Download pdf - Level and isomer systematics in even tin isotopes from 108Sn to 118Sn observed in Cd(α, xn) reactions

1.E.I: Nuclear Phystcs A134 (1969) 81 --109; (~) North-Holland Pt~bhshtng Co, Amsterdam

I.E.4: 3,A Not to be reproduced b) photoprmt or mtcrofilm ",~thout v*rttten permission from the pubhsher

LEVEL AND ISOMER SYSTEMATICS IN EVEN TIN ISOTOPES F R O M 1°8Sn TO 118Sn OBSERVED IN Cd(~, xn) REACTIONS

T. YAMAZAK1 -t and G. T. EWAN *t

Lawrence Radtatmn Laboratory, Unirerstty of Califorma, Berl~eley, CahJornia

Recewed 13 March 1969

Abstract: The gamma-ray spectra observed ~ hen separated cadmium ~sotopes (A = 106-116) are bombarded by 28-50 MeV ~-pamcles from the Berkeley 2.2 m c~clotron have been stu&ed with Ge(L0 detectors Levels m even tm isotopes produced by (~, xn) reactions are reported The p~ompt and delayed spectra have been stu&ed using nsec time analysis and the natural beam bunching of the cyclotron to gwe a zero of t~me. Isomers have bcen observed in ~SSn ~ ltb t_ I_ = 20 nsec, 230 nsec and > 500 nsec, m ~ ~6Sn with t~ 350 nsec and > 500 nsec and m J J4Sn ~ t h t½ > 500 nsec and m 11ZSn with t~ = 14 nsec. The level and ~somer systematlcs m the e~en tm ~sotopes are &scusscd Levels m odd tm ISOtOpes for A -- 113 to A ~ 119 are also discussed

NUCLEAR REACTIONS 106. 108. 110. 112. 114, I 16Cd(ct, xn?,~, 160(~, p), (~, n), (~, pn), E : 20-50 MeV; measured Er, c~T-dela2r 107. Joy, l,.lSn deduced E~,

~08, 1~0. ~x2. ~3. ~4. t~_~, ~16. ~7, xJSSn deduced levels, I, ~, T~. xs, 1OF, lONe level deduced T) Enriched targets

1. Introduction

I s o m e r i c s t a t e s p o p u l a t e d in n u c l e a r r e a c t i o n s ha~e b e e n ex tens ive ly s t u d i e d in

r e c e n t years . U s u a l l y s u c h e x p e r i m e n t s h a v e b e e n p e r f o r m e d u s i n g p u l s e d b e a m s f r o m

a n a c c e l e r a t o r , a n d t he r a n g e o f l i fe t imes c o v e r e d d e p e n d e d o n the c h a r a c t e r i s t i c s o f

t h e a c c e l e r a t o r b e i n g used. D i a m o n d , S t e p h e n s a n d c o - w o r k e r s h a v e d i s c o v e r e d m a n y

new i s o m e r s w i t h ha l f - l ives 1 m s e c u s i n g a h e a v y - i o n l i n e a r a c c e l e r a t o r 1 .z)

B r a n d i et al. 3) h a v e s t u d i e d i s o m e r s w i t h ha l l - l ives ~" 5 Ftsec p r o d u c e d b y (7, n )

r e a c t i o n s u s i n g a b e t a t r o n . I s o m e r s w i t h ha l f - l ives in t he r a n g e 5 - 1 0 0 0 ~tsec h a v e b e e n

s t u d i e d u s i n g p u l s e d b e a m s o f p r o t o n s , d e u t e r o n s a n d ~ - p a m c l e s f r o m c y c l o t r o n s 4 - 7).

I s o m e r s in t he Itsec r a n g e h a v e a l so b e e n s t u d i e d u s i n g a p u l s e d b e a m f r o m a t a n d e m

a c c e l e r a t o r 8) In all t h e s e m e a s u r e m e n t s , t he h a l f - h v e s h a v e b e e n in t h e / t s e c r a n g e

o r l onge r . I t w o u l d be a n o b v i o u s i m p r o v e m e n t i f t he r a n g e c o u l d be e x t e n d e d to

s h o r t ha l f - l ives e n a b l i n g h i g h e r - e n e r g y t r a n s i t i o n s a n d t r a n s i t i o n s o f l o w e r m u l t i p o l e

o r d e r to be s tud ied .

In r e c e n t l e t t e r s 9.10), we r e p o r t e d a s i m p l e m e t h o d for t he s y s t e m a t i c o b s e r v a t i o n

o f i s o m e r i c s t a t e s w i t h l i f e t imes l o n g e r t h a n a few nsec p o p u l a t e d in (pa r t i c l e , x n )

r e a c t i o n s T h e m e t h o d d e p e n d s o n t he n a t u r a l p h a s e g r o u p i n g o f b e a m pu l ses f r o m

t Now at Depa r tmen t o f Physics, Umvers l ty o f Tokyo, Bunkyo-ku , Tokyo, Japan. *t Permanent address: Chalk River Nuclear Laboratory, Chalk Rwer, Ontario, Canada

81

82 T. Y A M A Z A K I A N D G . T . E W A N

a cyclotron and the fast timing properties of Ge(Li) detectors. We have used this method to study isomeric states in a number of Sn and Po isotopes 9, l1-33). In the present paper, we report a survey study of the level and isomer systematics of even Sn isotopes revealed in Cd(c~, xn) reactions with separated cadmium isotopes.

Previously energy levels in 116Sn and 1 a asn populated following radioactive decay have been studied by several workers 14-a 7), and the levels in 1 a 2Sn ' 1 ~4Sn and 1 a 6Sn

populated in (3He, 3n) reactions have been studied by Betigeri and Morinaga as). A detailed study of the 1 a6Sn isomeric state has been performed by Chang, Hageman and Yamazaki ~9). A brief description of our results on Sn isotopes from 108 to 118 has appeared earlier 13). In this paper, the experimental methods, the analysis of experimental data and the comparison with theoretical predictions are described

in more detail.

2. Experimental method

2.1. ELECTRONIC ARRANGEMENT

In our experiments, the natural beam bunching in a cyclotron is used to give a zero of time, and the time distribution between the pulses of ),-rays from a target is studied with Ge(Li) detectors. Although this method has not previously been used for ),-ray studies, a similar technique has been used in neutron time-of-flight measurements where the neutron distribution is studied between beam pulses. Such measurements have been reviewed by Bloom 2o).

In the present experiments, the external beam from the Berkeley 2.2 m sector- focussed cyclotron has been used. The beam bunches from the cyclotron are typically 4 nsec wide, and the repetition interval is equal to the inverse of the r.f. frequency. This is in the range 100 to 170 nsec depending on the particle being accelerated and on the energy. In most of our experiments, 28-50 MeV s-particles were used to

bombard the targets. The ~-rays were studied with a Ge(Li) ),-ray detector. The fast timing properties

of Ge(Li) detectors have been discussed by several workers 2 a , 22,23). TO obtain fast timing, it is essential to use leading-edge rather than cross-over timing. This is because the pulses from Ge(L 0 detectors do not have a uniform rise time. The time resolution obtainable depends on the energy of the ),-ray being studied and on the size of the Ge(Li) detector. Wath 3 mm thick planar detectors, F W H M of 2.7 nsec has been obtained for 511 keV 7-rays 24). The half-slope on the edge of the curve can be as low as 1 nsec, although at ~ ~ peak height, there is usually a tail with a greater

slope. A simphfied block diagram of the present electronic arrangement is shown in

fig. 1. The ?-ray signal fi'om the Ge(Li) detector after amplification by a fast pream- plifier was split into one pulse for energy analysis and another for time analysis. After further amplification by a I nsec fast amplifier 25), the latter was used to trigger a fast discriminator to give a timing signal The threshold of the dascriminator was set slightly above the noise level. V, qth the planar detectors used in our experiments, this

Cd(~, xn) REACTIONS 83

trigger level corresponded to about 10 keV. For the large volume coaxial detectors, it corresponded to about 20 keV. The fast signal was used to generate the start signal for a time-to-height converter (EG & G Model TH200A). The stop signal was generated from the cyclotron r.f. oscillator using a fast discriminator and a suitable delay. For convenience, only every second pulse from the discriminator was used to generate a stop signal. This gives r~se to two prompt peaks with a time separation of the spacing of the beam bunches as the observed 7-rays could be associated with either beam bunch. The predicted time distribution is shown schematically in the reset to fig. 1.

' 2 - ~ ' ~ i ' d .... . . . . . . . . . ' l

,' t- q ~ : I ' I . . . . "o'o l ' ' A o,o,00 A i

Time pulse he~cjht

( 3 0 0 nsec F.S)

F~g. l . Slmphfied block dmgram of the electromc apparatus used m the present experiment. A schematic t ime distribution ~s also s h o ~ n

The output of the time-to-height converter was fed to a mixer amplifier, where a correction was applied to compensate approximately for the time "walk" with pulse-height associated with leading-edge timing. This correction was not critical as we were studying time distributions of individual y-ray peaks, and the "walk" only caused a shift in the position of "zero-t ime" for different energies. The energy signal from the Ge(Li) detector system and the time-signal u.ere analysed in a two- dimensional analyser usually with 256 energy channels and 16 time channels.

2 2. EXPERIMENTAL ARRANGEMENT

A schematic diagram of the experimental arrangement used with the cyclotron is shown in fig. 2 The analysed beam struck a thin target in a simple target chamber, and the transmitted beam was stopped in a well-shielded beam catcher. The ?,-ray

84 T. Y A M A Z A K I A N D O. T. E W A N

L[

$~TTFR~NG

DEF'NING ~mmm / Y : t , "

1

i

• t . - ~

1 LEAD SH,EL D

. . . . .

,02 ...... (" Irl ' ' ' ~

TARS£[ - _

COUNTER

SCATT-RING

ALU~ NUM w~co,~ h 2m m ih,ck

\L'~O SHI E~p_

DETAIL

BEAM TO HI GHINTENSrTy C~VE

ANAL~ SING SLITS_ i I 2mm~ 8mp

BEAM BENDING M%GNBT ~E AM H~r'ZQ'T%I ~'IDTI~ LI%~ITING COLLIMATOR

i 6m~ BEAM VERTICAL HE'GHT LIMITING COLLIMATOR

,6ram ' QLISP~ r':tE __ LLh5 • I , DCUBLET

STF~I_ S~IELnING

. .''/\ Po.~ ",CE ' 1

. BEA,~ ~LALY:'S MAGN_ET /

C~CLOTR^N MArNET t r G

m e ' e o I 2 3 ~ 3 E b~ ~ J

so,l,

F*g. 2. Schematic diagram of the experimental arrangement used at the Berkeley 2.2 m cyclotron.

Cd(~, xn) REACTIONS 85

spectrum was studied with a Ge(Li) detector surrounded by a thick lead collimator

to reduce the background from y-rays from sources other than the target. In the experiments described in this paper, either a 7 cm 2 z 10 mm thick planar detector or a 30 cc coaxial detector was used. The detector was usually placed at a distance of 12 cm from the target and at an angle of 126" to the beam direction in order to obtain 7-ray intensities free from angular &strlbutlon effects described by the P2 (cos 0)

term. The Cd targets used were prepared by depositing enriched isotopes of metallic Cd

on 800 izg/cm 2 thick mylar foil. The targets were typically 10-20 mg/cm 2 thick. With

these thicknesses bombarded with a beam of a few nA, the background counting rate from sources other than the target was less than a few per cent of the total cotmt-

mg rate from the target.

2.3. E X P E R I M E N T A L P R O C E D U R E

The y-ray spectra were initially surveyed either with 2048 channels for energy analysis and two time channels (prompt and ~ 50 nsec delayed) or with 1024 channels for energy analysis and four time channels (prompt, 25, 50 and 75 nsec delayed). These runs immediately revealed the existence of isomers although the half-li~es were not measured. The lower limit of observable half-lives in these runs was 7 nsec.

When delayed y-rays were observed in these survey scans, a more detailed time analysis was performed. The spectra were recorded with 256 channels for energy analysis and 16 channels for time analysis. The lower limit of observable half-lives in these runs was 5 nsec. The time spacing of the cyclotron beam bunches ( ~ 150 nsec) places an upper limit of 300 nsec on the measurable half-life. Longer lived

isomers are easily identified but their half-hves cannot be readily determined. The total counting rate in these runs was kept to about 3000 counts per sec and

the usual counting time was approximately 30 min for each run.

2 4 B A C K G R O U N D PEAKS IN T H E 7-RAY SPECTRA

In the y-ray spectra, there are some peaks which are due to the mylar backing of the targets, neutrons produced in the reaction and activity produced m the target Some of these peaks are present both in the prompt and delayed spectra. Some of these are identified in e.g. fig 4. The relative intensities depend on the bombarding

energy. When mylar backing is used, peaks are observed both m the prompt and delayed

spectra. Mylar contains hydrogen, carbon and oxygen. The peaks in the delayed spectra are due to reactions with 160. These arise from the 160(~, p)~°F, a60(c~, n)t9Ne and 160(c~, pn) 18F reactions. The transitions which appear in thc delayed spectra have energies of 184, 197, 238 and 939 keV. The locations of these transitions in the level schemes are shown on the right of fig. 3. The time spectra for the 197 and 238 keV y-rays are shown on the left of fig. 3 The half-hves measured in our experiment are consistent with previous determinations 26-2o). The measured half-hfe of the 184

and 939 keV transition in ~8F was ~ 130 nsec.

86 T. Y A M A Z A K I A N D G. T. E W A N

In all prompt spectra, a peak at 511 keV is observed. This is due to annihilation quanta emitted when high-energ~ 7-rays are absorbed by surrounding material by pair production. A peak at this energy is also usually present in the delayed spectra. This is due to the production of positon-emitting radioactive isotopes. For some heavy metallic targets such as Au and Pb used without backing material, the 511 keV y-rays have no delayed component.

I0

I0

2

o ~10

" - - -T ime (nsec} I00 50 0

' ' ' I ' ' ' ' ] ' ' ' ' I r

160 (,.,, p ) 19 F

1 9 7 ~ .

from 7 h i g h e n e r g y ?"

from ,8 +,~ l

, _~- ! ,_ ~ ~ I_• , , _ k _ 8 12 16

Channel number

160(,-, pn)18F

• 150 n s e c 5* % 1123

3 + - - ~ 939 939 E2

I + - - ~ - 0 18 f

160(a, p) 19F" 160(a,n) 19Ne

88 nsec 5•2+ ~ 238

I/2+- ~E20 1/2 + - ~ E 2 0

Fig. 3. Ttme distributions of low-energy delayed y-rays resulting from oxygen m mylar backing of targets. The relevant level schemes are shown on the right

The broad peak around 693 keV is characteristic of all v-ray spectra in nuclear reactions in which neutrons are produced. It is due to the (n, n ') excitation of the 691 keV 0 + excited state 3o) of 72Ge in the Ge(Li) crystal. This state de-excites to the ground state and produces internal conversion electrons which are fully absorbed in the crystal. The shape of this peak is not Gaussian but skew to the high-energy side due to a contribution from the recoil of the nucleus struck by high-energy neutrons As the half-life 31) of the 690 keV state of 72Ge is 422 nsec, this peak is also seen in the delayed spectrum.

3. Experimental results

The targets and energies of bombarding particles used in our experiments are given in table 1. For these energies, the predominant reactions are (c~, xn). The Q-values

Cd(a, xn) REACTIONS

TABLE 1 Targets and beams used

87

Target Beam Energy Predominant Residual (MeV) reactton nucleus

1 ~6Cd e 28 (e, 2n) 118Sn 40 (e, 3n) llTSn

114Cd ~ 28 (~, 2n) 116Sn 40 (~, 3n) 11SSn

1 t 2Cd o~ 28 (~, 2n) ll ,Sn ct 40 (~, 3n) x tZSn

'°Cd ct 28 (ct, 2n) 1'2Sn 40 (~, 3n) 1 1 lSn

l°aCd ~ 30 (~, 2n) xl°Sn 40 (~, 2n) + (~, 3n) x XOSn_i_ l OOSn 50 (~, 3n) l°gSn

~°6Cd ~. 30 (oq 2n) t°SSn cx 40 (~, 2n) + (~, 3n) ,OaSn + t O~Sn o~ 50 (0q 3n) 1°7Sn

HSln p 12, 14, 16 (p, 2n) HaSn ' laln p 12, 14, 16 (p, 2n) 112Sn

TABLE 2 Q-values (MeV) for Cd(g, xn) reactions

Target (g, n) (g, 2n) (g, 3n) (g, 4n) (g, 5n) (g, 6n)

ll6Cd 4.3 10.8 20.1 27.0 36.6 44.1 '14Cd 5.3 12.2 21.8 29 3 39.6 47 4 l'zCd 6.2 13.7 24.0 31 8 42.9 51.8 ll°Cd 7.7 15.4 26.5 35.4 46 6 55 9 l°8Cd 9.3 18.2 29 4 38 7 50.3 60.2 l°6Cd 11.1 20.4 32.1 41.9 54.1 64 5

for such reactions on the Cd isotopes used were estimated from the Myers-Swlatecki

mass table 32) and are given in table 2. For the heavier Cd isotopes, the a-particle

energies of 28, 40 and 50 MeV give nearly the max imum yields of the (a, 2n), (ct, 3n)

and (a, 4n) reactions, respectively. For the hghter isotopes, the Q-value is greater;

therefore the op t imum energy should be increased correspondingly. However, for

experimental convenience, we used energies of 30, 40 and 50 MeV which were below

the max imum of the yield curves. It was still easy to identify T-rays due to each reac-

t ion by compar ing their yield with various targets at different beam energies. In

co lumn 3 of table 1 the p redominan t reaction is gwen and in co lumn 4 the residual

nuclei with which p rominen t ),-rays are associated.

In addi t ion to (a, xn) reactions, other reactions such as (a, pxn) and (ct, ctxn)

could also occur. Excitat ion funct ions for various a- induced reactions in this mass region were studied by Fukush ima et aL 33,34) by means of the activation analysis on

8 8 T Y A M A Z A K I A N D G . T E W A N

1°rAg and l°9Ag targets. This work confirms the assignment of prominent 7-ray peaks to (e, xn) reactions but also shows that (c~, pxn) and (ct, c~xn) reactions are not negligible. It is difficult to identify y-rays arising from (~, pxn) reactions. The excitation curves for these reactions are similar to that for the (e, ( x + 1)n) reaction. In our experiments, they lead to In isotopes whose level schemes are largely unknown. However, these are odd nuclei in which a large number of weak 7-rays are usually observed rather than a few strong y-rays as is the case of even nuclei.

Since the first excited states of the Cd isotopes we used as targets are well known, it was possible to estimate the relative importance of (c~, e ' ) reactions from the yield of the y-ray from the first excited state to the ground state of the Cd target. For the heavier targets A = 110 to 116, the contribution from (ct, ct') reactions was negligible. For the lighter targets A = 108 and 106, evidence was observed for y-rays from (~, 7') reactions in both the prompt and delayed spectra. Such y-rays have a much smoother excitation curve than y-rays from (~, xn) reactions. They will be discussed individually when considering the observed spectra for these targets.

In the cases of 112Sn and ~4Sn, we also studied the y-ray spectra following the In(p, xn) reaction. This provided additional data and confirmed information obtained from the Cd(c~, xn) reaction. The targets and beam energies used in these studies are also listed in table 1.

3.1. THE 11aSn ISOTOPE

The decay of high-spin states in 1~8Sn has been studied by Bolotin et al. 14) in studies of the radioactive decay of 5.1 h l~SrnSb. They established the 7- , 5 - , 4 +, 2 ÷, 0 ÷ level sequence shown in fig. 5 and measured the half-lives of the 7- and 5- levels as 230_+ 10 and 20___3 nsec, respectively, lkegami and Udagawa ~ 5) confirmed these results.

The 7-ray spectra observed when 116Cd was bombarded by 28 MeV c~-particles and by 40 MeV or-particles are shown in fig. 4. The reaction at 28 MeV is predomi- nantly (cq 2n), while t}le reaction at 40 MeV includes some (c~, 3n) as well. In the delayed spectrum, peaks are observed at 1231.3, 1053.4, 477.9 and 254.3 keV. The time distributions of these y-rays are shown in fig. 5. The 254, 1053 and 1231 y-rays all have a long-lived component of ~ 230 -250 nsec. This lifetime is too long to permit accurate measurement in the present experiments. It is attributed to population of the 230 nsec 7- level reported from radioactive decay. The 40.7 keV y-ray also associated with this level was below the energy range covered in our experiment. The 1053.4 and 1231.3 keV ?-rays also showed a short component of ~ 20 nsec, which is ascribed to direct feeding of the 22 nsec 5- level.

The 477.9 keV y-ray was observed to have a long-lived component of > 200 nsec as well as a prompt component. This probably is due to a level of spin _>_- 8. A lower spin would permit alternative decay modes and hence a shorter lifetime. The higher spin assignment is also compatible with the relative yield of this y-ray as a function of bombarding energy. The fact that the 477.9 keV ),-ray has a prompt component

Cd(ct, xn) REACTIONS 89

shows that the isomeric transition must precede the 477.9 keV transition and that there is some prompt feeding of this transition as well. The isomeric transition may have too low an energy to permit observation in the present experiments ( < 100 keY).

i05 F - - I • I T r i T [ T - -

IIF IIBsn 116Cd ac 28xMeV c t 7cc Ge(L i )

' ~ ' ~ ~ (3) ,ev ["~t ", 184.4 I t 5It

I [I PROMPT 694 lies n ilSs n

i0~ ,wl,[ ~ ~ ' ~ ' ~ - W ''@, ,', rtl

' i l~ ~l _7_ . . . . . . . . £L c, ,a,

I0 0 I00 200 300 400 500 600 700 800 900 I000 I I00 1200

104

(.q i -

103 o o

C H A N N E L NUMBERS

105~ ' l l6cd + 4 0 M e V cl 3 0 c c G e ( b ) , i 9 F , - - ~ - - - ~

,,,.3 b) mF t84.4 IgNe 51tO

15910 i 2~ t8 .5 PROMPT ~ I

1 0 4 ! ~ i ~ ' ~2~54 3 13i9.Z / 4779 ' 'iTSn ilaSn 148 B [ I 8S~ I , 118- t

:>n = 18 F i i ? s n

776.9 t053.4 1278.9

:~ !,.jj , i. DELAYED " "~-" --.'.....-Jr 'v.¢ 0 ~'/ "-, ,'r ~.." I ! - -80 . . . . . 794 ~ ,7 - ' ~ ' ~ " ~ " ~ .'-.~-.4~

<-> - <-,W',. / ' i "t . r 'Pe~r~,4. t ~l 815 "~'~'+'~" ~ ~" 0, ~ ~,' ' ~ 11 a

IOZ I" ,,Wr,-w,,, h ~ . . ~ ,

1 I OlD 200 700 800 900 I000 3 0 0 4 0 0 5 0 0 6 0 0

C H A N N E L N U M B E R S

Fig. 4. Prompt and d e l a y e d T - r a y spectra observed in a) 116Cd@28 MeV ~ react ion and b) a 16Cd-~-40 MeV :~-reaction.

In the level scheme shown in fig. 5, the 478 keV transition is tentatively assigned as coming from an 8 + level at 3058 keV and the isomeric transition as arising from a 10 + level at slightly higher energy. The reasons for these tentative assignments are discussed later, when the systematic~ of levels in even Sn isotopes are considered.

3.2. THE ll6Srl ISOTOPE

Levels in a16Sn have been studied 16,17) from the radioactive decay of both l~6In and 116Sb. The electron capture decay of 60 min 116Sb populates a 7- level at 2913 keV and lower levels as shown in fig. 7. The spin assignments to these levels are based on 7-7 angular correlation measurements and K-conversion coefficients of

90 T. Y A M A Z A K I A N D G T £W'AN

v-rays between the levels. A long-lived 5 - level was observed at 2368 keV. The half- life of this level was first 16) measured as 200+20 nsec but a later 17) measurement gives a value of 350+20 nsec.

-.---Time (nsecl IO0 50 0

~ 116Cd (a, 2n) 118Sn Ect = 28 MeV 5~_ 254 ,eV

=250 nsec ~ ~,

A I0~-

i 5 k 1055÷1231 keV ~ ~'

2 ~ ~230 nsec ,.. ~, =20 nsec ,,'" 2+ 1231 !

, /

t 1231 >200 nsec E2 ~- 478 keY 1 1

I 0 I I I * I I ~ , I ~ , 118- 50bn68 4 8 12 16

Channe l number Fig. 5. Time dmtrtbutlon curves of l,SSn ~-rays and a proposed level scheme of ' tSSn.

missing (10+) . / >200 nsec

(8+) ~ 3058 478

7- 2~ 4~ 2580 230 nsec

$

5- ~ t , ~ 2326 22 nsec

4 + - - I ~1"~-12285 1053 E2

The ),-ray spectra observed when 114Cd was bombarded by 28 MeV and by 40 MeV a-particles are shown in fig. 6. Many v-rays are observed in the delayed spectrum. The time distributions of some of these v-rays are shown in fig. 7. They all show a long-lived component ( > 200 nsec), which is too long for accurate measurement in the present experiments. Chang, Hageman and Yamazaki 19) determined the half-life of th~s component to be 0.75 psec. The 100, 975, 1075 and 1293 keV v-rays are consistent with the established decay of the 350 nsec 5- level at 2368 keV. The 135, 408 and 544 keV v-rays have previously been established as decay from the 7- level at 2913 keV, whose lifetime 27) is less than 0.2 nsec. The long-lived components must be due to feeding of this level from an isomeric state at higher energy. The 117 keV, 319 keV and 584 keV v-rays were observed to be delayed, and the 319 and 584 keV transitions have a prompt component, thus indicating that they do not proceed directly from the isomeric state. Because of the observed intensity balance in the delayed spectrum, that is,

I~(319) ~ Ir(135)+I~.(544),

the 319 keV transition is readily placed on the 7- level. The 117 keV 7-ray could be

C d ( a b xn ) R E A C T I O N S 91

Z•IO 3

o

the transition from the isomeric state, but this is questionable as no such y-ray is observed at E= = 20 MeV (see the detailed study 19) of the lt6Sn y-rays). For this reason, the place for the isomeric transition is left open in fig. 7. The 319 keV transi-

tion is assigned as E l + M 2 in ref. ~9).

I ~ ~ 97.1.I.F3 114C(:1 + 2 8 M e V a 7CC Ge(L , ) ) l , , . . . ,s° a ~ 1844 2 "~1 5 319,2 l i l le . 116Srl |

~jI 1' F" .,,~,4- ~n.o 5e4o i0 4 [ ~ IL I * ~ . v { 116Sl 1 I il6sn 116Sh

I' J~ 117 ~-.~.~_ i I IlIsIM 1293.2

!, "tzGe ~-~,--~-~ ~_ , .~ ~ ~ ,! ~-w 694 DELAYED ~. .%~,.~.~,~ .~.~,~ r " ~ ' ~ jl

' ~,~ 1 / ~50 nile<: ~ 1075,0 " " ~ ' - +" . . . . . ~'"-',.,,.'/V / ,~t ' ~, 1 I l

f,/,+ , ~ , !1


105E - - - ~ 114Cd + 4 0 M e V e 30CC G e ( L I ) b) 115Sn 114,8 llS,Sn

157.1 11IS n 497 .6 1tSSn "SSn ~ 173.5 319.2 ' "SSn 97,5'0 n f 1 - ~ i0011 , 18 F 1504.6 1311.0

I ' ~j," 184.4 ; ' l l / 5 i 1 ° PROMPT 9~311~2 ~9780 293.2 Z 115Si1 |i

115Sn '~, ~ l l " ~ ' ~ - - " " -~ " ~ I 1380.0

- 'dl'J tJ ~"+ 3 * . . t , . , l J 1694 ,,6S n x" . , I 9 7 5 , 0

~ L • N I1[ i DELAYED I 116Sn ' "~ v.~"~" ' , ~ ~ =. 80 nsec , 1075.0

102 ',,~' ". i~ ~' " ~' J

i I' I ? #"tl"k" J"; "P t"4t:'ll'(' ! la l , . , , i ..... ,

I 00 I (~0 ~200 . . . . 560 400 500 600 700 800 900 I000 C H A N N E L N U M B E R S

Fig. 6. Prompt and delayedT-ray spectra observed in a) =~4Cd+28 MeV c~ reaction and b) t ~4Cd-t 40 MeV c~-reaction.

I04~ \

z ~ 10 3

o <..)

t

1 1200

In the prompt spectrum, a 1099 keV 7-ray was observed. This was placed between the 4 + and 2 + states in accordance with results from the radioactive decay of 116in" The population of the 4 + state in the 1 ~¢Cd (cq 2n) 1 ~ 6Sn reaction is much lower than that observed in the other nuclei. This is because higher-spin states decay to the 5- state, which lies below the 4 + state in ]~6Sn. For the same reason, the transition from the 3- to the 2 + state was only observed in ll6Sn

3 3. T H E ]l*Sn ISOTOPE

Only the first excited level at 1300 keV was previously established in 1~4Sn from studies 3s) of the radioactive decay of l'4Sb and '14In. This level was also observed in Coulomb excitation 35).

92 T Y A M A Z A K I A N D G. T EX, VAN

The 7-ray spectra observed when 112Cd is bombarded by 28 MeV and by 40 MeV 2-particles are shown in fig. 8. In the delayed spectra four prominent 7-rays are observed with energies 273, 628, 890 and 1299 keV. They all have equal intensities in the delayed spectra. The time distributions are shown in fig. 9. All of these 7-rays have prompt components as well as long-lived components. Their order in the level scheme was determined from the intensities of the prompt components. A 120 keV 7-ray was also observed in the delayed spectrum. No prompt component was observed, and it is assigned to a transition from the isomeric level. The half-life of the isomeric state is long ( ~ 300 nsec).

I0

~ 5

Time (nsec) I00 50 0

1 0 - - 7 - - , , , I ' ' ~ ' I ' ~ ' ' 1 1 ~ Y

5 H4Cd(a ,2n) 116Sn E a = 28 MeV -t

/

NSO0 n s e c ~ -~ 1293 keY ,~

- - 1075 keV ~ ,

976 keY ~'

2 4 * 4 4 + + * * + 4 * 4 * t I00 keY +

I0 i

5 ++++++~++++ /Similar to 155 \

! 319 keV ~408, 544, 584 keV)

IoL~L I I Z 1 J ~ I

4 8 12 Channel number

,.,,,,/--MISSING (10+) 075/~$ec. (8+) .--J;I +M2 3232

~,9 / '~ '29,3 7 - t

E2 408 ~- 1 2368 3so . . c .

/,00 ,099 I

975 El i

2+ - - ~ - - 1293

1293

l I 0-~ 0

16 116 Sn

5 0 6 6

Fig. 7. Time d~str~butlon curves of lZ6Sn ;v-rays and a proposed level scheme of 116Sn"

We have assigned 4 + to the second excited state. The other possible candidate, e.g the 3 - level, is known to be located at about 2280 keV as shown in inelastic scattering experiments 36). In the delayed spectrum, there are two weak 7-rays of 543 keV and 975 keV of equal intensity. The latter is assigned to a transition between the 3 - and the 2 + levels and the former to a transition between the 2817 keV and the 2274 keV 3- levels. The observation of the transitions from the 2817 keV state to both the 3- and 4 + states leads to the assignment of 5 - to this state.

Because very little had previously been established about the level scheme of x ~ ~Sn, we also studied the 7-ray spectrum observed in the laSIn(p, 2n)lJ~Sn reaction at Ep = 12, 14 and 16 MeV. The y-ray spectrum without time analysis is shown in

C,I(:(, xn ) REACTIONS 93

102~

t<i I0 - - .L _ _ _ O IOg

fig. 10. The ?-rays assigned to *l+Sn from the (~(, 2n) reaction are all observed in this spectrum

The population of a level of given spin depends on the angular momentum transfer in the reacUon. The relative yield of ?-rays is thus an indmation of the spins of the levels. These yields are more sensinve in (p, 2n) reacUons than in (~, 2n) reactions because of the lower exmtation energy (2-5 MeV) and the lower angular momentum

[ 19F II2cd + 28MeV Cl 7ccGe(Ll) _ _ ~ 1973 114Sn 272 8 O ) L I I 184ai 19Ne [ 114Sn 3Zl 8 114~n 104[ ._ i, 2~5 5110 PROMPT 628 4 r' - ~ I [ // 114Sn 1145 n ' " ~ ~ : i 8s9 6 120 ~ J k. ' , 1 2 9 9 , 4

" ~ i ,

ZOO 500 400 500 600 700 CHANNEL NUMBERS

1 O 5 - - T - - - - - ~ r I r • -- - -

l l S c d + 4 0 M e V C~ 50CC G e ( b ) 18F il4Sn

I 3 2 8 ll3Sn 104 !1976 27~.8 511 0 901.3

- ~ 1 9 N e l l i I ( 1 4 S i , ~ I I ' i 238J~1 'I i 6284 I PROMPT 114Snl IBF

t'-z 1031 l,'%~ ~' I 'l '

0

~) I0 L ' II 1021

I I 3 S n

1170 0

b)

t 1 4 S n

- ~ ] 12994 i13 St,

"',.le ! 172Ge ., "~.,'-, 2

, 694 ~' " -' **a*l .

,<.,, ~ , I DELAYED

, r ~ ' 1 ~ ~ 8 0 n s e c J

I

O IOO 260 .380 400 500 600 r 0 a 8oo soo ~oo0 C H A N N E L N U M B E R S

F t g 8 . P r o m p t a n d d e l a y e d y l r a y s p e c t r a o b s e r v e d m a ) ~ 2 C d T 2 8 M e V c< r e a c t m n a n d b ) ~ ~ 2 C d - - 4 0

M e V ~ - r e a c t l o n .

transfer 37). The variation of y-ray intensities with energy of bombardment is shown in fig. 11. Gamma rays associated with higher-spin states show a more rapid increase with energy than the 1299 keV 2 + ~ 0 + transition. The results are consistent with the spin assignments given in fig. 9. The relatively higher intensity of the 975 keV transition indicates that the 3- level is more highly populated in the (p, 2n) reaction. The relative intensities of the 543 and 628 keV transitions are independent of bombarding energy and are the same in the (p, 2n) as in the (cq 2n) reaction. This confirms the proposed assignment to ?-rays from the same level at 2817 keV.

94 T. Y A M A Z A K I A N D G . T . E W A N

I0

5

2

I0

5

2

~_1o 7

(..)

2

I0

Time (nsec) 150 I00 50 0

IOv I ' ' ' ' I ~ ' ' ' - I ' ~ ~ ' I ~ 112Cd (u ,2n) 114Sn

5 i L Ea = 28 MeV

I

I0

~ 5f ' 2 9 9 keVo

/

2 [-~ 890 keY

5i- ÷ ; ' 4 ~-4 f ~' 4 4 ~ 4

[ 273 keY e 2 L e * .,, . I . . e e

4 16 8 12 Channel number

.3210 >300 nsec ~120 --5090

273 ~ ' - ~ ' I 2 s , ?

543 628

t L:-22 , 3--- 2 / 2189 4 . . - -

975 l , r o 2+ 1299

1299 E2

O+ 1 - - 0

114Sn 50 64

Fig. 9. Time d~stnbution curves of x~aSn ?-rays and a proposed level scheme of a~aSn.

! I i 4 ~ i I J ~ I I

VlSIn (p, 2n ) U4sn " ' sn 7cc Ge ( M )

I ~ 0 115Srl 114S n 115 2 7 5

, / 1 ' " S n r ] ~J~Sn ~ 8 H4Sn

~ ]~ ! 4 9 8 , 8 9 0 L'4Sn

[ ~ - - . J ~ /"so * 1 "'Sn ,2,,

~, --~ ' Ep = 16 MeV /

2 Ep = 14 MeV ' ~ ,,~ ,.,~ } ~

lO . . . . . . . . ~ f t ~

I I I _ _ _ _ 1 ~ J _ _ ~ I I I I lO0 200 300 400 500 600 700 800 900 I00,

CHANNEL NdMBERS

]: lg. lO. Gamma- ray spectra observed ~hen ~L~ln ~as bombarded w i th 14 and 16 M e V protons. T h e m a m r e a c t ] o n l s ~S[n (p, 2n) "aSh.

Cd(zt, xn) REACTIONS 95

3 4, THE i~2Sn ISOTOPE

Only the first excited state was previously known 35) from studies of radioactive decay of 112Sb and from Coulomb excitation experiments. Betigeri and Morinaga 18) observed the 2 + ~ 0 ÷ y-ray in the l l2Cd(3He, 3n)112Sn reaction.

>- BE

n," l--

rr" ,:£

>..

H

IO

IO

- - - ' ('a) ~ - -

-li~Tn (p,2n)U4Sn

~ 6 2 ~273

J / X

- j<; j

5

I0

~%../ 9 7 5

(b) 4

"~In(p,2n)"ZSn i 1

4 I

I i

1 q

9 9 5 I

i 0 0 0 0 0 |

1299 1258 q

(NORMALIZED) (NORMALIZED) 1 I 1 ~ I I

12 14 16 14 16

Ep (MeV) Ep(MeV)

Fig. 11. R e l a t i v e e x c i t a t i o n f u n c t i o n s o f v - r a y s o b s e r v e d w h e n 115in a n d a ~ aln are b o m b a r d e d w i t h p r o t o n s . T h e v - r a y ln tens i tms g ]ven at e a c h b o m b a r d m e n t e n e r g y are re la t ive to that o f the 2 + -+ 0 +

v - r a y in tens i ty at that energy .

The y-ray spectra observed when ~l°Cd was bombarded by 28 MeV and by 40 MeV c~-particles are shown in fig. 12. The delayed spectrum revealed three v-rays of 302, 993 and 1258 keV of equal intensity. The time distributions of these v-rays are shown in fig. 13. All these y-rays have a half-life of 13 .6+0.6 nsec. The 1258 keV y-ray is the most intense in the prompt spectrum and is assigned as the 2 + --* 0 + transition in the level scheme in fig 13. The energy agrees with the previously known value. The 993 keV 7-ray is the next most intense in the prompt spectrum and is placed above the 1258 keV v-ray. The 302 keV 7-ray is apparently the isomeric transition. The half-life of 14 nsec is close to the Weisskopf estimate for E2. We ha~e tentatively assigned 4 + and 6 + to the 2251 and 2552 keV states, respectively. This assignment is partially based on the theoretical consideration of possible energy levels in this nucleus. The 806.6 keV ,/-ray observed in the prompt spectrum is placed

9 6 T . Y A M A Z A K I A N D G T . E W A N

above the isomeric state. The relative intensity of this 7-ray increased as the bombarding energy was increased to 40 MeV, thus indicating that it is probably a transition associated with a high-spin state at high energy.

: , ° 1 . 1 2 ~ r " " 104E - , ~ , , ._~ l J I PROMPT ] 113Sn H2S'1 112Sn

~ - ~ 1 ' 93 ;2 12577

103F r/""~-,,.,,..A ' DELAYED . . . . " ..... ~ - - ~ - , ~ , + , ~ ,' ~ ~ , = 5 0 nsec ~,~ " ~ " - ' , . < ~ ' - ~ . ~

I 0 1 . ,il , , _ _ , _ , . . . . . . ,I '@lh, . ~ i , l = , ' . . . . . 0 I(30 200 500 4 0 0 500 600 700 800 9 0 0 I000 I I 00 1200

CHANNEL NUMBERS

1 0 5 F . . . . q ~ ~ , - - - r - - ~ . . . . , , - - - - , ,

L 2385 IlOcd + 40MeV 30cc Ge(LI) k ~ i l- 19F I U/ I- 197.3 112Sn

18 F 301.9 112$n IIISn 112Sn 104~- ,84.4[ Iz~,~, 5 0 ,,, 806.6t " - ' s l ' L ~ ,,, ,,, ,,67.5

! PRo.P. . . . . , 3 , . 2 ,,.so z / ~ 1 ~ , ~ - ~ ~ ' ~ ~ - - . . . c . ' '< ~ II , ~ o ~ c , 1347.5

t ~ ~l I t 694 I ~* ~ DELAYED = r ~'"'~ I ,=80nsec 1 =

102b %"~ °"S~','P'~ '1 . <~ i'

[ " ' ' ! I , ~ , , i 1

o ,oo zoo soo 4oo soo ~oo 7oo Boo 900 ,ooo C H A N N E L NUMBERS

Fig. 12. Prompt a n d d e l a y e d 7 , - ray spec t ra o b s e r v e d m a) zZ°Cd-l-28 M c V = - r e a c t i o n a n d b) H ° C d q - 4 0 M e V ~ - r e a c t l o n .

The 7-rays of ~ 12Sn were also studied by the 113in(p ' 2n)X X2Sn reaction. The y-ray spectrum without time analysis is shown in fig. 14. The intensities of the observed transitions as a function of Ep shown in fig. 11 confirm the proposed order in the level scheme.

3 . 5 . T H E l l ° S n I S O T O P E

Nothing was known previously about the excited states of l~°Sn. In fig. 15, we show the y-ray spectra observed when ~°aCd was bombarded by 30 MeV, 40 MeV and 50 MeV c~-partlcles. Because of the higher Q-values, the (cq 2n) peaks appear with almost equal intensity in the spectra taken with 30 MeV and 40 MeV c~-particles

Cd(=, xn) REACTIONS 97

I0

JO 0

~ 2 g

g' io~- F

L

I0

-,,=--- Time (nsec) I00 50 0

' ' i ' ' ' ' t ,--r--r~-[~

I lOcd ((:Z, 2n) 112Sn

E,-, = 28 MeV

12625+_80 ; e V s e c /

993 keY T / ~

,ev / k / /

/ j ; !

8 12 Channel Number

I 16

807

(6+) 1 3O2

(4+) T

993

2+

1258

O+ 0 112 50Sn62

3359

2552

2251

1258

1 4 n s e c

Fig. 13. T ime dis t r ibut ion curves o f t t2Sn 7-rays and a proposed level scheme o f l t2Sn.

10 . . . . . ] ~ I I I i I I - - - m l I

5 IBIn (p, 2n) UZSn 7cc Ge(L,)

2 " Z S n

I0: - 302

' "Zsn 511 112 5 ~'L~ ~ " ~ - " } J n .377 .1 I,:SS~ 74G e 9i3Sn "2Sn1258

L-~ [~ 66L 694 112Sn ] 2 ~,,~. \~ ~ 807 t k- i I

"Alk . ,k~.~Lt, L \,

Ep= 14 MeV

I0 0 I/O0 I I I r _____ t i t 200 500 400 500 600 7(X:) 800 900 I000

CHANNEL NUMBERS

Fig. 14. G a m m a - r a y spectra observed when " 3 I n was bombarded with 14 and 16 MeV protons . The mare react ion is " 3 I n (p, 2n) '~2Sn.

98 T. YAMAZAKT A N D G. T. E W A N

1 0 5 ~ -

I I 0 4 ~

oO iI 102

i ] - r l - - T i

IgF 108Cd + 3 0 M e V a 3 0 c c Ge(L=) ,97.~ a )

18F l lO ( :n 1844 OSn I 281

60.o I 19273,[ 5 1 1 . 0 t l O s n i l O s n

1 ~51! 1 9. . 2 ,2,,5 1 k l ~ 632.8 PROMPT 1 IIISn 1 _~'~o'.~-, ~ . ~ 1 [ ,~ 7 1 0 8 5 3,

2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0


i0 L . . . . ~r Jl., ~, . . . . I.. t 0 I O0 9 0 0 I000

10 5-~ r ~ ~ - - ~ ~ ~ • , ~ ~ 1°8Cd + 4 0 M e V (1 3 0 c c G e ( L I ) 18 F - .

8Sr1184'4 I OSn ~ li,,/ I

4 16801 ~723~ I llOsn IO ~ \ I ~ RI 511.0 986 2

~ '~ ~ [ ,sF I ,,os. : : .~ ~ ,~ . ?l ,~ PROMPT 939.2 1211.5

o f ~ Jl_ ~ - " ~ - v , ~ . ~ . -

0 I 0 0 2 0 0 :300 4 0 0 5 0 0 6 0 0 7 0 0 BOO 9 0 0 ,000 C H A N N E L N U M B E R S

1 0 5 : - - 7

~8 F 1 0 4 : 184.4

o3 i - z D 103 O ¢_)

. . . . - l c)' I 108Cd + 5 0 M e V a 3 0 c c G e ( L I )

IOgsn 109Sn 5 1 1 . 0 6 3 4 2

! 3 3 6 . 9 109 In - 18 i 0 9 S r l I° '9 S n ' i I ,~-..,..-, PROMP. ,3~p ~ 1084.2 124~4~6".6 I~ '

- ~ - ~ _ l I F ,2,~31'257,° ,..260

:',} ~ ~ 69,. I ~ . . . . . i DELAYED

r~'~"t'6-~'~,4 • ,. -- II i~ = 70 n=ec •

I 0 0 I 00 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 I 0 0 0


F i g . 15. P r o m p t a n d d e ] a y e d 7-ray s p e c t r a o b s e r v e d m a) 1 ° 8 C d + 3 0 M e V ~ - r c a c t ] o n , b ) l ° S C d ~ 4 0 M e V ~ - r e a c t ~ o n a n d c ) 1 ° 8 C d + 5 0 M e V ~ - r e a c t l o n .

Cd(~t, xn) REACTIONS 99

but have almost disappeared in the spectrum taken with 50 MeV s-particles. All these spectra were taken with a large-volume coaxial detector. This had poorer timing properties than the planar detector particularly for low-energy v-rays. As a result, the delayed spectra shows small peaks which appear to decay rapidly: this can be attributed to poor time resolution. The most prominent prompt v-rays belonging to l~°Sn are 1212 keV, 987 keV and 281 keV in order of intensity. These y-rays are placed in a level scheme shown m fig. 16b. The 1212 keV and 987 keV v-rays show no delayed component. The 281 keV v-ray seems to have a delayed component but this may be due to the timing properties of the detector. As we have not measured the half-life of each v-ray, further studies are required. The half-life of the 281 keV v-ray is certainly considerably less than that of the 238 keV v-ray from 19Ne observed in this spectrum. A limit on the half-life is less than 7 nsec.

( 6 + ) ~ 2 3 6 5 <7 nsec

( 4 + ) ~ 2 l l l

906

2+ 1205

1205

O+ 0

IO8sn 50 58

(6+) -2480 "¢7 nsec

t 281 (4+) - - 2199

987

2 + - ~ 1 2 1 2

1212

O+ 1 - -0

IIOsn 50 60

Fig. 16 Proposed level schemes of l°SSn and 11°Sn

Weak delayed v-rays of 633, 872 and 1098 keV are observed in fig. 15a. These v-rays may be supposed to arise from 1°8Cd levels through inelastic excitation for the following reasons:

(i) These v-rays cannot belong to 11°Sn because of the lack of such a delayed component in the lowest 2 + --* 0 + transition.

(ii) They were not seen in the 11 oCd + 40 MeV s-spectrum, therefore the possibility that they belong to laXgn is excluded.

(iii) The first two energies are in excellent agreement with the known energies 633 keV and 872 keV of the 2 + --* 0 + and 4 + ~ 2 + level spacings in 1°8Cd, respectively 35). The half-lives of these y-rays were not measured, but a rough estimate gives ~ 100 nsec.

100 T. YAMAZAKI AND G T EWAN

i i r i - - - - - r i - T . . . . = ' l

'°°i ,=, o) ISNe" 106Cd + 5 0 M e V a 50cc Ge(L I )

L mF =Oesn 4 ~ 197.3 1~,3.7

I0 ~- ISF ~[:~Sn 109Sn 51 .0 . . . . zoe~ n .~. 184.4[ 1273.3 396 2 1 ~,',',',',',',',','~..T PROMPT 90~8 'UUSn "

1 ~ 1 " " 1206.0 IOeSn

/ A , ~ , " ~ ~ , , - ~ , . ~ - ~ k ~ - ~ . ~ I ~ I " I~L"I. ~ ' ° H ~ = n : z 103r ~ , " ~ .. l ~. 11257,0 I(~Sn = ~

o [~1? " ",d J II Jl ' i / ~ ""~ ~ . . . . . S [ / -u~,,~. j[ l ,. 6 ~ DELAYED .. . . ]

IO L . . . . . . . '" 'I'lP", " l V l ~ ! ~ j , ~ ; l ~ , . l l l h . , . ! 0 I00 200 300 4 0 0 500 6 0 0 700 800 900 I000


1 0 5 ~ - - , , ~ , , , , , ~ , --~ 106Cd + 40MeV u 30cc Ge(L~)

, . , = . . ,o.~ b ) ^4 I ~"F 1 09 4~9.~ u 11~''4 l - - ;~ loss, I 5,!.o .Oes, t

] I ~i '~'~ 39~1 "2 l 6:327 PROMPT 90p8 ,OIISn ,08Sn = . J ~ l ~ ~JI ~J~ • ~ i . ~ ' le F 1120.8 1 2 0 6 . 0 107Sn

I00 I00 200 300 400 500 600 700 800 900 I000 C H A N N E L NUMBERS

105F - -

I- z

103 o i l (.)

i

102

I . . . . . . - - - r - ' C ) ' ~ I I I F 1 ( ) 6 C d + 50 MeV a 30 cc Ge ( L t ) ,s44

459.3 4 1 4 ~ IO?Sn I I i o,o 395.4 633.9 1 0 0 3 . 9

= , PROMP~ 91% :1~Io t l

I0 0 - I1~0 2 0 0 3 0 0 4 0 0 500 6 0 0 7 0 0 8 0 0 9 0 0 I000 C H A N N E L NUMBERS

Fig. l 7 P I o m p t and delayed 7- ray spect ra observed in a) 106Cd_lt_30 MeV ~-reactlon, b) 1 °6Cd + 4 0 MeV c(-reactlon and c) 1°6Cd-}-50 MeV e-react ion.

Cd(~t, xn) REACTIONS l 01

The two delayed peaks at 675 and 1426 keV in fig. 15c were attributed to the 0.20 sec isomer 35, 3s) of 109in ' The ?-ray energies agree with the previously known values. This isomeric state is formed in the 1°8Cd(~, p2n)a°9In reaction and is believed to receive a considerable part of ?-decay flow. On the other hand, the 7-decay flow in 1°9Sn is split between many paths as shown by a number of peaks in fig 15c. The present identification indicates that the (c~, p2n) yield is relativel) small compared wath the (~, 3n) yield even at 50 MeV incident energy. This confirms our assignment

of prominent peaks to the (c~, 2n) product at 30 MeV incident energy.

3 6. THE l°SSn ISOTOPE

There is no previous information on any level of lOaSn" The ~-ray spectra observed when 1°6Cd was bombarded with 30 MeV, 40 MeV and 50 MeV a-particles are

shown in fig. 17. The most intense 1205 keV and 906 keV peaks were identified as belonging to l°SSn These 7-rays have no delayed component. The 253 7 keV y-ray is the next most intense ?-ray. These three y-rays are placed in a level scheme shown in fig. 16a.

Two delayed y-rays of 633 and 875 keV might be attributed to ~ °6Cd for similar reasons to those given in subsect. 3.5.

3.7 ODD Sn NUCLEI

Prompt and delayed y-rays belonging to odd Sn nuclei were observed in the spectra at E~ = 40 MeV. However, the construction of level schemes is very difficult because of many possible y-decay branches. In fig. 18, we show tentative level schemes indicating the presence of isomeric states in 1 aVSn ' 1 lSSn and 113Sn

The first four levels in 1178n are previously known. However none of the observed ),-rays m our delayed spectra corresponds to transitions between these levels. The 777 keV, 1310 keV and 1279 keV ),-rays are equally intense in the delayed spectra, but the prompt antensines increase in the above order These transitions are considered to be in cascade as shown in the figure. They are placed above the -12 ~- level, as no evidence

3 + 1 + transitions. was observed for the z ~ 2 The 159 l~sec isomeric state of l lSSn was identified at 726 keV by lvanov et al. 39)

with the 115in(p ' n)11SSn reaction From a detailed coincidence study, they concluded

that the 117 keV peak consists of two transitions at 107 keV (M2) and 120 keV (E2) m cascade. The total conversion coefficients they determined were 5.4 and 0.78, respectively. The y-ray intensity ratio is 1: 4. In the delayed spectrum in fig. 6b, the 114 8 keV and 497.6 keV y-rays originating from this isomer are observed, but the doublet peak is not resolved Therefore, it is assumed in the present level scheme that both y-rays have almost the same energy of 115 keV. Recently Ivanov and Magda (ref. 40)) determined the half-hfe of the 619 keV state to be 3 2 6 + 0 0 8 /~sec. The (d, p) experiment by Schneid, Prakash and Cohen 41) revealed the presence of the

7+ ~ - - ½+, ~+, z and one-quasiparticle states of energies as indicated in fig 18. In fig 8b, a new delayed y-ray of 661.2 keV is assigned to 113Sn This energy is in

102 T. YAMAZAKI AND G. T. EWAN

good agreement with the spacing between the 11- and {+ levels revealed in the (d, p) experiment 4t). This would be an M2 transition similar to the 159 itsec M2 transition in 115Sn. The half-life was not measured. Allowing for the difference in energy, it would be m 20 nsec if similar to the 159 psec isomer state in 1~5Sn. Another delayed peak at 312 keV is also seen in fig. 8b, but as it is weak, its assignment to l~asn is suspect. There is no delayed v-ray identified in odd nuclei lighter than ~ ~3Sn.

3/2+ 1030

\ %

P

(23/2+) / 3683

I

2906 i I

1310

i 1596 I

? 4, (7/2+) 840 159p.s 7 3 0 . . . / 1 1

11/2- f 740 .~ ,l/2- / 726 7/2+ l % - _ _ 115'M2 ' ~s" l % I'/Z+ f 6191( I I

.;3 ,.%3,2+ (M2) i i / 3 3p.s \ \ \% I 14d

/ l , , l l /2- ~ ' 317 , I I I 498 \ \ • \ ] 20m / I x3/2+ 2~Od

7/2+ V "(911 / 15S ' ' ~ ' x l I / 2 - - 89 I/2+ 0 I/2+ y 0 I/2+ 0 " , .3/2+ 24

I /2+ 0 113Sn 115Sn I I?sn 119SR

F~g 18 The isomer systemat=cs of odd Sn isotopes. The present s tudy has revealed a h=ghly excited isomer of t17Sn and a 740 keV -~ - isomer of 113Sn.

4. Discussions of the level and isomer systematics

The Sn isotopes are single-closed-shell nuclei in which 50 protons constitute an inert closed shell. The properUes of such nuclei have been studied extensively with the microscopic theories of nuclear structure since the first BCS calculation of Kiss- linger and Sorensen 42). Arvieu and co-workers 43.44) performed detailed calculations using the pairing scheme and a Gaussian-type residual interaction. Plastino, Arvieu and Moszkowski 45) used a surface delta interaction (SD[). These calculations involve only excitation up to two-quasiparticle (seniority 2) and are termed the quasiparticle Tamm-Dancoff method (QTD). Recently Ottaviani et al. performed calculations including four-quasiparticle (semority 4) excitation with short-range residual interactions 46) and also with Gaussian and Q-Q interactions 47). More recently,


G m i t r o e t al. 48) per fo rmed similar calculat ions with realist ic forces. They showed

tha t it is i m p o r t a n t to project ou t spur ious states due to the number non-conserva-

t ion in t roduced by the Bogol iubov t r ans fo rmat ion , and that the 0 + and 4 + states

involve apprec iab le four-quas ipar t ic le components . These calculat ions are called

the quasipar t ic le second T a m m - D a n c o f f me thod (QSTD) .

(h q,z) 2 \ /(ds/z) 2 \ /

\ I I(g~'/z )2 \ 1 1 V / / />0 5ps

/fO.5~s \ >0 5/zs / / 3232

/ / / ~ 7 ~ . . . 3058 ~ / - x "x. U-g-O S / / / 5 _ _ _ 6 _ 2 9 1 3 \ "~ / /.t. / . / 1- ~ 2817~ 2777 x , ~ , ~ z . / ~ 20ns / l i k e s

< 7 o s J ~ - - z - g ' ~ ' , - , ~ t - , . ~ . . . . " / - o . , ~ - ' i - ~ - - Co+ , " ~,-+e,u 2274 . ~ " ~ - . 2 - ~ 2 ~ " ~ ' " / " ' ' °

4 ~ 2L89 2180 2150-2100

2+ 1 2 0 5 - - ] " ~ - - - I - ' - ~ " 8 - " 1299 1293 ~1231 ~ 1171 ~-1140 t151

O * m 0 0 0 0 0 0 0 0 0

108 I10 112 114 ll6 118 120 122 124 50Sn58 50Sn60 50Sn62 50Sn64 50Sn66 50Sn68 50Sn70 50Sn72 50Sn74

Fig. 19. The level and isomer systematics revealed m the present study. Levels of the same spin and parUy are connected by broken hnes. Previously known levels g~ven m the compdat]on of Lederer

et al. as) are included

Calcula ted level energies depend on the parameters chosen for the forces, and

compar i son o f exper imenta l level energies with the calculat ions may not be a cri t ical

test of a theory if only appl ied to one or two isotopes. As a result of the present and

earl ier work, in fo rmat ion is now avai lable on a large range of even Sn nuclei f rom

A = 108 to A = 124. These have a closed p ro ton shell Z = 50 and values of N from

58 to 74. The s ingle-par t ic le neu t ron orbi ts in the d÷ and g~ orb i ta l s and the second

in the N = 50-82 region are g rouped into two regions: the first has neutrons in the

s~, d3 and h e orbi tals . These two regions are divided by a soft closed sub-shell at

N = 64. C o m p a r i s o n o f theoret ical predic t ions over this large range of i sotopes is

a good test of the vahd i ty o f the theory. The level systematics revealed in the present work cover the region below the sub-

shell b o u n d a r y at N = 64 and the b o u n d a r y region. Earl ier results are avai lable on

104 T. Y A M A Z A K I A N D G. T. E W A N

the heavier region. The level and isomer systematics are summarized in fig. 19. We shall make some qualitative comparisons o f these levels with theoretical predictions.

Quasiparticle energies calculated and used by Arvieu 44) are illustrated in fig. 20 as functions of mass number. It is apparent that the d{ and g~ orbits dominate the lighter isotopes (A < 114), while the s~, d~ and h~ orbits dominate the heavier isotopes (A > 114).

I i I L i i I

QUASI - PARTICLE / 3000 ENERGIES x /

(ARVIEU) / /

~>_ asoo ~, d ~ / / /

w°: ~ ~gT/~ 2000 i s o o ~ /

SI/2 i o o o i i i i I I i

58 60 62 64 6 6 68 70 72 74 N Fig. 20. Neutron quaslpartlcle energies calculated and used by Arvleu 44).

4.1. FIRST 2 + STATES

The experimental level energies are plotted in fig. 21. It is seen that the energy increases with N f rom N = 58 to N = 64, where it reaches a maximum, and then starts to decrease. This fact seems to reflect a slight shell effect at N = 64. The Q T D calculations o f Arvieu e t al. 43,44) gave somewhat higher energies and a more distinct maximum at N = 64 as shown in fig. 21. The QSTD calculations of Ottaviani e t al. 47) with a finite-range force was performed for A = 116, 120 and 124, and their results are shown in the same figure.

4.2. FIRST 4 + STATES

The experimental level energies are plotted m fig. 22. The energy of the first 4 + level shows a dependence on N similar to that o f the 2 + level energy except at N = 64, where there is an appreciable depression of energy. The calculations of Arvieu e t

al. 43.44) is shown in fig. 22 with predominant two-quasiparticle components in the

2000 -.

>

v

>.- CD ,-y W Z ILl

d W > L~ d

1500

- - 0 0

IO0( -

* STAT E

\ 0

\ \ o o

QSTD

(OTTAVIANI et al)

I I I I I I _ I I _ _ _ L _ 5 8 60 62 64 66 68 70 72 74

F~g 21. Comparmon of the experimental energies of the first 2 + states shown as open c~rcles, v~lth the QTD calculation by Arv]eu et al 4a.~4) and the QSTD calculation by Ottawanl et al. 47)

[ I I I I I I I I

4 0 0 0 z I ~ g7/2 4+ STATE /

- - g 7 / 2 $ 1 / Z

ssoo / ,,/'

>- ~ g-t/2su, z ,ooo / / \

] o ~ SDI o o 2 o o n h QSTD (PLASTINO et ol)

~ / ( ? T T A / I A N I j el e l ~ ) T , ~ , I

5 8 6 0 6 2 6 4 6 6 6 8 70 72 7 4

N

Fig 22. Comparmon of the experimental energies of the first 4 + states shown as open c~rcles, with the QTD calculations by Arvleu et al. 4a, 44), the SDI calculation by Plastmo et al 45) and the QSTD calculation by OttavJam et al. 47) The maln two-quaslpart~cle configurations are shown for the

four 4 + states calculated by Arvleu et al. 4a.~4)

106 T . Y A M A Z A K I A N D G . T E W A N

two regions. The energy dip at N = 64 is not reproduced in the calculations of Arvieu et al., but a qualitative understanding of the dip may be obtained by considering the structure of the first 4 + state. Below N = 64, its structure is dominated by the (d~_) 2, (g~)2 a n d d~g~ components and above N = 64 by the (h~) z component. At N = 64, all components contribute approximately equally and lead to a lowering m the energy of the state. The first 2 + state is less affected because more configurations are available. It is interesting that the SDI calculations of Plastino, Arvieu and Moszkowski 45) reproduced the experimental behaviour at N = 64 and 66 (see broken lines in fig. 22). The QSTD calculations with a finite-range force yielded a lower 14+ energy because of four-quasiparticle components, but this has been performed only for N = 66.

4000

= 3 5 0 0

w z w

3 0 0 0

w

2 5 0 0

I i i i i i

NEGATIVE - PARITY STATES

• 6- ~- EXPERIMENTA o z - j

QTD ( A R V l E U )

\ \ ~ d3/?hl l /z

• • ~ d3/2h l l /2 7 -

t 614 1 I I 12 I 2 0 0 0 62 66 68 70 7 74

N

Fig. 23. Comparison of the exper=mental energies of the negative-parity states w~th the calculatmn of mrv~eu et al ,*3, 44). The maln-two-quasipartlcle configurations are shown.

4.3. NEGATIVE-PARITY STATES

The experimental levels are presented in fig. 23 with Arvieu's energies. The 5- state (s~h~ and/or d~h~) is isomeric for A = 116, 118 and 120 but not for A = 114, where the energy difference between the 5- and the 4 + levels is too great for the E1 transition to have a lifetime measurable in our experiments. The 7- state (predom- inantly d~h~) is isomeric for A = 118 and 120 due to the highly hindered E2 decay but not for A < 116 as it feeds the 6- state via a fast MI transition. The 7- level for A = 114 could not be identified, presumably because it lay too high for observation. The systematics of these negative-parity states are consistent with the expected behaviour of the h~ orbit.

Cd(at, xn) REACTIONS 107

4.4. ISOMERIC STATES ABOVE THE 7- STATES

In add i t i on to the negat ive-par i ty isomers, a t least one long-l ived isomer (-" 0.2

#sec) has identif ied for each isotope in the region N > 64. On the o ther hand, no such isomer was seen in the region N < 64. The half-hfe of the i somer in l l 6 S n has

been de te rmined by Chang, Hageman and Y a m a z a k i 19) to be 0 .75/~ec .

This fact may be explained by the fol lowing conjecture. F r o m the quasipar t ic le

states avai lable , these long-l ived isomers are tentat ively assigned to the 10 + state o f

the h~ configurat ion. F o r N > 64, the 8 + and 10 + states o f the h~ conf igurat ion

are expected to lie a round 3 MeV and, as no o ther conf igurat ion of senior i ty 2

cont r ibu tes to these high-spin states, they should remain relatively unper turbed . The

energy spacing between the 8 + and 10 + levels is es t imated to be abou t 100 keV, and

the 10 + state could be expected to have a half-life of a b o u t l /~sec. The conjectured

isomeric E2 t ransi t ions were not observed in the present survey, and further exper-

iments are required to establish their existence. In the region N < 64, the h~ energy

becomes so large tha t this conf igura t ion m a y no longer be the sole con t r i bu to r to the

8 + and 10 + states. Four -quas ipar t i c le states and v ibra t iona l states may also cont r ib-

ute. This m a y explain why the long-hved Isomer does not exist in this region.

TABLE 3 Experimental B(E2, 6 + ~ 4 +) values for 1°SSn, l~°Sn and ~2Sn

A E~, T~ B(E2, 6 + ~ 4 + I B(E2)/B(E2)w (keV) (nsec) (e 2 fm 4)

112 302 14 16.3 0.25

110 281 < 7 > 46 > 07

108 254 < 7 > 77 > 1.2

4.5. THE 6-- STATES FOR N < 64 AND B(E2, 6 + -~-4 +)

It is interest ing tha t for N < 64 the highly popu la ted levels te rmina te at 6 +. Thlq

is consis tent with the fact that no higher-spin state is avai lable at tow energy with the

d r and g÷ orbi ts

The half-life of the 6 + state in 112Sn was found to be 14 nsec, which is comparab l e

with the Weis skopf es t imate for an E2 t ransi t ion. It is s t r iking tha t no delayed state

was observed for the hghter nuclei, a l though the level s t ructure is unchanged. The

B(E2, 6 + ~ 4 +) values deduced f rom these exper imenta l da t a are shown in table 3.

The B(E2) value increases rapid ly as N decreases This seems to be in d isagreement with a s imple pa i r ing theory which predicts tha t the B(E2, 6 + ~ 4 +) value should

decrease due to the ( U U - V V ) 2 factor. This poin t has, however, to be examined with

a more extensive calculat ion which takes into account possible conf igurat ion mixing and coupl ing with the collective mode.

4.6. POSS[BLETHREE-QUASIPARTICLE ISOMER IN l lVSn

The present s tudy revealed the presence o f a highly excited isomeric state m 117Sn

with energy ~ 3.68 MeV. No such isomer was found in other nuclei where only

108 T. YAMAZAKI AND G T. EWAN

the s ingle-par t ic le states and the low-lying isomeric state (h~ state) are present The

isomeric state in 117Sn could be a three-quas ipar t ic le state involving large- j orbits .

A candida te for the high- lying and high-spin state m a y be a [ (h~) 2 10 +, d÷]23/2+

state, in which the th i rd quas ipar t ic le is coupled stretchwise with the (hq ) / 10 ÷ state

appea r ing in the even isotopes. Of the mul t ip le t of the [ (h¥) z 10 ÷, d~]J configura-

t ion, the stretched state of J = - ~ is expected to be the lowest- lying if the S = 0

coupl ing of like par t ic les is energet ical ly favoured. The general feature of coupl ing

schemes resul t ing in i someric states is discussed elsewhere by Y a m a z a k i 4 9 ) . A more

deta i led exper imenta l s tudy is required to establish the propertxes of this i somer in

117Sn" Perhaps s imilar states might also be observed in other odd Sn isotopes with N_> 67.

5. Concluding remarks

We emphasize the possible presence of (h~) 2 10 + ~ (h~.) 2 8 + isomers in the

region N > 64, which preserve pure quasipar t ic le states. In the region N < 64, such

an isomer is not observed, and the g round band terminates at a spin of 6 +. The

d iscont inuance of the g round band structure in the region N < 64 is also seen in the

Z = 52 Te isotopes s tudied by Bergstr6m et al. so). These facts raise a quest ion a~

to the real nature of the g round band o f levels and the re la t ion between the so- called v ibra t ional sequence and the lowest-senior i ty scheme.

It should be noted that the present s tudy was a survey of energy levels and isomeric

states. There remain many exper imental ambigui t ies which require deta i led investiga-

tion. Of par t icu lar impor tance is the observa t ion of the low-energy isomeric transi-

t ions themselves, which were outs ide the range of this survey.

The au thors are grateful to Dr. S. G. Prussin for coopera t ion dur ing the exper iments ,

to Drs. J. M. Ho l l ande r and B. G. Harvey for suppor t of this exper imenta l p rog ram

at the 2.2 m cyclot ron at Berkeley and to F. S. Gou ld ing and D. Landis for help

with the electronic equipment required for these experiments . They would also like

to thank Professors B. R. Mot te l son and T. Udagawa for s t imula t ing discussions

and Professors I. Per lman and J. O. Rasmussen for the hospi ta l i ty o f the Lawrence Radia t ion Labora to ry .

References

1) R M. Diamond and F. S. Stephens, Nucl Phys 45 (1963) 632 2) J. Burde, R. M. Diamond and F. S Stephens, Nucl. Phys. 85 (1966) 481 3) K. Brandl, R Engelrnann, V. Hepp, E Kluge, H. Krehblel and U. Meyer-Berkhout, Nucl Phys.

59 (1964) 33 4) A. L. McCarthy, B. L. Cohen and L. H Goldman, Phys. Rev 137B (1965) 250 5) E. A. Ivanov, Phys. Lett 23 (1966) 265 6) A. L. McCarthy and T. W. Conlon, Phys. Rev. 147 (1966) 881 7) T W Conlon, Nucl Phys. A100 (1967) 545 8) S. Bjornholm, J Borggreen, H J. Frahm and N. J. S Hansen, Nucl. Phys 73 (1965) 593 9) T. Yamazakl and G. T Ewan, Phys Lett. 24B (1967) 278


10) T. Yamazakl and G T. Ewan, Nucl. Instr. 62 (1968) 101 11 ) T Yamazakl, Proc. 1967 Enrico Fermi Int School of Physics at Varenna, Course XL (Academic

Prcss, New York, to be published) 12) T. Yamazakl, E. Matthxas, S. G. Prussm, C. M Lederer, J. M Jaklevlc and J. M. Hollander,

Contribution to Int. Conf. on nuclear structure, Tokyo (September 1967) 13) T. Yamazakl, G. T. Ewan and S. G Prussm, Phys Rev. Lett. 20 (1968) 1376 14) H. H Bolotm, A. C L1 and A Schwarzschdd, Phys Rev. 124 (1961) 213 15) H lkegamI and T Udagawa, Phys. Rev 124 (1961) 1518 16) H. H Bolotln, Phys. Rev. 136 (1964) B1557, B1566 17) E. Bodenstedt et al., Nucl. Phys. 89 (1966) 305 18) M G Betlgerl and H Morinaga, Nucl. Phys. A95 (1967) 176 19) Ch Chang, G B. Hageman and T. Yamazakl, Nucl. Phys. A134 (1969) l l0 20) S D. Bloom, in Nuclear reactions, Vol. II, ed by P. M. Endt and P B. SmRh (North-Holland

Publ. Co , Amsterdam, 1962) p l 21) G T Ewan, R. L Graham and I. K. McKenzle, IEEE Trans Nucl. So NS-13 (1966) 297 22) J Plgne~et, J. J Samueh and A. Sarazm, IEEE Trans. Nucl. Scl. NS-13 (1966) 306 23) M. G. Strauss, R. N Larsen and L L. Sifter, IEEE Trans. Nucl. Scl. NS-13 (1966) 265 24) S Gornl, G. Hochner, E. Nadav and H Zmora, Nucl. Instl 53 0967) 349 25) H. G Jackson, Nucl Instr. 33 (1965) 161 26) C. M. P. Johnson, Phil. Mag. 1 (1956) 573 27) R Barloutaud, P Lehmann, A Leveque, G. C. Phdhps and J Quldort, Compt Rend. 245

(1957) 422 28) K W Allen, D. Eccleshall and M. J. L. Yates, Proc. Phys. Soc. 74 (1959) 660 29) J A Becker, J. W. Olness and D. H. Wdkmson, Phys. Rev. 155 (1967) 1089 30) D C. Camp, Nucl. Phys. A121 (1968) 561 31) E. Elchler, P. H. Stelson and J. K. Dickens, Phys. Lett., to be published 32) W D. Myers and W. J. Swiateckl, UCRL-11980 (May, 1965) 33) S. Fukushlma, S. Hayashl, S. Kume, H Okamura, K. Otozal, K. Sakamoto and Y. Yoshlzawa,

Nucl Phys. 41 (1963) 275 34) S. Fukushlma, S. Kume, H. Okamura, K. Otozai, K Sakamoto,Y. Yoshlzawa and S. Hayashl,

Nucl. Phys. 69 (1965) 273 35) C M. Lederer, J. M. Hollander and I. Perlman, Table of isotopes, 6th ed. (John Wiley and Sons,

New York, 1967) 36) O Hansen and O. Nathan, Nucl. Phys 42 (1963) 197 37) M. Sakal, T Yamazakl and H E llrl, Nucl. Phys. 74 0965) 81 38) K. F Alexander, H F. Brlnekmann, C. Helser, G. Lang, W. Neubert and H. Rotter, Phys

Lett. 17 (1965) 322 39) E. Ivanov, A Alevra, D Plostinaru, N. Martalogu and R. DumRresu, Nucl. Phys. 54 0964) 177 40) E. A. Ivanov and M. I. Magda, Rev. Roum. Phys. 12 (1967) 227 41) E. J Schneld, A. Prakash and B. L Cohen, Phys Rev. 156 (1967) 1316 42) L. S. Klsshnger and R. A. Sorensen, Mat. Fys Medd. Dan. Vtd. Selsk. 32, No. 9 (1960) 43) R. Arvieu, E. Baranger, M. Venerom, M. Baranger and V. Glllet, Phys. Lett 4 (1963) 119 44) R Arvleu, Ann. de Phys. 8 (1963) 407 45) A. Plastlno, R. Arvleu and S. A. Moszkowskl, Phys. Rev. 145 (1966) 837 46) P. L. Ottavlani, M. Savola, J. Sawlckl and A. Tomasml, Phys. Rev. 153 (1967) 1138 47) R. L. Ottawam, M Savom and J Sawlcki, Nuovo Clm. 56B (1968) 149 48) M. Gmltro and J. Sawlckl, Phys. Lett. 26B (1968) 493;

M. Gmxtro, A. Rtmlm, J. Sawlckl and T Weber, Phys. Rev Lett 20 0968) 1185 49) T Yamazaki, to be pubhshed 50) I Bergstrom, C. J. Herrlander, A Kerek and A Lukko, Contribution Int. Conf on nuclear

structure, Tokyo, Japan (September 1967)