David A. Lind

Studies of gamma rays from neutron inelastic scattering

Abstract Measurements of the gamma rays excited by neutron inelastic scattering in B11, Na, Si, K, Sc, Zr, In, I, Ta, Au, Tl, and Pb206 have been carried out by use of the conventional ring scatterer geometry within a neutron energy range from 0.2 Mev to 3.5 Mev. The studies were used to obtain information on level structure, to determine cross sections for gamma-ray excitation, and to make a test of the predictions of inelastic scattering cross sections computed by a combination of the statistical and optical models. New levels and information on the decay schemes have been found in Zr, I, Au, and Pb206. Excitation functions in terms of absolute cross sections are presented for the prominent gamma rays in the spectra of all the elements studied except Sc and Tl. Studies of the gamma rays from Pb206 have given experimental verification of the level scheme for the low spin states calculated by True and Ford. Statistical model calculations are presented which give reasonably good agreement with the experiments on Pb206 provided the penetrabilities are calculated using parameters obtained from an analysis of elastic scattering data specifically for Pb206. The analysis shows that the imaginary part of the optical potential is abnormally low for Pb206 compared to that for neighboring nuclei at comparable energies. The agreement in magnitude and shape between the experimental and calculated excitation functions can be improved if the imaginary part of the potential is permitted to increase with neutron channel energy.

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.

Rotational states of 158Dy, 164Er, 168Yb and 174Hf excited by evaporation reactions

Abstract A series of rotational bands in 158 Dy, 164 Er, 168 Yb and 174 Hf were studied by excitation with neutron evaporation reactions initiated by protons, deuterons, 3 He and α-particles with energies between 16 and 28 MeV. The resulting prompt γ-ray spectra were analyzed with a curved crystal diffraction spectrometer and with lithium-drifted germanium detectors. Transitions from levels in the ground state bands were seen up to 10 + in 168 Yb, 12 + in 158 Dy and 174 Hf and 14 + in 164 Er. The γ-vibrational bands in three of the nuclei were strongly excited and several β-bands were weakly excited. The energy levels deduced from the measurements were successfully fit by the empirical formulas of Holmberg and Lipas and of Sood for the ground state band as well as for the higher bands. The cross sections for transitions in the ground state band agree with the statistical model calculations made. Within the validity of the calculations, no large angular momentum deficit was observed.

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.

A Facility for Studying Neutron Energy Spectra at Intermediate Energies

Apparatus for measuring neutron energy spectra in the 50-200 MeV range is described. The apparatus, installed at the Indiana University Cyclotron Facility, consists of a beam swinger to change the angle of incidence of the beam on target, 100 m flight paths, large, subnanosecond neutron detectors, and a system for deriving phase stabilized timing signals.

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.

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 .