A Measurement of the Longitudinal Spin-Dependent Total Cross Section Difference, Delta Sigma[subscript L], in the n-d System

Abstract

Three recent experiments in the nd system, each with a different observable, have produced results that do not agree with theoretical predictions. The discrepancies have typically been ascribed to the poorly determined strength of the three-nucleon force. However, in the case of the recent high precision n-d scattering length measurement, the validity of some of the nucleon-nucleon force models has been questioned.

To study these disagreements, we have initiated a program of measurements of the longitudinal spin-dependent total cross section difference, Δσ[subscript L], using a polarized neutron beam and a polarized deuteron target for incident neutron energies of 1.18, 5.0, 6.88, and 9.0 MeV. Calculations have shown that Δσ[subscript L] should be sensitive to the three-nucleon force.

We performed these measurements at Triangle Universities Nuclear Laboratory in Durham, NC. A polarized neutron beam was produced as a secondary beam from polarized proton or deuteron beams. The charged particle beams were created in the TUNL Atomic Beam Polarized Ion Source and were accelerated by a tandem Van de Graaff accelerator. The charged particle and neutron polarizations were determined from elastic scattering measurements.

The deuteron target was polarized via dynamic nuclear polarization. A ³He-⁴He dilution refrigerator cooled the target to 250 mK and a superconducting magnet produced the 2.5 T magnetic field that is necessary to polarize the target. The target was irradiated with 69 GHz microwaves to achieve 20-30% polarization. The polarization was monitored by nuclear magnetic resonance and calibrated by reference to a Δσ[subscript L] measurement at 1.18 MeV.

The measurements of Δσ[subscript L] show good agreement with potential models at 6.88 and 9.0 MeV neutron energy, indicating no large disagreements with theory. However, the results are not precise enough to rule out the presence or absence of three nucleon forces.