Extreme deformations at high spin

Experimental nuclear structure and decay data are evaluated for all the 17 known nuclides of mass 149 (Xe, Cs, Ba, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb). Detailed compiled and evaluated spectroscopic information is presented for each reaction and decay dataset, and recommended values are provided for level properties, α, β and γ radiations, and other spectroscopic parameters, based on an evaluation of all the available experimental data for A=149 isobaric nuclides. Although large amounts of nuclear spectroscopic data are available for nuclides of A=149, yet large gaps in knowledge exist, as described below. For the lowest atomic number nuclide ¹⁴⁹Xe, only the isotopic identification has been made, with no data for its ground-state half-life. For ¹⁴⁹Cs, ¹⁴⁹Tm and ¹⁴⁹Yb information is available for only the respective ground states. For ¹⁴⁹Ba, ¹⁴⁹La and ¹⁴⁹Er, limited data exist for excited states. Many of the decay schemes of radioactive nuclei of A=149 are considered as incomplete, either due to large energy differences between the highest observed excited states in daughter nuclides and the respective Q-values, or due to the lack of confirmed γ-ray data, as listed below: ¹⁴⁹Cs → ¹⁴⁹Ba, ¹⁴⁹Ba → ¹⁴⁹La, ¹⁴⁹La → ¹⁴⁹Ce, ¹⁴⁹Ce → ¹⁴⁹Pr, ¹⁴⁹Pr → ¹⁴⁹Nd, ¹⁴⁹Tb(4.17 min) → ¹⁴⁹Gd, ¹⁴⁹Ho(21.0 s and 56 s) → ¹⁴⁹Dy, ¹⁴⁹Er(4 s and 9.6 s) → ¹⁴⁹Ho, and ¹⁴⁹Tm → ¹⁴⁹Er. No data exist for the decay of ¹⁴⁹Yb to ¹⁴⁹Tm. Data for half-lives of the excited states in this mass chain are generally lacking as given below by the number of excited levels of known half-life / approximate number of known levels in a nuclide: 2/17 for ¹⁴⁹Ba, 0/18 for ¹⁴⁹La, 3/53 for ¹⁴⁹Ce, 3/44 for ¹⁴⁹Pr, 17/110 for ¹⁴⁹Nd, 9/90 for ¹⁴⁹Pm, 10/210 for ¹⁴⁹Sm, 2/125 for ¹⁴⁹Eu, 6/270 for ¹⁴⁹Gd, 5/200 for ¹⁴⁹Tb, 3/80 for ¹⁴⁹Dy, 3/90 for ¹⁴⁹Ho, and 3/14 for ¹⁴⁹Er. This work supersedes earlier evaluations of A=149 nuclides published by 2004Si16, 1994Si18, 1985Sz01 and 1976Ho17.

The experimental results from the various reaction and decay studies leading to nuclides in the $\mathrm{A}=150$ mass chain have been reviewed. These data are summarized and presented, together with the adopted level schemes and properties, for the nuclides from Cs(Z=55) through Lu(Z=71). This evaluation replaces the previous evaluation by E. der Mateosian and J. K. Tuli (1995De28), which appeared in Nuclear Data Sheets 75, 827 (1995).

Nuclear spectroscopic information for known nuclides of mass number 151 (Cs,Ba,La,Ce,Pr,Nd,Pm,Sm,Eu, Gd,Tb,Dy,Ho,Er,Tm,Yb,Lu) with Z = 55 to 71 and N = 96 to 80 have been evaluated and presented together with adopted energies and Jπ of levels in these nuclei. No excited state data are yet available for ¹⁵¹Cs, ¹⁵¹Ba and ¹⁵¹La; only the isotopic identification has been reported, without information about their half-lives or decay modes. The ¹⁵¹Hf nuclide has not yet been identified. The radioactive decays of ¹⁵¹Ce, ¹⁵¹Ho (35.2-s and 47.2-s activities), ¹⁵¹Er (23.5-s and 0.58-s activities), ¹⁵¹Yb (two activities of about the same half-life) are not well established. For the decay of ¹⁵¹Lu, only the half-life is known.

Superdeformed structures are well established in ¹⁵¹Dy and ¹⁵¹Tb with a total of five SD bands in ¹⁵¹Dy and ten SD bands in ¹⁵¹Tb. In ¹⁵¹Gd, six SD bands have been tentatively assigned. In ¹⁵¹Tb, yrast SD-1 band has now (2008Ro23) been linked to normal-deformed structures through two well-defined high-energy γ transitions and possibly four other tentative γ rays.

This evaluation represents a revision of earlier (1997Si03,1988Si15,1976Ha35) evaluations of A = 151.

Nuclear spectroscopic information for known nuclides of mass number 149 (Cs,Ba,La,Ce,Pr,Nd,Pm,Sm,Eu, Gd,Tb,Dy,Ho,Er,Tm,Yb) with Z=55 to 70 and N=94 to 79 have been evaluated and presented together with adopted energies and Jπof levels in these nuclei. No excited state data are yet available for ¹⁴⁹Cs, ¹⁴⁹Ba, ¹⁴⁹La, ¹⁴⁹Tm and ¹⁴⁹Yb. The existence of ¹⁴⁹Cs is reported from mass separation of products from U(p,X) reaction and that of ¹⁴⁹Yb from proton decays of ¹⁵⁰Lu g.s. and ¹⁵⁰Lu isomer. There are no half-life data for ¹⁴⁹Cs and ¹⁴⁹Yb. In ¹⁴⁹Gd, 12 excited superdeformed bands have been reported, in addition to the yrast (g.s.) superdeformed band. In ¹⁴⁹Tb, five superdeformed bands are known, whereas in ¹⁴⁹Dy there is some evidence of superdeformation, but no discrete gamma rays have been found. The half-life of fully-ionized ¹⁴⁹Dy isomer at 2661 keV has been reported to be 11 s, as compared to 0.49 s for the neutral atom. This evaluation supersedes previous full evaluations of A=149 by Nucl. Data Sheets 46 (1985) 1 and Nucl. Data Sheets 19 (1976) 337 and a midstream (in update mode) evaluation by Nucl. Data Sheets 73 (1994) 351

The entrance channel effect was studied for the decay of ¹⁷⁰W formed in fusion reactions with different beam-target combinations. The average number of emitted neutrons suggest a lower effective excitation energy in the (α,xn) decay channel when more mass-symmetric reaction is used, especially at the highest angular momenta. The results are in qualitative agreement with predictions of the dissipative fusion model.

Abstract:Nuclear spectroscopic information for known nuclides of mass number 151 (Cs, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) with Z=55, 58 to 71 and N=96, 93 to 80 have been evaluated and presented together with adopted energies and Jπ of levels in these nuclei. No excited state data are yet available for¹⁵¹Cs,¹⁵¹Ce, and¹⁵¹Lu. The existence of¹⁵¹Cs is reported from mass separation of products from U(p,X) reaction but there is no information about its half-life.

In¹⁵¹Tb and¹⁵¹Dy, Several excited superdeformed bands have been reported, in addition to the yrast superdeformed band. This evaluation represents a revision of earlier (88Si15) Nuclear Data Sheets for A = 151.

Cutoff Date:Literature available up to January 15, 1997 has been consulted.

General Policies and Organization of Material:See the January issue of Nuclear Data Sheets.

Acknowledgments:The evaluator thanks M. J. Martin at Oak Ridge National Laboratory for a detailed and constructive review of the mass chain, especially, concerning adopted γ-ray branching ratios.

General Comments:The statistical analysis of γ-ray data and deduced level schemes is carried out through computer codes available at Isotopes Project, Berkeley and Nuclear Data Center, Brookhaven. The methodology and procedures for some of these codes are described by 86BrZQ and 86Br21. A general 3% uncertainty is assumed is quoted theoretical internal conversion coefficients taken mainly from 68Ha53. The values of μ and Q are from compilation by 89Ra17, when available.