M.A. Hofstee

OCTUPOLE EXCITATIONS IN VIBRATIONAL NUCLEI AND THE SDF INTERACTING BOSON MODEL

Abstract Proton and deuteron inelastic scattering experiments, performed with an energy resolution of 12–15 keV, have been used to study negative-parity states of vibrational and transitional nuclei with mass between 98 and 150. The analysis has been focussed on the isovector components, on the quadrupole-octupole two-phonon states and on the fragmentation of the octupole strength. This latter displays a regular dependence on the product of proton and neutron valence particle numbers and is satisfactorily reproduced by IBM-1 + f-boson calculations. Other features of the experimental spectra, as the relative positions of the 3− states, exhibit a dependence on the ratio of valence particle numbers and indicate that a IBM-2 approach might be more appropriate.

HEXADECAPOLE STRENGTH DISTRIBUTIONS OF VIBRATIONAL NUCLEI IN THE A = 100 MASS REGION

Abstract Proton and deuteron inelastic scattering data taken in a high-energy resolution measurement have been used to study J π =4 + states of some vibrational and transitional nuclei with mass between 94 and 112. Most of the levels up to about 4 MeV excitation energy have been resolved and several new 4 + assignments are proposed by comparing measured angular distributions with coupled-channels calculations. The hexadecapole strength is found to be highly fragmented and tends to concentrate in a cluster of levels around 2.5–3 MeV in approaching the more collective nuclei in an isotopic series. This is in contrast with the behaviour of the quadrupole and octupole strengths in the same nuclei which are mostly concentrated in the first 2 + and 3 − levels, respectively. The hexadecapole distributions are compared with IBM-1 calculations in the sdg boson scheme.

STRENGTH DISTRIBUTIONS IN NEODYMIUM ISOTOPES - A TEST OF COLLECTIVE NUCLEAR-MODELS

Abstract Excited states in even Nd isotopes, up to excitation energies of 3–4 MeV, were investigated in proton- and deuteron-scattering experiments performed with high-energy resolution. More than 300 transitions were studied. For several new excited states spin and parity assignments have been suggested. Reduced transition probabilities were extracted for natural-parity states from O + up to 6 + . The experimental strength distributions have been compared with the predictions of the interacting boson model (IBM) and of the quasi-particle-phonon model (QPM). The octupole transition probabilities are well described in both models as produced by the fragmentation of the f-boson or of E3 phonons. IBM-sdf calculations seem to account also for the transitions to the low-lying 1 − states. Quadrupole and hexadecapole distributions are well described in the QPM. The leading configurations are due to 6–8 low-lying one-phonon states. The two- and three-phonon states play an important role especially in 146 Nd. The failure of IBM quadrupole and hexadecapole calculations clearly points out the need of introducing additional bosons lying at high excitation energies. QPM evaluations account also for other features of the experimental data, as the E5 and E6 strength distributions and the isovector components. The limits of the two models are discussed.

Particle-hole and particle-particle correlations in neodymium isotopes

Abstract Excited states in 140, 142, 144, 146 Nd nuclei, up to an excitation energy of about 5 MeV, were investigated by (p,t) experiments performed with a good energy resolution. These data, together with proton and deuteron scattering data from a previous experiment, are compared with Quasi-Particle Phonon Model evaluations, in which the competition between particle-hole and particle-particle residual interactions is considered. The B (Eλ) distributions are satisfactorily reproduced. The 146, 148 Nd(p,t) reaction data are well accounted for, while difficulties are found in reproducing those for 142, 144 Nd(p,t). Limitations and improvements of the model are discussed.

AGORFIRM, the AGOR facility for irradiations of materials

AGORFIRM is an activity of the KVI, a cyclotron laboratory in the Northern part of the Netherlands. Central in the institute is the superconducting cyclotron AGOR. AGORFIRM is a facility that uses a dedicated beam line of the AGOR cyclotron for irradiations with protons and carbon ions in air. The facility is on a regular basis available for radiation damage and radiobiology studies. This paper gives an overview of the facility, the various beams and services available and of recently conducted characterization measurements and modeling.

A high resolution study of stretched-spin states in 116Sn with the 115In(3He, d)116Sn and 115In(α, tγ)116Sn reactions

Abstract The excitation-energy range 3.5–7.2 MeV in the nucleus 116Sn has been investigated via the proton stripping reactions 115In(3He, d)116Sn, 115In(α, t)116Sn and 115In(α, tγ)116Sn. Detailed information is obtained on the distribution of the spectroscopic strength for the different l-transfers. By combining this information with a study of the gamma decay following the (α, t) reaction, important parts of the (nearly) stretched-spin configurations Jπ = 10−, 9− and Jπ = 7+ and several Jπ = 3+ and Jπ = 5+ states have been identified. The natural-parity configurations are strongly subject to spreading into a continuum, while unnatural-parity states exhibit much less fragmentation. A comparison is made with large basis model calculations.

Neutron decay of deep-hole states in 89Zr populated by the 90Zr(3He,α)89Zr reaction at 102 MeV

Abstract Neutron decay of excited hole states populated by the 90 Zr( 3 He,α) reaction at an incident energy of 102 MeV has been investigated. The α-particles were analysed in a magnetic spectrograph positioned at 1.4° and detected with a multi-wire drift chamber. Excitation energies in 89 Zr up to 26 MeV were studied. An array of neutron detectors was positioned around the target at angles ranging from 68° to 209°. The neutron data were analysed in terms of decay into final states, using statistical model calculations as a tool to extract non-statistical decay.

Neutron decay of the isobaric analog state in 208Bi

The isospin-forbidden neutron decay of the isobaric analog state in 208Bi has been measured, following its excitation via the 208Pb(3He, t)“*Bi reaction at 61.2 MeV. In contrast to the proton decay, which is direct, the neutron decay spectrum has a statistical shape. Its branching ratio is determined to be (37 f 3)%. The results suggest that the damping of the isospin impurity is fast. Therefore, neutron decay results mostly from the final stage of the damping process and does not contribute to the width of the IAS.

Non-spin-flip (3He, t) charge-exchange and isobaric analog states of actinide nuclei studied at θ = 0°, E(3He) = 76 MeV and 200 MeV

The (He-3, t) charge-exchange reaction has been studied at theta = 0-degree-C and bombarding energies of E(He-3) = 76.5 MeV and 200 MeV. Spectra were measured using magnetic analysis for target nuclei of C-12, C-13, O-16, F-19, Si-28, Si-29, Si-30, Zr-90, Sn-117, Sn-120, (nat)Ta, (nat)W, Au-197, Pb-208, Th-230, Th-232, U-234, U-236, U-238 and Pu-244. The measurements at 76.5 MeV concentrated on the isobaric analog states of several actinide nuclei, particularly on their widths and the branching ratios for proton decay. Cross sections, Q-values and total widths were determined for the transitions to the isobaric analog states. Coulomb displacement energies derived from the measured Q-values display the influence of deformed nuclear shapes. Escape widths GAMMA-up and spreding widths GAMMA-down of the isobaric analog states in five actinide nuclei were deduced from the measured proton-decay branching ratios. They were found to be in agreement with predictions which postulate isospin mixing via the Coulomb force with the (T0-1)-component of the isovector giant monopole resonance. The measurements at 200 MeV were concerned with transitions to isobaric analog states in both light and heavy nuclei, including several actinide nuclei, but Gamow-Teller resonances and transitions to numerous other states were also observed. The measured cross sections for several transitions to isobaric analog states from Si-30 to Pb-208 were used to extract the effective interaction V(tau) for non-spin-flip (He-3, t) charge exchange at E(He-3) almost-equal-to 200 MeV. The interaction strength V(tau) decreases by a factor 0.6 when compared to previously measured values for the energy range E(He-3) = 65 to 90 MeV. An angular distribution from theta(t) = 0-degree to 16-degrees for the transition to the isobaric analog state in Sb-120 measured at E(He-3) = 200 MeV was found to be in very good agreement with microscopic calculations.

Neutron decay of deep hole states and isobaric analog states in Sn-115 populated by the (He-3,alpha) reaction at 102 MeV

Neutron decay of excited hole states and isobaric analog states (IAS) populated by the Sn-116(He-3,alpha) reaction at an energy of 102 MeV has been investigated. The alpha -particles were analysed in a magnetic spectrograph positioned at 1.4 degrees and detected with a multiwire drift chamber. Excitation energies in Sn-115 up to 23 MeV were studied. An array of neutron detectors was positioned around the target at angles ranging from 68 degrees to 209 degrees. The neutron data were analysed in terms of decay into final states, using statistical model calculations as a tool to extract lower limits of nonstatistical decay. Nonstatistical decay was observed from states up to an excitation energy of about 18 MeV. The neutron emission of the IAS is compatible with 100% statistical decay