Gamma decay of 172Yb after the (ρ, ρ′) inelastic reaction

Nuclear Physics A496 (1989) 255-268 North-Holland, Amsterdam

GAMMA

D E C A Y O F 172yb A F T E R T H E (7, ~") I N E L A S T I C R E A C T I O N

A. ATA(~, M. GUTTORMSEN, J. REKSTAD and T.S. TVETER

Department of Physics, University of Oslo, Oslo, Norway Received 9 December 1988 (Revised 19 January 1989) Abstract: The y-decay following the ~72yb(~',r') reactions with ET= 45 MeV has been studied. Enhanced

transitions with energies of E~, ~ 1 MeV were found in the y-decay pattern. Around 50% of this radiation consists of known transitions originating from the 1-2 MeV region and feeding into the ground-state band at low spins. However, unresolved y-ray lines constitute a considerable fraction of the bump. The y-decay from the highest excitation energies (E x= 5-8 MeV) is found to be well described taking into account the transition probability imposed by the giant dipole resonance. NUCLEAR REACTION '72yb(3He, 3He'), E = 45 MeV; measured O-(E3He), Ez, , I~,,(3He)ycoin. ~72yb deduced y-ray multiplicity, branching ratios. Enriched target, Ge, Nal, Si counters.

I. Introduction

In g e n e r a l , a v a i l a b l e l i t e r a t u r e indicates that h e a t e d nuclei are well d e s c r i b e d by statistical m o d e l s , at least, in the a s y m p t o t i c F e r m i gas limit. H o w e v e r , recent studies using the (3He, ay) r e a c t i o n ~-4) have s h o w n d e v i a t i o n s from this s i m p l e statistical picture. In p a r t i c u l a r , for the e v e n - e v e n rare earth nuclei, the y - r a y s p e c t r a d i s p l a y a 1 M e V b u m p s u p e r i m p o s e d on the statistical p a r t o f the s p e c t r u m . The m a i n c o n t r i b u t i o n to the b u m p originates from the lowest excitation region ( E × = 1-2 MeV), w h e r e several v i b r a t i o n a l b a n d s are known. This region, which we call the vibrational region, p r e s e n t s a m u c h h i g h e r level d e n s i t y t h a n the g r o u n d - s t a t e b a n d . T h e r e f o r e , a s u b s t a n t i a l p a r t o f the y - c a s c a d e s goes via this region. In e a r l i e r e x p e r i m e n t s the e n h a n c e d e m i s s i o n o f 1 M e V r a d i a t i o n was o v e r l o o k e d , m a i n l y b e c a u s e the entry regions d i r e c t l y p o p u l a t e d in the r e a c t i o n s e m p l o y e d s h o w e d c o n s i d e r a b l e s p r e a d with r e s p e c t to spin. The a i m o f the p r e s e n t s t u d y is to investigate the role o f the v i b r a t i o n a l region in the y - d e c a y o f low s p i n states. W e have used the 172yb(r, r ' ) inelastic scattering, which p r e s u m a b l y p o p u l a t e s the v i b r a t i o n a l region s t r o n g e r t h a n the (3He, or) reaction. In 172yb, as m a n y as 14 b a n d h e a d s are k n o w n in the 1-2 M e V region, m a k i n g this nucleus e x c e l l e n t for the s t u d y o f d e c a y routes via the v i b r a t i o n a l region. Since the v i b r a t i o n a l r e g i o n is strongly p o p u l a t e d , discrete y - r a y s can be studied. 0375-9474/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

256

A. Ataf et aL / Gamma decay of 172yb

The direct feeding into the vibrational region and the intensity of the 1 MeV radiation is also explored as a function of excitation energy.

2. Experimental method and reaction gross properties

The experiment was carried out with 45 MeV r-particles delivered by the MC 35 cyclotron at the University of Oslo. The 2.2 m g / c m 2thick ~72ybtarget was isotopically enriched to 97%. The charged particles from the reaction were detected in a set-up consisting of four Si particle telescopes placed at an angle of 52 ° relative to the beam direction. The thicknesses of the front and end counters were 150, 3000 jxm, respectively. The energy resolution of the particle telescopes was 0.3 MeV (FWHM), and the total solid angle was 0.26 sr. The ejectiles were recorded both as singles and in coincidence with y-rays from the reaction. The y-rays were measured with one Ge counter and seven 12.7 x 12.7 cm Nal counters. The Ge counter has an efficiency of 20% and a resolution of 2.1 keV at 1.3 MeV of y-energy. The experimental technique is described in detail elsewhere 1,2). The spectrum of inelastically scattered r-particles in coincidence with y-rays detected with the NaI counters is shown in the upper part of fig. 1. The lower part displays the y-ray multiplicity given by (Mr) = (,Ex)/(E~), where the average y-energy is taken from the unfolded Nal spectrum. A y-energy threshold of 0.43 MeV was chosen in order to obtain a total detection probability approximately independent of the y-energy. Since the ground-band transitions up to the I ~ = 8 + state have energies below this threshold, only a negligible part of the rotational decay is included. Thus, the multiplicity curve mainly represents statistical y-transitions. The lower excitation part of the r-spectrum displays pronounced structures. Two particle groups are centered at excitation energies of 1.3 and 1.9 MeV, respectively. They are mainly due to the excitation of known y- and octupole vibrational states. Furthermore, at Ex = 2.7 and 3.7 MeV we find two broad structures which probably represent more composite collective excitations. For excitation energies above 4 MeV the intensity of the r-spectrum is fairly constant. The sudden drop at gx = 8 MeV is caused by the onset of neutron emission*. The neighbouring 171yb isotope is then populated at low excitation energy and the y-ray multiplicity is low. The immediate decrease in yield shows that the neutron channel dominates as soon as it becomes energetically possible. In this work we will only deal with the excitation region below the threshold of neutron emission.

3. Discrete transitions below Ex = 2 M e V

The lower excitation level scheme of w2yb has been extensively studied using a variety of experimental techniques. From (d,p), (d,t), (d, d'), (7, c~) and (p, c~) * The neutron binding energy for w2Yb is B,, = 8.02 MeV.

A. Ata( et al./ Gamma decay of t72Yb Ex ( M e v ) i

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Fig. I. T-particles measured in coincidence with y-rays (upper part) and the y-ray multiplicity of statistical transitions (lower part) as a function of excitation energy in 272yb. A y-ray threshold of Ev >0.43 MeV was used in the Nal detectors. p a r t i c l e r e a c t i o n s s 7) m o r e t h a n 14 b a n d h e a d s have been identified. The y - d e c a y p r o p e r t i e s o f s o m e o f these b a n d s have b e e n i n v e s t i g a t e d in (c~, 2n) reactions s,9), n e u t r o n c a p t u r e lO) a n d e l e c t r o n c a p t u r e w o r k ~). Fig. 2 shows a p a r t i a l level s c h e m e o f ~72yb b a s e d on o u r w o r k a n d previous d a t a 5 ]1). T h e d e c a y p a t t e r n refers to the y - d e c a y o b s e r v e d by putting an energy gate on the scattered r - p a r t i c l e s from the 2.0-8.0 M e V excitation region. The corres p o n d i n g y - e n e r g i e s a n d intensities are listed in table 1. In fig. 3, G e y - r a y s p e c t r a from various excitation regions o f ~72Yb are shown. The c o r r e s p o n d i n g e x c i t a t i o n bins in the c o i n c i d e n t r - s p e c t r u m are d i s p l a y e d as A, B, C, D a n d E in the insert. Since this c o i n c i d e n c e s p e c t r u m is taken with the G e detector, its s h a p e d e v i a t e s s o m e w h a t from the one m e a s u r e d with the N a I d e t e c t o r (fig. 1 ). The s p e c t r u m l a b e l l e d A in fig. 3 shows the y - r a y s from the energy interval 0.2-0.5 MeV, where o n l y the 4 + ~ 2 + (181 keV) g r o u n d - b a n d t r a n s i t i o n is present. The 2 + ~ 0 + (79 keV) g r o u n d - b a n d t r a n s i t i o n falls b e l o w the d i s c r i m i n a t i o n t h r e s h o l d o f the G e detector. The next s p e c t r u m , l a b e l l e d B, shows also the 6 + ~ 4 + (280 keV)

A. Ataf et al. / Gamma decay of 172yb

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Fig. 2. Partial level scheme of LvzYbwith transitions observed in the decay from the E x = 2.0-8.0 MeV excitation region. transition. The c o i n c i d e n t r - p a r t i c l e yield s h o w n in the insert for the 6 + state (B) is a b o u t 20% o f the yield for the p o p u l a t i o n o f the 4 + (A). This r o u g h l y agrees with the (d, d') cross-sections o f 8.4 ixb/sr (6 +) a n d 61 i~b/sr (4 +) o b s e r v e d at 90 ° with 12 M e V b e a m energy 5). F o r the e x c i t a t i o n e n e r g y region 0.9-1.3 M e V (C) the p r o m i n e n t y - l i n e s are the Er = 181,962 a n d 1094 keV transitions. In this region o n l y the I, K = = 3, 1 state at 1222 keV a n d the 4, 3 + state at 1263 keV are p o p u l a t e d with significant strength in the (d, d') r e a c t i o n 5). Thus, we i n t e r p r e t the 962 keV y - l i n e as the t r a n s i t i o n from the 1222 keV state into the 4 + (260 keV) g r o u n d - b a n d state. The 1094 keV y - l i n e originates f r o m the t r a n s i t i o n b e t w e e n the I, K ~ = 3, 3 + (1172 keV) b a n d h e a d a n d the 2 + (79 keV) g r o u n d - b a n d state. The f o r m e r state is p o p u l a t e d by an u n o b s e r v e d 91 keV t r a n s i t i o n from the /, K ~ = 4 , 3 + (1263 keV) state 8). T h r e e a d d i t i o n a l y - l i n e s a p p e a r in the 1.3-1.6 M e V (D) region. The 1387 a n d 1466 keV t r a n s i t i o n s o r i g i n a t e from the s t r o n g l y p o p u l a t e d I, K ~ = 2, 2 + (1466 keV) y - v i b r a t i o n a l state 5) w h i c h d e c a y s into the 0 + a n d 2 + g r o u n d - b a n d states. The w e a k 723 keV y - l i n e is i n t e r p r e t e d as the direct b r a n c h from the I, K ~ = 4, 3 + (1263 keV) state to the 6 + g r o u n d - b a n d state. The c o i n c i d e n t r - p a r t i c l e yield shows a m a x i m u m for the e x c i t a t i o n region 1.6-2.0 M e V (E). In this p a r t i c l e g r o u p we find the I, K ~ = 3, 2 - (1821 keV) o c t u p o l e

A. Ata¢ et al. / Gamma decay of 172yb

259

TABLE 1 G a m m a ray energies and intensities (relative to the 181.6 keV transition) observed in the decay from E x - 2.0-8.0 MeV Transition energy E~, (keV)

181.6 203.7 246.9 279.6 371.8 599.9 623.1 629.8 722.7 858.2 912.1 961.8 1004.1 1077.8 1093.8 1119.6 1290.5 1387.6 1466.9 1578.8 1743.0

(1) (1) (1) (2) (10) (6) (6) (3) (7) (4) (4) (5)) (10) (1) (6) (7) (4) (8) (8) (14)

Relative intensity I~,

100 (5) 12 (2) 8 (3) 30 (3) 9 (2) 8 (3) 2 (1) 2 (1) 5 (2) 3 (2) 7 (2) 5 (3) 2 (1) 8 (3) 51 (6) 6 (4) 9 (3) 11 (4) 4 (3) 4 (2) 9 (3)

vibrational state. The main decay o f this state is found through the 1742 keV transition which populates the 2 + ground-band state. We also observe a branch into the I, K " = 2, 1 (1199 keV) state with a transition energy o f 622 keV. The 602 keV y-line originates from the decay o f the /, K ~ = 2 , 2 - ( 1 7 5 7 k e V ) state into the 1, 1(1155 keV) state. The former state is probably fed from the /, K ~ = 3 , 2 - (1821 keV) state by an unobserved 64 keV transition. From fig. 3 we can estimate the average spin distribution o f the side feeding into the ground-band states. The side feeding for the 2 +, 4 +, 6 + and 8 + states follows the ratios* ( > 81) : 100: 30: 13, respectively. Since the 2 + ~ 0 + ground-band transition was discriminated in the experiment, the total feeding into the 2 + state is u n k n o w n . The lower limit of 81 is based on the intensities o f the 1094 and 1120 keV lines only, and is probably very conservative. Thus, if we a s s u m e a m o r e realistic estimate o f 100-150 for the 0 + and 2 + states, the average spin distribution will be ( 2 . 5 ± 0 . 5 ) h . Provided that the y - d e c a y within c o n t i n u u m is o f dipole type with Al = ±1 or 0, w e expect a similar spin distribution for the states directly populated in the inelastic reaction. This means that the reaction m e c h a n i s m excites low-spin states also in the higher excitation region. * Normalized to the side feeding of the 4 + state.

260

A. Ata 9 et al. / Gamma decay of t72yb A:

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Fig. 3. Gamma-ray spectra from the Ge detectors taken in coincidence with the 7-particles. The directly populated excitation regions of '72Yb are indicated. Our study o f discrete y-lines (see fig. 3) verifies earlier investigations 5-,,). For the lower excitation region the (T, ~") inelastic scattering populates the same states with roughly the same relative intensities as observed in high-resolution (d, d') work. We will therefore not go into detail concerning the specific structure of these states. It is, however, proper to c o m m e n t on the structure o f the K = = 3 + b a n d with its b a n d head at 1172 keV, which plays an important role in the decay pattern. For instance, we find for the y - d e c a y of states directly p o p u l a t e d in the inelastic scattering in the Ex = 2.0-8.0 MeV region that about 50% o f the feeding from discrete lines into the 2 + g r o u n d b a n d state originates from the I, K '~= 3, 3 + state at 1172 keV (E~ = 1094 keV). One can argue that this b a n d is the yrare band, but nevertheless it is surprising that the b a n d s with K ~ = 0 + and 1-, which are located less than 50 keV above the 3 + band, carry a negligible fraction o f the total y - d e c a y strength. Already in 1965 Giinther et al. ,2) measured a positive nuclear magnetic m o m e n t for the l 1 7 2 k e V state indicating admixtures o f protons. Furthermore, in-band g-factors measured in the (a, 2n) reaction 9) also imply n e u t r o n / p r o t o n mixing. Recently, Burke et al. 7) have used the (p, a ) reaction to investigate the proton

A. Ataf et aL / Gamma decay of 172yb

261

content in the wavefunction. They conclude that the state consists of (73+ 10) 25 [ 5 1 2 ] + I [521] two-quasineutron and (27+ 10) 27+[404]-½+[411] two-quasiproton configurations. Their data also reveal significant couplings to the bands based on the 1466, 1609 and 1663 keV band heads. The admixture of protons and the interaction with these higher-lying states, in particular the collective I, K '~ = 2, 2 + (1466 keV) y-vibrational state probably explain why the 1172 keV state appear at such a low excitation energy. Including the strong 1094 keV transition, table 1 reveals that the ground band is fed by several y-transitions with energies around 1 MeV. This means that the last step in the statistical decay tends to go from the vibrational region at 1-2 MeV to the ground band. This feature will be more closely examined in the next section. 4. Gamma radiation with E~ ~- 1 MeV

Based on the detailed knowledge for the low-energy regime, we will in the following interpret the data for the higher excitation energies. The overall )'-decay pattern is best studied with the Nal detectors, which have nearly constant efficiency as a function of )'-energy. In fig. 4 the ),-ray spectrum for the excitation region from 2.0 up to 8.0 MeV is shown, where we expect the decay to be governed by statistical laws. The spectrum in the lower part of the figure is produced by unfolding the raw spectrum (upper part) with the NaI response functions. For the lowest )'-energies we recognize the ground-band transitions. Furthermore, we find that the 1 MeV b u m p actually consists of two substructures. The first is centered at Ev = 1.05 MeV with a width of 0.21 MeV (FWHM) which is higher than expected for a single ),-line. Here, a significant part of the structure can be ascribed to the 1094 keV ),-line. The other structure of the 1 MeV b u m p is broader ( F W H M = 0.30 MeV) and is centered around 1.45 MeV. Previously, a similar splitting into 0.9 and a 1.2 MeV )'-ray substructures was found in the '62Dy nucleus 3). In fig. 5 the background subtracted 1 MeV b u m p is displayed for various excitation regions. The background is taken to be a quadratic function of the )'-energy, with its m a x i m u m at Ev = 1.9 MeV. It is clear that the shape of the 1 MeV b u m p is rather constant with respect to excitation energy. This fact indicates that the enhanced 1 MeV radiation originates mainly from the lowest excitation region. The strength of the 1 MeV y-radiation varies with the initial excitation energy. In fig. 6 we show the average number of 1 MeV )'-rays per cascade as a function of excitation energy. This quantity is computed by N =f~(M~),

(1)

where f~ is the fraction of the )'-intensity of the background subtracted 1 MeV b u m p relative to the total intensity of the unfolded NaI spectrum. The second factor, the average )'-ray multiplicity, is taken from fig. 1. The solid curves of fig. 6 depict the corresponding theoretical values based on a simple statistical-decay model. The )'-transition probability depends on the number

A. Ataf et al. / Gamma decay of t72yb

262 6000

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of available final states and the y-energy. Thus, the transition probability for y-rays depopulating states at E× is given by

P( E~) oc E~p( E~- E , ) ,

(2)

where p(Ex- E~) is the level density in the excitation region fed by the y-emission. For Ex~ Eyra~t+2A we use the Fermi gas level-density function given in ref. 13). For initial states located within the Fermi gas regime, E1 decay is assumed to dominate completely. The exponent n is taken to be a function of the y-energy, according to Axel ~4). Below the Fermi gas regime, M1 and E2 transitions have also been taken into consideration. The upper curves of fig. 6 show the number of 1 MeV y-rays per cascade, using a broad energy definition (Ev = 0.7-1.7 MeV) which covers the total 1 MeV bump. The experimental data (histogram) reveal significant contributions to the bump from the whole excitation region between 1 and 8 MeV. The data give a value close to unity around E, = 1.45 MeV. This can easily be verified from the level scheme, which shows that all decay routes in this excitation region must include one and only one transition with y-energy between 03 and 1.7 MeV. Furthermore, the curve shows a

A. Ata¢ et al. / Gamma decay of

172Yb

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is

dip centered around Ex = 3.5 MeV, where only every fifth cascade involves an enhanced 1 MeV y-transition. This is partly a background subtraction effect. Besides, it might be that level density and 3,-energy considerations favour other decay routes for this specific excitation region. At higher excitation energies the intensity increases and reaches a level of 0.37 for Ex~>4.3 MeV. The theoretical model predictions (solid line) and the experimental data show some discrepancies, in particular around Ex = 2 MeV, which are analyzed below. The middle part of fig. 6 displays the contributions from the 1.05 MeV substructure. Both experiment and theory gives a pronounced particle group around 1.3 MeV of excitation energy due to the direct population of the I, K ~ = 4, 3 + state at 1263 keV (see sect. 3). For the next peak, which experimentally is found around E× = 2.1 MeV, the theory strongly underestimates the number of transitions. This is an indication of the peak being due to effects not included in the model, possibly the direct population o f two p h o n o n states. The lower part of fig. 6, representing the 1.45 MeV substructure, shows a shape similar to that found for the 1.05 MeV structure. Here, the theoretical curve reproduces the experimental data rather well, except for the flat region in the higher

A. Ata~' et al./ Gamma decay of 172Yb

264

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Fig. 6. The number of 1 MeV transitions N per cascade in the y-decay from various excitation regions. The plotted values are background subtracted. The upper curve includes the total 1 MeV bump defined by 0.7 < E v < 1.7 MeV. The middle and the lower curve shows the number of transitions in the Ev = 1.05 and 1.45 MeV substructures, respectively.

excitation region where the 1.45 MeV intensity is somewhat overestimated. The first particle group is centered around E~ = 1.6 MeV, and is partly due to the strong inelastic excitations of the 1466 and 1711 keV states, seen in (d, d') workS). The peak corresponding to two coincident 1 MeV transitions in cascade is clearly observed at Ex = 2.8 MeV. The splitting of the 1 MeV y-ray bump is probably connected with variations in the level density and the character of the states in the Ex = 1-2 MeV region. About 50% of the strength of the 1.05 MeV structure is accounted for by the 1094 keV line, and a major part of the rest is also due to the decay of collective low-lying bands. The broad 1.45 MeV bump can mainly be explained by unresolved y-transitions from the 1.5-2 MeV excitation region. Thus, it is clear that also several twoquasiparticle states participate in the decay. For the higher excitation region, the amount of possible routes including 1 MeV radiation increases. This is clear from table 2 which summarizes the various contributions to the 1 MeV bump in the y-decay from Ex = 4.3-8.0 MeV. We find that the relative contribution of continuum lines to the total 1 MeV bump is about 30%.

265

A. Ataf et al. / Gamma decay of 17"-yb

TABLE 2 Number of 1 MeV transitions per cascade from the 4.3-8.0 MeV excitation region Type of 1 MeV transition The total 1 MeV bump The 1.05 MeV structure The 1.45 MeV structure Discrete lines") The 1094keV line Continuum lines

Gamma energy E~, (MeV)

Number of transitions per cascade

0.7-1.7 0.7-1.2 1.2-1.7 0.7-1.7 ~ 1.09 0.7-1.7

0.37 (3) 0.26 (2) 0.11 (1) 0.27 (6) 0.12 (3) 0.10 (5)

~) Defined as the lines of table 1 with E~, =0.7-1.7 MeV. Probably, most o f these transitions originate from the decay o f the 14 bands k n o w n in the 1-2 MeV excitation region. In average, each o f the bands gives rise to a p p r o x i m a t e l y 6 transitions which feed the ground-state b a n d (see decay scheme o f ref. s)). In addition, there might exist e n h a n c e d 1 MeV radiation from higher excitation regions. However, this contribution is small.

5. High-energy ~/-transition probabilities A significant part o f the states populated in the inelastic scattering feeds the ground-state b a n d directly. The high density o f states in the vibrational region also makes it possible to observe y-transitions which feed the E~ = 1-2 MeV region directly. In the u n f o l d e d y-ray spectra we find the c o r r e s p o n d i n g decay intensities as one b u m p a r o u n d E~ = Ex and another b u m p a r o u n d E~ = E x - 1.3 MeV. The existence o f these two b u m p s provides an o p p o r t u n i t y to investigate the y-ray transition probability as a function o f y-energy. In order to avoid absolute normalization problems, we define the following quantity

P,,

(3)

R(EO - - Pv+ Pg'

where Pg and Pv are the intensities of the y-ray b u m p s representing direct feeding o f the ground-state b a n d and the vibrational region, respectively. The e x p o n e n t n o f eq. (2) can in principle by y - d e p e n d e n t , and we define different values for the different regions: ng = n(E~ = Ex) and nv = n(E~ = E × - Ev). Here, Ev determines the excitation energy for the vibrational region and equals Ev ~- 1.3 MeV in our case. The theoretical estimate of R and its asymptotic limits at high excitation energies are then given by (E~-E~)"pv E/E~0 R(Ex) - ( E x - Ev)""pv+ E~gpg

fl

v/(pv+pg)

if n g > n v if rig= n~ if n g < n v

(4)

266

A. Ata~" et al. / Gamma decay qf ~r2Yb

The level density p a r a m e t e r s pg a n d Pv are d e t e r m i n e d by c o u n t i n g k n o w n levels. Provided a flat d i s t r i b u t i o n of initial spins with 1 ~ = 1±, 2 ±, 3 ± a n d 4 ± (see sect. 3) we find that the n u m b e r of available states in the statistical decaY is 6 in the g r o u n d b a n d a n d 44 in the v i b r a t i o n a l region. These values d e p e n d on the spin distribution. However, the ratio p v / p g , which is used as i n p u t in eq. (4), d e p e n d s only weakly on the specific d i s t r i b u t i o n used. The e x p o n e n t n in eq. (2) is very i m p o r t a n t a n d governs the y - e n e r g y d e p e n d e n c e of the t r a n s i t i o n probability. This p a r a m e t e r has been widely discussed in the literature 14-17) a n d values r a n g i n g from n = 3 to 6 have b e e n suggested. The lower estimate o f 3 is based on general q u a n t u m theory for electromagnetic radiation, giving n =2A + 1 (for dipole transitions A = 1). A r g u m e n t s for a p p l y i n g higher n-values are based on a d d i t i o n a l structural overlap factors in the t r a n s i t i o n matrix elements associated with the tail of the giant dipole resonance. In fig. 7 the e x p e r i m e n t a l a n d theoretical values of R are displayed. The experimental points show an e n h a n c e d feeding of states in the 1.5-2.5 MeV excitation region. This is not i n c l u d e d in the model using r e a s o n a b l e ng a n d n, values. The effect c o u l d be due to the direct p o p u l a t i o n of states closely related to the vibrational states. T w o - p h o n o n states c o u l d p r o d u c e this high t r a n s i t i o n probabilities a n d are the most likely candidates. This c o n c l u s i o n is also s u p p o r t e d by the e n h a n c e d emission of 1 MeV r a d i a t i o n a r o u n d E~ = 2 MeV (see m i d d l e part of fig. 6).

rr >, 4J .r-I c

P, P, + &

90 .

֥ ,

c

/

>

30-

r~

,+,jo •

I, •

"'1•

.

.

n=3

• o o]

~ ~ ~o~ ~o~ i 60-

.

" "--k "AXE<

•1_ •

,p.up

I II, ~11// ///

n-

O

I 2

I 4

Exc±tation

I 6

energy

I 8

(MeV)

Fig. 7. A comparison between the observed (full circles) and theoretical (dashed curves) ratio R = P,./(P, + Pg). The predictions labelled n - 3, 4 and 5 assume a E ~ function where n is independent of the y-energy. The asymptotical value for these solutions is also indicated at R - 0.88. The curve labelled Axel is based on ref. ~4) and takes into account the tail of the giant dipole resonance (n varies from 4 (E~= 1 MeV) to 5.4 (E~-8 MeV)).

A. Ataf. et aL / Gamma decay of tTeyb

267

In the higher excitation region the experimental points exhibit large statistical fluctuations. However, it is clear that the data require an E~ dependent n-value with ng> nv. Previous work by Axe114) on neutron capture has shown that the y-decay is well described taking into account the influence of the giant dipole resonance. This approach is tested also on our data. The parameters used for the resonance are a centroid of ER = 80A -1/3 MeV and a width of FR = 5 MeV. The resulting theoretical values for R, labelled Axel in fig. 7, are in good agreement with the experimental points. Thus, it is satisfying to confirm that the conclusions of Axel 14) also apply to the y-decay of states with 2-3 h higher spin values. 6. Conclusions

The importance of the vibrational region, defined for E~ = 1-2 MeV, has been explored in the y-decay following the 172yb(r, "/"y)172yb reaction. The average spin populated in the reaction process is low ((I) = 2.5 + 0.5), also in the higher excitation region. The average multiplicity of statistical y-rays was measured as a function of excitation energy, and a m a x i m u m value of ~ 3 transitions per cascade was found around the neutron binding energy (E× = 8 MeV). From the vibrational region several discrete y-transitions with energies around 1 MeV are identified. The unfolded NaI spectrum recorded in coincidence with T-particles from the Ex = 4.3-8.0 MeV region shows a pronounced 1 MeV y-ray bump. Totally, every cascade involves 0.37 enhanced 1 MeV transitions on an average. A substantial part of the 1 MeV b u m p (30%) consists of unresolved y-lines, which probably originate from one of the 14 bands in the vibrational region. Enhanced 1 MeV radiation from higher excitation energies plays only a minor role. The direct feeding from high excitation energy into the vibrational region and the ground band is used to test the E~ dependence of the exponent n in the expression for the y-transition probability. Around Ex = 2 MeV we observe a strong feeding into the vibrational region, which could reveal large overlaps between the initial and final wavefunctions. In order to fit the transition probability expression to the data at higher excitation energies, it is necessary to use a n-value increasing with y-energy. The strength function imposed from the giant dipole resonance 14) gives a reasonably good description of the data. However, other transition probability functions with ng> nv cannot be excluded. The present study has shown that the vibrational region plays an important role in the y-decay from low spin states in the excitation region up to the threshold for neutron emission. It would be a challenge to investigate whether the direct y-feeding into the vibrational region decreases relative to the feeding into the ground band for E×> 8 MeV. Here, future experiments could verify the influence of the giant dipole resonance upon the value of n. Financial support from "Norges allmennvitenskapelige forskningsr~d (NAVF)" is acknowledged.

268

A. Ataf. et al. / Gamma decay of 172Yb

References 1) A. Henriquez, J. Rekstad, F. lngebretsen, M. Guttormsen, K. Eldhuset, B. Nordmoen, T. Rams0y, R. RenstrCm-Pedersen, R.M. Aasen, T.F. Thorsteinsen and E. Hammar~n, Phys. Lett. B130 (1983) 171 2) J. Rekstad, M. Guttormsen, A. Ata% F. Ingebretsen, S. Messelt, T. RamsCy, T.F. Thorsteinsen, G. L0vh0iden and T. R0dland, Phys. Scripta 34 (1986) 644 3) J. Rekstad, A. Ataq, M. Guttormsen, T. Rams0y, J.B. Olsen, F. lngebretsen, T.F. Thorsteinsen, G. L0vh¢iden and T. Rodland, Nucl. Phys. A470 (1987) 397 4) A. Ataq, J. Rekstad, M. Guttormsen, S. Messelt, T. Rams0y, T.F. Thorsteinsen, G. L~vh0iden and T. R0dland, Nucl. Phys. A472 (1987) 269 5) D.G. Burke and B. Elbek, K. Dan. Vidensk. Selsk. Mat. Fys. Medd. 36 (no. 6) (1967) 1 6) R.A. O'Neil and D.G. Burke, Nucl. Phys. A182 (1972) 342 7) D.G. Burke and J.W. Blezius, Can. J. Phys. 60 (1982) 1751 8) J.R. Cresswell, P.D. Forsyth, D.G.E. Martin and R.C. Morgan, J. of Phys. G7 (1981) 235 9) P.M. Walker, S.R. Faber, W.H. Bentley, R.M. Ronningen and R.B. Firestone, Nucl. Phys. A343 (1980) 45 10) W. Gelletly, J.R. Larysz, H.G. B6rner, R.F. Casten, W.F. Davidson, W. Mampe, K. Schreckenbach and D.D. Warner, J. of Phys. Gil (1985) 1055 11) D. Novfikovfi et al. Czech. J. Phys. B33 (1983) 1070 12) C. Giinther, H. Blumberg, W. Engels, G. Strube, J. Voss, R.M. Lieder, H. Loig and E. Bodenstedt, Nucl. Phys. 61 (1965) 65 13) M. Guttormsen, A. Ataq, F. Ingebretsen, S. Messelt, J.B. Olsen, T. Rams0y, J. Rekstad, L.A. R0nning, G. L0vh~iden, T. R0dland and T.F. Thorsteinsen, Phys. Scripta 33 (1986) 385 14) P. Axel, Phys. Rev. 126 (1962) 671 15) J.M. Blatt and V.F. Weisskopf, Theoretical nuclear physics (Wiley, New York, 1967) p. 583 16) R.J. Liotta and R.A. S~rensen, Nucl. Phys. A297 (1978) 136 17) S.M. Sie, J. Newton and R.M. Diamond, Nucl. Phys. A367 (1981) 176, and references therein