High-spin states in 69Ge from the 55Mn(16O, pn) reaction

1.E.I: 3.A

Nuclear Physics A330 (1979) 216-224 © North-Holland Publishing Co., Amsterdam

Not to be reproducedby photoprint or microfilmwithout written permissionfrom the publisher

HIGH-SPIN STATES IN 69Ge FROM THE 5SMn(160, pn) REACTION T. PARADELLIS Nuclear Research Center Demokritos, Athens, Greece

G. J. COSTA and R. SELTZ Centre de Recherches Nucl~aires et Universit~ Louis Pasteur, Strasbourg, France

C. LEBRUN, D. ARDOUIN and F. GUILBAULT [nstitut de Physique de Nantes, France

and M. VERGNES and G. BERRIER Institut de Physique Nucl~aire, Orsay, France

Received 28 March 1979 (Revised 20 June 1979) Abstract: Electromagnetic properties of excited states of 69Ge have been studied using the 5SMn(160, pn) reaction. Investigation of the decay scheme included branching ratios, mixing ratios and angular distributions. Assuming that 69Ge is described by a system consisting of a 68Ge excited core coupled with a neutron in the g9/2 state, we obtain a striking similarity in the two modes of decay.

NUCLEAR REACTION 55Mn(160, pny), E = 47.5, 42.5 MeV; measured E v, Iv(E), Iv(0), yy-coin. 69Ge deduced J, y-branching, y-mixing ratios. Natural target.

1. Introduction T h e l o w e r - l y i n g states of 69Ge h a v e r e c e n t l y b e e n e x t e n s i v e l y s t u d i e d by F o r s s t e n e t al. 1) a n d b y E b e r t h e t al. 2,10) using the 66Zn(o~, n'y)69Ge r e a c t i o n . V e r y d e t a i l e d

i n f o r m a t i o n for l o w - l y i n g l o w - s p i n states has also b e e n o b t a i n e d b y P a r a d e l l i s et al. 3). A l l the a b o v e - m e n t i o n e d w o r k s give s u p p o r t to a d e s c r i p t i o n of 69Ge in t e r m s of a t h r e e - p r o t o n cluster c o u p l e d to a v i b r a t i n g nickel core, at least up to an e x c i t a t i o n of a b o u t 1.5 M e V [ref. 3)]. In c o n t r a s t , i n f o r m a t i o n c o n c e r n i n g t h e s t r u c t u r e of highspin states r e m a i n e d scarce a n d l i m i t e d to t h e o b s e r v a t i o n of levels with j r < ~ ÷ [refs. 1,2,10)]. In the p r e s e n t w o r k , the 55Mn(160, p n y ) r e a c t i o n was used to p o p u l a t e h i g h - s p i n states in 69Ge. Since v e r y d e t a i l e d w o r k b y d e L i m a et al. 4) has r e v e a l e d states in 68Ge up to an e x c i t a t i o n of a b o u t 7 M e V , it was felt t h a t a c o m p a r i s o n of t h e i r results with results f r o m t h e p r e s e n t w o r k c o u l d give s o m e b e t t e r u n d e r s t a n d i n g 216

HIGH-SPIN STATES IN 69Ge

217

of the mechanism of particle-core coupling at higher excitation energies in the mass region around A = 70. During the revision of this paper, the results of a similar study by Zobel et aI. 5,6) appeared, where states up to 4595 keV excitation are observed.

2. Experimental details

The targets used were 1 m g / c m 2 pure 55Mn deposited by vacuum evaporation onto 5 and 10 ~g of Au. The 5* 160 beam was supplied by the Strasbourg MP tandem accelerator. Various Ge(Li) detectors with 2-2.5 keV resolution have been used to study the y - r a y spectra, excitation functions, y - y coincidences and angular distributions. 25/2-(29~-)

6592.9

6505,4

58

27/2-

~

5836.7 2

~

2"9 [

23/2-

UN

21/2- 3 2 7

[1248

4598.5

I

25/2-

I 872 I

19/2-

525 318~J--~ 19/2-

3750.4

,

4068.7

592

17/2+

17/2+

1662 I

3606.2 2.5--

T3

3158.0

13 5

15/2 (-

3076.8

313

t22

573

2756°0 1140

1348

I

(~7/2.~.~

J 99z

5007.7

39

j

845

5_] 518

~

I I

~

4268.6

5595.3

5467,0 - - --; - --

|

242

{5802) 19/2"~21/2-, 23/2-

13/2 + --

~_~

2483.7 1669

Y 2018.6 113

IT--'-

6

668 13/2 +

1407.6 11/2 +

°°t -r

9/2 +

1350o9

153 398.0

5/2-

% 32 @ Fig. 1. Level scheme of 69Ge. Relative intensities of the 7-rays are indicated in the arrows. The dashed horizontal lines are extension of the other levels.

28

5

591.8(3)

2.5

1122.5(5)

994.4(3)

3.5 d) 3158.0

3750.4

3750.4

3606.2

3158.0

3076.8

2756.0

2483.7

673.3(5)

7 d)

1076.0(2)

2483.7

8

8

1133.0(2)

2018.6

1139.4(5)

11

667.7(2)

1407.6

12

72

1009.6(2)

1350.9

1669.3(2)

24

952.8(2)

398.0

37

100

398.0

Ei

1348.5(2)

l v a)

E-~ (keV)

1

3158.0

2756.0

2483.7

2483.7

2018.6

1407.6

1407.6

1407.6

1350.9

1350.9

398.0

398.0

0

Ef 5

13+

13+

17+

15+

~ --,~+

~---, ~+

(~7) _., ½5+

T ~T

17+

~+ --, ~+

~ ...,.~3+

T --'Y

T ~T

15+

~+ --, ~+

13+ ~- ~11+

~+ __,9+

~+-~ ~+

~ -~

9+

J~- ~ J [

(i) -0.22(2) (ii) -0.22(3)

(i) -0.25(2) (ii) -0.24(2)

(i) -0.37(10)

(i) 0.26(4) (ii) 0.26(5)

6)-0.21(3) (ii) -0.23(5)

(i) 0.29(2) (ii) 0.31(2)

(i) (ii) 0.26(8)

(i) 0.27(2) (ii) 0.31(5)

(i) 0.25(2) (ii) 0.24(2)

(i) 0.26(1) (ii) 0.25(1)

(i) 0.39(1) (ii) 0.37(2)

Az b)

0.03(3) -0.04(4)

0.05(3) -0.01(3)

0.10(11)

-0.10(5) -0.09(6)

0.00(3) -0.01(6)

-0.12(2) -0.13(3)

0.14(10)

-0.06(3) -0.11(6)

0.06(2) 0.04(3)

-0,06(1) 0.06(1)

0.09(1) 0.07(2)

A4

0.72(3)

0.73(3)

0.62(2)

0.65(5)

0.69(5)

0.75(5)

0.59(3)

0.59(2)

0.55(2)

OL2

OL4

0.65(5)

0.67(5)

0.50(5)

0.50(5)

0.67(5)

0.70(5)

0.45(5)

0.28(5)

0.37(5)

Angular distributions and mixing ratios of transitions in 69Ge (Ep = 47.5 MeV)

TABLE

E1

E1

(M1) (M2/E1)

E2

E1

E2

E2/M1

E2

E2/M1

E2

E2/M1

(j¢~)

-0.01 ±0.02

-0.03 :i: 0.02

-0.1±0.1

-0.01 ± 0.04

0.35±0.15

0.45±0.10

0.65±0.12

~ c)

7"-

),

tO

2 a)

11.5

5

4

5

5

10

7

6

662.4(5)

326.9(2)

844.8(4)

526.7(3)

739.1(3)

1326.7(5)

1241.2(5)

1247.4(5)

662.5(5)

6592.9

6505.4

5842.9

5836.7

5595.3

5007.7

4595.5

4595.5

4595.5

4268.6

4268.6

4068.7

4068.7

3750.4

5836.7

5842.9

4595.5

4595.5

4268.6

4268.6

4068.7

3750.4

4268.6

3606.2

3750.4

3750.4

2756.0

3076.8

17T

23-

~ - ~ z~29 T ~ 27

23 23 2---"T T19~ T2 3 -

T "-'T

27

(~9-) _~~ (~-)~( ~ - ) -, ~ -

T ~ T2 1 -

25-

~---, ~ -

~-~-

~ - ~ z~-

~-

21- --'T 19T

~--~ ~-

~9 __,½7+

19- ~ TisT

(ii) 0.05(7)

(i) 0.21(4)

(i) -0.17(5)

(i) 0.27(6)

(i) 0.15(8)

(i) 0.28(5)

(i) 0.35(6)

(i) 0.27(7) (ii) 0.28(9)

(i)-0.39(3) (ii) -0.38(4)

(i) -0.10(1) (ii) -0.08(3)

(i) -0.25(5)

(i) -0.26(5)e)

0.07(8)

-0.09(4)

0.06(6)

-0.08(6)

0.05(8)

-0.13(6)

-0.08(7)

-0.06(8) -0.14(10)

0.13(5) 0.01(5)

0.04(2) 0.01(4)

-0.06(6)

-0.07(5) e)

0.80(5)

0.80(5)

0.80(5)

0.80(5)

0.78(3)

0.78(3)

0.75(3)

0.75(3)

0.75(5)

0.75(5)

0.75(5)

0.75(5)

0.75(5)

0.75(5)

0.70(5)

0.68(5)

E2/M1

E1 or M1 E1 or M1

E2

E2/M1 E2/M1 E2/M1

E2

E2

E2

E2/M1

(E2)

E2/M1

E1

E2

-0.15±0.05 0.17±0.04

0.03±0.03 0.00±0.03

-0.23±0.06 -0.40±0.10 -0.25 ± 0.05

-0.11±0.04

0.08±0.01

-0.02 ± 0.04

Intensities are from the 47.5 MeV experiment. (i) refers to the 47.5 MeV experiment while (ii) refers to the 42 MeV experiment. The Krane and Stetten phase convention is used. Doublets. Doublet. The measured angular distribution coefficients are A2 = 0.18 + 0.02, A4 = -0.06 ± 0.02. The quoted result is after subtraction of the assumed M1, 0.3 in y-ray intensity from the deexcitation of the 3158 keV level.

a) b) c) a) ~)

13.5

518.2(2)

756.3(5)

1 d)

2

1312.8(5)

318.0(1)

8 a)

673.3(3)

7~

t-~

7~

,-] >

6~

220

T. PARADELLIS et al.

G r e a t effort has been paid in studying in detail the 55Mn(160, x n y p y ) reaction in the energy interval from 37.5 to 50 M e V bombarding energy. Nine exit channels have been identified in the reaction and their formation cross sections have been determined. This study was based mainly on the results of the 3'-7 coincidence experiments and the yield m e a s u r e m e n t s from radioactivity and on-line experiments. The results of this work will be reported in a separate publication 7). In summary, the two dominant exit channels in this reaction are the pn and 2pn channels. The y - y coincidences were studied at 47.5 M e V bombarding energy while angular distribution data were taken at 42.5 and 47.5 MeV. The decay scheme determined in this work for 69Geis shown in fig. 1. It is mainly based on the results of the y - y coincidence experiments and partly on energy and intensity considerations. In determining the spin and parities the following arguments were used: (a) Since all the y-rays shown in fig. 1 have been observed in the coincidence experiments with a resolution of 25 ns we assume that the half-life of the level involved is shorter; (b) the ratio R of the intensities of the y-rays at 42.5 and 47.5 M e V b o m b a r d i n g energies (normalized to the 398 keV y-ray); and (c) the results of the angular distributions. In analyzing the angular distribution data the formalism of Yamazaki*) was followed. The attenuation coefficients ce2 and OL4 were determined for levels deexciting by pure E2 transitions, according to the method of Samuelson et al. 9). These values were then used for neighbouring levels taking into account the deorientation due to y - r a y emission. The procedure was repeated until a consistent set of a2 and a4 values was obtained. For levels above 4268 keV, due to a lack of sufficient data, the a2 and a4 values deduced from the 4268 keV level are used. In table 1 the results of the angular distributions and their analysis are presented for the 42.5 and 47.5 MeV experiments. The angular distribution data at the lower energy were very useful in cases where peaks were masked at 47.5 M e V by y-rays belonging to other nuclei. 3. The level s c h e m e of

69Ge

The 398, 1350.9, 1407.6 and 2018.6 keV levels have well-established spins and parities 1-3). Our results (table 1) agree with the previous data. T h e 2 4 8 3 . 7 k e V l e v e l . In ref. 2), two 2484 keV levels are adopted to a c c o m m o d a t e 1133 and 1076 keV y-rays which deexcite a ~+ and a ~+ level. Although the 1076 keV y - r a y is masked in our 47.5 M e V experiments from an equivalent 68Ge y-ray, we have used the coincidence data to determine the relative intensity data of the two 1076 keV y-rays. Using the known distribution of the 68Ge peak 11,12), w e arrive at an angular distribution result identical with the 42.5 MeV one, where no 68Ge contamination exist. Both angular distributions do not support a 1076 keV y - r a y originating from a ~z+ level and are consistent with deexcitation of a ~+ level, as the 1133 keV y - r a y does. Noting the constant ratio of these two y-rays we observe at lower energies, we adopt only one 2484 keV ~+ level.

HIGH-SPIN STATES IN 69Ge

221

The 2 7 5 6 . 0 k e V level. This level was weakly excited in the work of refs. 1,2). F o r s s t e n et al. 1) p r o p o s e d ~ ÷ , ~ + or ~ + for this level. O u r yield data (table 2) a n d a n g u l a r d i s t r i b u t i o n d a t a f a v o u r a p u r e E2, 1348 k e V y - r a y d e e x c i t i n g the 2756 k e V ~ + level. TABLE 2 Relative yield ratios, normalized to the 398 keV y-ray E v (keV)

Level

398 a) 1009 953 668 1133 1669 1348 1140 994 592 673 1313 518 327 845 525 1242

398 1408 1351 2019 2484 3077 2756 3158 3750

756

1248 662 740 1327

R = I(47.5)/I(42.5)

5836

1.0 1.30 + 0.05 1.0 +0.10 1.1 ±0.10 1.4 ±0.10 1.4 +0.10 1.7 ±0.10 1.5 +0.20 1.85 ± O.10 ] 1.80±0.152 1.90±0.10) 1.9 ±0.2 2.3 ±0.1 2.5 ±0.1) 2.5 ±0.22 2.5 ±0.2) 4.0 +0.5

6593

3.5

±0.5

5842 6505 5007 5595

2.3 2.2 3.0 2.2

±0.3 ±0.3 +0.3 4-0.3

4069 4269 4595

Spin assignment 9 13 ~11 ~13 ~15 ~15 ~17 y 15 17 2, 2 192,217 17

19

21

2, 2, 2 21 2 ~ 19 2 232,221 27

29

2, 2

25 27 2 9 2, 2, 2 23 21 19

2, 2, 2 21 19 2, 2, 2 25 2 7 2, 2, 19 2 l 23 2, 2, 2

23

a) Normalizing y-ray.

The 3158 ke Vlevel. B o t h the a n g u l a r d i s t r i b u t i o n a n d excitation ratio indicate that the spin a n d parity of this level is ~ + , deexciting with a 1140 k e V E 2 transition. The 3 0 7 6 . 8 k e V level. This level has b e e n o b s e r v e d in refs. 1,2). It has b e e n assigned spin a n d parity ~ + f r o m the a n g u l a r d i s t r i b u t i o n data of the 1669 k e V y - r a y d e e x c i t i n g this level. T h e d e t e r m i n e d m i x i n g ratio of this y - r a y was 6 -- + 0.14 ± 0.05 [refs. 1,2)]. O u r result agrees with a J = ~ a s s i g n m e n t . T h e p r e s e n t l y d e t e r m i n e d mixing ratio 8 = - 0.01 + 0.04 a n d the u p p e r limit 5) of 0.5 ± 0.1 ps for the lifetime of this level are n o t i n c o n s i s t e n t at all with a n e g a t i v e parity a s s i g n m e n t . A positive parity a s s i g n m e n t to this level leads to a n E 2 c o m p o n e n t s t r e n g t h f o r ' t h e 1669 k e V y - r a y of less t h a n 0.3 W.u. / ,r17+ The 3750 keVlevel. This level deexcites m a i n l y to the 2756 K e v ~- state with the 994 k e V y - r a y . T h e a n g u l a r d i s t r i b u t i o n d a t a for this y - r a y a n d the 592 k e V y - r a y which deexcites also this level to the ~ + 3158 k e V a l o n g with R = 1.85 (table 2)

222

T. PARADELLIS et al.

allows a unique J = ~ spin assignment to this level. The mixing ratio of both transitions is zero, indicating a parity change. For reasons also connected with the physical interpretation of the levels in 69Ge (see discussion) we adopt negative parity for this state. The measured polarization of the 994 keV y-ray by Zobel et al. 5,6), p = 0.32 + 0.18, gives further confidence in the ~ - assignment to this state. Thus, fixing the spin and parity of this state to ~ - , the angular distribution of the 673 keV y-ray decaying to the 3077 keV level by an E2 transition results in a unique ~ - spin and parity assignment for the 3076.8 keV level. The 4 2 6 8 ke V level. The measured angular distribution of the 518 keV transition is consistent with ~ or ~z spin assignment of this state. The yield ratio favours the z~ spin assignment. The mixing ratio of 8 = 0 . 0 8 + 0 . 0 1 measured here is somewhat lower than the value measured (6 = 0 . 1 7 ± 0 . 0 2 ) by Zobel et al. 5,6), but both 6's, along with a lifetime r = 4.2 ps measured by the same authors, indicate an E 2 / M 1 character of the 518 keV transition, and thus a unique z~ spin and parity assignment can be made to the 4268 keV level. The 4 5 9 5 k e V 'level. This state decays mainly with the 327 keV y-ray to the 4268.6 keV ~ - level. The measured angular distribution of this transition is influenced at the 47.5 MeV experiment by the neighbouring 328 keV y-ray from 66Zn. This influence is smaller in the 42.5 MeV experiment. Both the angular distributions and the yield ratio favour a z~ spin assignment. The E2 character of the 845 keV transition to the 3750 keV ~ - state indicates a unique ~ - spin and parity. The 5836. 7 k e V level. The yield ratio of 4 + 0.5 suggests J > z~ for this level. The angular distribution of the 1242 keV y-ray deexciting this level to the z~ , 4595 keV level is consistent with an E2 transition and thus a ~ - spin and parity is assigned to this level. The remaining levels. From the other levels a definite spin and parity assignment can be made to the 4068 keV level (1~-) and to the 5007 keV level (z~-). For the remaining levels no definite assignment can be made. 4. Discussion In fig. 2, the levels of 69Ge through which the bulk of deexciting y-rays proceed are shown in comparison with levels of 68Ge through which also the main bulk of decay occurs 4.11). Assuming that 69Ge is described by a system consisting of a 68Ge excited core coupled with a neutron in the g9/2 state we get a striking similarity in the two modes of decay. Thus the ~+ and ~+ states at 1.4 and 2.75 MeV may be described as 2 + Q g9/2 and 4 + ~ g9/2 states. The measured mean lives of these two states 10) indicate B(E2) values of 1 9 + 6 and 3 3 + 7 W.u. for ~ + ~ + and ~ ÷ _ 9 + respectively and are to be compared with the corresponding 4 + - 2 + and 2++ 0 ÷ B(E2)'s of 13 and 9 W.u. respectively 11).

HIGH-SPIN STATES IN 69Ge

5439 27/2-

223

~

(9-__)--5331

25/2-

4810

4198 23/2-73 6 9 6 6 " J 5 -, 3649

3871

+ 4054

4958

3883

3352 19/2----'

'~15~- 2679 2358 17/2+

1010 13/2"1"

9/2 + ¢ :2SGe ® (g 9/2)

4+.__~t_~____

2+

0

~

~ J L - -

-~-2649

12268

1016

f

% 32 e

Fig. 2. Comparison of the two modes of decay in 69Ge and 68Ge.

The 3750 keV state has been assigned negative parity in the present work on the grounds of 8 = 0 transitions deexciting this level. If this state was ~+ it could result only from a 6 + <~ g9/2 coupling. But then the transition to the ~+ state should contain a large fraction of an E2 component since the corresponding 6+-->4 + transition in 68Ge has B(E2) = 12 W.u. The observation of a ~+ -->~+ transition with • = 0 would then be incomprehensible. It is also obvious that in a (6- or 7-) <~) f5/2 description of a possible 3750 keV state it cannot decay strongly to a 4 + Q g9/2 state. Thus, trusting our ~ - parity assignment, this state can be interpreted as 5- ~) g9/2 which indeed should decay strongly by an E1 transition to the 4 + ~ g 9 / 2 . The other ~ - state at 4068 keV can also be considered as a g9/2 neutron coupled to the second 5- state observed at the same excitation energy (fig. 2). The two states at 4268 and 4595 with ~ - and z~- may very well be described by coupling the g9/2 state to the 6 - and 7states respectively.

224

T. PARADELLIS et al.

T h e state at 5837 k e V ~ - m a y b e d e s c r i b e d as a 9 - (~) g9/2 state. A state, with p o s s i b l e spin a n d p a r i t y 9 - , has b e e n o b s e r v e d at t h e right e x c i t a t i o n e n e r g y in 68Ge [ref. 4)]. T h e s t a t e at 3076 k e V , a s s i g n e d h e r e a ~ - spin a n d p a r i t y , m a y b e the 3 - @ g9/2 c o n f i g u r a t i o n . T h e 3 - state d e c a y s to the 2 + state with a 1633 k e V t r a n s i t i o n with 11) +2 r = 2.5 -1.5 ps a n d ~ = - 0.16 + 0.20 while t h e m e a s u r e d r for t h e 3 1 5 8 k e V ~ + level is r = 0 . 5 + 0 . 1 ps. T h e o t h e r l o w - l y i n g states also o b s e r v e d in t h e p r e s e n t w o r k m a y b e d e s c r i b e d by c o u p l i n g a g9/2 n e u t r o n to o t h e r 68Gestates. It is u n f o r t u n a t e t h a t t h e ~ - state which s h o u l d r e s u l t f r o m t h e 6 + @ g9/2 state has n o t b e e n l o c a t e d . It is p o s s i b l e t h a t this s t a t e b e i n g s o m e w h a t l o w e r t h a n the [ 6 - , 7 (~) g9/2] ~-, ~- c o n f i g u r a t i o n s c a n n o t c o m p e t e with t h e h i g h e r spins a n d thus all d e c a y s p r o c e e d t h r o u g h the ~ - a n d spin states. T h e s i m i l a r i t y b e t w e e n t h e 69Gea n d 68Gem o d e s of d e c a y c o u l d b e c o r r e l a t e d with t h e e x i s t e n c e of d e c o u p l e d b a n d s p r e v i o u s l y o b s e r v e d 12) in o t h e r o d d nuclei of this region. T h e o c c u r r e n c e of t h e h i g h - s p i n g9/2 o r b i t a l , which b e g i n s to fill a r o u n d N = 3 8 40, c o u l d t h e n b e r e s p o n s i b l e for t h e o b s e r v a t i o n of states with a n g u l a r m o m e n t a given b y t h e a d d i t i o n of t h e c o r e plus t h e p a r t i c l e m o m e n t a , the C o r i o l i s i n t e r a c t i o n being maximum. T h e n e g a t i v e p a r i t y states of 68Ge ( 3 - , 5 - , etc.) built with o n e p a r t i c l e in the 1 g o r b i t a l s , s e e m to give rise to similar effects in t h e e x p l a n a t i o n of the 69Geo d d p a r i t y levels. O n e of the a u t h o r s (T.P.) g r a t e f u l l y a c k n o w l e d g e s the h o s p i t a l i t y e x t e n d e d to him b y the C R N S t r a s b o u r g a n d the G r o u p e d e s Basses E n e r g i e s .

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12)

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