The level structure of 183W has been studied using gamma–gamma coincidences from thermal neutron capture in 182W accompanied with the reaction (d→,p). From these data and those of previous studies a total of 76 levels and about 490 connecting γ-transitions have been established for energies below 2.4 MeV. An analysis based on the standard distorted-wave Born approximation (DWBA) provides the lj-angular momentum transfers and spectroscopic factors for 36 levels up to 2.2 MeV excitation energy. A large number of particle transitions indicate an influence of strong mixing between particle and probably hole states. The extra exchange of phonons across the Fermi surface leads to a fine structure in the fragmentation of most single particle strengths and at the same time has the effect of breakdown of the individual properties of Nilsson states. The extracted l=1 and 2 (d, p) sum below 2 MeV has about the same magnitude as in 185,187W. It comprises a rather small fraction of the expected strengths. The observed states below 2 MeV are compared with predictions of the quasiparticle–phonon model. Of particular interest is the identification of “quasi-bands” with inverse spin sequence based on the state 9/2+ at 622.8 keV; this indicates some transition aspects from rigid rotors of light W to γ-soft nuclei in the Os-Pt region.

Low–high and low–low energy γγ-coincidences, following the Re187(n,γ)Re188 reaction with thermal neutrons, have been measured. Spectra have been recorded in the energy range from 50 to 2600 keV, and from 4500 to 6500 keV. The earlier known 188Re level scheme is corrected and extended up to ∼800 keV excitation energy and 62 levels by including 20 new levels. For 11 levels, the earlier interpretation has been changed. The complete level scheme is presented for up to ∼400 keV energy and spins ⩽5. Core plus two-quasiparticles model calculations have been performed. Some 188Re two-quasiparticle states display features indicating the nuclear shape phase transition from axial-deformation to γ-soft form.

Thermal neutron capture gamma-ray spectra for Re-187(n,gamma)Re-188 reaction were measured. Singles and coincidence spectra were detected in order to develop the level scheme. The evaluation is in progress, of which the first results are obtained from the analysis of coincidence spectra, allowing to check the level scheme below 500 keV excitation energy. Seven low-energy negative parity bands are developed in order to find better energies for rotational levels. With a good confidence, a few positive parity bands are developed as well. Rotor plus two quasiparticle model calculations, employing effective matrix element method are performed for the system of six negative parity rotational bands.

The level structure of 187W has been studied using prompt and delayed gamma–gamma coincidences from thermal neutron capture in 186W and also measuring the (d,p) reaction. From these data and those of previous studies a total of 170 levels (121 connected by γ transitions) have been established for energies below 2.35 MeV. Some of these levels have been grouped into rotational bands built on 14 intrinsic states of quasi-particle and quasi-particle plus phonon character. Of particular interest has been the identification of “quasi bands” with inverse spin sequence based on the newly established isomeric 11/2+ state at 410.1 keV. Although the DWBA analysis permitted definite spin–parity assignments for most states a large number of particle transitions have “anomalous” angular distribution shapes with respect to the DWBA which indicate an influence of strong mixing between particle and probably hole states. The exchange of phonons across the Fermi surface leads to a fine structure in the extra fragmentation of most single particle strengths and, at the same time, it produces the effect of breakdown of individual properties of Nilsson states. The total extracted l=1, 2 and 3 (d,p) strengths below 2 MeV have about the same magnitudes as in 185W that comprise a rather small fraction of the expected strengths. The observed states below 2 MeV are compared with predictions of the quasi particle phonon model. The discussion focuses on transition aspects from rigid rotors of light W to γ-soft nuclei in the Os, Pt region.

The level structure of W-187 has been studied using prompt and delayed gamma-gamma coincidences from thermal neutron capture in W-186 and also measuring the (d, p) reaction. From these data and those of previous studies a total of 170 levels (121 connected by gamma transitions) have been established for energies below 2.35 MeV. Some of these levels have been grouped into rotational bands built on 14 intrinsic states of quasi-particle and quasi-particle plus phonon character. Of particular interest has been the identification of "quasi bands" with inverse spin sequence based on the newly established isomeric 11/2(+) state at 410.1 keV Although the DWBA analysis permitted definite spin-parity assignments for most states a large number of particle transitions have "anomalous" angular distribution shapes with respect to the DWBA which indicate an influence of strong mixing between particle and probably hole states. The exchange of phonons across the Fermi surface leads to a fine structure in the extra fragmentation of most single particle strengths and, at the same time, it produces the effect of breakdown of individual properties of Nilsson states. The total extracted l = 1, 2 and 3 (d, p) strengths below 2 MeV have about the same magnitudes as in W-185 that comprise a rather small fraction of the expected strengths. The observed states below 2 MeV are compared with predictions of the quasi particle phonon model. The discussion focuses on transition aspects from rigid rotors of light W to gamma-soft nuclei in the Os, Pt region. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.

The isotopes in vicinity of the closed shell Z=50, the long chain of odd Te isotopes from Te-119 to Te-131, Sn-125 and Sn-123, have been investigated by means of (d,p), (d(pol),p), (d(pol),t), (3He,alpha) and (n,gamma) reactions. The experimental data were interpreted within the framework of Interacting Boson Fermion Model (IBFM) and Quasiparticle Phonon Model (QPM).

The nuclear structure of 127Te has been investigated with the Te126(n,γγ)Te127 reaction using thermal neutrons and with the Te126(d→,p)Te127 reaction at Ed=20MeV. About 190 levels were identified in a region to 4.1 MeV excitation energy, in most cases including spin, parity and γ-decay. The γ-decay scheme after neutron capture is essentially complete containing about 100% of the population of the 11/2− isomer and of the ground state. The thermal neutron capture cross section and isomer production of the 11/2− state at 88.3 keV were determined to be 0.44(6) b and 0.069(10) b, respectively. The neutron binding energy was determined to be 6287.6(1) keV. A significant number of the (d,p) angular distributions of cross section and asymmetry are anomalous with respect to the distorted-wave Born-approximation calculations and could be accounted for by inelastic multi-step mechanisms. The observed strong correlation of the (d,p) and primary (n,γ) strengths gives evidence for the direct neutron capture process which is mainly responsible for the primary population of 16 levels. The experimental level scheme is compared with predictions of the interacting boson–fermion model and of the quasiparticle phonon model.

Reliable information on level density and radiative strength functions for the excitation energy region with density of excited states more than 100 levels per 1 MeV and higher can be obtained now only by its model-free extraction from intensities of two-step cascades proceeding between compound states and few low-lying levels. Full model-free determination of these parameters in any method is possible only if one can extract from additional experimental information about general trend in dependence of ratio of strength functions of emitted reaction products of a given type on excitation energy of the nucleus under study. Analysis of the available experimental data for 125Te shows that the peculiarities observed earlier in other nuclei are also inherent to this nucleus.

The level structure of 185W has been studied using the prompt and delayed gamma–gamma coincidences from thermal neutron capture in 184W accompanied with the one-nucleon transfer reactions (d,p) and (d,t) with polarized beams. From these data and those of previous studies a total of 183 levels has been established for energies below 3 MeV. Many of these states have been grouped into rotational bands built on 28 intrinsic states of quasiparticle and quasiparticle-plus-phonon character. Although the DWBA analysis permitted definite spin–parity assignments for most of states a large number of particle transitions have ‘anomalous’ angular and asymmetry shapes with respect to the DWBA which indicate an influence of strong mixing between particle and hole states. The extra exchange of phonons and the significance of configurational ΔN=±2 mixing across the Fermi surface lead to a fine structure in the fragmentation of most single-particle strengths and at the same time has the effect of breakdown of the individual properties of Nilsson states. The accumulated l=1 (d,p) sum is about a factor two smaller than the equivalent (d,t) strength. Thus, the previously observed loss of the (d,p) strength in the W nuclei with A=184,185 is presumably because of their redistribution amongst particle- and hole-type states. The observed states below 2 MeV are compared with predictions of the quasiparticle–phonon nuclear model.

The nuclear structure of Te-127 has been investigated with the Te-126(n, gamma gamma) 127Te reaction using thermal neutrons and with the Te-126((d) over bar, p) Te-127 reaction at E-d = 20 MeV. About 190 levels were identified in a region to 4.1 MeV excitation energy, in most cases including spin, parity and gamma-decay. The gamma-decay scheme after neutron capture is essentially complete containing about 100% of the population of the 11/2(-) isomer and of the ground state. The thermal neutron capture cross section and isomer production of the 11/2(-) state at 88.3 keV were determined to be 0.44(6) b and 0.069(10) b, respectively. The neutron binding energy was determined to be 6287.6(1) keV A significant number of the (d, p) angular distributions of cross section and asymmetry are anomalous with respect to the distorted-wave Born-approximation calculations and could be accounted for by inelastic multi-step mechanisms. The observed strong correlation of the (d, p) and primary (n, gamma) strengths gives evidence for the direct neutron capture process which is mainly responsible for the primary population of 16 levels. The experimental level scheme is compared with predictions of the interacting boson-fermion model and of the quasiparticle phonon model. (c) 2005 Elsevier B.V. All rights reserved.

From the comparison of absolute intensities of the two-step gamma-cascades and known intensities of their primary and secondary transitions, the cascade and total population of about  100 levels of 181Hf and 184,185,187W excited in thermal neutron capture was determined. These experimental results and intensities of two-step cascades to the low-lying levels of mentioned nuclei were reproduced in calculation using level densities with clearly expressed step-like structure. Radiative strength functions of the primary transitions following gamma-decay of these compound nuclei to the levels in the region of pointed structure are considerably enhanced. Moreover, population of levels below 3 MeV can be reproduced only with accounting for local and rather considerable increase in radiative strength functions of the secondary transitions to the levels in vicinities of break points in energy dependence of level density and significant decrease of that to lower-lying states. Simultaneous change in both level density and strength functions in the same excitation region of a nucleus corresponds to the definition of the second-order phase transition.

Systematics of quasiparticle and vibrational states in the chain of W isotopes ((179)w, W-181, W-183, W-185, W-187) is presented. Levels in W-183, W-185, and W-187 have been studied up to 2 MeV with the (d,p) or (d,t) and (n,gamma) reactions. One-quasiparticle states below 2 MeV are interpreted in the framework of the quasiparticle-phonon model (QPM), rotational degrees of freedom using the particle-rotor model (PRM) including Coriolis interaction.

The level scheme of Dy-164 up to about 2.6 MeV energy is analyzed, using experimental data obtained in (n,gamma), (n,e(-)) and (n,ngammagamma) reaction measurements at the high-flux reactor ILL Grenoble, as well as the (n,n'gamma) reaction data, measured at the IRT reactor in Salaspils. The structure of low-lying levels of Dy-164 is interpreted in terms of two-quasiparticle states, interacting with collective excitations of axially-deformed core.