A study of the 16O(6Li, t)19Ne reaction

Nuclear Physics A196 (1972) 145--155; (~) North-HollandPublishiny Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

A STUDY OF THE 160(6Li, t)19Ne REACTION A. D. P A N A G I O T O U t a n d H. E. G O V E tt

Nuclear Structure Research Laboratory, University of Rochester, Rochester, N Y 14627, USA t t t Received 28 F e b r u a r y 1972 (Revised 16 A u g u s t 1972) Abstract: T h e reaction x60(6Li, t) t 9Ne h a s been e m p l o y e d in the s t u d y o f 19Ne with 35 a n d 36 M e V 6Li ions. Triton spectra were obtained covering an excitation energy in 19Ne up to 10 MeV. Some new strongly excited states were found. A n g u l a r distributions were obtained between 6 ° a n d 39 °, in 3 ° steps. O n the basis o f the a n g u l a r distributions a n d cross sections the following spin a n d parity a s s i g n m e n t s were m a d e : ~ + , 4.61 MeV; .~+, 5.41 MeV; ( ~ - , ~ - ) , 6.27 M e V a n d ( ~ - , ~ - ) , 6.83 MeV. A c o m p a r i s o n with the m i r r o r nucleus 19F is made.

El

NUCLEAR

R E A C T I O N S 160(6Li, t); E = 35, 36 MeV; m e a s u r e d tr(Et, 0). 19Ne deduced levels, J, ~, l.

1. Introduction The (6Li, d) and (6Li, ~) reactions have been shown 1-3) to proceed mainly by a direct mechanism, and this has been understood on the picture of 6Li = ~ + d and the small energy (1.472 MeV) required for the break-up of 6Li into an a-particle and a deuteron. A number of studies 4-6), however, have also shown appreciable t + 3He clustering in the ground state of 6Li. On this basis one might expect the reactions (6Li, t) and (6Li, 3He) to proceed with a substantial direct component. This direct component is expected to be larger the higher the incident energy, by analogy with the (6Li, d) reaction. It may therefore be possible to assign angular-momentum transfer values to the triton angular distributions and to extract spectroscopic information for the residual nucleus. The present experiment employed the reaction (6Li, t) to study the levels of 19Ne" Since the levels of the analogue nucleus ~ 9F are known up to about 8 MeV, a comparison between the analogue states of 1 9 N e and ~ 9 F is of some interest. + Present address: University o f Oxford, D e p a r t m e n t o f Nuclear Physics, Keble Road, Oxford, England. tt Present address: Laboratoire de P h y s i q u e Nucl6aire et d ' l n s t r u m e n t a t i o n Nucl6aire, BP 20 C R O , 6 7 - S t r a s b o u r g 3, France. ÷÷e Research s u p p o r t e d by the National Science F o u n d a t i o n . 145

146

A. D. PANAGIOTOU AND H. E. GOVE

2. Experimental procedure A 6Li-1 beam was obtained from the duoplasmatron source of the University o f Rochester's MP tandem accelerator. More than 1 pA of 6Li + 3 was analyzed and more than 700 nA were available on target. However, at most half of this amount was actually used. The target was self-supporting A 1 2 0 3 , 60 pg/cm 2 thick. A surfacebarrier detector positioned at 45 ° to the beam served as a monitor. The reaction particles were momentum analyzed and detected on the focal plane of the Rochester split-pole magnetic spectrograph with a 50 cm wire-proportional counter. Deuterons, tritons, 3He and 4He particles were separately identified by their energy loss in the counter. The momentum spectra of the four types of particles were stored in four buffers of the on-line PDP-8 and PDP-6 computers. This particle identification was particularly important for tritons and deuterons because, owing to the negative Q-value ( - 7 . 3 5 MeV) for the 160(6Li, t) reaction and the positive Q-value for the 160(6Li ' d) reaction, the triton and deuteron groups leading to the ground states of 19Ne and 2°Ne had approximately the same momentum. Two incident energies were used, 35 and 36 MeV, the latter being about the limit for bending the triton group leading to the ground state of 19Ne in the spectrograph. Angular distributions fbr the strongly excited levels in ~9Ne up to 6.83 MeV excitation were obtained between 6 ° and 39 ° in the laboratory, in 3 ° steps. The beam current was integrated and this together with the known target thickness allowed the calculation of absolute cross sections to about + 20 ~o. Excitation energies are accurate to about ± 4 0 keV.

3. Results As discussed above, one would expect the reaction (6Li, t) to proceed mainly by a direct mechanism at these high energies. Thus, starting with a closed 160 core, the addition of the three nucleons will populate the positive-parity (sd) 3 levels and the negative- and positive-parity states arising from the configurations [(sd)Zf I] and [(sd)lf z] in 19Ne. The latter two configurations, however, should appear at excitations of very roughly 7 MeV and above xs). The cross section to negative-parity (p-shell hole) levels will give some indication of the compound-nucleus contribution to the reaction mechanism. The 2p-2h and 4p-4h impuritxes of the 16 0 ground state will also contribute to the population of the hole states but only to about 20 °o [ref. 7)]. Furthermore, the addition of three nucleons to the closed core of 160 will strongly populate only the (60) configuration, i.e. the leading SU(3) representation of the maximum orbital symmetry [f] = (3). This has been found to be the case for the 160(6Li ' d)2ONe reaction s), which populates strongly the positive-parity states, (sd) 4, which belong to the (80) representation only. Fig. 1 shows triton spectra obtained at 12 ° with E6Li = 36 MeV, and covering an excitation energy in ~9Ne up to 10 MeV. The strong selectivity of the reaction can be clearly seen. Only the states built on the 160 core are strongly excited, as indicated by

160(6Li, t ) l g N e

147

the known levels at low excitation. The weak excitation of the negative-parity hole states indicates that the main component of the reaction is direct. It is also interesting to note that among the known levels of the ground-state band, the ~+ state at 2.78 MeV is the most strongly excited whereas the l + ground state is very weakly populated, indicating that the reaction favours high-spin states in the residual nucleus. The levels above 5.4 MeV have not been seen previously 9) with this reaction, and on the basis of the systematics of the reaction, it is probable that they are high-spin states with J __> -2 5. 6OO,

9.38

E ~'LI = 36 M~tV eto b ~ 12"

8.70

S.41

300

6.,3

J ~J Z Z

il 6.27

!

"r U

79

II

I 6.12

i

461

f'l

"~

n, iii n ,

u) IZ 3 0 U

'

,

0

,

,

,

;

,

I

,

,Oi...L

I I00

500

600

2.77

5.41

300

4.61 Ii

6.83 7 55 I • 7 20 J ' I II

O

6 27 6 '2 " ,i [l

~1 II 4 2 0 "l

114"38,1~ 4.1S

500

1538

0.241

J i

1.61 ~]

,

CHANNEL

IIOO

Fig. 1. Triton spectra obtained at 12 ~ in the lab system from the reaction 160(6Li, t)lgNe, at E%i = 36 MeV.

Since there is considerable parentage for 6Li = t + 3 H e in a relative s-orbit, and because the reaction mechanism is mainly direct at this incident energy, the positiveparity (sd) 3 levels in 19Ne will be populated via an even angular-momentum transfer. That is: J + ( ' O N e ) = 1. . . . +½+,

148

A . D . P A N A G I O T O U A N D H. E. G O V E

where ½+ is the spin o f the transferred 3He. Thus the ½+ g r o u n d state will be p o p u lated b y a n I = 0 transfer, the~ + a n d ~ + levels with l = 2, the ½+ a n d ~ + with 1 = 4, etc. Fig. 2 shows the angular distributions obtained for the k n o w n positive-parity states o f the K = ½+ band. The difference between the l = 0 and l = 2 transitions is evident, not only f r o m the shape o f the distributions but also from the magnitude o f the cross

1.0

\.

\

\"k

~60(%+.t I '~N¢ E6Li = 36 McV

"\.\ \

L.

0.1

b ',

\

\\ ._

-..

\'~

/',.

\

',,,/ \ ",.:

\o~'-4(~-

,+ "

2.77McV)

+...v,

),/ ',,

! t l

i

OX31

!

'~ p ~'-0(~ ,+; g.s.}

:

,

Oo

i tO o

210 o

3'0 o

4tO o

5JOo

6tO o

Oc.l'it.

Fig. 2. Angular distributions of the four lowest positive-parity states, assigned to the K -- I + band.

section. The peak cross section for the ½+ ground state is about twice and four times smaller than that for the ~3 + and ~ + states respectively. The differences between the 1 --- 2 and 1 = 4 transitions are also evident, f r o m the intensity and the shape o f the distributions. The l = 4 transfer appears to have a more slowly decreasing distribution than that of the I = 2, while the cross section is about twice as large. It is also interesting to note that the angular distributions with 1 > 2 vary smoothly as a

160(6Li ' t) 19Ne

149

I0"0 I •r--..~"

I('0( 6Li" t )19N¢

;F-..:'"---.

F

E('L, = 36 MeV

E "'%P " " - t .

F

I = 1.0 t

"--...,,

"'\.

,a

• ,,

~",~..

\

"'... k,,,.

"'~

"---,.~ ~'.,=.t,

6 (4.61 MeV )

e=(s.61(6.83 M,vl

""',,,, ~

t

.# =

03 l

" " " "~"" '~

=4 (5.41

\,,...j /

I

b

I I0 °

Oo

I 20 °

l 30 °

MeV)

.e = (3)(6.27MeV)

I 40 °

l SO °

t

60 °

ec.m.

Fig. 3. Angular distributions o f the four strongly excited states between 4.5 and 7 MeV excitation.

I0

t60 (6Li,t)19Ne

" . - ~ .....

I'--

~O~Q~.~..

z

MeV

C,?o

n,"

0 I,--

E 6L i = ~

•

"'""D.

\o ),..g%., × "~

I.O

"o~

laJ U3 (,/3

•£ =2

o¢r"

o/~

6

× /e-4 o~o

0.1

o°

t

,o°

L

2o°

3'o-

Co°

5b°

6~"

OCM. Fig. 4, Comparison o f the angular distributions for the different/-transitions. The 1 = 2 and l = 4 distributions are those o f the ~+ (0.24 MeV) and ]+ (2.77 MeV) respectively. The I = 6 distribution is for the J,2-+ (4.61 MeV) state.

function of angle but have a characteristic slope for each l. That is, the l = 2 distribution has the largest rate of change of cross section with angle. The I = 4 has a more gradual slope at forward angles (0 < 25°), while the l = 6 distribution has the smallest overall slope.

150

A.D. PANAGIOTOU AND H. E. GOVE

Fig. 3 shows the angular distributions for the four strongly excited levels at 4.61, 5.41, 6.27 and 6.83 MeV. These distributions and cross sections, when c o m p a r e d with those o f the known states, indicate the following angular m o m e n t a for the transitions and the corresponding spins: 4.61MeV,

1 = 6,

j ~ = - x11+. 1.3+, - ,-~-

5.41 MeV,

l = 4,

J " = ~+" 9z+

6.27 MeV,

1 = (3),

J~ = ( ~ - ; ~ - ) ,

6.83 MeV,

l = (5, 6),

J~ = ( 3 - ; 11.- )7. (11+;

½3 .).

In fig. 4 the shapes o f the angular distributions for the different/-transitions normalized to the most forward angle are compared. 3.1. THE 4.61 MeV LEVEL The angular distribution o f this level indicates an l = 6 transfer corresponding to a spin o f 12-~+ or 1@+. The 1@+ member o f the K = ½+ band in 1.9F is k n o w n to be at 4.65 MeV [ref. 1.o)], lying lower than the J~-+ member. In view o f this the level at 4.61 MeV is assigned a spin of ~ - + and considered as the member o f the K = ½+ band o f 1.9Ne. A similar assignment has been made 9) for the same level through a comparison o f the a60(6Li, t)19Ne and 1.60(6Li, 3He)1.9F spectra. 3.2. THE 5.4l MeV LEVEL This level has an 1 = 4 angular distribution and thus has J'~ = ~+ o r 3 +. Its cross section is smaller than that o f the 3 + level at 2.77 MeV. Furthermore only one ~}+ state is expected at low excitation, that o f the K = ½+ band. Based on this evidence the level at 5.41 MeV is assigned the spin o f ~ +. A level at 5.43 MeV has been given the same assignment 9) and due to the absence o f any other strong positive-parity states in the immediate region the two levels are unquestionably the same. 3.3. THE 6.27 MeV LEVEL The angular distribution for this state does not appear to have either a g o o d l = 2 or l = 4, for the angular m o m e n t u m transferred. The correct assignment may be l = 3, although there are no k n o w n 1 = 3 angular distributions with which a comparison can be made. Such an assignment would mean that the state has negative parity and a spin o f either ~ or ~, obtained by raising one particle into the f~ shell. Estimates 1.5) o f the excitation energy for such a configuration,[(sd)Z(f)1.], place it roughly at about 7 MeV. It is likely, therefore, that the level at 6.27 MeV arises from a single-particle excitation into the f~ shell. 3.4. THE 6.83 MeV LEVEL The angular distribution leading to this level corresponds to either l = 5 or 6. This ambiguity is not resolved by considering the magnitude o f the cross section,

160(6Li, t) ~9Ne

151

because the two different /-values suggest different configurations and therefore different intensities. An l = 5 assignment would mean an [(sd)2f 1] structure with negative parity and a spin of ~ or -12 -1-. In view of the likely assignment for the 6.27 MeV level discussed above such an assignment is a distinct possibility. An 1 = 6 assignment on the other hand, constrains the state to be either 121-+ or -~23-+.The -~-+ spin assignment would be excluded by the prediction 15) that the second -~-+ level lies at about 9.3 MeV. An J2 ~ + assignment for this state compared with the 1~+ level at 6.5 MeV in 19F, the lowest known 12-1-+ level [ref. lz)], creates a difference in energy between the analogue states of about 330 keV, which would be difficult to account for theoretically.

4. Discussion and conclusions

Much theoretical work has been done regarding the properties of the energy levels of 19F [ref. 13)]. Since 19Ne is the mirror nucleus the same calculations apply. It was suggested 13) that in 19 F the first -}+ level at 4.39 MeV does not belong to the groundstate band, ()40 = (60), but rather to the (2/0 = (22) SU(3) symmetry. This was experimentally verified by its weak (18 %) transition to the 9 + (2.78 MeV) member of the band. In the spectra of ~gNe, shown in fig. 1, as well as in those of ref. 9), the corresponding ½+ state at 4.38 MeV is very weakly excited, probably on account of its low (sd) 3 admixture. The ~+ member of the K -- ½+ band in ~9F is the 5.47 MeV 7+ state. The analogue level in 19Ne is assigned to be the z state at 5.41 MeV. Up to this excitation energy, all but the ~ - + member of the K = 1+ band in 19Ne have been found. It is interesting to note that up to the Q+ member at 2.77 MeV, the analogue states in 19Ne are at about the same energy as those of 19F, with the exception of the ~+ state. The 1@+ member in 19Ne ' however, appears to be about 40 keV below its analogue in ~9F, and the ~}+ member about 60 keV lower. It seems that at higher excitation, the analogue states of 19Ne lie at lower energies than those in 19F. This may be due to the fact that particle decay channels open at lower energy for 1'~Ne than for ~9F. If this trend persists one would expect the .~ + .member . of . the K - ~ + band in 19Ne to lie about 50-60 keV below the _1~_+ member of the same band in 19F. The lowest known 1~_+ state in 19F at 6.50 MeV has been assigned 13) to the (41) SU(3) symmetry and not to the K = ½+ band of the (60) symmetry. This has also been predicted by other full intermediate-coupling calculations ~4. 15), which produce more than one ½+ and one 121-+ state in the region 5-7 MeV in 19F, and which further predict that the lowest -~+ and J~ + states have a poor overlap withthe ~+ and J2 A+ members of the K = zl+ band. The analogue of the 19F -a2-1-+ state (6.50 MeV) should occur at about the same energy (or slightly less) in 19Ne. However, no strong level is observed in the vicinity of 6.50 MeV in the 19Ne spectrum. Very recently a second -~-+ level in t9F was found a 6) to lie at 7.94 MeV excitation. This level, however, although it decays 90 %

152

A. D. P A N A G I O T O U A N D H. E. G O V E

B60 (6Li,t)lgNe K=~/2+ ROTATIONAL

BAND

PEAK DIFFERENTIAL CROSS-SECTION E6t.i = 3 6 M c V 0 0

X

o o 1= i=

EC,Li =24 MeV

E~L ~ =35 MeV

, oJ

.llO IO0 .gO .80 70 60 50 40 30 20 I0

I00 90 8o

--~

70 60

40 30 20 I0

,/2 ,~= ,~ 7~ % ,,~ % J

Fig. 5. Peak cross section for the members o f the K = ½+ band at three different energies relative to that for the ~+ level at 2.77 MeV.

'60(6Li,t) rgNc ÷

K" 112R O T A T I O N A l B A N D INTEGRATED C R O S S - S E C T i O N E6t.i = 3 6 M e V 6o_<0_~ 3 9 °

E6, I ~35 M e V 6o_<# _<2 4 °

q

I00-

90. 0

o X

I00 • 90

80.

70 60-

.70 .60

bib

.SO

40

50. 30"

I

30 20

N ....

I0"

,10

O Fig. 6. Integrated cross section for the members of the K = ½+ band at two incident energies relative to that for the {+ level at 2.77 MeV.

~60(6Li, t ) l g N e

153

t~'N¢

t9F

K = '/~BAND

K-

(x.) = (6o) i L- 0.2.4.6

~ ~0."

107

>.-

19 9.-

9.-

kt.lz 8

. j~(.i/2+ )

8.-

.4

~.

L) 3:

3."

2."

2.-

L-

t-

ttl

'/~BAND

( x . ) - ( 6 o ) i L- 0,2.4 ' 6

O 0

6 2

20 4

42 6

L (L+I) L

~

,i,z+

~+J

/

~

0

6

20

42

0

2

4

6

Fig. 7. Plot o f the energy of the m e m b e r s o f the K = ½+ band in 19Ne and 19F ( F o r the definition of L see text.)

versus

L(L+I ) L

L(L=-I).

to the 1_~_+ (4.65 MeV) and 10 % to the ~+ (2.77 MeV) members of the K = ½+ band, was assigned to a K = ~+ band. It should be noted that this state lies about 30 keV above the level at 7.91 MeV in 19Ne" It is then conceivable that the two states are analogues, and may be the 1_~_+members of the K = ½+ band of 19F and 19Ne respectively. Figs. 5 and 6 show the peak and integrated cross sections for the members of the K = ½+ band of 19Ne at different incident energies. The peak cross section for the 24 MeV incident beam energy is taken from ref. 9). The anomalously small cross section to the -~+ and ~3+ levels was suggested 9) to be due to the Q-dependence of the reaction mechanism. Such an effect evidently does not occur in this experiment where the incident energy is 11-12 MeV higher. To summarize the present results, the levels found at 4.61 MeV and 5.41 MeV are assigned as the 13_+ and ~+ members respectively of the K -- ½+ band. Fig. 7 is a plot of the energy of the members of the K = ½+ band of both t9Ne and 19F versus L(L+ 1), where L is the orbital angular m o m e n t u m allowed for the K L = 0 + band of the (60) SU(3) symmetry. The 1-~-+ level at 7.94 MeV in 19F is assumed to be the 1_~/_+member of this band. A possible candidate for the analogue level in ~9Ne is the state at 7.91 MeV. This suggestion is derived from the systematics of the K = ½+ bands in both ~9F and a 9Ne. The level at 6.27 MeV in 19Ne is either a 25-or ~ - state, which together with the state at 6.83 MeV ( ~ - , / ~ - - ) and the other highspin states at excitations higher than 7 MeV may belong to bands with [(sd)2f 1] configurations. Finally, the negative-parity states at 1.61, 4.15 and 4.20 MeV as well as others not

154

A . D . P A N A G I O T O U A N D H. E. G O V E

identified arise most probably from the 2p-2h and 4p-4h impurities of the 160 ground state. 9.38

8.70

-

-

7.91

3/2'

7.55

-

(11/2-;912-)

-

- -

(512-~7/2-) -

-

7. 6 2

11/2" 7/2 + 5/2 +:

-

6.50 .6.34

720

6.83

b. 2 7 -

-

~6-29

5/2 b.17 ~ - - ' 6 , 1 0 S/2 " 1 ' "0.O7 "S -94

6.12

7/2 +

5.4l . . . . . . .

-

-

7/2 + /

S.63 \5.47

7/2y

\5.42

I/2 --, 5-34 512 ~ - - ' 5 . 1 0

1312

5/2 13/2 t 3/2 -

4.61

*

( 7 / 2 +) -

9/2 +

~

-

4.20 4.15

2,77

3/23/2*

1,61 1,54

5/2 +

0.241

I]2 +

9.s.

. . . . . . .

{gNe PF:tESENT EXPERIMENT

-

4.68 4.65 4.55 4,38

-

9/2,. . . . 7 / 2 - - 3/2(+} -

4.04 4.00 3.91

9/2 + -

-

2.78

3/2* 3/25/2-

-

I.S6 1.4b 1.3S

-

5/2 + I/2- I/2 + -

O.t9B O.Oll 9.$.

'~F

REE 12

Fig. 8. The i9N energy levels excited in this experiment. The ]9F states are taken f r o m ref. 12). Dashed lines are drawn between analogue levels in the two nuclei.

160(6Li ' t) 19Ne

155

Fig. 8 s h o w s t h e e n e r g y levels o f 1 9 N e e x c i t e d i n t h i s e x p e r i m e n t , a n d c o m p a r e s t h e m w i t h 19F. D a s h e d l i n e s a r e d r a w n b e t w e e n t h e a n a l o g u e levels i n t h e t w o n u c l e i . O n e o f t h e a u t h o r s , A . D . P., w i s h e s t o t h a n k

Professor K. W. Allen and Drs.

P. J. Ellis, D . S t r o t t m a n a n d D . K . S c o t t a n d M r . D . J. M i l l e n e r f o r v a l u a b l e discussions and comments.

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16)

K. Bethge, Ann. Rev. Nucl. Sci. 20 (1970) 255 I-[. I-[. Gutbrod, Y. Yoshida and R. Beck, Nucl. Phys. A165 (1970) 240 A. D. Panagiotou, H. E. Gove and S. Harar, Phys. Rev., to be published J. K. Perring and T. H. R. Skyrme, Proc. Phys. Soc. A69 (1956) 600 A. M. Young, S. L. Blatt and R. G. Seyler, Phys. Rev. Lett. 25 (1970) 1764 S. L. Blatt, A. M. Young, S. C. Ling, K. J. Moon and C. D. Porterfield, Phys. Rev. 176 (1968) 1147 G. E. Brown and A. M. Green, Nucl. Phys. 75 (1966) 401 R. Middleton, B. Rosner, D. J. Pullen and L. Polsky, Phys. Rev. Lett. 20 (1968) 118 H. G. Bingham, I-L T. Fortune, J. D. Garret and R. Middleton, Phys. Rev. Lett. 26 (1971) 1448 K. P. Jackson, K. Bharuth-Ram, P. G. Lawson, N. G. Chapman and K. W. Allen, Phys. Lett. 30B (1969) 162 D. D. Tolbert, P. M. Cockburn and ]7. W. Prosser, Jr., Phys. Rev. Lett. 2I (1968) 1535 J. H. Aitken, K. W. Allen, A. E. Azuma, A. E. Litherland and D. W. O. Rogers, Phys. Lett. B28 (1969) 653 T. Engeland and P. J. Ellis, Nucl. Phys. A179 (1972) and references therein H. G. Benson and B. H. Flowers, Nucl. Phys. A126 (1969) 305 D. Strottman, private communication W. R. Dixon, R. S. Storey, J. H. Aitken, A. E. Litherland and D. W. O. Rogers, preprint