D. D. Warner

IBA consistent-Q formalism extended to the vibrational region

Abstract The IBA-1 consistent- Q formalism developed by Warner and Casten is here extended to encompass the U(5) vibrational region. The extended model closely parallels the neutron-proton IBA (IBA-2) in form and parameter values. The transitional nucleus 154 Gd is used as a test case.

Effective g factors and proton-boson numbers in the vicinity of proton subshell closures

Abstract New experimental results on g factors of 21+ states are analyzed together with existing data in the Ba-Dy region. It is shown that a simple relation, based on constant values of the proton and neutron boson g factors can describe a broad range of nuclei. Moreover, the extracted values yield a much improved description of the B(M1) strengths to the newly discovered collective isovector excitations in the rare earth region. The anomalous values of g(21+) that occur in the region of the Z = 64 subshell closure are then described in terms of changes in the effective number of proton bosons Nπeff which take part in the collective motion, and the deduced values of Nπeff are compared with recent microscopic calculations. A similar approach is used to predict g(21+) values in the region of the Z = 40 subshell near A = 100.

Magnetic moments of 21+ states as a probe of the effective proton boson number near the Z = 64 subshell

Abstract The g -factors of the 2 1 + states in 144, 146 Ba have been measured using the perturbed angular correlation method. The results are combined with existing experimental data for the neutron-rich Nd, Sm and Gd isotopes and discussed in the framework of the neutron proton interacting boson model. It is shown that the assumption of a drastic change in the effective proton boson number around N = 88, proposed in an earlier study, allows all the results to be explained in a simple framework involving constant values of the boson g -factors.

The rotational structure of 227Ra

Abstract The level structure of 227Ra has been studied using the (n, γ), (d, p) and ( t , d) reactions and the β− decay of 227Fr. A model-independent level scheme was established including 28 levels below 1500 keV. Cross sections and excitation energies have been measured for 72 levels below 2.5 MeV and analysing powers have been measured for 25 levels below 1.4 MeV. The level structure is interpreted in terms of the Nilsson model. The ground-state configuration is 3 2 + [631↑] and the first excited band [633↓] starts at 1.7 keV. Several octupole vibrations coupled to single-particle configurations are tentatively assigned above 280 keV. The neutron binding energy was determined to be 4561.41 ± 0.27 keV. The half-life of 227Fr was measured to be 148 ± 2 s.

Nuclear structure of 198Au studied with (n, γ) and (d, p) reactions and interpreted with the IBFFM

Abstract The 197 Au(d, p) 198 Au reaction was measured with the high-resolution Q3D spectrograph at the Munich Tandem Accelerator at θ lab = 35°, using 20 MeV deuterons. Average resonance neutron capture gamma rays were measured with a filtered neutron beam at the Brookhaven National Laboratory. Using also secondary (n, γ) data a level scheme of 198 Au was established with 66 levels up to an excitation energy of 1600 keV mostly with spin and parity assignments. The level energies, E2 and M1 branching ratios and (d, p) transfer data were compared with new model predictions of the interacting boson-fermion-fermion model (IBFFM). This model provides a basic understanding of 198 Au.

g Factors in heavy nuclei and the proton-neutron interaction

Abstract It is shown that the experimental values of g (2 1 + ) for nuclei in the mass range A =70−200 are well described within IBA-2 using a simple linear relationship. Values of g Π , g v , the effective proton, neutron boson g factors, were extracted for six groups of nuclei in this mass range. The results indicate that g π tends to increase, and g v to decrease with A . No obvious distinction is observe d between spherical and deformed nuclei. A large deviation in the systematics is observed for Er, Yb isotopes. This anomaly is explained in terms of a change in the effective number of valence particles, due to the K dependence of the p-n interaction in the deformed field.

Low-lying levels in 194Os and the prolate—oblate phase transition

Abstract Gamma-rays have been observed deexciting the 2 + 1 and 2 + 2 levels in 194 Os following two-fold neutron capture on 192 Os using the high-resolution bent crystal spectrometers and high neutron flux available at the ILL. The results indicate that rather different structural changes take place in the osmium and the platinum nuclei, and that the prolate-oblate transition occuring near 192 Os leads to a substantially developed oblate character in 194 Os.

Average resonance capture studies of 198Au: A test of symmetry schemes for odd-odd nuclei☆

Abstract The low-lying levels in 198Au have been studied by means of the 197 Au ( n ,γ) 198 Au reaction, using the technique of Average Resonance Capture. The results yield a complete set of states with Jπ=0−−3− up to an excitation energy of 1 MeV. The data are used to examine the validity of a recently proposed symmetry scheme for odd-odd nuclei which simultaneously reproduces the spectra of the neighboring even-even, odd-proton, and odd-neutron nuclei. It is concluded that the existing theoretical framework is too restrictive to provide an acceptable description of 198Au.

IBA calculations near the Z = 64 subshell

Abstract It has recently been shown that the effects of the Z = 64 subshell are important for nuclides with N ⩽ 88 but disappear as N approaches 90, due to the influence of neutron-proton interactions. IBA-2 calculations show excellent agreement with experimental energy systematics if the effective reduction in valence bosons caused by the Z = 64 subshell closure is explicitly included. The deduced parameters then behave as predicted by the microscopic basis of the IBA-2 model, in contrast with previous studies.

Levels in 154Eu populated by (n, γ) and (d, p) reactions

Abstract The structure of the doubly-odd nucleus 154 Eu was investigated using neutron capture and (d, p) reactions on 153 Eu. The low-energy γ-ray and conversion electron spectra from thermal neutron capture, as well as the γ-ray spectrum for primary energies up to 6.5 MeV, were measured with precision instruments at the Institut Laue-Langevin, Grenoble. The multipolarities were determined for most of the detected low-energy transitions. The (n res , γ high ) spectrum measurements with 2 keV, 2 eV and 3 eV neutrons were performed at Brookhaven National Laboratory for primary transitions leading to the 0–600 keV excitation energies. The 153 Eu(d, p) 154 Eu reaction was measured with the Q3D spectrograph at TU Munich. A level scheme below 600 keV excitation energy comprising 99 levels was deduced and the parities and most probable spin values were determined from the experimental data. The low-lying levels were interpreted as due to the two-particle Nilsson configurations, taking into account the proton orbits 5 2 + [413], 5 2 − [532], 3 2 + [411] , and the neutron orbits 11 2 − [505], 3 2 + [651], 3 2 − [521], 3 2 − [532], 5 2 + [642], 3 2 + [402], 5 2 − [523], 1 2 + [400], 1 2 − [530] . The 145.3 (± 0.3) keV level observed in the (d, p) reaction can be identified with the isomeric level with T 1 2 = 46 min . The neutron separation energy for 154 Eu was determined to be 6442.0 ± 0.3 keV.