“Anomalous” signature splitting in the yrast band of the odd-odd 160Ho nucleus

Volume 137B, number 1,2

PHYSICS LETTERS

22 March 1984

"ANOMALOUS" SIGNATURE SPLITTING IN THE YRAST BAND OF THE O D D - O D D 16°Ho NUCLEUS J.A. PINSTON 1 Centre d'Etudes NucMaires - DRF/Chimie Physique NuelEaire, 85X.38041 Grenoble Cedex, France

S. ANDRE, D. BARNEOUD, C. FOIN, J. GENEVEY Institut des Sciences NucMaires (IN2P3), Grenoble, France

and H. FRISK Department o f Mathematical Physics, Lund Institute o f Technology, Lund, Sweden

Received 6 December 1983

The (11 B, 5n) reaction has been used to study the yrast band in 16°Ho up to high spin states (I = 24). A careful analysis of the experimental odd-even staggering of the [i 13/2] n[h 11/2 ]p band reveals in "anomalous" signature splitting for this configuration. This effect is compatible with a small triaxial deformation y = 5°.

Very recently the high-spin states have been studied in six o d d - o d d nuclei at the beginning o f the deformed rare-earth region: 152Eu [1], 154'156Tb [2], 156'158Ho [3,4] and 158Tm [5]. The common feature in all these nuclei is the presence of the decoupled [i 13/2] n [h 11/2]p band always strongly fed in the heavy-ion reactions. It was already shown [1 ] that the [i 13/2]n[h 11/2]p configuration carries an aligned angular momentum comparable with the value measured in the S band of the neighbouring e v e n even nuclei. This fact fully explains the strong feeding of this band and the simplicity o f the level schemes of these o d d - o d d nuclei. But the most striking feature of the [i 13/2]n[h l l / 2 ] p band is that the experimental routhians, corresponding to the two signatures observed, are crossing each other at a rotational frequency, ~a)c, characteristic o f the considered nucleus [2,6]. It was already proposed in ref. [6] that the "anomalous" signature splitting is due to a positive triaxial deformation of the core. Up to now, the o d d - o d d nuclei studied are situat1 Chercheur C.NRS. 0.370-2693/84/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

ed in a region of small quadrupole deformation. It is then interesting to know if the signature crossing is present for larger deformations. For this purpose we have reinvestigated the N = 93 160Ho nucleus where the yrast band was already known up to 1591 keV excitation energy [7]. High-spin states in 160Ho were populated in the 154 Sm(11 B, 5n) reaction with the 66 MeV 11B beam from the Grenoble isochronous cyclotron. A Sm203 target enriched to 98% in 154Sm was used. The 7 - 3 ' coincidence experiment was made using two HP-Ge detectors of 40% efficiency. The beam-Ge detector plane was surrounded by twelve large hexagonal (6" × 8") NaI detectors [8]. Coincidence events in the Ge detectors were accepted when four or more of the twelve NaI detectors fired. Fig. 1 shows two examples of coincidence spectra. The level scheme of 16°Ho presented in fig. 2 is based on coincidence data. Up to 1594 keV (1591 in ref. [7] ) it agrees well with a previous investigation of Leigh et al. [7]. The decay of the 118 keV level (the yrast band-head) to the 5 + and 6 + states of the groundstate band is only compatible with the spin assign47

Volume 137B, number 1,2

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ments I = 5 or 6. The previous choice [7] between these two values, I = 5, was based on the comparison with 162Ho. However following the natural assumption that the yrast band has the configuration 48

[i 13/2] n [h 11/2] p like the neighbouring o d d - o d d nuclei it is possible to extract this spin from simple theoretical considerations. In the odd mass nuclei near 160Ho, the [h 11/2] p

Volume 137B, number 1,2

PHYSICS LETTERS

24-

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22 March 1984

cleus, is due to the [h 11/2]p state. One can predict that, for a deformation 3, ~< 0, the even spins will be favoured for all frequencies, while for 3' > 0 only the even spins located above the signature inversion frequency will be favoured [1,2] (see below). In 16°I-Io a clear signature splitting is observed above h~o = 0.19 MeV (see fig. 3) and it is then possible to identify the levels which correspond to even spins in the z2d = 1 yrast band. Consequently only the spin value I = 6 can be deduced for the bandhead o f the [i 13/2]n X [h 11/2] p band at 118 keV. This spin assignment is coherent with (i) the fact that the alignment in the considered band is in very good agreement with the sum of the neutron and proton contributions (fig. 4), (ii) the spins I = 7/2 and 5/2 respectively measured for the ground state of the [h 11/2] p and [i 13/2] n bands in the neighbouring nuclei 16trio [9] and 159Dy [10]. Moreover the absence of any gain in alignment in the yrast band up to h w = 0.35 MeV agrees with the blocking of the [i 13/2] n orbit in this band. It was previously shown that the experimental routhians of the [i 13/2; a = 1/2]n[h 11/2; a = +1/2] p

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Fig. 2. Level scheme o f 16°Ho. The spin assignments are partially based on theoretical considerations and then are tentative. The widths of the arrows indicate the transition intensities.

level shows a relatively small signature splitting while the [i 13/2; a = 1/2] n level is strongly favoured compared to the [i 13/2; a = - 1 / 2 ] n level. It can then be deduced that only the configuration [h 11/2; a = - 1 / 2 ] p X [i 1 3 / 2 ; a = 1/2] n , corresponding to even spins and the configuration [h 11/2; c~ = 1/2 ] p X [i 13/2; c~ = 1/2 ] n, corresponding to odd spins, will be observed in experiment. The signature splitting between these two configurations, in the o d d - o d d nu-

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Fig. 3. Energy staggering in the [i 13/2]n[h l l / 2 ] p and [h 11/2]p bands. Below the crossing frequency h w c = 0.19 MeV the staggering in 16°Ho is out o f phase with the one in 161 no.

49

Volume 137B, number 1,2 I

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PHYSICS LETTERS I

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22 March 1984

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Fig. 4. Experimental aligned angular m o m e n t u m as a function o f to for the [i 13/2]n[h l l / 2 ] p band in 16°Ho and for the [i 13/2]n and [h 11/2]p band in the neighbouring ] s9 Dy and 161 Ho. The symbols a and • correspond to the sum o f the neutron and proton alignment for the total signatures a = 1 and c~ = 0 respectively.

configurations are crossing each other in the N = 89 and N = 91 o d d - o d d nuclei [2,6]. This behaviour was not expected because it is not observed in the [h 11/2; a = -+1/2 ] p configurations of the nearlying odd-Z nuclei. A plot E ( I ) - E ( I - 1) versus I is the most simple way to identify the crossing o f the two signatures; the effect shows up as a reversal of the phase of the staggering. But this effect is small in 160Ho compared with the situation in N = 89 and 91 nuclei and to magnify the "anomalous" signature splitting we have plotted (fig. 3) the quantity E(I) - E(I+ E(I -

1 ) - [E(I + 1 ) - E ( / ) 1) - E ( / - 2)]/2

as a function of the rotational frequency: hw(I ~ I -

1) = [ h w ( I + 2 ~ 1) + h c o ( I -+ I - 2)

+he(I+ 1 ~I-

1) + P t w ( I -

1 ~I--

3)]/4.

In fig. 3 it is obvious that the staggering in the [i 13/2]n[h 1 1/2]p band of 160Ho is out of phase with the one in the [h 11/2] p band of 161Ho below the crossing frequency h w c = 0.19 MeV. By fitting the theoretical routhians to the experimentally observed signature splitting in 160Ho we found a quadrupole deformation e = 0,187 and a triaxial deformation 7 50

= 5 °. The 7 deformation is small compared to those fitted in 156'158Ho (ref. [11] ). That was expected since 160Ho is more deformed and therefore more stable against 7 deformations. The present (11 B, 5 n ) experiment has established the levels of the yrast band in the o d d - o d d nucleus 160Ho up to 4184 keV excitation energy. By analogy with the known o d d - o d d nuclei of the rare-earth region the configuration [i 13/2] n [h 11/2] p has been assigned to this well developed band. The spin 1 = 6 of the ground state of the yrast system was assigned on theoretical considerations, this value differs by one unit from the value given in the previous investigation. From a careful analysis of the o d d - e v e n spin staggering in the [i 13/2] n [h 11/2] p band a crossing between a = 0 and a = 1 signatures has been evidenced at a rotational frequency fi~c = 0.19 MeV. This effect is very small in 16°Ho compared to the situation of the o d d - o d d nuclei with N = 89 and 91 neutrons. If this effect is the result o f the triaxility in 160Ho a deformation 7 = 5° can be deduced. R eferen ces [1] J.A. Pinston, R. Bengtsson, E. Monnand, F. Schussler and D. Barneoud, Nucl. Phys. A361 (1981) 464. [2] R. Bengtsson, J.A. Pinston, D. Barneoud, E. Monnand and F. Schussler, Nucl. Phys. A389 (1982) 158.

Volume 137B, number 1,2 [3] [4] [5] [6]

PHYSICS LETTERS

G. LCvh¢iden, Physica Scripta 25 (1982) 459. N. Rizk and J. Boutet, J. Phys. Lett. 37 (1976) 197. S. Drissi et al., Z. Phys. A304 (1982) 293. R. Bengtsson, F.R. May and J.A. Pinston, Proc. XX Intern. Winter Meeting on Nuclear physics (Bormio, 1982). [7] J.R. Leigh, F.S. Stephens and R.M. Diamond, Phys. Lett. 33B (1970) 410.

22 March 1984

[8] H. E1-Samman et al., to be published. [9] L. Funke, K.H. Kaun, P. Kemnitz, H. Sodan and G. Winter, Nucl. Phys. A170 (1971) 593. [10] H. Beuscher, W.F. Davidson, R.M. Lieder, A. Neskakis and C. Mayer-Boricke, Nucl. Phys. A249 (1975) 379. [ 11] R. Bengtsson and H. Frisk, to be published.

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