S. A. Wender

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.

A high efficiency bunching system for polarized beams

Abstract A three-stage bunching system has been constructed for use with a Lamb-shift polarized ion source. The first stage consists of a linear voltage ramp of approximately 100 V peak-to-peak applied to the anode of the duoplasmatron. This voltage ramp prebunches the beam at 4 or 2 MHz into bursts approximately 70 ns wide. The beam is compressed further with a two-stage double-drift buncher system. With this system we have been able to transport 75% of the dc beam to the target and bunch up to 90% of this beam into pulses approximately 2.0 ns wide (fwhm). Average pulsed currents of up to 150 nA have greatly enhanced our capability to perform ( p , n ), ( d , n ), ( n , n ) and ( n , γ) experiments.

Measurements of Thermal Neutron Diffraction and Inelastic Scattering in Reactor-Grade Graphite

Abstract High-resolution Bragg-edge transmission measurements were conducted on granular as well as solid samples of graphite to understand the basis for a bulk measurement of the diffusion length 24% larger than predicted by MCNP5 for bulk reactor-grade graphite. High resolution enabled a measurement of the total diffraction cross section from 1 to 200 meV. This was subtracted from the total cross section to find the inelastic cross section in the same energy range. Small-angle scattering, which has been thought to contribute to the total cross section in the region of the lowest Bragg edge, is shown not to be present in our measurement or in those of others claiming to find it. Instead, neutron total reflection from the surface of graphite microcrystals is shown to contribute to the cross section at low energies. Reactor-grade graphite is shown to have an inelastic scattering cross section over most of the energy range larger by at least 10 than the nearly perfect crystal structure of pyrolytic graphite. The ratio of inelastic scattering to diffraction at 25 meV for our graphite is inferred to be twice as large as that of graphite manufactured 50 yr ago, and we believe that our larger diffusion coefficient is rooted in this difference. The distortions in the microcrystalline structure introduced in the manufacturing of the graphite studied here at 24°C are found to be equivalent to the uncertainty in atom positions seen in heating perfect crystal graphite to a temperature of ~1800°C.

Photon production cross section for /sup 181/ta

The /sup 181/Ta(n,chi..gamma..) reaction has been measured for neutron energies E/sub n/ = 2 to 100 MeV and for gamma-ray energies E/sub ..gamma../ = 2 to 25 MeV using an array of bismuth germane detector and the pulsed neutron source at the Los Alamos Meson Physics Facility. The integrated photon production cross section reaches a maximum at --7.5 MeV. Above 20 MeV, the cross section increases slowly with energy. The angular distributions of the photon production cross sections for the different neutron energies are isotropic. At all measured neutron energies the gamma-ray spectra have the simple evaporation form.

The analyzing power for the 2H(d, n)3Heg.s. reaction from 5.5 to 11.5 MeV

Abstract The analyzing power A y ( θ ) for the 2 H(d, n) 3 He g.s. , reaction was measured from 0° to about 150° (c.m.) at 5.5, 7.0, 8.5, 10.0 and 11.5 MeV. The experiment was conducted with a newly developed pulsing and bunching system for the polarized deuteron beam and used standard time-of-flight neutron detection. The present results disagree in some angular regions with measurements of the same observable reported recently by a Zurich group who detected the 3 He particles, but agree with the limited angular distribution measured at 10 MeV by Hilscher and Liers, who also detected 3 He particles. The present results are reported in terms of associated Legendre polynomial expansions of the product σ ( θ ) A y ( θ ).

The analyzing power Ay[(θ) for 12C(n, n0,1)12C between 8.9 and] 14.9 MeV neutron energy

Abstract The analyzing power A y ( θ ) for 12 C(n,n) 12 C elastic scattering and for inelastic scattering to the first excited state ( J π = 2 + , Q = −4.44 MeV) of 12 C was measured in the energy range from 8.9 to 14.9 MeV in 1 MeV steps. A pulsed polarized neutron beam was produced via the su2rmH( d , n ) 3 He polarization transfer reaction. Monte Carlo simulations were used to correct the data for finite geometry and multiple scattering effects. The A y data, together with published crosssection data, were analyzed in the framework of the spherical optical model and in the coupled-channels formalism. A good description of the data has been achieved.

The 1H(7Li, n)7Be reaction as an intense MeV neutron source☆

Abstract Heavy ion bombardment of hydrogen provides a number of new possibilities for mono-energetic and quasi mono-energetic neutron beam production. Close to the threshold, the strong kinematic focussing for negative Q-value reactions gives rise to intense well-collimated beams of MeV neutrons. An experimental study of 0° neutron production in the 1H(7Li, n) reaction (Q = −1.64 MeV) has been carried out from the threshold energy, 13.15 MeV, up to 22 MeV. Measured cross sections are compared to prediction based on known 7Li(p, n) cross sections. At 13.5 MeV the effective cross section reaches 4 b/sr, and the neutrons are confined within a cone of half angle 9°. Cross section predictions for 9Be, 11B and 13C bombardment of hydrogen are also presented.

A linear voltage ramp generator for a three stage bunching system

Abstract A circuit capable of modulating the energy of a beam oor use with a three-stage bunching system has been designed and tested. This circuit charges up the anode of a duoplasmatron ion source through a resistor and discharges it through high power FETs at repetition rates of 4 MHz or 2 MHz. This new design represents an improvement over previous circuits in simplicity, reliability, and performance.

The analyzing power for elastic scattering of 9.9, 11.9 and 13.9 MeV neutrons from Ca

Using the polarization transfer reaction 2H(d↘,n↘)3He at 0° as a source of polarized neutrons, the analyzing power in elastic neutron scattering from Ca has been measured. Optical‐model predictions based on global analyses differ appreciably from our present data. Preliminary optical‐model studies indicate a lower strength and a much smaller diffuseness of the spin‐orbit potential than usually assumed.

Vector analyzing power for the 2H(d,n)3He reaction

The use of the newly developed pulsed polarized deuteron beam for obtaining (d,n) analyzing power data was investigated. Results are given for the 2H(d,n)3He reaction between 5.5 and 11.5 MeV.