G. Mertens

Toward a resolution of the neutron-neutron scattering-length issue

Abstract We report a high-precision determination of the 1 S 0 neutron–neutron scattering length ( a nn ) using the 2 H( π − , nγ ) n reaction. The value obtained in the present work is −18.50± 0.05 (statistical) ± 0.44 (systematic) ± 0.30 (theoretical) fm, which is consistent with the values from previous measurements. Combining our result with previous measurements reduces the total uncertainty in the world average of a nn to ±0.4 fm, matching the accuracy to which the charge-symmetric parameter a pp is determined.

Verification of the space-star anomaly in nd breakup

Abstract Cross-section measurements of a collinear configuration, the space-star and the coplanar-star configurations in nd breakup at E n = 13.0 MeV are reported. The present measurements for the collinear configuration are in good agreement with pd and nd data. Our coplanar-star data are consistent with theoretical predictions and resolve the reported problem with this configuration. The previously observed large discrepancy between theory and nd cross-section data for the space-star configuration is confirmed in the present work.

The low-energy neutron-deuteron analyzing power and the 3P0,1,2 interactions of nucleon-nucleon potentials

Data for the analyzing power Ay(θ) for the elastic scattering of neutrons from deuterons have been measured at 5.0, 6.5 and 8.5 MeV to an accuracy of ±0.0035. Surprisingly large differences have been observed at these low energies between the data and rigorous Faddeev calculations using the Paris and Bonn B nucleon-nucleon potentials. The Ay(θ) data provide a stringent test for our present understanding of the on-shell and off-shell 3P0,1,2 nucleon-nucleon interactions.

Implications of the space-star anomaly in nd breakup

Abstract Cross-section measurements of six exit-channel configurations in nd breakup at 13.0 MeV are reported and compared to rigorous calculations. Except for the coplanar-star configuration, our data are consistent with previous data. The present data for all configurations, with the exception of the space star, are in good agreement with theoretical predictions. The previously observed large discrepancy between theory and data for the space-star configuration is confirmed in the present work. The inclusion of the Tucson-Melbourne 2π exchange three-nucleon force with a cutoff parameter that correctly binds the triton only changes the predicted cross section by 2%, a factor of 10 smaller than the amount needed to bring theory into agreement with data.

Resolution of discrepancy between backward angle cross-section data for neutron-deuteron elastic scattering

High-accuracy differential cross-section data for neutron-deuteron elastic scattering at centre-of-mass backward angles in the range from 140° to 178° have been measured to accuracies better than ±3% for incident neutron energies of 8.0, 10.0, and 14.0 MeV. The measurements were made by detection of the recoil deuterons and protons from a mixed CD2-CH2 foil. The new data are compared to existing data and to three-nucleon calculations with the Bonn-B nucleon-nucleon potential. We conclude that the present measurements unambiguously resolve the discrepancies among previously reported data of the same type.

Discrepancy between three-nucleon calculations and neutron-deuteron elastic analyzing power data at 8.5 MeV

Abstract The analyzing power A y ( θ ) in neutron-deuteron elastic scattering was measured at E n = 8.5 MeV. Comparison of the data with realistic three-nucleon calculations reveals a pronounced difference at the maximum of A y ( θ ), which is most likely due to either an inadequate knowledge of the 3 P components of the nucleon-nucleon interaction or to three-body effects in the three-nucleon system.

Measurement of the neutron-proton spin correlation coefficient Ayy at 90° c.m. by elastic scattering of 13.7 MeV polarized neutrons from a polarized proton target

Abstract The neutron-proton spin correlation coefficient A yy was measured at 90° c.m. by elastic scattering of 13.7 MeV polarized neutrons from a polarized proton target. The target polarization of about 60% was produced in a LMN single crystal by dynamic nuclear orientation using 70 GHz microwaves. The target was thin enough to allow the detection of the recoil protons in coincidence with the associated scattered neutrons. Our result for A yy is the first experimental determination of a neutron-proton spin correlation parameter at such a low energy. To determine the influence of A yy on the 3 S 1 - 3 D 1 mixing parameter e 1 , a single-energy n-p phase shift analysis was performed using the experimentally determined value of A yy = 0.078 ± 0.014 along with analyzing power and cross section data. As expected, the result for e 1 changes drastically upon the inclusion of A yy . An unexpectedly small value of e 1 = −0.16°±0.54° has been obtained in the present analysis.

Scattering length measurements from radiative pion capture and neutron-deuteron breakup

The neutron-neutron and neutron-proton {sup 1}S{sub 0} scattering lengths a{sub nn} and a{sub np}, respectively, were determined simultaneously from the neutron-deuteron breakup reaction. Their comparison with the recommended values obtained from two body reactions gives a measure of the importance of three-nucleon force effects in the three-nucleon continuum. In order to check on the result obtained for a{sub nn} from the two-body {pi}{sup {minus}}-d capture reaction, a new measurement was performed at LANL. Preliminary results of the three experiments are given.

Spin-flip probability in the inelastic scattering of 16.9 MeV neutrons from 12C

Abstract The neutron spin-flip probability S ( θ ) for inelastic scattering of unpolarized 16.9 MeV neutrons to the 4.44 MeV state of 12 C has been determined by measuring the absolute directional correlation between the scattered neutrons and the subsequent E2 deexcitation γ- radiation emitted perpendicular to the scattering plane. Time-of-flight techniques with carbon recoil detection in a plastic scintillator were used to separate elastically and inelastically scattered neutrons. The neutron spin-flip data were found to be in close agreement with 20.0 MeV proton spin-flip results. Comparison of the measurement is made with the predictions of a microscopic antisymmetrized distorted wave calculation in which the direct reaction mechanism is supplemented by a two step resonance contribution. This theoretical analysis reveals the presence of a strong quadrupole resonance at 20.5 MeV excitation in 12 C. The results are also influenced to a lesser extent by the E1 giant dipole resonance.

Multiple scattering effects in fast neutron polarization experiments using high-pressure helium-xenon gas scintillators as analyzers

Abstract In order to study multiple scattering effects both in the gas and particularly in the solid materials of high-pressure gas scintillators, two asymmetry experiments have been performed by scattering of 15.6 MeV polarized neutrons from helium contained in stainless steel vessels of different wall thicknesses. A Monte Carlo computer code taking into account the polarization dependence of the differential scattering cross sections has been written to simulate the experiments and to calculate corrections for multiple scattering on helium, xenon and the gas containment materials. Besides the asymmetries for the various scattering processes involved, the code yields time-of-flight spectra of the scattered neutrons and pulse height spectra of the helium recoil nuclei in the gas scintillator. The agreement between experimental results and Monte Carlo calculations is satisfactory.