Lifetime measurements in g92 decoupled bands

Volume 61B, number 1

PHYSICS LETTERS

1 March 1976

L I F E T I M E MEASUREMENTS IN g9/2 DECOUPLED BANDS ~ B. HEITS, H.-G. FRIEDERICHS, A. GELBERG, K.P. LIEB, A. PEREGO*, R. RASCHER, K.O. ZELL and P. Von BRENTANO lnstitut fiir Kernphysik der Universitiit zu Kdln, KOln, Germany

Received 23 January 1976 Lifetimes of positive-parity states in 71,73As produced by the reactions 56'SaFe(~aO, p2n)71'73As have been measured by the Doppler recoil-distance method. The ratios of the measured reduced transition rates to the corresponding ones in the core nuclei 7°'72Ge can be explained by assuming that the coupling of a proton in the 1/21440] state increases the deformation of the core.

Recent investigations have strongly suggested the existence of moderate deformations in lg9/2 odd nuclei [1,2]. Since the deformation is not large, the Coriolis mixing of Nilsson intrinsic states should be taken into account [3]. Decoupled bands can occur at high spin under certain conditions [4]. Such bands have been e.g. found in 71,73As [5] and in 81,83Rb [6]. Since electromagnetic transition probabilities are quite model sensitive, lifetime values can be compared with predictions of various models, in particular of the rotation aligned coupling (RAC) model [3]. A similar experiment has been carried out in order to compare lifetimes in 81Rb and the 80Kr core [6]. The purpose of this paper is to describe and interpret measurements of lifetimes of positive-parity high-spin states in 71As and 73As. High-spin states in 71,73As have been studied by Protop et al. [5]. The existence of positive parity decoupled bands has been established by means of 7-7 coincidences, 7-ray excitation function and angular distribution measurements. Partial level schemes are given in fig. 1, together with the corresponding levels in the core nuclei 7°Ge and 72Ge. One can notice that the energy spacings of the odd nuclei are considerably smaller than those of the respective even-even cores (see table 1). This conclusion remains valid also if one takes as core energies the average between the two even-even neighbours. Lifetimes were measured by the recoil distance * Supported by the Deutsche BMFT. * On leave of absence from Istituto di Fisica A. Garbasso, Florence, Italy.

1730

~"

(1712° )

1949

4"

1363

1037

2"

635

~1 428 73As

0"

835 13/2"°1

0

0"

9/2 °

72Ge

0 745e

3789

6" 2468

4" ~15,2"1

2791

, ,

(1712") 2"

1040

0

0• 70Ge

1637

2691

(1112") 1312*

1905 I

.....~.~__

2+

1715

0° 71As

862

Io

72Se

Fig. 1. Partial level schemes of 71 ,TaAs and of neighbouring even-even nuclei. Relative intensities of transitions in 71,73As are given.

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Volume 61B, n u m b e r 1

1 March 1976

PHYSICS LETTERS Table 1 Lifetimes, reduced transition probabilities and deformations.

Nucleus

E 3, (keV)

I i -~ If

rexp(ps)

B(E2)ex p (fm4e 2)

~exp

7OGe

1040 1114 714 975

2 ÷---, 0 ÷ 4÷--* 2 ÷ 13/2+~9/2 ÷ 17/2 ÷ ~ 13/2 ÷

1.92 0.9 5.7 0.9

352 +- 10" 531 -+ 300 775-+80 1034 -+ 400

0.224 *- 0.0003

72Ge 73As

835 609

2+~0 + 13/2+~9/2 +

4.54 _+0.1 12.0 -+ 0.8

8°Kr 81Rb

617 623

2+~0 ÷ 13/2+~9/2 +

12.7-+0.7 8.7_+0.5

;/lAs

± 0.05 +- 0.4 -+0.6 -+-0.3

445 -+ 10" 815 -+ 50 722-+40 1004-+60

0.28 0.30

+-0.02 -+ 0.05

0.247 +- 0.003 0.28 -+ 0.01 0.25 0.25

-+0.01 -+0.01

* C o u l o m b excitation data from P.H. Stelson and L. Grodzins, Nuclear Data A1 (1965) 21.

Doppler-shift technique by means of the plunger device described in ref. [7]. The investigated nuclei were produced by the reactions 56Fe(180, p2n)71As and 58Fe(180, p2n)73As. The 180 beams with energies 60 and 52.5 MeV, respectively, were provided by the K61n FN tandem accelerator. Natural iron targets were used in the 71 As experiment; targets enriched to 65.1% 58Fe were used for 73As. The 0.2 mg/cm 2 thick targets were vacuum evaporated on a 2 mg/cm 2 thick Au foil. Both recoil nuclei and beam were stopped in a 20 lain Ta foil. The flight distance has been varied in the interval 1-6000 ~m. Gamma-rays were detected by two Ge (Li) detectors having an energy resolution of 2.0-2.2 keV at 1332 keV. The detectors were placed at angles 0 ° and 30 ° for 71As (55 ° for 73As) relative to the beam axis. The data were analysed according to the method described by Lieb et al. [7]. The 301 keV Coulomb excitation peak in the 181Ta stopper was used for intensity normalization. The variation of the Doppler shifted and unshifted lines (fig. 2) were fitted) by assuming a superposition of two components with different lifetimes. The influence of delayed feeding from other levels was considered very carefully. To this end, the intensities of all relevant transitions were determined from angular distribution data taken at the same effective beam energy. As shown in fig. 1, the presumed 15/2 ~ 13/2 and 21/2 ~ 17/2 transitions are very weak and do not affect the fits. The influence of finite feeding time associated with 34

7-rays leading to the observed band has been neglected; recent investigations in similar cases indicate that the feeding time does not exceed 0.1 ps [8]. A long-lived component with z = 618 -+ 60 ps was found in the analysis of both transitions in 71 As; it obviously belongs to an unidentified higher lying state. No similar lifetime has been found in 73As. The reduced transition probabilities have been compared with RAC predictions. If we deal with unique parity states and neglect contributions of terms not diagonal in ~2, the reduced transition probability is B(E2; I i "+ ] f ) - 5e2Q2 167r ~ c21(1i2~2011f~)12

(1)

where Qo is the core intrinsic quadrupole moment and cs~ is the amplitude of the I I ~ ) component of the wavefunction. If we take the amplitudes cf~ from the RAC model, we get the ratios of transition rates in the odd nucleus and the even-even core. B(E2, 13/2 + -+ 9/2+)= 1.4; B(E2, 2 + -+ 0 +)

(2)

B(E2, 17/2 + ~ 13/2 +) B(E2, 4 + ~ 2 +) = 1.08. As can be seen in table 1, the transition probabilities in the odd nuclei are larger than predicted by RAC. We should, however, not forget that in this simple model we assume that the core deformation and

Volume 61B, number 1

~'

PHYSICS LETTERS

4000

3000[

1 March 1976

"

~

52.5 MeV

>~ 3000 56Fe(1BO,p2n)71 1 1715-1001 keV /~\

'°°°t / 0~.~, 10

50 100 500 1000 Fright distance [lam]

1

i Dpopier sh?ed peak 5 10

50 100 500 Flight distance [Hm]

Fig. 2. Intensities of Doppler-shifted and unshifted peaks versus recoil distance.

consequently the value of Qo are not modified by the addition of a particle. This simplifying assumption is not always valid. The decoupled state consists mainly ofa K = 1/2 component. If we look at the 1/21440] state on the Nilsson diagram [9], we see that it goes steeply down with increasing deformation, thus favouring a larger stable deformation. This also explains why the energy spacings are smaller in the odd nucleus than in the core. A simple estimate of the non-diagonal, i.e., single-particle contributions to B(E2; 13/2 -~ 9/2) has also been carried out in the approximation of unique parity states. This contribution is smaller than 15% and thus cannot explain the observed discrepancy. Values of/3 extracted from the measured reduced transition probabilities are given in table 1. Data concerning 81Rb are also shown there. It can be seen that in this case the ratio of the transition probabilities in the odd nucleus and in the core are in good agreement with RAC. Hartree-Fock calculations have been recently carried out by Sharma [10] for even Ge and Kr isotopes. The results indicate deformations around/3 = 0.2. Moreover, the increase in deformation induced by a particle in the 1/2 [440] state should be stronger for 70Ge than for 72Ge, the former nucleus being softer; this is in agreement with our data.

Since the investigated nuclei beong to a transition region, it is reasonable to assume that there is no unique choice of a model. However, the evidence coming from odd nuclei is quite consistent with a description by means of the unified model with strong Coriolis coupling, as suggested by Scholz and Malik [ 11 ]. An alternative description could be based on particle-vibration coupling [ 12]. Even if these models will eventually prove to be equivalent in the case of transition nuclei, the deformed picture has the advantage of simplicity.

References [1 ] P. Van Brentano, B. Heits and C. Protop, in Problems of vibrational nuclei, ed. G. Alaga, V. Paar and L. Sips (North-Holland, Amsterdam, 1974) p. 155. [2] S.L. Heller and J.N. Friedman, Phys. Rev. C10 (1975) 1509 and C12 (1975) 1006. [3] F.S. Stephens, Revs. Mad. Phys. 47 (1975) 43. [4] F.S. Stephens, R.M. Diamond and S.G. Nilsson, Phys. Lett. 44B (1973) 429. [5] C. Protop et al., Z. Physik 271 (1974) 65, and to be published. [6] H.-G. Friederichs, A. Gelberg, B. Heits, K.O. Zell and P. Van Brentano, to be published; H.-G. Friederichs et al., Phys. Rev. Lett. 34 (1975) 745.

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Volume 61B, number 1

PHYSICS LETTERS

[7] K.P. Lieb, M. Uhrmacher, J. Dauk and A.M. Kleinfeld, Nucl. Phys. A223 (1974) 445. [8] J. Kolata, private communication. [9] S.G. Nilsson, Dan. Mat. Fys. Medd. 29 No. 16 (1955).

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1 March 1976

[ 10] S.K. Sharma, private communication. [11] W. Scholz and F.B. Malik, Phys. Rev. 176 (1968) 1355. [12] G. Alaga, in Problems of vibrational nuclei, eds. G. Alaga, V. Paar and L. Sips (North-Holland, Amsterdam, 1974).