S.M. Perez

Application of spin-dependent sum rules for spectroscopic factors to transfer reactions on 45Sc

Abstract For f 7 2 neutron transfer on 45 Sc the sum rules form a set of 8 equations connecting 16 partial sums of spectroscopic factors for stripping and pick-up to states of different spin. Using existing spin assignments, fits to experimental data enable further spin assignments to be made to states in 44 Sc and 46 Sc. The fits are consistent with an accuracy of 4% in relative spectroscopic factors. Tests performed on modified sum rules suggest that the absolute normalizations of the relevant DWBA cross sections are accurate to 10% and that there is not more than 10% missing single particle f 7 2 strength.

The exotic structure of heavy nuclei

We analyse the exotic decays and the spectra of the ground state bands of even–even nuclei in the mass range 222 ≤ A ≤ 242. We find that the data are well reproduced by a cluster model using either a mass-asymmetric or a mass-symmetric form of the core–cluster interaction, provided that other pertinent quantities are treated consistently with the form of the interaction.

Exotic clustering in Ce and Nd isotopes

Abstract We propose exotic binary cluster models for the intermediate mass nuclei 146,148,150Ce and 148,150,152,154Nd. The optimum choice of cluster and core is determined by joint consideration of the penetrability for exotic cluster emission, the deviations of cluster and core binding energies from underlying liquid drop values and the Q-values for breakup. It is noticeable that the core neutron number selected in this way is at or near the N = 82 shell closure. We use a standard form of the cluster-core potential to calculate the energies and wave functions of states in the ground 0+, 2+, 4+, … and lowest lying negative parity 1−, 3−, 5−, … bands. The theoretical spectra and electromagnetic properties are compared with measurements, and their general trends are reproduced satisfactorily.

Possible clustering in isotopes of Ba

Abstract A local potential cluster model describing 140Ba as 136Xe + α and 146Ba as 132 Sn + 14 C provides a good account of the energies and electromagnetic properties of the lowest bands of these two nuclei.

Exotic clustering in 232,234,236,238U

Abstract We account for the energies and electromagnetic E2 transitions of the J π = 0 + , 2 + , 4 + … ground state bands of 232,234,236,238 U, using a model in which these bands are treated in terms of a 24 Ne cluster orbiting the appropriate Pb core. The cluster-core potential parameters used are identical to those employed in a previous study of 222,224,226 Ra. In addition to the simultaneous fitting of energies and B (E2) strengths, we also obtain good agreement with the measured half-life for the 24 Ne decay of the 0 + ground state of 232 U and 234 U as well as with the measured B (E4 ↑; 0 + → 4 + ) transition strengths in 234,236 U. We find that a small parity dependence of the cluster-core potential is required in order to give a good account of the energies of the J π = 1 − , 3 − , 5 − ,… states of the lowest negative parity band of 232,234,236,238 U, and of the B (E3 ↑; 0 + → 3 − ) strengths in 234,236,238 U.

Can we obtain nuclear structure information from exotic decays of heavy nuclei

Abstract A simple cluster model has shown itself to be outstandingly successful in describing the half-lives for emission of heavy fragments ranging from 14 C to 32 Si from heavy even-even nuclei. However, its predictions for the half-lives of similar decays from odd-even nuclei are systematically below the corresponding measured values or lower limits set by experiment. In view of recent experimental results on the 223 Ra → 209 Pb + 14 C system, we suggest that many of the exotic decays of odd-even nuclei so far observed, or searched for, may be proceeding preferentially to low-lying excited states of the daughter nucleus. We show that all the discrepancies between our model predictions and the presently available data for 14 C and 24 Ne emission would be consistently removed if this should prove to be the case. An opportunity to extract nuclear structure information from exotic decay measurements would thus be available.

Negative parity bands in even-even isotopes of Ra, Th, U and Pu

There are negative parity bands, conveniently labelled Kπ = 0−, 1− and 2−, with very similar patterns of excitation energies in many of the even–even isotopes of Ra, Th, U and Pu. Furthermore, the 3− states in many of these bands decay to the 0+ ground state with reduced B(E3) transition strengths closely similar to one another and comparable to the strengths found in the isotopes of Pb. We propose a Pb core plus exotic cluster model explanation of these observations. Such a treatment accounts for the persistence of these features across many isotopes and gives a good description of the excitation energies and E3 rates for all of them. The wavefunctions generated are intermediate between the weak and strong coupling limits. They suggest that the Kπ = 1− bands consist of doublets, possibly inverted, and thereby give rise to a novel proposal for a unification of some of the disparate band structures seen in 238U.

Cluster model investigation of the structure of 223Ra

Abstract We extend our previous exotic cluster treatment of heavy even-even nuclei by modelling 223 Ra as a 208 Pb + 14 C + n system. The neutron occupies 1 g 9 2 , 0i 11 2 , 0j 15 2 , 2d 5 2 , 3s 1 2 , 1g 7 2 and 2d 3 2 states with energies taken from the known spectrum of 209 Pb, while the 14 C cluster occupies 0 + , 2 + , 4 + , … or 1 − , 3 − , 5 − , … states whose energies are taken from the known levels of 222 Ra. The 14 C-n interaction mixes together the various combinations of neutron and 14 C orbital motion states coupled to total angular momentum I . Diagonalization of the Hamiltonian matrices for each I -state then yields energies and eigenvectors for comparison with experimental data. Once the Pauli principle has been satisfied by excluding the odd neutron from orbitals already occupied by neutrons in the 14 C cluster we generate low-lying bands with K π values (in order of increasing energy) 3 2 + , 3 2 + , 5 2 + , 5 2 + , 1 2 + and 1 2 + . We calculate strong E2 transitions (∼100 W.u.) within a given band, and much weaker ones (≲1 W.u.) between different K -bands. We give a good account of the relatively strong E1 transitions between bands of equal K but opposite parity (≲0.001 W.u.) and the much weaker ones ( −4 W.u.) between opposite parity bands with different values of K . Our description of in-band M1 transitions is in good accord with the data, and we predict equally strong cross-band transitions. Our calculation indicates that the ground state of 223 Ra is constructed principally from a 0i 11 2 neutron coupled to the 208 Pb 14 C relative motion, in qualitative agreement with the observation that the exotic decay of 223 Ra goes preferentially to the first excited state of 209 Pb with spin 11 2 + .

Normal and superdeformed cluster bands in heavy nuclei

Abstract We apply a principle of maximum stability to determine the most favoured core-cluster decompositions of 194 Hg, 236 U and 240 Pu. The results suggest two distinct modes of clusterization for each nucleus, one of which can be associated with the ground state band, and the other with an excited superdeformed band. We compare the calculated properties of these bands with the corresponding data available in all three nuclei.

RECENT DEVELOPMENTS IN THE THEORY OF ALPHA DECAY

We examine the extensive experimental data on partial half-lives for alpha-decay, and discuss some interpretations, with emphasis on the use of a simple three parameter cluster model. In this model, the alpha-particle moves in a potential well of fixed depth, in an orbit with a large value of a global quantum number. With the model parameters fixed at values consistent with those suggested by the underlying nucleon orbits, we obtain excellent agreement with the data, and reproduce the discontinuities associated with shell closure in a natural way.