Cross-Section Measurement of Deuterium(n,np)n at 16 MeV in Symmetric Constant Relative Energy Configurations.

摘要

The neutron-deuteron (nd) breakup reaction serves as a fertile testing ground for theories of three nucleon dynamics and meson exchange descriptions of nuclear systems. The three-body kinematics of the nd breakup reaction allow observables to be studied in a variety of exit-channel configurations to test nucleon-nucleon potential models as well as three-nucleon force models. Over the last two decades there have been significant advances in modeling three-nucleon dynamics using empirical nucleon-nucleon potential models. These calculations have shown excellent agreement with most experimental data. However, there remain some exceptions where serious discrepancies arise. We have undertaken new cross-section measurements to provide further insight into one of these discrepancies, the space-star anomaly.;The space-star configuration is a special case of the symmetric constant relative energy (SCRE) configuration in nd breakup. The SCRE configuration occurs when the three outgoing nucleons have the same energy and are separated by 120° in the center-of-mass frame. The space-star configuration occurs when the plane containing the outgoing nucleons is perpendicular to the incident beam. The other SCRE configuration measured in this experiment is the coplanar-star, in which this plane contains the incident beam.;The space-star anomaly is a discrepancy between theoretical predictions and experimental measurements for the nd breakup differential cross sections; the data are systematically higher than theory at all energies where measurements have been taken. This anomaly has been established by eight previous measurements taken at neutron beam energies of 10.3, 13.0, 16.0, and 25.0 MeV. Three of these experiments were performed in Germany at Bochum and Erlangen, one at the Chinese Institute of Atomic Energy, and four at Triangle Universities Nuclear Lab at Duke. All previous measurements were taken with essentially the same experimental setup, the common features being: (1) the scatterer was a deuterated scintillator, (2) two neutrons were detected in coincidence, (3) the target-beam integrated luminosity was determined through nd elastic scattering by detection of the scattered neutron. To determine if there could be a common experimental error in previous measurements, our experiment utilizes a technique similar to the one developed by Huhn et al. to measure the neutron-neutron scattering length. The primary distinctions between our technique and those used in previous SST measurements are: (1) the deuterated target was a thin foil, (2) a neutron was detected in coincidence with the scattered proton, and (3) the integrated target-beam luminosity was determined by nd elastic scattering via detection of the scattered deuteron.;To compare the results of this experiment with theoretical predictions, a Monte-Carlo simulation of the experiment was developed which averaged point-geometry Faddeev calculations over the finite geometry of the experimental apparatus. Along with this averaging process several other effects were simulated including: the energy loss and attenuation of charged particles in our system, background events from low energy neutrons in the beam, the time resolution of the detectors and electronics, and kinematic constraints in breakup events. The effective experimental cross section produced by the Monte-Carlo simulation was then used to predict the number of counts expected as a function of detected neutron energy which could be directly compared with the experimental measurement. This method has the advantage that statistical and systematic uncertainties are clearly separated between experimental measurement and theoretical prediction, respectively.