J.L. Ullmann

Excitation of analogue isovector resonances via the (n, p) reaction at 60 MeV?

The (n,p) charge-exchange reaction is used as a selective tool for studying T+1 isovector excitations. Here, 60 MeV neutrons have been used to measure (n,p) spectra from a range of nuclei. For 6Li and 7Li large components of l=1 dipole strength, not seen in photoneutron data, have been found at higher excitation. In the cases of 6Li, 12C and 16O comparison with photonuclear and (d,2He) data indicates that most of the l=1 strength appears to be s=0. Gamow-Teller (GT)-type transitions have been measured in 6Li, 12C and 28Si and a measure of nu sigma tau , the isovector spin-dependent component of the effective nucleon-nucleon interaction, has been obtained at 60 MeV. In the Ni isotopes, increasing blocking of the GDR with N-Z is seen. A structure of substantial strength in 209Bi(n,p)209Pb, originally thought to be isovector quadrupole, may more probably be evidence of a 2h(cross) omega sigma tau -type excitation.

Excitation of analog isovector giant resonances via the (n, p) reaction on 14N and 16O at 60MeV

Abstract Data were accumulated for the (n, p) reaction on 14N and 16O using 60MeV neutrons. Angular distributions of the cross sections for eight proton groups were extracted and compared to distorted-waves Born approximation (DWBA) calculations using a simple macroscopic model. Enhancements in the spectra were seen in each case in the region of excitation corresponding to the giant dipole resonance (GDR) of the target nucleus. DWBA calculations show that these excitations are primarily dipole and favor the use of the Goldhaber-Teller form factor. Comparison with photo-absorption measurements suggests that there may also be significant contributions to the (n, p) reaction from “spin-flip” dipole excitations. The other proton groups could be correlated with the results of other experiments and include excitations of analogs of the giant isovector “magnetic” dipole mode of 14N.

Analog giant dipole Ni(n, p) at 59.6 MeV

A broad enhancement above the continuum observed in 58, 60, 62, 64Ni(n, p) at 59.6 MeV is interpreted as the parent analog of the T> component of the giant dipole resonance. A continuum parameterization is used to extract the resonance cross section. The angular distributions for the state are most consistent with the Steinwedel-Jensen model for the giant dipole. The sum-rule strength is somewhat greater than observed with photonuclear measurements. A measurement of the blocking of the cross section with neutron excess is made, and is consistent with theoretical predictions.

Electron capture by protons in a neutron beam facility

Abstract Relative measurements of electron capture probabilities for protons of 42.5 and 61.8 MeV are presented. Attention is drawn to some uses of the resulting beam in a 0° neutron beam facility.

3H+d⇌n+4He measurements and absolute neutron-polarization determination at 50 MeV

Abstract The reaction 3 H(d, n ) 4 He is used to produce 50 MeV polarized neutrons, which at Θ n = 29.7° lab are analyzed in the inverse reaction 4 He( n , d) 3 H. At Θ d )(deut) = 25° lab the two reactions are time reversed (or more specifically, reciprocal), corresponding to the same center-of-mass angles and energies, so that the neutron polarization, P , and the (inverse reaction) neutron- 4 He analyzing power, A , are equal. Thus the measured asymmetry e = P 2 and the absolute neutron polarization at 50 MeV is determined as P = 0.480±0.016. A ( Θ d ) and σ d ( Θ d ) are also measured.