uncertainties on the order of 1%. This compact, laboratory-scale technique has the potential to address gaps and discrepancies in existing cross section libraries at a fraction of the cost of large-scale dedicated facilities, with direct applications in active neutron interrogation, detector calibration, nuclear fusion science, and Monte Carlo simulations, among others. We demonstrate the technique through proof-of-concept experiments on thin and thick samples of natural Fe (1 mm, 8 mm) and natural C (2 mm, 10 mm). For Fe, gamma-ray production cross sections were measured for the 846.78 and 1238.33 keV transitions from the first and second excited states of 56Fe. For C, the (n,n'g) cross section was measured for the 4438.91 keV transition from the first excited state of 12C. Measurements were performed at 110 and 48 degrees relative to the neutron beam to characterize gamma-ray anisotropy. For the thin samples, the measured cross sections are 723 +/- 70 mb (56Fe, 846.78 keV, 110 deg), 303 +/- 42 mb (56Fe, 1238.33 keV, 110 deg), 755 +/- 106 mb (56Fe, 846.78 keV, 48 deg), 321 +/- 149 mb (56Fe, 1238.33 keV, 48 deg), 141 +/- 16 mb (12C, 4438.91 keV, 110 deg), and 279 +/- 105 mb (12C, 4438.91 keV, 48 deg). Uncertainties are dominated by counting statistics and detector efficiency calibration, both of which can be reduced in future experiments to achieve overall uncertainties of 5% or better.