Lifetimes and spin-parity assignments of excited states in 69Ge populated in the 66Zn(α, nγ) reaction

Nuclear Physics A321 (1979) 189-206; (~) North-Hoiland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

LIFETIMES AND SPIN-PARITY ASSIGNMENTS OF EXCITED STATES I N 69Ge POPULATED IN THE 66Zn(~z, nT) REACTION A. A. ALEXANDROV, M. P. KUDOYAROV, I. Kh. LEMBERG and A. A. PASTERNAK

A.F.Ioffe Physical-Technical Institute, The USSR Academy of Sciences, 194021 Leningrad, USSR Received 6 February 1978 (Revised 19 July 1978) Abstract: The lifetimes of twenty levels in 69G-e have been determined by use of the Doppler shift

attenuation (DSA) method. In order to reduce the uncertainty in the lifetimes of these states, an evaluation of the stopping power in zinc for Ge ions was made by a new method using a semithick target. Measurements of y-ray yields and y-ray angular distributions following the 66Zn(~t, ny) reaction have been made. Beam energies of 11.8, 14.0 and 16.5 MeV were used. New spin and parity assignments have been made for ten levels. Multipole mixing ratios are given for many of the observed transitions. Extracted B(E2) and B(M1) values are compared with Alaga model calculations.

NUCLEAR REACTIONS 66Zn(~t, ny), E =

11.8, 14.0, 16.5 MeV; measured It(E),

ly(O), DSA. 69Ge deduced levels, T1/2, B(E2) and B(M1) values, 7-branching ratios, J, t$, Enriched target.

1. Introduction Due to the general interest in the f-p shell nuclei at about Z = 28 and N = 40 where in some cases quasirotational and quasivibrational states are expected to be observed in the same nucleus, the electromagnetic properties, spins and parities of the excited states have recently become the subject of extensive studies, both experimental and theoretical. In addition, odd nuclei having three holes in the 2p~, 2p½ and lf~ subshells offer an opportunity to make use of the Alaga model in treating their properties. T h e 6 9 G e level scheme was studied in the fl-decay of 69As [ref. 1)], and also in the reactions 7°Ge(p, d) [ref. 2)] and 6SZn(3He, d) [ref. 3)]; however, the main body of spectroscopic information has been obtained by ),-spectroscopic techniques. The singles spectra of the 69Ge 7-rays following the (p, nT) reaction were measured in ref. 4); as a result of internal conversion studies, spin and parities have been assigned to the five lowest excited states. Positive parity states populated in the reaction 66Zn(~, n~) were studied in ref. 5) and on the basis of 7-ray angular and polarization measurements, spins and parities up to J~ = ~ + have been assigned to a number of them. The most complete level scheme of 6 9 G e reported in ref. 6) is based on the 189

190

A. A. ALEXANDROV et

al.

measurement ofy-y coincidences, y-ray angular distributions, excitation functions and linear polarization. This enabled the authors to assign spins and parities to a number of levels and to determine multipole mixing ratios for a number of transitions involved; several new levels have also been identified. Lifetimes (z) of the relatively long-lived 87 and 233 keV states in 6aGe were reported in ref. 1). For seven more excited states (at 812, 862, 1350, 1407, 1431, 2755 and 3076 keV) lifetime measurements were made in ref. 7) by the Doppler shift attenuation method (DSAM), and B(E2) and B(M1) values for a number of transitions have been deduced from measured z-values and y-ray angular distributions. Thus the information on the electromagnetic transition probabilities for a number of the known germanium levels was absent. The main purpose of the present study was the determination of lifetimes in 69Ge by use of the DSA technique. Data handling involved corrections of the electronic and nuclear stopping power and corrections for the finite population time of states from a continuous spectrum. Lifetimes of twenty excited states in 69Ge have been measured, thirteen of which were not previously reported. Apart from the lifetime measurements, y-ray angular distributions (AD) for twelve transitions have been studied. These studies enabled us to assign spins and parities not known before to eight levels and to determine multipole mixing ratios and B(E2) and B(M1) values for the transitions involved. The results are compared with calculations carried out in the framework of the Alaga model s).

2. Experimental procedure The 69Ge levels were populated in the reaction 66Zn(0t,ny) at projectile energies E = 11.8, 14.0 and 16.5 MeV. A thin self-supporting zinc foil, 1.95 mg/cm 2 thick, enriched in 66Zn to 90 ~/o,was used as a target. T o determine energy loss parameters for the recoils slowing down in the target substance, an additional experiment was carried out at an incident energy of 13.5 MeV using a target with thickness of 0.28 mg/cm 2. The y-ray singles spectra were measured with a 50 cm a Ge(Li) detector having energy resolution of 3.5 keV for y-rays of 1330 keV. The detector was placed at a distance of 10 cm from the target. In experiments employing 11.8 MeV a-particles, spectra were taken at 0% 30°, 90 °, 125 ° to the beam axis; at 14.0 MeV beam energy spectra were taken at 0% 25 °, 40 °, 50°, 60 °, 75 °, 90 °, 102 °, 117°; and in those with 16.5 MeV, spectra were taken at 0 °, 45 °, 90 ° and 125 °. The stability of the analyzing system was maintained using two reference y-lines from 22Na and say sources. The y-ray transitions from 22Na, 6°Co and say sources provided the energy calibration of the spectra. The relative efficiency for the y-detector was measured with calibrated lS2Eu and Say y-sources fixed in the target position. The y-ray peak centroid positions and peak areas with associated errors were obtained using a computer program 9) which provided separation of partially overlapping y-ray

STATES IN 69Ge

191

peaks making use of Doppler line shapes observed at different angular positions of the detector. Measured y-ray peak centroid positions and full Doppler shifts derived from kinematics were utilized to derive DSA factor (F(z)) values via least square fitting 10). Fig. 1 is a typical example of these data. Deduced peak areas were also used in y-ray angular distribution studies. / / 74P'

779 key

/

/

/

/

iO59keV

/

/ /

/

7W

/

7110

/

/ 8l$

8t2 a,v

/

/

/

/

/

/

/ /,3~ key

to ~q

/

/

t~83

/

/

/ ~okeV

/

/

/

fOtl

/

/669key /

ie~O

/

~fTd

/

4670

Fig. 1. Peak centroids for some of the transitions in 69Ge plotted against the cosine of the 7-ray detector angle. The dashed lines indicate the maximum possible centroid shifts; the solid lines are least squares fits to data.

3. Corrections of the electronic and nuclear stopping parameters: The F(~) calculations

The accuracy of DSA lifetime measurements is crucially dependent upon an adequate choice of fe and f, defined by d~=f~

dpp = + f -

dp

"

The quantities (de/dp)= and (de/dp). denote electronic and nuclear components of the total stopping power, respectively, as given by the LSS theory 11). The f= and f, values may fluctuate considerably depending on the particular choice of the stopped nucleus and stopping medium. For this reason in DSAM experiments, it is desirable to determine f= and f, from additional measurements, keeping

192

A. A. ALEXANDROV

et al.

experimental conditions identical to those of the main experiment. Several methods have been developed for this purpose [see e.g. refs. 12,13)]. However, application of the first 12) is restricted to proton capture reactions and the second 13) is valid only for the combinations Z 1 = Z 2 (Z 1 and Z 2 are the recoil and target charges respectively). In the present study a technique is used which is similar in principle to that of ref. 13). The technique is based on a DSA measurement for a relatively long-lived state and a semi-thick self-supporting target whose thickness is comparable with the projected range of the recoiling ions. In this case DSA effect is essentially independent of z, since the lifetime is much longer than the characteristic slowing down time of the recoils and is entirely determined by the stopping process in the target. Thus by measuring the target thickness and DS attenuation factor one can derive the combination of fe and fn values, which correspond to the experimental F(z) for the relatively long-lived state chosen. The use of the technique proposed was preceded by the development of a new computer program which correctly takes into account the electronic and nuclear stopping contributions, multiple scattering of the recoils in the target itself and in its backing, reaction kinematics, the energy loss of projectiles in the target, and the effect of cascade feeding of the level involved from the upper levels. It is impossible to define independently the fe and fn values from a single measurement of F(z). Nevertheless the numerical calculations made with the program mentioned above show that the variations of fe even by 30 ~o combined with a correlated choice of corresponding fn value give practically the same F(z) plots for a thick target, and therefore the proposed method for correction of the f~ and fn is applicable to lifetime measurements. T h e 66Zn target of 0.28 mg/cm 2 was used to determine the f~ and fn values. Its thickness was determined by measuring the energy loss of :t-particles from 226Ra with an accuracy better than 5 ~. The Doppler shift of the ground state transition from the 862 keV level was measured. Its lifetime of about 3 ps is considerably longer than the slowing down time of Ge ions in Zn. The value of the attenuation factor turned out to be F = 0.49_ 0.03. Calculated values of F(z) corresponding to different f~ and fn combinations and the experimental F-value are shown in fig. 2. The values obtained (f~ = 0.75___0.05 and f , = 0.55+0.05) were used in further calculations of z. The choice of the electronic and nuclear component of stopping power with the method proposed above is somewhat dubious, as has discussed before. It is seen however from fig. 2 (curve 8) that the LSS theory prediction (f~ = f , = 1) does not agree with the experiment in this case. At the bottom of the same figure F(z) plots for the 862 keV y-rays are compared with the experimental F-value obtained at E = 11.8 MeV with the target thickness of 1.95 mg/cm 2. It is seen that the error in the z-value resulting from the uncertainties in the fe and fn values is less than 10 Yo. Calculations take into account the cascade feeding from the upper levels at 1.92 and 2.25 MeV. The lifetime of the 862 keV level

STATES IN 69Ge

193

7k J,

0.5

0.~

~

19- ~0.7o \

o.,

oj

8-D:l.o

0

o,oi

0,02

i.= t.o

_

i

o.03 02o 0,06

~

,

l

oJ

72

"d"t,t~

Fig. 2. Calculated plots of F(z) corresponding to different fe and f . combinations and a range of experimental F-values for the 862 keV level. Curves 1 4 are for the target thickness 0.28 mg/cm 2, curves 5-7 for the target thickness 1.95 mg/cm 2, and curve 8 corresponds to LSS theory calculations.

has been found to be 9 R+o.9 ps. W i t h o u t taking into account the cascade feeding this value is consistent with the value 3.4 +__0.7 ps obtained independently in ref. 7) where recoils were slowed down in gold and cascade feeding was neglected. The F(z) plots calculated with corrections for the fraction of nuclei recoiling into vacuum from the 1.95 mg/cm 2 thick target at incident energy 11.8 MeV are shown - ' ~ - 0 . 6

t,

¢tz El i

E01

0.02

i

I

t

i

t

0,03 o.o~/ a06 o.08 a#

,

aZ

o.3 on

aa

as ~

~

3

o

-

-rT-~e¢

Fig. 3. Calculations of F(x) plots for the level without cascade feeding: (1) with corrected and (2) with uncorrected f, and f , values.

194

A. A. A L E X A N D R O V et aL

in fig. 3. The shaded area corresponds to variations of fe and f , within the limits of their errors. The curve for fe = f , = 1 also shown in fig. 3 indicates that neglecting the corrections for f¢ and fn would considerably change the lifetime deduced. 4. Corrections for finite side feeding time at E . = 14.0 and 16.5 MeV: The results of lifetime measurements

In DSAM experiments using a compound nuclear reaction it is of great importance to take into account the finite time of population of the states from the continuous spectrum. It is known 14-16) that when the highest possible excitation energy of the residual nucleus considerably exceeds the energies of states studied ( ~> 5-10 MeV), the effective population time (so called "effective side feeding time" zs.f.) becomes comparable to the characteristic slowing down time and substantially reduces the DSA factor. Therefore the most reliable results come from experiments where the energy of the highest level studied is slightly below the maximum excitation energy and the states involved are populated without cascades from the continuous spectrum. At the incident beam energy 11.8 MeV the highest possible excitation energy (Emax) is equal to 3.7 MeV and the highest possible angular momentum imparted to the compound nucleus amounts to 4.6. In the "grazing" collision approximation 17) the expected range of the most probable excitation energy and angular momentum values for the residual nucleus at the moment just after the neutron emission lies inside a parabola (curve 1 in fig. 4). This range contains practically all the levels observed below 2.5 MeV excitation energy. (The levels which are essentially not populated at E = 11.8 MeV are denoted by an asterisk). Since the cross section for the formation of the residual nucleus reaches its maximum at an energy approximately 2 MeV lower than gmax' * the discrete levels in 69Ge under consideration are directly populated at E~ = 11.8 MeV and the effect of ~s.f. can be neglected at this energy. The regions of excited states in 69Ge reached after neutron emission are shown in fig. 4. It is easily seen that the higher-lying high spin states in 69Ge are not and cannot be populated at 11.8 MeV. This was a reason for additional experiments to be carried out at higher a-particle energies. At E = 14.0 and 16.5 MeV the maximum value of the angular momentum imparted to the compound nucleus amounts to 6.6 and 8.5 and E*ax to 5.8 and 8.2 MeV, respectively. If the neutrons carry away an average kinetic energy of 2 MeV, the lengths of the unobserved ),-ray cascades are about 3 and 5 MeV, introducing a finite time delay. Under these conditions the ~.f. value has to be estimated and used for F(z) calculations at E~ = 14.0 and 16.5 MeV. In order to estimate ~.f. the lifetime of the 1431 keV level measured at E~ = 11.8 MeV was used as a reference value. This was preceded by calculations of F(z) corresponding to different z~.f. values in the "single level approximation", i.e. on the assumption that the level in question is entirely populated by a single cascade transition from a certain effective level. Fig. 5 calculated for E~ = 16.5 MeV shows the dependence of the lifetime on the z~.e. value adopted. From the value

STATES IN 69Ge

195

(3)e.,=~~ / ~:~ \-~ Z "~'::~ /

Fig. 4. Ranges of probable population of 69Ge levels for E~ = 11.8 MeV (curve l), 14.0 MeV (curve 2) and 16.5 MeV (curve 3) calculated in the "grazing" collision approximation. Levels denoted by an asterisk are not populated at E~ = I 1.8 MeV.

~z7 o.7 o.6

\

o.g

\

o.2

o.,f

.i

•

Q.05 o.~ pser.

Fig. 5. Determination ofT, s" values at E, = 16.5 MeV using the reference x-value for the 1431 keY level.

A. A. ALEXANDROV et al.

196

F = 0 . 1 5 +- - 0 . 0 1 m e a s u r e d a t 16.5 M e V a n d the t r u e lifetime z(1431) = ~1' - -'9 +0.3 ps, -0.2 a feeding t i m e o f z~.e. = 0 . 2 + 0 . 1 ps w a s d e r i v e d . A v a l u e o f Zs.f. = 0.12-t-0.07 at 14.0 M e V was d e r i v e d in the s a m e way. T h e s e c o r r e c t i o n s w e r e t h e n i n t r o d u c e d e q u a l l y i n t o the a n a l y s i s o f all D o p p l e r shifts a t 14.0 a n d 16.5 M e V for all levels n e g l e c t i n g the d e p e n d e n c e o f z~.f. v a l u e s o n t h e i r spins a n d energies. T h i s a p p r o a c h is p o s s i b l e b e c a u s e the e n e r g y difference b e t w e e n the i n v e s t i g a t e d levels a n d the reference level d o e s n o t e x c e e d 1.5 M e V , a n d as follows f r o m o u r d a t a the z~.f. i n c r e a s e o f the u n o b s e r v e d ~ - c a s c a d e is n o t l a r g e r t h a n 0.04 p s / M e V . T h e r e f o r e the v a r i a t i o n s o f z~.f. a r e less t h a n the i n t e r v a l of p o s s i b l e z~.f. v a l u e s d e t e r m i n e d in the p r e s e n t w o r k . T h e d e p e n d e n c e o f Zs.f. o n level spins c a n be n e g l e c t e d d u e to the s t a t i s t i c a l n a t u r e of the u n o b s e r v e d ~-cascade, as c a n be seen f r o m fig. 4. T h e results o f lifetime m e a s u r e m e n t s are listed in t a b l e 1. It is seen f r o m fig. 5 a n d the z~.f. v a l u e s d e r i v e d f r o m m e a s u r e m e n t s at E~ = 14.0 a n d 16.5 M e V t h a t the e r r o r s o f z c o n n e c t e d w i t h the u n c e r t a i n t y of Zs.f. i n c r e a s e w i t h E . T h e r e f o r e the r - v a l u e s w e r e d e r i v e d a t o p t i m a l b e a m e n e r g y w h i c h was c h o s e n n o t t o o l a r g e TABLE I

Lifetimes of levels in 69Ge El" (keV)

F(O

z ") (psec)

812 862 933 995 1196 1350 1407 1431 1433 1479 1591 1613 1920 2018 2248 2483.5 2483.6 2755 3076

0.12+0.01 ") 0.09 +0.01 ") 0.07+0.03 ") 0.20 + 0.03 ") 0.13 _+0.03 ") 0.16_+0.02") 0.13___0.02 ~) 0.16-+0.02 ") 0.12-+0.03 a) 0.30__+0.15 ") 0.18_+0.03 ") 0.04_+0.03 b) 0.08_+0.03 b) 0.01 _+O.04S) 0.19+0.02 b) -0.13+0.02 b) -0.17+0.02 b) _ 0.21 +0.02 b) -0.08_+0.03 b)

1.3+°:2 a) 2.8+°: 9 ") > 2.5 0.90_+°:13° ") 1.4+~:o ,) 0.70+0:255 ") 1.7+00:~ ~) 1.2+0123 a) 2.0+~17s ,) 0.5+~i~5 a) 1.1 +o:~ ~) > 1 b) > 1.5 b) > 2 b) 0 " 7 +0.4 b~/ -0.3 1" ~,;+o.s b~ -0.6 J 0 . 9 - o . +o.~ 3 b) 0 " 7 -+0.`* b~ 0.3 J

") Data obtained at E~ = 11.8 MeV. b) Data obtained at E~ = 14.0 MeV.

°) d) ") f)

Data Data Data Data

obtained at E~ = 16.5 MeV. taken from ref. 7). obtained at optimal E~ values. obtained at other E~ values.

> 1 b)

z e) (psec)

¢ a) (psec) 1.8 +0.36 3.4 +0.68

1.4+~:8 b) 1.9+20:~ b) 1.2+0123 b,c)

1.1 _+0.22 1.4 _+0.28 0.73_+0.15

1.7+~:~ b) > 1 ~) > 1 ") > 1.5 ") 0 " 8 +0.5 ") -0.4 > 0.4") ~) 0 - 8 - 0 . +1.0 ,, 1" 2 -+i'° ~) 0.6 > 1 ¢)

0.48+0.10 -0.46_+0. t0

STATES IN 69Ge

197

(to prevent an increase of zs.f) but sufficient to maintain 7-line intensity at the level needed for accurate Doppler shift measurements. The F(z) and z values derived from our measurements at optimal beam energies are listed in cols. 2 and 3 of table 1. (The superscripts a, b and c correspond to the data obtained at E = 11.8, 14.0 and 16.5 MeV, respectively.) The lifetimes derived at other beam energies are listed in col. 4. These data are in agreement with those given in col. 3 but their errors are larger. This is why calculations of the electromagnetic transition probabilities were made with the data presented in col. 3. In the case of the 812, 862, 1196, 1350 and 1407 keV states, F(z) calculations involved corrections for cascade feeding from the upper states in accordance with the decay scheme reported in refs. 6.7). This scheme including some branching ratios and spin-parity assignments from the present study is shown in fig. 6. The relative intensities of the 69Ge transitions measured at different beam energies and needed for the calculation of cascade and side feeding are listed in cols. 5-7 of table 2.

(keV)

J~"

~076

~.q/g*

2q8~.6 2qt~

~ o o ~ , o o - - - •,o.-I

2248

20f8 #920 t891

~o--~q

,6,3

-,,

!

too I

i

--7,,

z5

~',s~;~"

f~/2*

I,,~

~¢,~" ~'/~"

,

z --s!-28---~--t rt--L ~62 -~2 ~ s -J--L

99S

~27

,,,

, I "~ I I - - ? a ~ ¢6--¢a

i +

3Yq ~zt-l~...

--,,L i o ,,1

87

s/z- , 9,/z-

-+7/~+9, ~ , 'It., ~/2•

too

fqo7

233

~31z+

3~.~-

52 '

¢350 , 119~ - - ~ ' ~ ' ~ ' 6

8f2

/s/g* ~ ; #/2-

B

#f91

14~9

I

~., _ ,.,

#7/2÷

o-A-

r~,

-~+ -.r~~,~"

~.~.%-

Fig. 6. Level scheme for 6aGe. The branching ratios are from this work. The levels not discussed in text are not shown. For spins and parities, see text.

762* 995 256 1068 822* 1196 952 1010 1431 620*

995

1479

1350 1~7 1431 1433

1196

1068

933

1105

398 414" 438 812 629 862 559 846 933

398 812

862

E 7

(keV)

Elev

(keY)

~~3-

5+

i~~+ ~+ ~~l~+ 2 t~+ 2 3~+ 3+

~2+ {+ 3+ ~+ 2 ~5~~~

J;

<2

1.2±0.3

~+

]-

1.5±0.4 3.8±0.8 0.8±0.3 6.0±0.7 3.0±0.5 2.0±0.5 7.6+0.8 -5.3+0.7 -7.2±0.6

100 2.9 _+0.2 4.3±0.3 18.0±0.5 1.2±0.6 12.8±1.0 1.7±0.4 2.9±0.5 5.7 ± 0.6

½~{+ ~{i9+ ~ ~+ ~-

~~9+ ½~½~~½~-

11.8 MeV

I~sso}

2 ±1

1.5±0.2

1.4±0.3 3.4±0.7 1.0±0.3 6.2±0.6 3.9±0.3 2.8±0.4 28.9+1.0 -26.0+1.5 -15.0±1.2

100 2.9+_ 0.2 4.2±0.3 18.8±0.7 2.0±0.5 23.8±0.8 2.1 ±0.4 3.4±0.5 6.9 ± 0.9

14.0 MeV

l~sso)

3.5±1.2

1.6±0.2

1.0±0.2 2.9±0.3 1.1±0.3 6.8±0.6 3.8±0.3 2.8±0.3 29.8+0.8 -35.0+0.8 -16.6±1.0

100 2.8 _+0.2 4.2±0.3 19.0±0.7 2.2±0.4 26.0±0.9 2.3±0.4 3.6±0.4 7.3 ± 0.6

16.5 MeV

l~5so)

-0.14+0.05

0.53+0.04 0.36+0.06 0.34 _ 0.03

- 0 . 4 t +0.11 -0.71 +0.07

- 0.44 + 0.08

0.24 + 0.04 0.46 + 0.05

0.14+0.06

-0.06+0.06 - 0.08 + 0.08 -0.12+0.03

-0.16+0.03 0.15+0.07

0.15+0.10

0.12+0.04 0.03 + 0.06

0.14+0.04

0.12+0.04

0.28 -{-0.04 0.45 + 0.02

0.07-t-0.10

A4

0.13___0.08

(present work)

A2

Data on 7-ray intensities and angular distribution measurements

TABLE 2

--

-1.0

-0.5

"

-0.10

04+0.06

+0.1 0.7-0.07

'

b)

a)

0 7 +0.35 b)

--~"

0.9_+ 002 •a ) n 7 +0.45 b)

0 "4 -+00.3 a) .1

1" 2 -+0.7 a~:' 0.4 © <

X >

> > >

oo

1133 1076 1348

1669 (1139)*

3076 ?

_

t~5 + 2

x3 + 2

2

L5+ 2

L3+

11 + T 2 13+

_

25+ ~7+ 23 + 2

~-

l~+ 2 ~+

~2

2z_

~_

~-

~-

:~+

:~-

0.2

1.9+0.5

2.7 +0.5 -0.5 +0.5

1.1 +0.5

0.5_+0.~ 1.1 _+0.5

2.9+0.5

3.2+0.7

4.6_+0.6

4.6_+1.5

.

71.+_0.6 3.8_+0.4 5.2+0.3 5.9+0.4 . 5.9-+0.5 1.9+_0.4

6.1 _+0.6 2.0_+0.3 4.5+_0.2 3.1+0.2 . 1.7-+0.2 0.5-t-0.5

.

11.1 + 0.3 2.8_+0.5

.

9.4 + 0.4 2.3_+0.5 .

4.3+0.3

4.1 +0.4

1.2+0.5

3.2+0.4

3.4+0.4

5.4+0.3

10.5+_2.2

2.4+0.3

0.7+0.4 2.9+0.4

3.2+0.4

3.3_+0.5

5.6+0.3

8.1+_2.0

.

-0.06+0.08 . -0.50-+0.08 0.37-+0.11

0.29_+0.05 0.44_+0.05

0.19_+0.13

0.32 + 0.03 0.25_+0.10

0.46+0.10

--0.65+0.07

-0.30+0.20

--0.19+0.06

0.28+0.09

-0.35-+0.15

0.16_+0.08 0.10-+0.20

0.0 +0.11

--0.13_+0.08 0.19+0.10

0.13_+0.10

0.07 + 0.03 0.05_+0.10

--0.07+0.10

0.19+0.09

0.10__+0.20

0.08+0.09

-0.03+0.11

0.01_+0.15

"

- 0 . 0 7

0 ' 67 +0.07 c~ -0.06 +0.o6 c/ _0.10_o.o 5 ) -0.17___0.02 a)

0.35_+ohz~ b)

0 "58 +- 00.07 . 0 4 ,~ !

0"6--0"4 b)

1.0_+~:a5b) __0.8_+01~Sb)

!

0 5 +0.'0 a~ - 0 . 7 b)

--

0.6+~f b) 0.5_+o~iO5b)

The results on angular distribution measurements for transitions marked with an asterisk have been obtained for the first time. The sign of 6 is chosen in accordance with ref. is). ") Data taken from ref. 6). b) Data from the present work. °) Data taken from ref. 5).

2248

2483 2484 2755

~-

1920"

L3+ 2 1__3+ 2 ~l~-

¢j-

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&3

A. A. ALEXANDROV et al.

200

Fig. 7 demonstrates the importance of corrections for cascade feeding. The F(z) plots in this figure correspond to the cases when correction for cascade feeding of the 812 keV state from the 1433 keV (r = 2 ps) and 1591 keV (z ~ 1.7 ps) levels were taken into account (curve 2) and were not taken into account (curve 1). It is easily seen that cascade feeding corrections reduce z from 2.4 ps down to 1.3 ps. In the case of the 812 keV state, the F-value was determined for the ground state transition.

f(t-j t2# 0,3.

0.2.

o

•

0j

0

.

•

,

0.'z 0.3 0,4 0,6 0,8 1

[

j

r p,,,

Fig. 7. Influence of cascade feeding for the 812 keV level. The calculations were made with (curve 2) and without (curve 1) taking into account the cascade feeding.

F o r the 869, 933, 16t3, t920 and 2Ot8keV Icvcls, F(e)vatues listed in-table 1- are t h e result o f averaging over the different transitions involved. In the case of the 995, 1196 and 1479 keV levels, attenuation factor measurements were made for the 762, 822 and 1479 keV transitions, respectively, since it was impossible to separate the y-lines due to the other transitions from the background y-ray peaks. Data reported in ref. 7) are presented in col. 4. For the 812, 862, 1350, 1407 and 2755 keV levels the results of the present study are consistent with those of ref. 7). In the case of the 1431 and 3076 keV levels there is some disagreement.

5. The y-ray angular distribution studies: Spins and parities of the ~9Ge levels At E~ = 14.0 and 16.5 MeV angular distribution coefficients A 2 and A , were extracted from the intensities of the 7-ray lines. For y-ray intensity corrections connected with a change in the beam intensity, the 69Ge 287 and 1431 keV lines with known angular distributions were used as reference lines. Their A 4 values are small and of opposite sign (0.05 +0.02 and - 0 . 1 0 +0.08, respectively) and the A 2 values are of opposite sign (-0.10+__0.01 and 0.27-t-0.05, respectively). These y-lines are among the most intense in the 69Ge spectrum and can be reliably separated from the background. For the normalization, almost isotropic combinations of

STATES IN 6aGe

201

these y-lines given by i

x I55°(287) (287)--f

-A2(287) X

-

-/(1431),

~

--55°(14-31)

were used where I(E~) is the intensity of the y-peak with energy equal to E7. Some examples of the angular distributions measured at E~ = 14.0 MeV are given in fig. 8. To provide an independent test, A 2 and A 4 values for eleven y-transitions have been measured and compared with those reported in refs. s. 6). The results obtained are given in cols. 8 and 9 of table 2 and in general are consistent with the previous measurements. Using the technique described above we have measured angular distributions for twelve transitions (listed in col. 2 of table 2 and labeled by asterisks) which were not studied previously. In order to deduce spin-parity assignments for the levels involved and to determine E2/M1 mixing ratios we made use of the

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Fig. 8. Angular distributions for some of the 7-ray transitions in 6aGe. The solid lines correspond to a least square fit to the function W(O) = Ao+AzP2(cosO)+A4P4(cosO ).

202

A. A. ALEXANDROV et al.

tabulations from ref. 1a). The values of the angular distribution attenuation factors, ~2, as a function of level spin were adopted to be the same as in ref. 6). The 995 and 1196 k e V states. Due to large negative A 2 values for transitions to the 233 keV (3-) and 374 keV (~-) states the spin assignments ½, 3 and -~ are excluded unambiguously. Therefore spin values for the 995 and 1196 keV states are i5- . (When data from lifetime measurements are involved parities opposite in sign to those of final states in this and other cases considered here are excluded because the M2 transitions involved are too highly enhanced.) The 1433 k e V state. Due to a negative A 2 value for the 620 keV transition the spin assignments ½ and 9 are excluded. The spin value { is more probable because A 4 > 0 only in this case. The 1479 k e V state. The results o f angular distribution measurements allow excluding unambiguously the spin assignments ½ and 9 for this level. The 1591 k e V state. Due to the large A 2 value the spin assignments ½ and 3 are excluded unambiguously. When the results of lifetime measurements are included the 9 spin assignment becomes the least probable one because for J~ = 9+ the enhancement factor for the pure E2 transition to the I ÷ state would a m o u n t to about 100. The 1613 k e V state. In agreement with the results of ref. 6), in addition to j,~ = ~5 - , a d~ = i3 - assignment is also allowed. The results of the present angular distribution studies are consistent with both allowed J~ assignments. The 1920 k e V state. Transitions from this state to the J~ = 3 - levels (at 724 and 1920 keV) and to the J~ = 7 - level (at 1059 keV) are'observed. Hence, the allowed J~ values are ~ - , -~-, 7 - and 9 - . Large negative value of A 2 for the 1059 keV transition excludes J~ = 3 - and 7 - . Due to the value ofA 4 for the transition 1059 keV, the spin assignment 9 is m o r e probable. The 2248 k e V state. N o n e of the J~ - 3- 11- assignments can be excluded on the basis of angular distribution data. However, if J~ < 9 - then an M 1 transition to ground state would be observed. The results of our lifetime measurements and the absence of the 2248 keV ground state transition suggest that the partial lifetime with respect to such a transition is much longer than 1 ps, which implies that the hindrance factor is essentially larger than 103 . For this reason the most probable J~ values are 9 - and ~ - . This statement is confirmed by the fact that, as follows from table 2, the intensity of transitions with J < 9 increase less than two times with E increasing from 11.8 MeV to 16.5 MeV, while the 1386 keV transition intensity increases by four times. Besides that we observed the 1139 keV transition at E = 16.5 MeV. The fact that the 1139 keV transition belongs to the 69Ge decay scheme cannot be considered as established definitely without y-), coincidence measurements. However there are some indirect arguments in favour of this assumption. These arguments are based on a consideration of alternative possibilities. One of them is that the 1139 keV transitions are connected with reactions involving 64Zn and 68Zn nuclei which are

STATES IN 69Ge

203

the most significant impurities in the enriched 66Zn target. For checking this possibility, y-spectra from 64, 68Zn(~ ' ny) reactions were studied. The 1139 keV y-line was found in the case of 6 8 Z n ' which corresponds to the known transition 19) in 7iGe. However the intensity of this transition had to be ten times smaller than observed with our 66Zn target, containing only 2 ~ of 6aZn. Another possibility is that the 1139 keV transition is connected with other reaction channels such as 66Zn(ct, py)69Ga, 66Zn(~, 2nJ6aGe and 66Zn(ct, pnj68Ga. But there are no known y-lines from 68Ge and 68Ga nuclei in our 3:-spectra. We observed some ~-transitions due to 69Ga in our spectra, but the 1139 keV transition is absent in the known 69Ga decay schemes [-see for instance ref. 20)]. The angular distribution data are consistent with the assumption that this transition is of pure E2 type. The intensity of this y-line rapidly decreases with decreasing bombarding energy. All these considerations indicate that the transition 1139 keV feeds one of the 69Ge high spin levels. 6. Discussion

Lifetime measurements for essentially all the 69Ge levels experimentally observed and additional y-ray angular distribution studies carried out in present work allow obtaining detailed information on the electromagnetic properties of the 69Ge excited states. The reduced E2 (in W.u.) and M1 (in mW.u.) transition probabilities calculated from z(E2) and z(M1) values are given in cols. 4 and 5 of table 3. The electromagnetic properties of the 69Ge low-lying states have been qualitatively treated in ref. 7) in terms of the weak coupling model by coupling the f~ neutron hole and the g~ neutron to the quadrupole core vibrations, respectively. In ref. s) the electromagnetic transition probabilities obtained from the lifetime measurements have been discussed in the framework of the Alaga model. Negative parity states have been treated as arising from the coupling of three neutron holes in the (2pl, lf~, 2P½) subshell to the collective core vibrations. Other modes of excitation (e.g. five holes and two particles) which could also give rise to such states have been neglected. Positive parity states have been interpreted in the same paper s) as arising from the coupling of the g~ single particle state or a (g})3 cluster to the phonon spectrum. Two possible interpretations have been proposed for the nature of these levels based on a consideration of a "4h-lp" type neutron cluster or a "6h-3p" type cluster. However, coupling one particle and three particles to core vibrations, respectively, have been accepted as a first approximation. Since the core in this case, in contrast to that of negative parity states, is not even approximately closed (it may have four or six holes depending on the initial assumption), calculations related to the properties of positive parity states cannot be expected a priori to be successful. Nevertheless, calculations involving three valence particles account better for the observed number of states.

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STATES IN 69Ge

205

TABLE 3 (continued)

Ele , (keY)

2248 2483 2484 2755 3076

J~

91A-2 x~+ 2 Iv+ L3+ 2 15+ ~x~+ 2

Jr"

Er

B(E2),xv

B(M 1).~p

(keV)

(W.u.)

(mW.u.)

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1386

1~+ 2 ~13+

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1348

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1669

2

+a.s 1"5-12 14_+~° 17 +'2 -6 38+,~ -,v 4 8 +5'' • - 2.2 < 6 < 0.13

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B(M1)cale

(W.u.)

(mW.u.)

max

rain

38

16

max

min

15+is -a

13_+61a 19_+~a < 7

7.2 0.6 0

3.6 1.8 0

10 5 0.6

2 1 0

The m a x i m u m and m i n i m u m calculated B(E2) and B(M 1) values chosen from ref. a) are listed in cols. 7 and 8. Calculations a) were made with different sets of effective charge values and gyromagnetic ratios. The z-values for levels marked with an asterisk are taken from ref. '); the 6-value for the 233 keV transition is taken from ref. 6).

In cols. 6 and 7 of table 3, B(E2) and B(M1) values calculated in ref. 8) are presented. Only in one case is there a considerable discrepancy (by more than a factor of 3~1) between theory and experiment. The pure E2 transition (5+ 2 - 2 9% ~ from the 812 keV level is essentially larger than the theoretical prediction. The transition energies 1076 and 1010 keV are close to transition energies 4 ÷-2 ÷-0 ÷ in the adjoining nucleus 7°Ge (1017 and 1040 keV, correspondingly). Enhancement factors for E2 transitions between these states determined from our measurements are 40 and 20. These values are in agreement with the enhancement factors for yrast bands of adjoining even-even nuclei. This fact gives some evidence in favour of the existence of a decoupled band in 69Ge. The decoupled band in 69Ge based on the 9+ level, as supposed by us, is of interest in connection with the article by Alaga and Paar 21). They discuss the particleanharmonic vibrator coupling model applied to states with unique parity (in the case of 69Ge such a state is g~t)" They state that the occurrence of normal bands (J, J + 1, J +2, etc.) or decoupled ones (J, J +2, J + 4, etc.) is determined by the sign of the product of the particle quadrupole moment Q(j) and the core quadrupole moment Q(2+). The decoupled band occurs if this sign is positive. Just such a situation is observed in 6 9 G e , where both Q(j) and Q(2 ÷) are negative. Our data may be considered as additional support to the conclusion 21) that the occurrence of decoupled bands based on a state with unique parity can be understood without the hypothesis of rotational alignment. On the whole, comparison of the experimental results, both for states with positive and negative parity, with predicted B(E2) and B(M1) values shows that calculations performed in terms of the Alaga model reproduce the experimental data obtained fairly well, at least in cases where levels observed can be identified unambiguously.

206

A. A. ALEXANDROV et al.

The same situation arises in the case of 67Zn [ref. 22)] which also has three neutron holes in the N = 40 subshell. These facts suggest that the Alaga model can be expected to be effective in treating not only the structure of excited states in nuclei with a closed proton or neutron shell, but also in the case of a closed subshell.

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

S. Muszynski and S. K. Mark, Nucl. Phys. A142 (1970) 459 T. H. Hsu, R. Fournier, B. Hird, I. Kroon, G. G. Ball and F. Ingebretsen, Nucl. Phys. A179 (1972) 80 C. M. Fou, R. W. Zurmiihle and J. M. Joyce, Nucl. Phys. A97 (1967) 458 A. Isoya, T. Kuroyanagi, Y. Nakajima, T. Maki, T. Nakashima, N. Kato, I. Sugimitsu, K. Kimura, S. Mitarai and S. Vehera, J. Phys. Soc. Japan 35 (1973) 340 K. Forssten, A. Hasselgren, Ph. Monseu, A. Nilsson and Z. P. Sawa, Phys. Scripta 10 (1974) 51 U. Eberth, J. Eberth, E. Eub¢ and V. Zob¢l, Z. Phys. A273 (1975) 411 U. Eberth, J. Eberth, E. Eube and V. Zobel, Nucl. Phys. A257 (1975) 285 V. Paar, U. Eberth and J. Ebcrth, Phys. Rev. C13 (1976) 2532 A. A. Alexandrov, I. Kh. Lemberg and A. A. Pasternack, Abstracts, 28th Meeting on nuclear spectroscopy and nuclear structure (Nauka, Leningrad, 1978), in print M. F. Kudoyarov, I. Kh. Lemberg and A. A. Pasternack, Izv. Akad. Nauk USSR (ser. fiz.) 40 (1976) 2103 J. Lindhard, M. Scharff and H. E. Schi~tt, Mat. Fys. Medd. Dan. Vid. Selsk. 33 (1963) no. t4 A. Antilla, M. Bister and M. Piipariner, Nucl. Instr. 96 (1971) 141 I. Kh. Lemberg and A. A. Pasternack, Nucl. Instr. 140 (1977) 71 D. G. Sarantites, J. H. Barker and N. Lu, Phys. Rev. C9 (1974) 603 J. Urbon, D. G. Sarantites and C. C. Rutledge, Nucl. Instr. 126 (1973) 49 I. Kh. Lemberg and A. A. Pasternack, Izv. Akad. Nauk USSR (set. fiz.) 40 (1976) 2052 H. V. Klapdor and H. Willmes, MPIH-1976-v6 E. Der Mateosian and A. W. Sunyar, Atomic Data and Nucl. Data Tables 13 (1974) 407 D. T. Kelly, P. W. Green and I. A. Kuchner, Nucl. Phys. A289 (1977) 61 M. Ivascu et al., Nucl. Phys. A225 (1974) 357 C. Alaga and V. Paar, Phys. Lett. 61B (1976) 129 M. F. Kudoyarov, I. Kh. Lemberg, A. A. Pasternack and L. A. Rassadin, Yad. Fiz. 27 (1978) 577