J.N. Mo

Measurements on prototype Ge and BGO detectors for the Eurogam array

Abstract This paper presents the results of tests, both with sources and in-beam experiments, on prototype Ge and BGO detectors of the Eurogam array. Timing and energy resolution data are reported along with results for various methods of ballistic deficit correction. The first measurements with the new “shared suppression” technique are presented.

Configuration dependent pairing from band crossing frequencies

Abstract Data are presented on the crossing frequencies of odd-parity rotational bands in 161 Er. The crossing frequencies are shown to be correlated with the sign and magnitude of the quadrupole moments of the valence neutron orbits in odd- N rare earth nuclei. This can be understood in terms of configuration-dependent “blocking” of pairing-energy contributions requiring the inclusion of quadrupole terms in the pairing-correlation energy.

Angular momentum dependence of giant dipole vibrations in hot dysprosium nuclei

Abstract The spin dependence of the giant dipole resonance built on excited states in 156 Dy has been studied with a new technique using Ge-BGO spectrometers. The observed structure of the GDR and the measured γ-ray anisotropies are consistent with a change in nuclear shape from prolate to oblate with increasing spin. No evidence is found for centroid shifts nor for superdeformed shapes at high temperature.

Electromagnetic properties in 165Lu

Abstract Relative electromagnetic transition rates in 165Lu94 have been measured to high spin ( I≤ 63 2 ) for the two signature components of a band built on a π h 11 2 shell-model state. M1 transitions are observed through two band crossings. A DSAM measurement indicates that full collectivity persists for the π h 11 2 configuration up to I ≤ 53 2 and that for higher values of I a loss of collectivity could explain the observed increased values of B(M1, I → I−1)/B(E2, → I−2). The effects of alignment, shape changes and rotationally induced deoccupation of antialigned configurations are discussed. In the measured values of the ratio of transition quadrupole moments Q1/Q2 the influence of the vi 13 2 alignment is observed for the first time. The data are discussed within the context of triaxial nuclear deformation. Comparisons are made with the nuclei 161,163Lu.

The identification of the 12+[660] proton orbitals at high spins in rare-earth nuclei

Abstract Decay sequences based on the 1 2 + [660] proton orbital have been identified in 171 Ta and 177 Re based on spin, parity, and large alignment. This decay sequence is observed higher in energy than predicted in cranking calculations based on modified oscillator potentials. Similarly known 1 2 − [541] decay sequences in these and other neighbouring isotopes are observed lower in energy than predicted. A reduction in the strength of the spin-orbit potential for protons is suggested as a solution to these problems.

Neutron orbit sizes in the isotopes of tin

Root-mean-square radii of 3s12, 2d32, 1g72 and 1h112 neutron orbits in the isotopes of tin with A = 113, 115, 117, 118, 119 and 120 have been determined from a DWBA analysis of cross section data from the sub-Coulomb (t, d) reaction. The differential cross sections for the (t, d) reaction were measured at lab angles in the range from 120° to 160° for triton energies in the range from 4.25 MeV to 5.75 MeV. Spectroscopic factors of the populated states were obtained from a sum rule analysis of published neutron stripping and pickup reactions on the isotopes of tin. The measured rms orbit radii were used in a calculation of the rms radii of the neutron excess in the even tin isotopes, and the resulting values were compared with those extracted from an analysis of Coulomb displacement energies. The behaviour of neutron orbit size and parameters of the neutron bound state potential within the isotopic sequence of final nuclei is discussed. The systematics of matter, charge and neutron radii are discussed.

Construction and performance of a multiparameter focal plane detector for use on a QMG/2 magnetic spectrometer

The design, construction and performance of a modular, two pressure focal plane detector, 550 mm long, 1000 mm deep and with an active height of 60 mm are described. The detector has been operated with ions ranging from 50 MeV 1H to 200 MeV 36S. Typical results achieved are 0.5 mm and 0.3° for the spatial and angular resolutions, and 0.7% and 3% for the total energy and energy loss measurements. A height signal is also provided.

GDR decay in hot Hg isotopes. An approach to improve the sensitivity to GDR decay following fusion reactions

Abstract The giant dipole resonance (GDR) decay from the compound nucleus 190Hg∗ at E ∗ ≈ 52 MeV is studied in coincidence with mass separated evaporation residues. It is demonstrated that the selection of decay channels with few-neutron emission, combined with a precise matching of the input excitation energy enhances the experimental sensitivity to the details of the GDR strength function by a factor of ten or more.

The radial distribution of the 1f72 neutron in 41Ca from the 40Ca(13C, 12C)41Ca reaction

Abstract The reaction 40Ca(13C, 12C)41Ca leading to the ground and low-lying 3 2 − levels has been studied at bombarding energies of 18.5 and 19.0 MeV, close to the Coulomb barrier. The cross sections have been analysed using the DWBA with previously measured calibrations to obtain the rms radii of 1f 7 2 and 2p 3 2 orbits in the 41Ca levels. The rms radius of the 1f 7 2 point neutron orbit in the ground state relative to the 40Ca core was determined to be 3.89 ± 0.12 fm. This is compared with various theoretical predictions based on Hartree-Fock theory. The 1f 7 2 orbit radius gives the point neutron excess size to a good approximation for comparison with the value of 3.45 fm deduced from Coulomb energy differences. The consistency of the results with 40Ca(t, d)41Ca experiments using a (t/d), zero-range normalization of 3.26 × 104 MeV2 · fm3 is demonstrated, and trends in neutron size parameters through the Ca isotopes are discussed.

Three-dimensional γ-ray correlations and the study of rotational damping in 167,168Yb

Abstract A new method of analyzing E γ1 ∗E γ2 ∗E γ3 correlation data by rotational plane mapping is discussed and applied to high spin triple data on 167,168Yb formed in 48Ca+124Sn reactions. Values for the rotational damping width Γrot=124±20keV and 96 ±30 keV are found for transition energy intervals 820–980 keV and 980–1140 keV corresponding to I≈35 h and ≈45 h , respectively.