Measurement of the Neutron-Neutron Quasifree Scattering Cross Section in Neutron-Deuteron Breakup at 10.0 and 15.6 MeV

Neutron-neutron (nn) quasifree scattering (QFS) in neutron-deuteron (nd) breakup is the kinematic configuration in which the proton remains at rest in the laboratory frame. The cross section for this process is sensitive to the 1S0 component of the nn interaction. Recent measurements of the cross section for nn QFS at 26 and 25 MeV indicate that rigorous ab-initio theory calculations underpredict the data by more than 15%. This discrepancy can be resolved by increasing the strength of the 1S0 component of the nn interaction; however, this solution creates a level of charge-symmetry breaking in the effective range parameter thatis much larger than currently accepted.

The goal of this work is to investigate the discrepancy between ab-initio three-nucleon calculations and data for nn QFS cross section in nd breakup. Two measurements of the nn QFS cross section were performed at the Triangle Universities Nuclear Laboratory tandem accelerator facility. Standard time-of-flight techniques were used to determine the energies of the two breakup neutrons detected in coincidence. The integrated beam-target luminosity was determined from the nd elastic scattering yields measured simultaneously with the coincidence yields from nd breakup.

In the first measurement, the breakup reaction was induced with an uncollimated beam of 10.0-MeV neutrons and the emitted neutrons were detected with two heavily shielded detectors placed at equal angles on opposite sides of the beam. This measurement was optimized to be insensitive to changes in the strength of the nn interaction in order to validate the technique for determining the integrated

beam-target luminosity. In the second measurement, a collimated beam of 15.6-MeV neutrons was used with eight unshielded detectors. Two nn QFS configurations were measured, one with detectors placed at equal angles (40°) and one with detectors placed at asymmetric angles (26° and 56°) on opposite sides of the beam. A configuration near the nn final-state interaction was also measured. Several other nd breakup configurations were measured but are not discussed in this dissertation.

Theory calculations of the nd breakup cross section were averagedover the energy spread and finite geometry of the experiment using a Monte-Carlo simulation. The simulation was also used to compute the average detector efficiencies, the average neutron transmission factors, and corrections to the raw yields for multiple scattering.

In the first measurement conducted at 10.0 MeV, our results agree well with theory, validating our experimental technique, in particular for measuring the integrated beam-target luminosity used to normalize the nn coincidence yields to determine the absolute breakup cross section. The results of the second measurement conducted at 15.6 MeV agree with the standard theory calculations, in contrast to the previously observed discrepancies. However, the systematic uncertainties are too large to rule out the previously observed discrepancy between theory and data with high confidence level.