Average resonance capture studies of 185, 187W: The Nilsson model and the SU(3) Bose-Fermi symmetry scheme

doi:10.1016/0375-9474(87)90432-5

Nuclear Physics A

Volume 465, Issue 2, 6 April 1987, Pages 221-239

https://doi.org/10.1016/0375-9474(87)90432-5 Get rights and content

Abstract

Complete sets of low-lying $12^{−}$ and $32^{−}$ levels in ¹⁸⁵W and ¹⁸⁷W have been obtained using measurements of primary γ-rays following average resonance neutron capture at mean incident neutron energies of 2 and 24 keV. The results are discussed in terms of both the Nilsson model and the SU(3) boson-fermion symmetry scheme appropriate to this region. The data highlights the advantages and deficiencies of both frameworks, and shows that neither is able to describe the complete spectrum of low-lying low-spin energy levels. The two approaches are outlined and compared and the role of the missing degrees of freedom in each is discussed.

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Cited by (13)

Nuclear Data Sheets for A = 187
2009, Nuclear Data Sheets
Evaluated spectroscopic data and level schemes from radioactive decay and nuclear reaction studies are presented for ¹⁸⁷Hf, ¹⁸⁷Ta, ¹⁸⁷W, ¹⁸⁷Re, ¹⁸⁷Os, ¹⁸⁷Ir, ¹⁸⁷Pt, ¹⁸⁷Au, ¹⁸⁷Hg, ¹⁸⁷Tl, ¹⁸⁷Pb, ¹⁸⁷Bi, and ¹⁸⁷Po. This evaluation for A = 187 supersedes the earlier one by R.B. Firestone (1991Fi02) published in Nuclear Data Sheets 62, 159 (1991) and the ¹⁸⁷Tl evaluation by C.M. Baglin (1999Ba24) published in Nuclear Data Sheets 86, 487 (1999).
Highlights of this publication are the following:
This evaluation includes first identifications of ¹⁸⁷Hf, ¹⁸⁷Ta (1999Be63, 2000PoZY) and ¹⁸⁷Po (2006An11) nuclides.
A B(E2)↑ = 0.080 12 was determined by 1967Bi10 for the 9/2+ state at 845 keV in ¹⁸⁷Re Coulomb Excitation. Using this B(E2) value and the adopted 845γ ray properties, a $T_{1 / 2} = 54$ fs 34 can be deduced for the 845 keV level of ¹⁸⁷Re. This T_1/2 yields a large $B(E2)(W.u.) \approx 1.3 \times 105$ for the 263γ (depopulating the same level) indicating either substantial unobserved feeding or incorrect g-ray branching intensities of this level.
The decay of g.s. and isomer of ¹⁸⁷Tl are poorly known (please see ¹⁸⁷Tl ε decay (51 s+15.60 s)).
Based on ¹⁹¹Po α decay studies, 2002An19 establishes a ¹⁸⁷Pb isomeric level energy at 2(15) keV and assigns $J π = 13 / 2 +$ for the ¹⁸⁷Pb g.s. and $J π = 3 / 2 -$ for the isomeric state (inverse of the systematics of odd-mass Pb nuclei in this region). From ¹⁸⁷Pb and ¹⁸⁷Pb^m mass measurements, 2005We11 determined the isomeric level energy at 33(13) keV.
Nuclear structure of 187W studied with (n, γ) and (d, p) reactions
2008, Nuclear Physics A
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.
Low-spin mixed particle-hole structures in 185W
2005, Nuclear Physics A
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 $Δ N = \pm 2$ 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.
Nuclear data sheets for A = 185
2005, Nuclear Data Sheets
Evaluated spectroscopic data for all nuclei with mass number A = 185 are presented together with the relevant level schemes from radioactive decay and nuclear reaction studies. This evaluation contains new data for ¹⁸⁵Ta, ¹⁸⁵Re, ¹⁸⁵Os and ¹⁸⁵Tl from heavy ion experiments, and data for ¹⁸⁵Tl, ¹⁸⁵Pb and ¹⁸⁵Bi from α-decay work. The re-analyzed result of ¹⁸⁵Pt level structure from ¹⁸⁵Au ɛ-decay is also included. The present evaluation supersedes the earlier one by E. Browne (1995Br04), published in Nuclear Data Sheets 74, 165 (1995).
Nuclear levels in 187W
1997, Nuclear Physics A
Levels in ¹⁸⁷W have been studied up to 2.2 MeV with the (d, p) and (n, γ) reactions. The (d, p) studies were performed at deuteron energies of 24 and 28 MeV and at two angles of 30° and 50° by using the high resolution Q3D spectrograph in Munich. Primary and secondary γ-rays and γ-γ coincidences were studied with Ge detectors. The combination of these results with the previously known ones allows us to extend the existing level scheme and refine some of its structure assignments. Most of the low-lying states have been classified in terms of the Nilsson model with the extension of some vibrational degrees of freedom. The possibility for the coexistence of triaxial or oblate shapes in ¹⁸⁷W is observed in the form of anti-aligned states with a bandhead of $92^{−}$ at 364 keV. The polarization effects towards the triaxial shape produced by the odd neutron in specific Nilsson orbitals have been studied. The calculated polarization energies for some γ-driving orbitals are comparable with the prolate-oblate energy difference which indicates the possibility of the stabilization of triaxial or oblate shapes in these states.
Level structure of 191Os: Coexisting oblate and prolate configurations
1991, Nuclear Physics, Section A
An extensive level scheme for the transitional nucleus ¹⁹¹Os has been constructed on the basis of (n, γ), (n, e⁻) and (d, t) reaction data. γ-rays following thermal capture and average resonance capture (ARC) were detected with pair spectrometers. Secondary γ-ray spectra were obtained with the GAMS bent crystal spectrometers. The high precision of the resultant energies was the critical factor in developing a level scheme on the basis of Ritz combinations. γγ coincidences were obtained from both primary and secondary transitions. Conversion electrons, detected with the BILL spectrometer, gave multipolarity information. The (d, t) reaction measured with a Q3D magnetic spectrometer gave additional information on the hole states in ¹⁹¹Os. A discussion of the resulting level scheme briefly reviews the fragmentation of negative-parity strength but focuses mostly on a rare example of an anti-aligned decoupled set of coexisting positive-parity states. For these, triaxial particle-plus-rotor calculations are carried out. Reasonable agreement is obtained and the calculations also explain the unusually strong population in (n, γ) of this set of mostly medium spin states.

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^☆: Research has been performed under contract DE-AC02-76CH00016 with the United States Department of Energy.

¹: Present address: Niels Bohr Institute, Risø, DK-4000 Roskilde, Denmark.

²: Present address: Daresbury Laboratory, Warrington WA4 4AD, U.K.

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Nuclear Physics A

Abstract

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Cited by (13)

Nuclear Data Sheets for A = 187

Nuclear structure of <sup>187</sup>W studied with (n, γ) and (d, p) reactions

Low-spin mixed particle-hole structures in <sup>185</sup>W

Nuclear data sheets for A = 185

Nuclear levels in <sup>187</sup>W

Level structure of <sup>191</sup>Os: Coexisting oblate and prolate configurations

Average resonance capture studies of 185, 187W: The Nilsson model and the SU(3) Bose-Fermi symmetry scheme☆

Abstract

Nucl. Phys.

Nucl. Instr. Meth.

Nucl. Phys.

Phys. Rev. Lett.

Phys. Rev.

Nucl. Data Sheets

Nucl. Phys.

Mat. Fys. Medd. Dan. Vid. Selsk

Mat. Fys. Medd. Dan. Vid. Selsk

Phys. Rev. Lett.

Phys. Rev. Lett.

Phys. Rev. Lett.

Phys. Rev. Lett.

Phys. Rev.

Average resonance capture studies of ^{185, 187}W: The Nilsson model and the SU(3) Bose-Fermi symmetry scheme☆