Twin backbending in 156Dy

Volume 49B, number 2

PHYSICS LETTERS

1 April 1974

TWIN B A C K B E N D I N G IN 156Dy R.M. LIEDER, H. BEUSCHER, W.F. DAVIDSON, A. NESKAKIS and C. MAYER-BORICKE Institut ffir Kernphysik, Kemforsehungsanlage J~ilich, D-5170 Jiilich, West Germany Y. EL MASRI, P. MONSEU, J. STEYAERT and J. VERVIER lnstitut de Physique Corpusculaire, B-1348 Louvain-La-Neuve, Belgium Received 9 October 1973 The ground-state and ~-bands in lS6Dyhave been definitely established up to 20+using the (a, 8n) and (p, 4n) reactions. They cross at 16+. Two backbendings were observedand attributed to the crossing of a third band with these two bands. The ground-state bands (gsb) of several rare-earth nuclei show irregularities in the level spacings at high spins [1]. These effects result from a sudden increase of the nuclear moment of inertia at a critical angular momentum. This phenomenon is generally named the backbending effect, since a plot of the nuclear moment of inertia 0 versus the square of the rotational frequency co generally exhibits an S-shape. Recently anomalous level spacings have also been reported for the/3-band in 156Dy [2,3]. The gsb and ~-band in 156Dy are both definitely established to 14 + [ 2 - 4 ] . A plot of 20~ 2 versus h 2 w 2 shows backbending at 10+ for the/3-band; however, in contrast, a steady increase in 0 for the gsb up to 14 + was observed. It has been suggested by Szymanski and Krumlinde [5] that there might exist a third band which crosses the /3-band and subsequently the gsb, thereby causing two backbendings. We have studied the high-spin states in 156Dy to test this prediction. The high-spin states in the nucleus 156Dy were popdated via the (a, 8n) reaction by bombarding a metallic 16°Gd target (~ 5 mg/cm 2, 99.9% enrichment) with 98 MeV c~-particles from the Jiilich isochronous cyclotron JULIC. The low-spin states were populated via the 159Tb(p, 4n)156Dy reaction at the CYCLONE isochronous cyclotron at Louvain-La-Neuve, with 39 MeV protons on a metallic ~- 5 mg/cm 2 terbium target. Excitation functions, "y-ray angular distributions and 3,-0, coincidences were measured. The procedures used for spin assignments have been described elsewhere [6,7]. In fig. 1 two background-corrected 'y-'y coincidence spectra from the (c~,8n) reaction are shown. In the up-

per portion a summed coincidence spectrum is displayed + where the three spectra gated on the 10~ -+ 8~', 12g 10~- and 14~ ~ 12~ have been added. Three new "ylines appeared at 635.7,655.4 and 680.9 keV which were respectively assigned as the 16~--* 14g, + 18~'~ 16~" and 20~- ~ 18~" transitions; 7-lines at 684.7 and 527.4 keV, and a doublet at 610 keV, are also evident. A spectrum coincident with this latter doublet is shown in the lower portion of fig. 1. Since this gating peak does not vanish, the two 7-lines comprising this peak must be in coincidence. In this coincidence spectrum it can be seen that the 527A keV 7-transition appears strongly. From the study of all coincidence spectra, the 527.4 keV 'y-transition was assigned as the 18~ -~ 16~-, the 684.7 keV 7-transition was assigned tentatively as the 22"~ -~ 20"~ transition, and the doublet was found to consist of a 609.9 keV 20~" ~ 18~-transition and a 611.4 keV 16~"~ 14~" interband transition. The lower members of the ~band up to 14~-were clearly observed in the (p, 4n) reaction. This reaction also allowed the 7-band in 156Dy to be established up to 9 +. In fig. 2, a level scheme resulting from the present work is shown. The gsb and the ~-band were deFinitely established to 20 + (the tentative level at 22 + is also shown). The assignments for the states above 14+ to the gsb and ~-band, respectively, are based on the reduced branching ratio in the decay of the 16~ state: the reduced transition probability for the 16~"~ 14~ 432.8 keV transition is four times greater than that of the 16~ ~ 14~" 611.4 keV transition. In the 'y-band only those 'y-transitions with I 7 > 0.02 are shown. 161

Volume 49B, number 2

PHYSICS LETTERS 1

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1 April 1974

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rived from the ?-ray angular distribution measurements with a high-resolution Ge (Li) spectrometer. Starting at 16 + the higher spin members o f the 13-

Volume 49B, number 2

PHYSICS LETTERS

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I56Dy Fig. 2. Level scheme of lS6Dy. The energies are accurate to ± 0.3 keV. The transition intensities of v-transitions observed in the (a, 8n) reaction are given; the error of the intensity given in brackets is the uncertainty in the last digit.

band become yrast states indicating that the gsb,and ~-band cross around 16 +. Indication for a similar band crossing has been observed for the neighbouring nucleus 154Gd by Khoo et al. [8 ]. The close proximity of the two 16 + states (24.3 keV) in 156Dy is an interest. ing feature. The 16~ state deexcites by a strong interband transition of 611.4 keV to the 14 + state. This transition is the strongest of the I~+ ~ (~-2)~ transitions. A 18~-+ 16~"transition was not observed (in-

1 April 1974

tensity < 0.02). No interband transitions of the type Ig+ ~ (I-2)~ were observed (intensity < 0.02). In order to understand more about the nature of the bands at the crossing region around 16 +, a twoband mixing calculation was performed in which the mixture between the/3-band and gsb was calculated from comparison of theoretical and experimental B(E2) ratios of interband to intraband transitions. The results are summarized in table 1. Reasonable agreement for the interband to intraband B(E2) ratios was obtained. The discrepancy at 12 + is not surprising since the admixture is very small and other effects may have a dominant role. It is concluded that the gsb and the fl-band both retain their respective characters throughout and that a significant mixing of these bands occurs only at the crossing region at 16 +. If attention is devoted purely to the intraband ?transitions in the gsb and/3-band as given in fig. 2, then on construction of the usual 0 versus w 2 plot, a backbending is observed for the/3-band whilst no such backbending exists for the gsb (fig. 3a). However, in the framework of the third-band hypothesis [5 ] briefly described above, the data must be interpreted from a different point of view. Essentially the levels of the third band are then identified as the (12~), 14~, 16J, 18J, 20~ and 22J of fig. 2. The corresponding results are shown in fig. 3b. According to this hypothesis the backbending behaviour of the/3band between 10 + and 14+ results from an interception of the third band with the 13-band. The resulting band is designated as "upper band" in fig. 3b. The subsequent interception of the third-band with the gsb around 16 + produces a second backbending. The band which results from this interception is the yrast band, Table 1 Interband to intraband B ( E 2 ) ratios from two-band mixing calculations 4"

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0.16 0.35 0.027 0.0023

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a) The admixture is defined as the square of the amplitude o f the fl-band states in the gsb states, and vice versa.

163

Volume 49B, number 2

PHYSICS LETTERS

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tion of the third-band with the gsb, not only the yrast band is formed but also correspondingly a higher-lying band originates [5 ], which is the continuation of the "upper band" to higher-spin states beyond 14~- (fig. 3b). This portion comprises the 16g, 18~- and 20~ states of fig. 2. It is essential to note that from the two two-band mixing calculation, the connecting 16~" 14~- transition (fig. 2) is expected to have an intensity of 0.005, which is four times less than the experimentally detectable upper limit; this 16~"-~ 14~- transition could therefore not be observed in this experiment. To summarize, the present results indicate that the backbending features in 156Dy can be understood in the framework of the third-band hypothesis [5]. Correspondence and discussions with Professor Z. Szymanski and Dr. J. Krumlinde are gratefully acknowledged. Professor A. Faessler is thanked for advice concerning the two.band mixing calculation. The technical help and assistance during the data collection given by Mr. H.M. J~'ger is gratefully appreciated.

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Fig. 3. Nuclear moment of inertia versus square of rotational frequency for a) the experimental gsb and #-band in lS6Dy, b) the yrast band and "upper band" as obtained from an interpretation of the data in the framework of the third-band hypothesis [5].

which in 156Dy can be identified with the gsb below the crossing point around 16 + and beyond with the third band. The connecting 7-transition between these two branches is the 16~"~ 14~ 611.4 keV transition shown in the level scheme of fig. 2. In fig. 3b it can be seen that the yrast band displays a pronounced S-shape. In most other backbending nuclei [1 ] only this backbending yrast band was observed. Due to the intercep-

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1 April 1974

References [1] A. Johnson and Z. Szymanski, Phys. Rep. 7C (1973) 183; R.A. Sorensen, Revs. Mod. Phys. 45 (1973) 353. [2] D. Ward, H.R. Andrews, J.S. Geiger and R.L. Graham, Bull. Am. Phys. Soc. 18 (1973) 630. [3] Y. El Masri, P. Monseu, J. Steyaert and J. Vervier, Proc. Intern. Conf. on Nuclear physics, Vol. I, Munich (1973) p. 185. [4] H. Ryde et al., Nucl. Phys. A207 (1973) 513. [5] Z. Szymanski and J. Krumlinde, private communication. [6] R.M. Lieder et al., Z. Physik 257 (1972) 147. [7] Y. El Masri, P. Monseu, J. Steyaert and J. Vervier, Rapport annuel 1972 de l'Institut de Physique Corpusculaire, p. 15, and to be published (see also ref. [3]), [8] T.L. Khoo, F.M. Bernthal, J.S. Boyno and R.A. Warner, preprint Michigan State University, Cyclotron Laboratory.