The identification of the 12+ [660] proton orbitals at high spins in rare-earth nuclei

Volume 133B, number 1,2

PHYSICS LETTERS

8 December 1983

THE IDENTIFICATION OF THE 1/2+[660] PROTON ORBITALS AT HIGH SPINS IN RARE-EARTH NUCLEI

CX. YANG a, j. KOWNACKI b, J.D. GARRETT, G.B. HAGEMANN, B. HERSKIND The Niels Bohr Institute, Copenhagen, Denmark

J.C. BACELAR c, J.R. LESLIE, R. CHAPMAN, J.C. LISLE, J.N. MO, A. SIMCOCK, J.C. WlLMOTT University of Manchester, Manchester, England

W. WALUS d, L. CARLI~N, S. JONSSON, J. LYTTKENS, H. RYDE University of Lund, Lund, Sweden

P.O. TJOM University of Oslo, Oslo, Norway

and P.M. WALKER Daresbury Laboratory, Warrington, UK Received 27 September 1983 Decay sequences based on the 1/2+[660] proton orbital have been identified in 171Ta and 177Re based on spin, parity, and large alignment. This decay sequence is observed higher in energy than predicted in cranking calculations based on modified oscillator potentials. Similarly known 1/2-[541 ] decay sequences in these and other neighbouring isotopes are observed lower in energy than predicted. A reduction in the strength of the spin-orbit potential for protons is suggested as a solution to these problems.

Rotational bands based on single-quasiparticle intrinsic configurations can be studied at large angular momentum in (heavy-ion, xn) reactions leading to oddN and odd-Z nuclei. The spectrum of such high-spin states can be compared [1,2] with calculations of independent-particle motion in a rotating deformed potential (e.g. cranking-model calculations [3]). Furthermore, such spectra can be taken as the basis of constructing multiple-quasiparticle states [1,4]. Whereas considerable high-spin data exist for single-quasineutron configurations (see refs. [1,2] and references a Permanent affiliation: Institute of Atomic Energy, Beijing, China. b Permanent affiliation: Institute of Nuclear Research, Swierk, Warszawa, Poland. c Present address: The Niels Bohr Institute, Copenhagen, Denmark. d Permanent affiliation: Inst. Fizyki, Krakow, Poland.

0.031-9163/83/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

therein), there is a scarcity of data for quasiproton configurations in rotational odd-Z rare-earth nuclei (see, however, refs. [5-8]). This letter reports new data for 171Ta98 and 1777Re102 in which decay sequences associated with the rotational aligned component of the i13/2 protons*l are identified for the first time in rare-earth nuclei. It is found that in order to reproduce simultaneously the observed positions of this 1/2 + [660] orbital and the 1/2- [541] orbital (which also is derived from shell-model configurations that are above the Z = 82 closed shell) in cranked shell model calculations [3] based on the modified oscillator (Nilsson ,1 At zero rotational frequency and in the limit of large deformation this configuration corresponds to the 1/2+[660] proton Nilsson orbital which at zero deformation lies above the Z = 82 shell closure. Under rotation the configuration of this orbital remains relatively pure, hence it is referred to as the 1/2÷[660] proton orbital in this letter.

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Fig. 1. Partial decay schemes for 171 Ta and 177Re. The highly-aligned sequence feeding the "5/2+[402] sequence" is suggested (see text) to be based on the 1/2+[660] orbital. For tTl Ta the excitation energies given for the "5/2[402] and 1/2+[660] sequences" are relative to the 5/2 + [402] band head and those for th e "7/2 + [ 404 ] and 9 / 2 -[ 514] sequences " arerelatwe • to the 7/2 + [40 4 ] band head. Similarly for 177Re the excitation energies of the " 9 / 2 - [ 514 ] sequence" are given relative to its own band head. The transition connecting the bands based on the 9 / 2 - [ 514 ] and 7/2+[404 ] configurations in 171Ta are delayed b y 42 -+ 3 ns [ 13 ].

Volume 133B, number 1,2

PHYSICS LETTERS

model) potential [9], it is necessary to reduce the s p i n - o r b i t strength relative to standard values [9,10]. The decay schemes, shown in fig. 1 were established from 3 ' - 7 coincidence measurements following the 159 Tb (16 O, 4n) 171 Ta and 165 Ho (16 O, 4n) 177 Re reactions. Beams o f 160 ions about 85 MeV were provided by the Niels Bohr Institute tandem accelerator, and ")'-rays were detected in an array o f five anti-Compton spectrometers similar to that described in ref. [11]. For 177Re the decay sequences associated at low spin with the 5/2 ÷ [402], 1 / 2 - [541], and 9 / 2 - [514] orbitals [12] have been extended to higher spins. The spin and parity assignments o f the added transitions are based on angular distribution and conversion electron measurements. A new highly-aligned positiveparity, signature, a, 1 / 2 ( 1 - a mod 2) decay sequence is observed feeding into the 17/2 + and 21/2 + members o f the "5/2 + [402] sequences". Little information was available for the decay scheme o f 171Ta prior to the present w o r k ; P r = 5 / 2 had been suggested for the ground state from decay studies [12]. Based on the systematics o f heavier Ta isotopes [14], the ground state had been identified with the 1 / 2 - [541] proton orbital. Transitions connecting the band heads of the "5/2 + [402] and 7/2 + [404] sequences" in 171Ta and the " 9 / 2 - [514] sequence" in 177 Re to the respective ground states were not observed in either prompt or delayed coincidence 7 - X or conversion electron studies also conducted in this laboratory. Therefore, such transitions must be of very low energy, ~<40 keV. The p r assignments for the " 9 / 2 - [514] and 7/2 + [404] decay sequences" are based on the observed 42 -+ 3 ns E1 transition connecting these two bands and on the systematics o f neighbouring odd-Z isotopes [14,15]. The association o f the remaining decay sequence with the 5/2 + [402] orbital also is based on systematics. At 1 = 33/2 + a large increase in alignment is observed in the a = 1/2 portion o f the "5/2+[402] decay sequence", therefore, the upper portion o f this decay sequence is shown as a separate rotational sequence in fig. 1. In both 171 Ta and 177Re a highly aligned (rr, a) = (+, 1/2) decay sequence feeds into the a = 1/2 portion o f the "5/2 + [402] decay sequence", see figs. 2 and 3. The only candidate in this'mass region for such a highly-aligned proton sequence with the appropriate parity and signature is that corresponding to the 1/2 + [660] orbital, see figs. 2 and 3. (The 1/2 + [660]

8 December 1983 •9/~-[514i

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Fig. 2. Comparison of experimental and calculated intrinsic frame excitation energies [3] (or routhians), e',for 171Ta as a function of the rotational frequency, ~to. A "reference" configuration of moment of inertia J = J0 + J1 co2 with ,TO= 29 MeV-t~ 2 and J1 = 90 MeV-ah 4 has been subtracted from the experimental values (points). The small uncertainties (< 40 keV) in the band-head energies for the 5/2+[402], 7/2+[404] and 9/2-[514] configurations (see fig. 1) are not included. "Standard" parameters [9,10] e2 = 0.236, e4 = 0.013, 3' = 0; pairing gap Ap = ,Xp (odd-even) = 1.13 MeV; K = 0.0620 and = 0.604 were used in the calculation (curves). The a = 1/2 levels are designated by solid points and full curves, and the a = -1/2 levels are shown as open points and broken curves. The Nilsson model coefficients are only valid in the limit of ~ o = 0 and large deformation.

orbital has a very large positive signature splitting, therefore, the a = 1/2 sequence is predicted much lower in energy than the a = - 1 / 2 sequence.) The level scheme information, transformed to the intrinsic frame, is compared in figs. 2 and 3 with cranked shell model calculations based on modified oscillator potentials [3] and standard parameters [9,10]. The predicted excitation energy o f the "1/2 + [660] configuration" is about 300 keV too low in both 171Ta and 177 Re. In contrast, the " 1 / 2 - [541 ] configuration" is predicted too high in these ( ~ 8 0 0 keV in 171Ta and ~ 3 0 0 keV in 177Re) and in other neighbouring [5,7, 12] odd-Z nuclei. Both orbitals are dominated by the gZ = 1/2 component o f a high-/configuration, i13/2 for the "1/2 + [660]" 41

PHYSICS L E T T E R S

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42

8 D e c e m b e r 1983

and h9/2 for the " 1 / 2 - [541 ]" and are, therefore, o f a similar prolate shape. Since both are predicted above the Fermi surface, a change of deformation will have a similar effect on the energies o f these orbitals: i.e. either an increase in e 2 or a decrease in c 4 will decrease the excitation energies o f both "prolate" orbitals. Similarly a decrease in e 2 or a an increase in c 4 will increase the excitation energy o f both "prolate" orbitals (see fig. 4). Occupying either of these orbitals has the effect o f increasing b o t h e 2 and e4 ; i.e. both orbitals have positive quadrupole and hexadecapole moments. Likewise, configuration dependent pairing [16] will have a similar effect on orbitals o f the same shape. A change o f strength of the l 2 term in the modified oscillator potential also will affect the excitation energy of both "prolate" orbitals in the same manner (see fig. 4). On the other hand, a change of the s p i n - o r b i t strength in the modified oscillator potential will have an opposite influence on the excitation energy of i13/2(j = l + 1/2) and the h9/2( j = l - 1/2) orbitals (see fig. 4). Indeed the "1/2 + [660] orbital" can be shifted to the correct relative excitation energy in these nuclei by a decrease o f the s p i n - o r b i t strength by about 16% for 177Re and 10% for 171Ta. Such a change also would improve the agreement for the " 1 / 2 - [541 ] orbital"; however, a larger change would be required to bring the calculated energy for this configuration into agreement with experimental data. More extensive calculations indicate that such a decrease in the s p i n - o r b i t potential for proton configurations improves the overall agreement between the band-head energies and the predicted single-particle spectrum of proton states in the rare-earth region. It, however, is beyond the scope o f this letter to refit the modified oscillator parameters for the protons in this mass region. It is only our intention to indicate: (i) that large discrepancies appear for the high-j, lowproton orbitals which are derived from shell-model configurations that are above the Z = 82 closed shell, and (ii) that such deviations apparently are associated with the s p i n - o r b i t term in the modified oscillator potential. These orbitals are the basis o f the proton band crossings observed in discrete line data in the Os nuclei [17] and the predicted [18] higher frequency crossings which probably are observed in continuum "),-rays [19]. Before realistic modified oscillator calculations can be applied seriously to these alignments the discrepancies pointed out in the present experiments

Volume 133B, number 1,2

PHYSICS LETTERS

must be addressed. For example, cranking calculations based on the modified oscillator potential and "standard" parameters [9,10], such as those shown in figs. 2 and 3, predict that the lowest quasiproton alignment in the heavy rare-earth nuclei should be based on i13/2 quasiprotons, whereas an extrapolation of the experimental routhians from the present data would indicate that "h9/2 quasiproton alignment" probably occurs at a lower rotational frequency. These experimental data for the high~/, low-~2 proton configurations are reproduced more satisfactorily [20] by cranking model calculations based on deformed Woods-Saxon potentials [21] than with modified oscillator calculations. It remains to be determined whether the deformed Woods-Saxon potential is a better basis for independent-particle motion than the modified oscillator potential, or whether the observed differences only reflect an improved choice of parameters for the deformed Woods-Saxon proton potential. The authors thank W. Nazarewicz and J. Dudek for communicating the results of their W o o d s - S a x o n potential cranking calculations prior to publication. Financial support from the Danish Natural Science Research Council, the Danish Ministry of Education, the Nordic Committee for Accelerator Based Research and the U.K. Science Engineering Research Council is acknowledged.

References

8 December 1983

[2] J.D. Garrett, in: Proc. XX Intern. Winter Meeting on Nuclear Physics (Bormio, Italy, Jan. 1982), Ric. Sci., ed. Educazione Permanente, Suppl. no. 25, p. 1. [3] R. Bengtsson and S. Frauendorf, Nucl. Phys. A327 (1979) 139. [4] S. Frauendorf et al., Univ. of Tenn. -NBI preprint (1983); J.D. Garrett, G.B. Hagemann and B. Herskind, Nucl. Phys. A400 (1983) 113c. [5 ] C. Foin, S. Andr6 and D. Barneoud, Phys. Rev. Lett. 35 (1975) 1697. [6] A. Neskakis et al., Nucl. Phys. A261 (1976) 189. [7 ] A.J. Larabee and J. Waddington, Phys. Rev. C24 (1981) 2367; R. Holzmann, J.K. Kuzminski, M. Loiselet, M.A. van Hove and J. Vervier, Phys. Rev. Lett. 50 (1983) 1834. [8] G.B. Hagemann et al., Phys. Rev. C25 (1982) 3224. [9] S.G. Nilsson et al., Nucl. Phys. A131 (1969) 1. [10] R. Bengtsson, J. de Phys. Colloq. 41 (1980) C10-84. [11] P.J. Twin et al., Nucl. Phys. A409 (1983) 343C. [12] J.R. Leigh, J.O. Newton, L.A. Ellis, M.C. Evans and M.J. Emmott, Nucl. Phys. A183 (1972) 177. [13] I. Reyanka, I.M. Ladenbauer-Bellis, F.M. Bernthal and J.O. Rasmussen, Phys. Rev. Lett. 25 (1970) 1499. [14] S. Andre, D. Barn~noud, C. Foin, B. Adler and N. Perrin, Nucl. Phys. A279 (1977) 347. [15] J. Bacelar, thesis, University of Manchester (1982); J. Bacelar et al., to be published. [16] J.D. Garrett et al., Phys. Lett. l18B (1982) 297. [17] A. Neskakis, R.M. Lieder, G. Sletten and J.D. Garrett, Phys. Lett. 118B (1982) 49; Contr. IUPAP Conf. on Nucl. Phys. (Florence, Italy, Sept. 1983), to be published. [18] T. Bengtsson, Phys. Lett. 126B (1983) 411. [19] M.A. Deleplanque et al., Phys. Rev. 50 (1983) 409. [20] W. Nazarewicz and J. Dudek, private communication. [21 ] S. Cwiok, W. Nazarewicz, J. Dudek, J. Skalski and Z. Szymanski, Nucl. Phys. A333 (1980) 139.

[1 ] N. Roy et al. Nucl. Phys. A382 (1982) 125.

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