R. L. Walter

An optical model for the scattering of nucleons from 4He at energies below 20 MeV

Abstract A real optical model potential of Woods-Saxon form is obtained which closely reproduces the observed scattering of neutrons and protons from 4 He at bombarding energies below 20 MeV. Two parameters are allowed to depend linearly on the energy. The spin-orbit coupling term peaks inside the central potential. There are indications of small differences between the potential strengths required for neutrons and protons. The phase shifts obtained from this model are presented in tabular form. Possibilities for further studies are discussed.

New Measurement of the {sup 1}S {sub 0} Neutron-Neutron Scattering Length Using the Neutron-Proton Scattering Length as a Standard

The present paper reports high-accuracy cross-section data for the 2 H(n,nnp) reaction in the neutron-proton (np) and neutron-neutron (nn) final-state-interaction (FSI) regions at an incident mean neutron energy of 13.0 MeV. These data were analyzed with rigorous three-nucleon calculations to determine the 1 S0 np and nn scattering lengths, anp and ann. Our results are ann = -18.7±0.6 fm and anp = -23.5 ±0.8 fm. Since our value for anp obtained from neutron-deuteron (nd) breakup agrees with that from free np scattering, we conclude that our investigation of the nn FSI done simultaneously and under identical conditions gives the correct value for ann. Our value for ann is in agreement with that obtained in � − d measurements but disagrees with values obtained from earlier nd breakup studies.

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.

Proton-induced X-ray emission analysis of thick and thin targets☆

Abstract We examine the problem of preparing materials of biological origin for trace element analysis using PIXE (proton-induced X-ray emission). We distinguish between targets in which beam energy loss and X-ray absorption are negligible (thin) and those in which they are not (thick). The computer code FUDGE which calculates X-ray absorption factors for thick targets is described and the results of calculations for several organic materials are presented. We report the results of analyses of both thick and thin targets of NBS SRM 1577 (bovine liver) and compare their accuracy, precision and detection limits.

Elastic and inelastic scattering of neutrons from 54, 56Fe and 63, 65Cu: (I). Measurements from 8 to 14 MeV and a spherical optical model analysis

Abstract Differential cross sections were measured at 8, 10, 12 and 14 MeV for elastic scattering of neutrons from enriched samples of 54 Fe, 56 Fe, 63 Cu and 65 Cu. Inelastic scattering to the first excited state in 54, 56 Fe was also observed. For the 63, 65 Cu isotopes, the inelastic cross sections for scattering to the combined group of the three (five) states were determined at 8 and 10 MeV (12 and 14 MeV). The elastic scattering data are compared to predictions of earlier global optical models. New spherical optical model representations were obtained. These data were combined with data for nickel, tin and lead to generate a new global parametrization. Comparisons of derived volume integrals for the potentials, total cross sections and potential radii are made to available information.

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.

Comparisons of vector analyzing-power data and calculations for neutron-deuteron elastic scattering from 10 to 14 MeV

High-accuracy analyzing-powerAy(θ) data forn-d elastic scattering at 12 MeV have been measured using the polarized-neutron facilities at the Triangle Universities Nuclear Laboratory (TUNL). The present data have been combined with our previousn-d measurements at 10, 12, and 14.1 MeV to form the highest-accuracyAy(θ) data set forn-d elastic scattering below 20 MeV. These data are compared to recent Faddeev-based neutron-deuteron (n-d) calculations which use the Paris and Bonn equivalent separable potentials PEST and BEST, as well as Doleschalls representation of theP- andD-wave nucleon-nucleon interactions. None of these models adequately describe the data in the angular region around the maximum ofAy(θ). Possible reasons for the discrepancies are discussed. The sensitivity of the present Faddeev-based calculations to various angular momentum components of the nucleon-nucleon interaction are examined.

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.

Analyzing power and differential cross section at 9.9, 11.9 and 13.9 MeV for Ca(n, n)Ca

Abstract The analyzing power and differential cross section for elastic neutron scattering from calcium have been measured at 9.9, 11.9 and 13.9 MeV using the 2 H(d, n) 3 He source reaction and neutron time-of-flight (t.o.f.) techniques to detect the scattered neutrons. Polarized neutron beams were produced via the polarization transfer reaction 2 H( d , n ) 3 He at θ = 0°. The data have been corrected for finite geometry and multiple scattering effects. None of the global neutron-nucleus optical model parameter sets usually referred to in the literature reproduces the present cross-section and analyzing power data. Individual as well as energy-averaged fits of the data resulting from new optical model searches are presented. It is shown that the quoted uncertainties of a recent empirical determination of the real part Δ V c of the Coulomb correction term are probably underestimated. Our imaginary Coulomb correction term Δ W c agrees quite well with both a very recent empirical determination and theoretical studies. Although the quality of the fits to the data can be improved by adding l -dependent potentials to the general optical potential, no definite conclusions can be drawn from the present data as to whether or not l -dependent potentials are important in neutron-calcium scattering in the energy range investigated. The data have also been analyzed using a Fourier-Bessel series description of the real central optical potential. Comparing the X 2 values, the experimental data are better reproduced by the Fourier-Bessel method than by our Woods-Saxon optical model analyses. The Fourier-Bessel potentials obtained show strong deviations from the standard Woods-Saxon shape but are in good agreement with calculations using the nuclear structure approach.

Computer analysis of proton induced X-ray emission spectra

Abstract A computer code TRACE for fitting spectra obtained in proton induced X-ray emission studies is described. The code provides rapid convergence when properly initialized, yielding the abundance for up to 18 elements in one to two minutes on a local computer (24-bit words, 1.9 μs cycle time). The code is designed around a least-squares routine which fits Gaussian shaped peaks to the characteristic peaks in the spectra.