C.D. Zafiratos

O+ states observed in Cd and Sn nuclei with the (3He, n) reaction

Abstract The ( 3 He, n) reaction has been studied using targets of 104, 106, 108, 110 Pd and 106, 110, 112, 114, 116 Cd at bombarding energy of 25.4 MeV. Angular distributions were obtained from 0° to 25° and the results compared with the predictions of the single-step zero-range DWBA theory. In addition to the ground-state transition, numerous L = 0 transitions were observed up to 5 MeV excitation in the residual nuclei. The pairing vibration state was found to be fragmented in all final nuclei and, except for 106, 108 Cd, a strong L = 0 transition was observed to a state at an excitation energy much below that predicted for the pairing vibration. If this state is assumed to be a fragment of the pairing vibrational state, then the total strength to excited 0 + states is in reasonable agreement with the strength observed in the corresponding Sn( 3 He, n)Te ground-state transition. However, the centroid energy of the pairing vibrational state is then much lower than expected. No transitions with L > 0 were excited with measurable strength.

Optical model analysis of quasielastic (p, n) reactions at 22.8 MeV

Quasielastic (p, n) differential cross sections have been measured for 29 nuclei ranging from 9Be to 208Pb at an energy of 22.8 MeV in approximately 7.5° steps from 10° to 152°. The results have been analysed with a distorted-wave Born approximation in terms of the generalized optical model due to Lane. Starting with a complex isospin interaction form factor, U1, deduced from the Becchetti-Greenlees global set of proton optical parameters, the shape of the surface-peaked, imaginary part of U1 was varied until good fits to the data were obtained. The shape of the real part of U1 and the ratio of the real to imaginary well depths were kept fixed at the Becchetti-Greenlees values. The resulting best-fit form factors had overall strengths 20–30 % less than the Becchetti-Greenlees value. Further, the resulting imaginary part of U1 was found to peak at a decreasing radius relative to the real part of U1 with an increasing width as A increased. A smoothed parameterization of the best-fit U1 is given for all nuclei with A > 40. The individual best-fit U1 is used to generate self-consistent neutron optical potentials from the Becchetti-Greenlees proton optical potentials as prescribed by the Lane model. Neutron elastic scattering angular distributions and reaction cross sections predicted by these self-consistent potentials are in good agreement with observed neutron scattering data.

A cyclotron time-of-flight facility☆

Abstract A description of the neutron time-of-flight facility at the University of Colorado 30 MeV isochronous cyclotron is given. The cyclotron is provided with a gated ion source to select one out of a predetermined number of rf cycles to accelerate ions. The system incorporates a beam swinger to rotate the direction of the incoming beam relative to a fixed flight path. Three detectors separated by 8° are used for a flight path of 9.5 m while an array of seven detectors are used for a single angle for a flight path of 30 m. The angular range available is 10°–160°. Beams of protons, deuterons and 3He are available with overall time resolution of 0.8–1.2 ns depending on the detailed circumstances. Examples of spectra from (p, n) and (3He, n) reactions are given.

Two-proton transfer near Z = 40☆

Abstract The ( 3 He, n) reaction has been studied on targets of 88 Sr, 90, 92, 94 Zr, 92, 94, 96, 100 Mo and 102 Ru at 25.4 MeV. Angular distributions were measured from 0° to 40° and compared with the predictions of DWBA calculations. Excited states were generally weak compared with the ground-state transition strength illustrating the superfluid nature of the nuclei investigated. No evidence for a proton shell closure at Z = 40 was found.

Ratio of the isovector effective interaction strengths ‖Jστ/Jτ‖2 between 200 and 450 MeV from the 14C(p, n) 14N reaction

Abstract The cross section at 0° for the reaction 14 C(p, n) 14 N has been measured at energies of 200, 300, 400 and 450 MeV using a nucleon charge-exchange facility recently commissioned at TRIUMF. Energy resolution of 1 MeV or better was obtained at each energy, permitting a reliable measurement of the cross section ratio for the Gamow-Teller transition to the 1 + state at 3.95 MeV and the Fermi transition to the 0 + state at 2.31 MeV. The deried values of ‖ J στ / J τ ‖ 2 are in clear disagreement with impulse approximation calculations using a free NN t -matrix.

The (3He, n) reaction on tellurium isotopes

Abstract The 122, 124, 125, 126, 128, 130Te(3He, n) reactions have been studied at an incident energy of 25.4 MeV using the University of Colorado 9 m neutron time-of-flight facility. Angular distributions of neutrons have been measured for all targets over the angular range 0°–22°. In addition to the ground state transitions relatively strong L = 0 transitions to excited levels in the region of 2.0 MeV are observed in all residual nuclei. The experimental results are compared to zero-range DWBA predictions. Pairing model calculations are able to account for the strongly excited low-lying Jπ = 0+ states observed.

The (3He, n) reaction on N = 82 targets and even isotopes of neodymium

Abstract Angular distributions of neutrons from the ( 3 He, n) reaction were measured at 25.4 MeV over the angular range 0°–22° for the N = 82 targets 136 Xe, 138 Ba, 139 La, 142 Nd and 144 Sm. In addition, measurements were carried out for the targets 144, 146, 148, 150 Nd spanning the transition from spherical to permanently deformed ground states. For each even A , N = 82 target, transitions to excited states were observed with considerable L = 0 and L = 2 strength. Weak transitions to excited states were observed for the 144 Nd and 139 La targets. Only the ground state transition was observed for the other targets. A careful search was made for the 0 + vibrational states excited in the (t, p) reaction at 0.747 MeV in 150 Sm and 0.686 MeV in 152 Sm, but these were not populated with measurable strength in the ( 3 He, n) reaction. The Q -value for the 144 Sm( 3 He, n) 146 Gd(g.s.) reaction was measured to be 0.977±0.030 MeV, yielding a ground state mass of 145.91832±0.00003 amu for 146 Gd.

The (p, n) reaction to the isobaric analogue state of high-Z elements at 25.8 MeV

Abstract Angular distributions have been measured for the (p, n) reaction to the isobaric analogue state of the targets 197Au, 206Pb, 207Pb, 208Pb, 209Bi, and 232Th at a proton energy of 25.8 MeV. Results of distorted wave Born approximation calculations indicated that angular distributions were predicted well by both real and complex form factors, although there was less sensitivity to an imaginary surface term than found in previous studies for lower-Z elements. An isospin correction to parameters for proton elastic scattering as determined from (p, n) quasi-elastic scattering was used to predict neutron elastic scattering in an application of the isospin model due to Lane. Although the isospin correction appeared necessary, it was not sufficient to provide adequate agreement with neutron elastic scattering data. A microscopic interpretation of the form factor provided a model in which angular distributions could be predicted well by a real form factor involving only two free parameters. This model was also used to determine the ratio of the rms radii of the neutron matter distribution to the proton matter distribution in 208Pb and gave the result 〈r n 2 〉 1 2 /〈r p 2 〉 1 2 = 1.07±0.02 .

Observation of Fermi and Gamow-Teller strength in the 800 MeV(p, n) reaction

Abstract Angular distributions have been measured for the (p, n) reaction on targets of Li, C, C, and N at 800 MeV with an energy resolution of 2.7 MeV allowing observation of discrete nuclear levels. As is the case at lower energies, this reaction is very selective, emphasizing Gamow-Teller (GT) and Fermi transitions. Absolute cross sections of pure GT transitions are reproduced by impulse approximation calculations. The ratio |Jστ|2/|Jτ|2 was extracted from C and N measurements at 800 MeV and from a C measurement at 31

Structure of 22Mg, 26Si, 34Ar and 38Ca via the (3He, n) reaction

Abstract The (3He, n) reaction has been studied on targets of 20Ne, 24Mg, 32S and 36Ar at 25.4 MeV using a time-of-flight spectrometer. Angular distributions were measured for neutron groups up to excitation energies ranging from 8.55 MeV in 22Mg to 7.47 MeV in 38Ca. New levels were found at 6.98 ± 0.08, 7.84 ± 0.09, 8.29 ± 0.04 and 8.55 ± 0.09 MeV in 22 Mg ; 5.62 ± 0.03 and 6.99 ± 0.05 MeV in 34 Ar ; and 7.47 ± 0.05 MeV in 38 Ca . DWBA analysis along with comparisons with results of (t, p) studies on the same targets permits a number of new spin-parity assignments. Comparison with results of (sd) shell model calculations shows generally good agreement for 22Mg and 26Si up to excitation energies of about 5 MeV. At higher excitation, transition strengths show the increasing importance of (fp) shell contributions. Results for 34Ar show significant deviations from model predictions for most excited states. For 38Ca, only the ground state transition shows appreciable strength for (sd) transfer. In all nuclides strong transitions to negative parity states are observed, and (fp) contributions to measured cross sections become important for even parity states at high excitation.