Rvf Janssens

Structure of 52,54Ti and shell closures in neutron-rich nuclei above 48Ca

Abstract The level structure of 5422Ti32 has been explored for the first time by combining β-decay measurements from fragmentation products with prompt γ-ray spectroscopy following deep inelastic reactions. The latter technique was also instrumental in tracing 52Ti30 to higher spin. The data provide new tests of effective interactions for full pf-shell calculations in neutron-rich nuclei above 48Ca. The data indicate the presence of a significant subshell gap at N=32 and comparisons between theory and experiment suggest an additional shell closure at N=34 in Ca and Ti isotopes.

Production of radioactive ion beams using the in-flight technique

Reactions with a heavy projectile incident on a light target can be used for the efficient in-flight production of secondary radioactive beams. An overview of this technique is given using data on 17F beams produced via the p(17O, 17F)n and d(16O, 17F)n reactions. With primary 16,17O beam currents of 100 pnA, intensities of up to 2×106 17F/s on target were achieved. Using this beam, the p(17F,α)14O reaction was measured.

Excited superdeformed bands in 191Hg

Abstract Two weakly populated rotational bands have been observed in 191 Hg with properties (energy spacings, moments of inertia and lifetimes) very similar to those of the previously reported superdeformed band. Based on cranked Woods-Saxon calculations, these structures are interpreted as the first excited bands in the superdeformed minimum of 191 Hg. Comparisons between the data and the calculations highlight the role of specific orbitals at large deformations.

PARTICLE-PARTICLE CORRELATION STUDIES FOR THE SYSTEM N-14+TB-159 AT 10 AND 12 MEV/A

Abstract Energy and angular correlations of p, n and α-particles in coincidence with projectile-like fragments have been studied for the reaction 14 N + 159 Tb at 10 and 12 MeV/ A . The measurements involved in-plane, and at one energy also out-of-plane, correlations. Position-sensitive detectors were employed to investigate sequential ejectile decays. The correlations indicate the presence of at least two processes: (a) formation in a binary process of an excited fragment that decays sequentially by light particle emission and (b) emission of a fast light particle at an early stage of the interaction. The second is characteristic of an “uncorrelated” process that can be described by a factorisation prescription. For the heavier fragments, the first process is associated with the quasielastic component while the second component corresponds to inelastic events.

OPENING UP THE A 180 K-ISOMER LANDSCAPE : INELASTIC EXCITATION OF NEW MULTI-QUASIPARTICLE YRAST TRAPS

Abstract New high-spin isomers in 175 Lu, 181 Ta and 186 W target nuclei have been strongly excited by pulsed 238 U beams. The use of inelastic excitation opens up an uncharted region in the neutron-rich K-isomer landscape. First results establish the key role of multi-quasiparticle configurations in the high-angular-momentum structure.

First excited 0+ states in the germanium isotopes via the Se(d, 6Li) reaction

Abstract A systematic study of the even-A germanium isotopes with mass 70 ≦ A ≦ 78 via the Se(d, 6Li) reaction has been performed at Ed = 45 MeV. The reaction products were momentum analysed and mass identified with a QMG/2 magnetic spectrograph and the accompanying focal-plane detector system. The main emphasis of this investigation was on the nature of the first excited 0+ states. The striking variation in strength from one isotope to the other already observed in the (p, t) reactions to the same final nuclei is also seen for the α-pickup reaction. Previously derived wave functions that assume the excited 0+2 states to be pure proton configuration states can also account for the present results.

K-SHELL IONIZATION OF DY ATOMS PRODUCED IN (ALPHA,CHI-N) AND (C-12,CHI-N) NUCLEAR-REACTIONS

Abstract The K-shell ionization cross sections, σ K Z , are determined for Dy atoms produced in bombardments of several Gd targets with α-particles in the energy range 47–130 MeV and of 148, 150 Nd targets with 12 C ions at 65 and 90 MeV. The determination is based on the measured intensity ratios of the characteristic K X-rays of the product atoms to those of the target elements. Analysis of the simultaneously measured discrete in-beam γ-ray spectra allows one to determine ionization cross sections for Dy product atoms, σ Z K ( der ), derived as a sum of the products of the γ-ray intensities times the corresponding K-shell internal conversion coefficients. The measured and the total derived values are found to be identical to within the experimental errors. The partial ionization yields per reaction are analyzed as functions of the average angular momentum left in each exit channel. Strong dependence of these yields on the nuclear structure is found. Moreover, the variation of the yields with the angular momentum is also strongly structure dependent. Properties of the quasicontinuum γ-radiation deexciting the reaction residues are considered. Upper intensity limits are determined for the low-energy magnetic dipole components in this radiation.

Isovector pairing in odd-odd N = Z 50Mn

High-spin states in the odd–odd N=Z nucleus 5025Mn have been investigated. A sequence of states up to Jπ=6+ has been assigned as the T=1 analogue of the yrast band in 5024Cr for the first time. The differences in energy between levels in these bands are interpreted in terms of rotational alignments and the effect they have on the Coulomb energy of the nucleus. Comparisons with shell model calculations show that the Coulomb energy difference between the T=1 analogue structures is an important indicator of the competition between isovector pairing modes in N=Z nuclei and their isobars.

Identification of excited states in 140Dy.

Excited structures in the proton-rich nucleus 140 Dy have been established following the decay of an 8 − isomer. The excitation energy of the isomer is established to be 2.16 MeV with a half-life of 7.3 ± 1.5 µs. The isomer decays into the yrast line at the 8 + state, revealing a rotational band with a deduced deformation of β2 = 0.24(3). The isotope 140 Dy is the daughter of the deformed proton emitter 141 Ho. The new information obtained here supports the role of deformation in proton emission and the previous assignments of single-particle configurations to the two proton emitting states in 141 Ho. In addition, the reduced hindrance factor measured for the isomer is consistent with the trend observed in the N = 74 isotones.  2002 Elsevier Science B.V. All rights reserved.

Feeding and decay of superdeformed states

Abstract The mechanisms for feeding and decay of superdeformed (SD) bands are examined. Data connected with both processes in 192 Hg are compared with model calculations. The calculations successfully reproduce the data, suggesting that the mechanisms for both processes are understood. Constraints on the energy of the SD band energies and on the well-depths at low and high spins have been obtained. At the point of decay around spin 10, we suggest that the SD band 1s 3.3–4.3 MeV above the normal yrast line and that the well depths at spin 10 and 40 are 0.5–1.3 and 3.5–4.5 MeV, respectively.