Excited nuclear levels in doubly even Xe isotopes populated in (α, 2n) reactions

l 1.E.I: 3.A

I

Nuclear Physics A123 (1969) 99--113; (~) North-HollandPublishing Co.,Amsterdam

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Not to be reproduced by photoprint or microfilm without written permission from the publisher

EXCITED N U C L E A R LEVELS IN D O U B L Y E V E N Xe I S O T O P E S P O P U L A T E D IN (~, 2n) R E A C T I O N S I. B E R G S T R O M , C. J. H E R R L A N D E R and A. K E R E K Research Institute for Physics, Stockholm and A. L U U K K O Department of Physics, University of Helsinki Received 24 September 1968 Abstract: Using in-beam technique, gamma radiation from excited levels in the even Xe isotopes A ~ 124, 126, 128, 130 and 132 has been studied. The nuclei are produced in (~, 2n) reactions on enriched Te targets at a b o m b a r d i n g energy o f about 28 MeV. F r o m intensity and angular distribution measurements, members o f the collective bands having spin and parity o f up to 8 + or l0 + are assigned. On the basis o f their angular distributions, a few dipole transitions are classified. These are tentatively interpreted as depopulating odd-parity two-quasi-particle neutron states. Indications o f a new isomeric state in 13°Xe are found. Because o f the g a m m a intensities observed, the transition order is changed in the decay o f the earlier k n o w n 8.4 ms

isomeric state in la2Xe. Preliminary results from similar experiments on Te isotopes using Sn (~, 2n)Te reactions are also reported. E[

N U C L E A R R E A C T I O N S 116,118,12°,122,1248n 122,12~, 126,12S,130Te(~ 2n7) g 28 MeV; t measured (r(Er, 0). 118,120,122,12,,126Te' 12,, 126,12s,13o, 132Xededuced levels, J, zl. Enriched targets. I

1. Introduction Collective levels in spherical nuclei often are considered to be due to harmonic vibrations about spherical equilibrium. This assumption is based on the approximately equal energy spacing of the first 2 + and 4 + levels and the fact that in many cases, three possible candidates of the expected two-phonon triplet have been observed as close-lying levels of spin and parity 0 +, 2 + and 4 +. On the other hand, the absence of the 0 + level in quite a few cases has stimulated models of asymmetric vibration 1) or rotation 2). Of particular interest are the regions of nuclei where the level sequence changes from a vibration-like to rotation-like sequence, and much experimental work has been devoted to these transition regions during the last few years. Several suggestions have been made for new deformed regions of nuclei 3). Sheline et al. 4) proposed that some neutron-deficient nuclei of Ba might have a stable deformation in the ground state. Originally, this proposal was based on a comparison between the experimentally found energies of the first excited 2 + levels in even Ba isotopes and the theoretical expectations of the positions of these levels if the nuclei 99

100

t. BERGSTROMet al.

were deformed. Later experiments by Chanda et al. 5) showed that for 126Ba and 128Ba, the E2+/E4~/E6÷ ratios provided some evidence for rotational bands in these nuclei and thus also for a new region of deformation. Recent experiments 6) using the in-beam spectroscopic technique on gamma spectra from nuclei produced in (HI, xn) reactions have confirmed the previous 126Ba results and have also been extending them to 124Ba, but no convincing arguments for ground state deformation have been presented. Using (e, xn) reactions, Morinaga and Lark 7) studied the collective levels in even Xe product nuclei and reported levels up to 8 + and tentatively 10 ÷ and 12 +. As in the case of Ba nuclei, the energies of the first 2 + levels were found to decrease with a decreasing number of neutrons. For all cases studied (124 __< A _~ 132), one could not assume that these nuclei are deformed in their ground states because of the positions of the even spin and parity levels. The very neutron-deficient nuclei 12°Xe and ~22Xe were later investigated by Clarkson et al. 6), who used (HI, xn) reactions. The results of these experiments agree well with the results of Morinaga and Lark 7), although the use of a Ge(Li) spectrometer allowed Clarkson et al. 6) to make better energy determinations than could be achieved by the NaI(T1) scintillation technique 7). Two more recent experiments of interest should be mentioned. Betigeri and Morinaga 8) studied excited levels in even Xe, Te and Sn isotopes using (3He, 3n) reactions. Except for a few isotopes, they did not observe levels with spins higher than 4. More recently, Ward et al. 9) used (HI, xn) reactions to populate high-spin states in even Ce isotopes. A weak point in all these measurements is that, to a great extent, the assignment of spins and parities is a guess from the vibrational model. Therefore, since no crossover transitions are observed, the experimentalists assumed that all gamma rays are due to cascading E2 transitions. As yet, no coincidence measurements have been carried out to confirm this assumption. Because of intensity reasons, the weaker gamma rays are assumed to precede the stronger ones. Several different groups 10-12) have shown that the product nuclei populated in (particle, xn) reactions preserve the spin orientation which is introduced into the compound nucleus by the orbital angular momentum of the projectile. Studies of the subsequent angular distribution of gamma rays can thus be used for multipolarity and spin assignments. Also, the attenuation of the expected anisotropy depends on the spin of the level feeding the transition. This attenuation is largest for small spin values and approaches unity for high-spin states la). However, attenuation of the anisotropy can also be introduced for other reasons, for example, by isomeric states having half-lives which are not small compared to the spin relaxation time. Therefore, studies of angular distributions offer a useful tool for making level assignments without introducing the more difficult coincidence technique. There were many reasons to repeat the experiments on the even Xe isotopes. The first and trivial reason was to improve the earlier results by using better energy resolution. Before this investigation, many of the transitions in Xe were studied only in

Te($, 2n)Xe REACTIONS

101

NaI(T1) spectroscopy. However, we studied these isotopes more carefully mainly to fill the need for more convincing spin assignments. We concentrated our attention on the reported 7) 10 + and 12 + levels in Xe, and the fact that these levels were observed above 3 MeV seemed remarkable. Since the energy gap for these nuclei is about 2 MeV, there must be several high-spin two-quasi-particle states in the region 2-3 MeV. These states would be competitors to the high-spin collective levels regarding the feeding via gamma rays following the neutron evaporation from the compound nucleus. Thus the strength of feeding to the high-spin collective levels would be significantly diluted. Moreover, some of the high-spin two-quasi-particle states are expected to have odd parity with one neutron in the h~ shell. Transitions between such an odd-parity state and members of the collective band might be strongly hindered E1 transitions. The pronounced difference in angular distributions between dipole transitions and quadrupole transitions (connecting the members of the collective band) should simplify the search for transitions between such two-quasi-particle states and collective states. A possible case of isomerism of the type discussed above has been discovered in 13ZXe by Brinckmann et al. 14), who report an 8.4 ms isomeric state. However, preliminary measurements of the internal conversion intensities Is) indicate that the isomeric transition is of a higher multipole order than dipole *

2. Experimentalprocedure In the product nuclei studied, the excited levels were populated in (~, 2n) and in a few cases in (c~, 3n) reactions on enriched Te targets. We used the external 42 MeV a-beam of the 225 cm Stockholm cyclotron. The targets were made by depositing enriched metallic Te powder ( ~ 10 mg/cm z) between thin formvar foils ( ~ 20 #g/ cm2).

In the (~, 2n) experiments, the a-particle energy was reduced from its maximum value of 42 MeV to about 28 MeV by introducing degrading foils at the focal points in the beam transport system. This energy change corresponds to a loss in beam intensity from 5-10 nA to about 1-2nA. For the (~, 3n) experiments, the energy was reduced to 38 MeV with small intensity losses. The experimental arrangement is schematically shown in fig. 1. At the target, the beam has about 5 mm diam. The last beam-defining slit is located about 10 m from the target. A 10 ° bending magnet is placed between this slit and the quadrupole doublet shown in fig. 1. The combination of this arrangement and the rather long Faraday cup partly embedded in concrete lowers the level of the gamma-ray background in the experimental area. * Preliminary reports le, 17) on the experiments described have been given at the Swedish Physics Conf. Kiruna, June 1967 and at the Conf. on Nuclear Structure, Tokyo, September 1967.

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Te(m, 2n)Xe REACTIONS

103

The long Faraday cup allows the use of relatively thick targets without the current integration being affected by Coulomb scattering in the target. In addition, the beam is monitored by two surface-barrier solid-state detectors mounted symmetrically ( + 2 5 °) relative to the beam axis (cf. fig. 1). The sum of the counts registered in the two detectors is proportional to the integrated beam current and was used as a measure of the total dose delivered on the target. The summed monitor pulses are fed into two parallel scalers, one of which is blocked with the dead-time pulses from the multichannel analyser. From the recorded number of counts in the two scalers, accurate dead-time corrections can be obtained. The gamma rays from the product nuclei were studied using Ge(Li) detectors of different sizes mounted about 12 cm from the target. This long distance was chosen to make angular distribution measurements with large detectors possible up to rather large angles (155°). Also, due to the relatively long distance, the finite solid-angle corrections are kept small; therefore, the correction factors QE and Q4 are about 0.95 and 0.85, respectively. The angular distributions were studied using a 16 cm 3 detector (resolution 5 keV at 1.33 MeV). Energy and intensity measurements, however, were carried out using a 2.5 cm 3 detector (1.8 keV at 122 keV gamma-ray energy). The relative intensity measurements were performed at an angle of 125 ° in order not to introduce errors due to the Pz term in the angular distribution. The pulse spectrum was analysed in a 4096-channel Inter-technique pulse-height analyser, and the experimental data were fed via paper tape into the T R A S K computer for evaluation. The centring of the target relative to the detector was checked with radioactive sources placed in the target position. Also, the photopeak efficiency was measured with standard calibration sources in the target position. Therefore, the efficiency calibration would contain a possible absorption from the thin (0.5 mm) brass window on the semicircular side of the scattering chamber. The position and area of the photopeaks observed were numerically determined by the T R A S K computer, and the corresponding energies and intensities were evaluated. The angular distributions were performed in angles ranging from 90 ° to 150 ° in steps of 10 °. The recorded peak intensities were fitted by least-squares analysis to Legendre polynomial expansions. The theoretical coefficients A~ ax (k = 2 and 4) for complete alignment have been tabulated by Yamazaki 1a). Since the alignment of the populated states is only partial, the angular distributions are attenuated, i.e. A~,xp = ak(Ji)A'~ ax. Yamazaki defines the attenuation coefficients ek(Ji) of the alignment as the ratio of the actual statistical tensor to the statistical tensor for complete alignment. Theoretically, the ek(Ji) coefficients depend on several quantities in the feeding process of the level de-excited via the studied gamma transition. However, several groups have found that C~k(Ji) is increasing with the spin of the gamma-ray emitting level and approaches a value of unity for high-spin states. In the present experiments, the attenuation coefficients have been determined experimentally, and the values obtained have been used in making spin assignments.

104

t. BERGSTROMet al. 3. Results

Before presenting the results of the experiments, we shall first survey the criteria used in the analysis of multipolarity, spin and parity assignments. On the basis of the experimental energy and intensity information, tentative level schemes for the individual isotopes were constructed. Energy data were normally evaluated from energy calibrations performed before and after the experiments. In a few cases (~24Xe and 13°Xe), the Ge(Li) detector was exposed to radioactive calibration sources during the experiment, thus giving a much more accurate energy determination (see fig. 2). The results found in using the different methods agreed well, thus suggesting that there was no appreciable drift in the electronics used. The tentative level schemes obtained were compared with information received from the angular distribution measurements which give information on the gammaray multipolarities as well as the spin value of the gamma-ray emitting level. Examples of the results from the angular distribution experiments are shown in figs. 3 and 4. Most of the transitions observed can be classified as electric quadrupole radiation because of the positive A 2 values. The positive A2 values only exclude the pure dipole radiation. Transitions with multipoles other than dipole or electric quadrupole have been assumed to be too slow compared to the spin relaxation time, and therefore the attenuation coefficients should be equal to zero and the angular distributions isotropic. A few cases of dipole transitions have been observed. Adopting the E2 assignment for the transitions having positive A 2 values, the attenuation coefficients have been calculated. The agreement between comparable ~2 values indirectly supports the level assignment. Also, the present ea values agree with those deduced from the results of similar experiments reported by Newton et al. ~ ) . Concerning higher-order terms in the angular distributions, the attenuation factors e4 here tend to diminish the A 4 terms much more severely than in the A2 case. All the deduced A 4 terms, however, seem to agree with the corresponding A2 values. The intensity measurements serve as a further check on the level assignments. The experimentally found intensities for corresponding transitions in different isotopes agree reasonably well with each other. Also, the intensities agree roughly with the recent calculations by J~iglre ~8). Using optical potential parameters, he has calculated the probable spin distributions in the compound nucleus and in the product nuclei populated via neutron evaporation. It seems possible to explain that the discrepancies between experiment and theory are due to high-energy gamma radiation following the neutron evaporation. Such gamma radiation was not considered in the calculation of J/igare. The quantitative information to be discussed in the following originates from (e, 2n) reaction experiments. However, complementary experiments have also been performed in order to identify gamma rays due to target impurities of neighbouring isotopes as well as gamma rays originating from other reactions than (~, 2n) reactions. Thus, experiments have been performed at higher bombarding energies (33 and 38 MeV) and also in one case (~25Te) using an odd-mass target.

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L BERGSTR()M et al.

108 Table

1 a n d fig. 5 s h o w t h e r e s u l t s o f t h e d a t a a n a l y s i s f o r t h e v a r i o u s X e i s o t o p e s .

In the following,

the level schemes

g i v e n i n fig. 5 will b e d i s c u s s e d

in more

detail.

The levels o f ~ 3 2 X e . T h e e x i s t e n c e o f a n i s o m e r i c s t a t e a t 2 7 5 5 k e V r e p o r t e d r e c e n t l y by Brinckmann et al. ~4) h a s b e e n c o n f i r m e d in t h e p r e s e n t e x p e r i m e n t s . S i n c e all t h e levels populated

in the isomeric

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TABLE l Results o f energy, intensity a n d a n g u l a r distribution studies Isotope lazXe

laoXe

l~sXe

l~eXe

a24Xe

E~. (keY) 175 539 600 630 668 727 773

±2 ±1 ±1 4-1 =El :L1 ±1

536.04-0.2 586.1 ==0.2 668.2--0.1 739.94-0.2 752.54-0.1 854.9:k0,1

lr 27±2 17~2 52±4 5± 1 100±7 12±1 82±7 1004-8 94-1 82±7 30-+-3 20±2 28±3

443 527 590 704 776 847 1196

:El ±1 ±1 4-1 4-2 ±2 ±2

1004-7 84-1 76:k6 44±3 184-2 9±1 154-2

389 492 554 693 801 880 965 1060

±1 ±1 ~1 i2 ±2 4-2 ~-*_~ :k2

1004-7 15±2 73±5 504-4 254-2 8±1 18±2 8±1

354.1 ± 0 . 2 400.7i0.2 524.84-0.2 589.9±0.2 669.3±0.1 782.2±0.2 893.3i0.2 1077 4-0.5

100±7 .7-L1 734-6 16±1 61 ± 5 17±2 7±1 10i2

A2

A4

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Assignment

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( 7 - --+ 5-) (10 + --->7-) (5)-+4 + 22+ --->~2+ 12+ --+ 0 +

0.18±0.02

--0.05±0.04

0.35±0.04

4 + -+ 2 +

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0.01 ± 0 . 0 2

0.224-0.03

0.25±0.02 0.21 ± 0 . 0 3 0.274-0.04 --0.33±0.04

0.00___0.02 --0.02±0.05 0.02~0.05 0.02±0.05

0.494-0.04 0.464-0.07 0.63±0.10 1.0 4-0.1

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0.294-0.03

0.05+0.03

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0.404-0.04 0.254-0.09 0.38±0.11 <0? --0.33=k0.11

0.054-0.04 --0.04±0,11 --0.14±0.14

0.78±0.08 0.55±0.20 0.9 ± 0 . 3

12+ ~- 0 + 22+ --~ 12+ 4 + ---* 12+ 6+--+4 ~ 8+ - + 6 +

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0.354-0.03

0.33±0.02 0.324-0.04 0.40±0.05 0.30±0.12

--0.05±0.03 --0.034-0.06 --0.06±0.07 --0.12±0.13

0.65±0.04 0.70±0.09 0.9 4-0.1 0.7 4-0.3

12+ -+ 0 + 22+ ~ 12+ 4 + -+ 12+ 6 + --~ 4 + 8 + -* 6-10 + - : - 8 +

--0.234-0.16

--0.11±0.23

0.30±0.02

0.01 ± 0 . 0 2

0.42±0.03

0.344-0.02 0.35~0.06 0.364-0.02 0.36:k0.06 0.50±0.14 <0

--0.054-0.02 --0.05±0.08 --0.05±0.03 0.03±0.06 0,25~0.16

0.674-0.04

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0.794-0.04 0.9 ± 0 . 1 >0.9

a2+ --> 0 + (32+ --* 12+) 4 + -~ a2 + 6 + -+ 4 + 8 + -+ 6 + 10 + ---* 8 +

T h e a t t e n u a t i o n coefficients % are calculated as the ratio o f experimental a n d theoretical A2 coefficients a s s u m i n g the given assignments.

Te (~t, 2n)Xe REACTIONS

109

present g~mma-ray intensities, which are the sum of p r o m p t and delayed radiation, give information on the transition order in the isomeric decay. Because of the intensities observed, the transition order of the first two g a m m a rays de-exciting the isomeric state as proposed by Brinckmann et al. ~4) should be reversed. This transition order has been pointed out earlier ~7) and has later been confirmed by Brinckmann et al. ~s). N o cross-over g a m m a rays were found. The anisotropies in the angular distributions were strongly reduced due to the transitions partly being fed via the 8.4 ms isomeric state. The spins and parities of the levels at 668 keV and 1441 keV are known 19) from radioactive decay to be 2 + and 4 + . On the basis of delayed conversion electron measurements by Brinckmann et al. as), it has been concluded that the multipolarity of the 600 keV transition is El, M1, E2 or a mixture of M1 and E2. Our angular distribution measurements indicated a negative value of A z , thus excluding the pure E2 transition as an alternative. Therefore, the spin of the 2041 keV level should not be higher than 5. 10"

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Fig. 5. Level scheme of doubly even Xe isotopes. The relatively large conversion coefficient of the 175 keV transition indicated ls) a multipole of E2 or M I + E 2 . The A 2 value observed (for this transition) did not contradict such an assignment. Finally, the conversion electron intensity of the isomeric transition of 539 keV indicated 15) that this transition is of M2 or E3 character. On the basis of their conversion coefficient measurements, Brinckmann et aL Is) have tentatively assigned the corresponding levels as 5 - (2041 keV), 7 (2216 keV) and I0 + (2755 keV), respectively. The isomeric state is supposed to have the neutron configuration (h÷)~o+. However, the relatively high g a m m a intensity observed for the 539 keV transition in the present experiments as well as in those by Brinckmann et aL 15) seems to be difficult to understand if the spin of the 2755 keV level is as high as 10. Further experiments on the isomeric decay are needed to clarify the level assignments. A 727 keV g a m m a ray, which is not observed in the isomeric decay, cannot be placed in the level scheme on the present amount of information.

110

i. BERGSTROMet al.

The levels o f l a°xe. In the angular distribution measurements, four transitions having energies 536, 668, 739 and 753 keV with A2 > 0 were observed. The corresponding levels have been assigned as 2 +, 4 +, 6 + and 8 + levels. The first three of these assignments are in accordance with decay studies involving angular correlation measurements 2 o). The anisotropies for these four angular distributions were all found to be surprisingly low (cf. table 2) thus indicating an isomeric state feeding this band. An attempt to find such an isomeric state has been made using an external beam-pulsing system. The available pulsing frequency ( ~ 1 kHz), however, was found to be too low, and weak delayed g a m m a rays of energies 536 and 668 keV could only be observed in the first time interval ranging from about 100/~s to 200/~s after the beam pulse. Since no similar g a m m a rays were observed during any of the consecutive 100 #s time intervals, the results are interpreted as an indication of an isomeric level having a half-life less than 50/zs. The 855 keV transition has previously been assigned as an 8 + ~ 6 + transition 7), but the negative A z value found in the present experiments excludes this possibility. The only level which can accept the strong feeding implied in the high transition intensity is the 4 + level. The large negative A 2 coefficient and the small A 4 coefficient are only compatible with a spin change of one unit. Since no transitions are observed from the proposed 2059 keV level to any levels other than 4 +, the 2059 keV level is assumed to have spin 5. Such a state might be interpreted as a two-quasi-particle state with a configuration (hq, s~)5-. Such a state is not observed in radioactive decay 20). The levels o f 128Xe. Also in this case, the angular distribution measurements showed that four of the prominent transitions are of E2 character because of their positive A 2 values. The relative intensities as well as the deduced attenuation coefficients are in accordance with the 2 + , 4 + , 6 + and 8 + level assignments. The angular distribution of the 704 keV (6 + ~ 4 +) transition is affected by the presence of the 695 keV g a m m a rays from neutrons inelastically scattered in the Ge detector. These 695 keV g a m m a rays cause a relatively large error in the angular distribution measurements as seen in tables 1 and 2. The (22+ ~ 1 2 + ) transition of 527 keV has previously been observed in decay studies 21). Besides the five transitions discussed above, two transitions of 847 and 1196 keV were observed. The 1196 keV transition shows a large negative A2 coefficient. Even though the relative intensity is lower than the intensity of the 855 keV transition in 13°Xe, the 1196 keV transition has tentatively been interpreted as a (5- ~ 4 +) transition. The 847 keV transition was earlier considered to be a 10 + ~ 8 + transition (ref. 7)), but the almost isotropic distribution contradicts such an assignment, and our experimental results obtained thus far do not justify an assignment attempt. The levels of 126Xe. Besides the pronounced E2 transitions originating from even-spin and parity levels ( I < 8) and the isotropic (22+ ~ 12+) transition 22), the observed spectra contain three transitions having energies 880, 965 and 1060 keV.

Te(~t, 2n)Xe REACTIONS

1I 1

The intensity as well as the Az coefficient of 880 keV transition is in agreement with a ( 1 0 + - * 8 +) assignment, which was also suggested by Morinaga and Lark v). Concerning the other two transitions, no definite assignment can be made, but on the basis of the negative A 2 value, it seems possible that the 1060 keV transition is of the same nature of the 1196 and 855 keV transitions in t 28Xe and 130Xe ' respectively. The levels o f 124Xe. As in 126Xe, levels with even-spin and parity values up to 10 + have been observed. The (22+ ---* a2+) assignment of the 401 keV transition is tentatively based on the isotropic angular distribution and the low intensity. These data agree with what has been found for the other even Xe isotopes, where the (22+ --* ~2 +) transitions are known from studies of radioactive isotopes. The 1077 keV transition which might be of dipole character cannot be placed in the level 1819 " - ' ~

1614. (49)

i

1205 .-.-,,L-1150,-.-[~, T 600

6":1775-5

6+ 1751

6÷ 1774--7--- 6÷ 6+ 1748 , 570 614. 499 J 414 (53) r (58) (64) 1430--r"'*'-- 2+ 1360--i--,-.-4" -~-- 2" 1326-I i 4.* 1249-4.o1191.5_L_,~-. 2" 1257-1181 I 6901 760 2+1161.5-- ~ 4.+ i I 646 723 601 631 617 i 693 I I I (84) (80)

-zz$

i

,

564. (loo)

603 1(lOO

564. 605 (100)

1560.5 i(100)

57o--i-t- 2" I

J

1670 I 1 1

0 0 + O_-t---O* 124._ 126.152/e86 52/e68 52/e72 52/e74 Fig. 6. Level scheme of doubly even Te isotopes.

0

~

I18--

0+

O'

~'

120_

0+

o 0" 122.[ 52 e7o

scheme. The A z coefficient of the 590 keV transition indicates that this is an E2 transition, but in spite of its relatively large intensity, it was not possible to insert this transition into the level scheme. Levels in even Te isotopes. Using the described technique, we have started an investigation of the levels in even Te isotopes. The results are presented in fig. 6. The gamma-ray spectra and the proposed level schemes are found to be similar in character for these isotopes (118 < A < 126). Angular distribution studies support the level assignments. No indications are found for states with higher spin values than 6 or for transitions of dipole character. Though the experiments on levels in Te should be considered as preliminary, the results are included in this paper because they are useful when discussing our measurements on levels in Xe isotopes (cf. fig. 7). 4. Remarks

The level schemes shown in figs. 5 and 6 exhibit a very systematic behaviour, which

112

L

BERGSTR()M et al.

supports our interpretations. As pointed out in sect. 3, multipolarity and level assignments are mainly based on angular distribution studies. Here not only the sign of the anisotropy has been considered as important but also the degree of attenuation. Table 2 summarizes the attenuation coefficients observed for the E2 transitions in the ground state collective band. For the first three isotopes (124'126'12SXe), the coefficients are found to be equal, whereas in the last two cases (t3°'132Xe), stronger attenuations are observed. These changes are interpreted to be due to long-lived states partly feeding the transitions discussed. TABLE 2 Attenuation coefficients o f the angular distributions observed for the g a m m a transitions assigned to the collective ground state b a n d Isotope

x2+ ~ 0 +

4 + ~ x2+

6 + -+ 4 +

8 + -+ 6 +

l~4Xe a28Xe 12aXe 13°Xe aa2Xe

0.42±0.03 0.35±0.03 0.41 4-0.04 0.22 4-0.03 0.13 4- 0.03

0.674-0.04 0.654-0.04 0.784-0.08 0.49-k 0.04 0.35 4-0.04

0.794-0.04 0.70±0.09 0.55±0.2 0.46 4- 0.07

0.9 4-0.1 0.9 4-0.1 0.9 4-0.3 0.63 4-0.10

10 + -+ 8 + > 0.9 0.7±0.3

E%

%

_ pure

rotational

s.o~

3.0 p

.<.."-4 ""

pure rotational.

zo~

Ce

5.0~ 2. =.

•

4.OF Te

"~" 3.01"

2.(1 6'6

'

7'o

'

'

NEUTRON NUMBER

'

66

7;

74

7?

NEUTRON NUMBER

Fig. 7. The energy ratio E~+/E~+ and E6+/E~+ for even ~Te, ~4Xe, 5eBa and 5sCe isotopes having 66 ~ N < 80.

The regularity of the even-spin and parity levels is shown in fig. 7, where the energy ratios E4+/E2+ and E6+/E2+ are plotted as a function of neutron number. As can be seen from fig. 7, which also contains data from neighbouring isotopes, the discussed levels become more and more equidistant (vibrational-like) when approaching the single-closed shells Z = 50 and N = 82. F o r the Te isotopes, the deviations from the vibrational values are only a few per cent. As mentioned in sect. 1, several attempts

Te(~t, 2n)Xe REACTIONS

113

have been made to understand the nature of the collective levels in these nuclei. However, at the moment, no description o f these levels which can be derived from a simple nuclear model seems to exist. Going further from the closed shell Z = 50, the level structure of the even nuclei becomes more rotational-like. This tendency is more pronounced for the neutron-deficient isotopes. However, it appears that the rotation-like structure disappears when going to even more neutron-deficient isotopes because of the neutron subshell occurring at N = 64. Preliminary results on Te isotopes having N < 66 also show very strange level patterns. Most interesting, perhaps, is the observation of dipole transitions. These transitions are interpreted as being connections between odd-parity states and members of the collective bands as discussed above. Some of these states may have measurable halflives, and attempts will be made to measure these. N o appreciable change in the attenuation of the angular distributions of the E2 transitions is observed because of these dipole transitions. Therefore, the half-lives of these transitions should be below the limit set for the proposed isomeric state in 13°Xe.

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22)

A. S. Davydov and A. A. Chaban, Nucl. Phys. 20 (1960) 499 R. M. Diamond, F. S, Stephens and W. J. Swiatecki, Phys. Lett. 11 (1964) 315 S. A. E. Johansson, Ark. Fys. 36 (1967) 599 R. K. Sheline, T. Sikkeland and R. N. Chanda, Phys. Rev. Lett. 7 (1961) 446 R. N. Chanda, J. E. Clarkson and R. K. Sheline, Reactions between complex nuclei (University of California Press, Berkeley, 1963) p. 397 J. E. Clarkson, R, M. Diamond, F. S. Stephens and I. Perlman, Nucl. Phys. A93 (1967) 272 H. Morinaga and N. L. Lark, Nucl. Phys. 67 (1965) 315 M. G. Betigeri and H. Morinaga, Nucl. Phys. A95 (1967) 176 D. Ward, R. M. Diamond and F. S. Stephens, Nucl. Phys. A l l 7 (1968) 309 H. Ejiri, M. Ishihara, M. Sakai, T. Inamura and K. Katori, Nucl. Phys. 89 (1966) 641 J. O. Newton, F. S. Stephens, R. M. Diamond, K. Kotajima and E. Matthias, Nucl. Phys. A95 (1967) 357 R. M. Diamond and F. S. Stephens, Ark. Fys. 36 (1967) 221 T. Yamazaki, Nucl. Data A3 (1967) 1 H. F. Brinckmann, C. Heiser and W. D. Fromm, Nucl. Phys. 96 (1967) 318 H. F. Brinckmann, C. Heiser, W. D. Fromm and U. Hagemann, Int. Symp. on nuclear structure, Contributions, Dubna, p. 20 and private communication I. BergstrOm, C. J. Herrlander, A. Kerek and A. Luukko, Ark. Fys. 37 (1968) 305 1. Bergstr6m, C. J. Herrlander, A. Kerek and A. Luukko, Proc. Int. Conf. on nuclear structure, Tokyo (1967) p. 659 S. J~igare, Nucl. Phys. A95 (1967) 481; S. Hultberg and S. J/igare, private communication R. Henck, L. Stab, P. Siffert and A. Coche, Nucl. Phys. A93 (1967) 597 P. Holmberg and A. Luukko, Soc. Sci. Fenn. Comm. Phys.-Math, 34:1 (1968) P. Holmberg and A. Luukko, Soc. Sci. Fenn. Comm. Phys.-Math. 33:7 (1967) 1. Asplund, L. G. StrOmberg and T. Wiedling, Ark. Fys. 18 (1961) 65