Volume 124B, number 6 PHYSICS LETTERS 12 May 1983

EVIDENCE FOR 3 p - 2 h ROTATIONAL BANDS IN 170

A. CUNSOLO, A. FOTI, G. IMME, G. PAPPALARDO, G. RACITI lstituto Nazionale di Fisica Nucleare, Sezione di Catania, Italy Centro Siciliano di Fisica Nucleate e di Struttura della Materia, Catania, Italy and Istituto Dipartimentale di Fisica dell 'Universit3 di Catania, Corso Italia 57, 95129 Catania, ltaly

and

N. SAUNIER

D~partement de Physique Nucldaire; Centre d 'Etude Nucl~aire de Saclay, B.P. 2, 91190 Gif sur Yvette, France

Received 14 May 1982 Revised manuscript received 17 February 1983

Comparison is made, in the weak coupling hypothesis, between the transitions to positive parity 170 states selectively ex- cited in the 14C(6Li, t)170 reaction at E(SLi) = 34 MeV, and those from the 160(6Li, t)19Ne reaction at E(6Li) = 36 MeV, leading to the K 7r = 1/24. rotation',d band states in 19Ne. The good results show the validity of the weak coupling hypothesis, and indicate the existence of two 3p-2h bands in the 170 nucleus. The candidates for L = 0, 2, 4 members of the T = 1/2 rotational band and for L = 4 member of the T = 3/2 band are suggested. These findings are confirmed by the extracted 3 He..spectroscopic strengths.

In a previous investigation [1] of the 14C(6Li,t)170 reaction at E(6Li) = 34 MeV, it was shown that the predominant reaction mechanism is a direct 3He- transfer. In particular, a dominant 3 p - 2 h configuration was suggested for the most strongly populated 170 positive parity states. In order to identify other 170 3 p - 2 h states, it seemed worthwhile to compare, in the weak coupling hypothesis [2--4], the previous results [1] with those obtained in the 160(6Li, t)19Ne reac-

tion at E(6Li) = 36 MeV [5]. In fact, this reaction

mostly excites, via a direct 3He-transfer, levels be- longing to the 3 p - 0 h band based on the 19Ne ground state (1/2+). Therefore,states in 170 belonging to a ( sd )3 (p l /2 ) -2 rotational band can be located by cou- pling the two Pl/2 h° les , Jn = 0+, T= 1 coupled, to the K 'r = 1/2 +, T = 1/2 ground state rotational band states in 19Ne [6,7] [19Ne ® 14Cgs]. In this hypothesis each I rr level of the 19Ne positive parity band should corre- spond to two 170 levels, with same spin and parity, but one with T = 1/2 and the other one with T = 3/2. The approximate energies of 3 p 2 h 170 states can be estimated by a simple weak coupling calculation [8].

In particular, the parameters a and b of ref. [8] were choosen by fitting the excitation energies of several Pl/2-holes states in 170 neighbouring nuclei (0.3 <~ a ~< 0.5 and 5 ~< b ~< 6), so getting an excitation energy of 4 MeV ~< E * ( 3 p - 2 h ) ~< 7 MeV for the lowest 3 p - 2h T = 1/2 170 state. In this excitation energy region the only known 1/2 + 170 state is at 6.36 MeV [9]. Analogously the known levels at 7.38 MeV, 8.89 MeV and 9.87 MeV [10] could correspond to the 5/2 +, 3/2 + and 9/2 + 3 p - 0 h levels of 19Ne, respectively. The 3 p - 2 h T = 3/2 states should be ~7.5 MeV shifted at higher energy from the corresponding T= 1/2 ones.

In fig. 1 the triton spectrum from the 14C(6L,t)ITo

reaction at 01a b = 5 ° is shown. In this spectrum the aforementioned T = 1/2 170 levels are observed with relative intensities similar to those of the correspond- ing (sd) 3 states in 160(6L, t)19Ne [5], except for the 8.89 MeV (3/2 +) level which is rather weakly populated. Moreover the peak at 7.38 MeV could contain contri- butions from closely spaced 5/2 + and 5 / 2 - 170 levels

[ 1 ], but we assume that it is mainly due to the excitation of the 5/2 + state only. Oil the other hand an unknown

0 031-9163 /83 /0000-0000 /$ 03.00 © 1983 North-ttolland 439

Volume 124B, number 6 PHYSICS LETTERS 12 May 1983

u3 I--- Z

O U

30C

200

100

c5

O

co t--

a5 r-

,-.,j

- ~5 ~ ~ 16 I ,h-~. , - I T -~ ~' i I , I ~

' , ' ' i T ' i., I ' ! l iI .

I

650 850 1050 1250 CHANNEL NUMBER

'~C(r'Li,t ) 0

E,, ,= 34 MeV

O~.t,= 5 °

el

i I

~ t ~ ° ~ ~. - I : ' I

~ , i I

, i I ~ i

l,I !i 1~,50 ]650

Fig. 1. Tri ton energy spectrum from tile 14C(6 Li,t) 170 reaction at E ( 6 Li) = 34 MeV and 01a b = 5 °. The underlined levels are the proposed 3 p - 2 h rotational band members.

spin level at 12,27 MeV is strongly excited. This level could be the 7/2 + T = 1/2 member which, in the light of the weak coupling model, is expected near 12.5 MeV. The T = 3/2 positive parity 170 states lying in the ap- propriate energy region should be the known [9] levels at 12.94 MeV, ( I /2+) , 13.6 MeV (5/2 +) and 16.3 MeV (9/2+). However, it can be seen in fig. 1 that only the 13.6 MeV and the 16.3 MeV levels are populated in the presently investigated reaction. The lack of the 12.94 MeV level in the spectrum as well as the weak- ness of the T = 3/2 states with respect to the T = 1/2 ones are accounted for by the angular momentum mis- match, that disfavours low/-values at high excitation energy, and by the Clebsch-Gordan coefficients of isospin coupling, that make final states with T = T O - 1/2 2T 0 times more probable than states with T = T O + 1/2, with T O being the isospin of the target nucleus. The lack of knowledge of spin and parity of 170 levels makes it difficult to locate the other 3 p - 2 h states.

The suggested correspondences between 3 p - 2 h 170 states and 3 p - 0 h 19Ne states are summarized in fig. 2.

• gNe , l 0

i I

i I

, ' I ' 1227 (7/2"1

" " 9/2" , ....... 987

6 3 s g s 1/2"

3p-0b

163 9/2~:~ !

z~

3p-2h

Fig. 2. Comparison o f the proposed 3 p - 2 h band states of tTo with the 3 p - 0 h rotational band of 19Ne.

440

Volume 124B, number 6 PHYSICS LETTERS 12 May 1983

"C( ' I , l ) C E ( L , ) = 3 4 M e V

" O ( L , t ] Ne E ( - ) - 3 6 N ' e V

f ,3 122/Mev "q 9~". '~3Ve., 15.0 ~ "" Ne ~M""eC"/Z¢ 1000 1... ° 'Ne ?8,ele.' t

50O'.-- '

5C . 50 "" •"

IOOC J :3,,/2 . t :

~0( ~ " '0 t3B'{}MP" I 1" ° . "4e ' ~,4 MeV , . . , 1 _.~ I00t " ~ . 5 0 ! % " " " ~000r5C~3~_~. c. "6 ; '*,' ..

f Y 5:?" f ~ "~ " °

1000r L b 7 ss re,' ' 0 . , -

, J : '}//2 ' [ 3 , " ;I 0 0 1003~ 50

". . 'x, t .~. 5 0 0 ~ , _ " ' ~ C ~3( 'Ve' 1

50? ~ --~ 530.- " : ' / " " ~':.:-"~.. 1 '2 S 3~ "4ev

,OK) L" ',J~ qs '0}- : ' 0 ~ - ',, . " . "~

!

'0 c. ~ 8": Me,

s[- ." : '%,-- sect ~

so- " • . ' .~. . .~J

a .. _ t~ . . . . . . . . . . . 8 20 ~.0 60 0 20 40 60

~ ~ :'deg

Fig. 3. Comparison of the triton angular distributions from 14C(6Li,t)IVo at E(6Li) = 34 MeV (filled circles) and 160(6Li,t)19Nc at E(6Li) = 36 McV (open circles) (from rcf. [5 ]). The full curves represent the incoherent sum of HF and EFR-DWBA cross sections for the 14C(6Li,t)170 reaction.

19Ne strength is exhausted by the added contributions of the 9/2 + 170 levels at 9.87 MeV (T = 1/2) and 16.3

MeV (T = 3/2). In order to extract more quantitative information on

the structure of the proposed 3 p - 2 h states in 170, we have analysed their angular distributions in the hypo- thesis that the contributions from the direct 3He- transfer and from the statistical compound nucleus

reaction mechanism add incoherently [ 1 ]. The statis- tical compound nucleus cross section has been estimat- ed by using the Hauser-Feshbach ( I iF) formalism [ 1 ]. The direct cross section was evaluated in tile ex- act finite range distorted wave Born approximation (EFR-DWBA) framework, in the hypothesis of direct transfer of 3He-cluster which moves with 2 N + L = 6 relative motion outside the 14C core. The used optical model parameters are the same as in ref. [ 1 ]. As shown in fig. 3 the experimental angular distributions are rather well reproduced by the theoretical predictions, except for the 13.6 MeV state whose angular distribu- tion seems better fitted assuming/-transferred values l = 3 or 4 rather than l = 2. This finding disfavours the presently observed 13.6 MeV state as the T = 3/2, 5/2 + member. On the other hand the well accounted angular distribution of the 12.27 MeV level corrobo- rates this state as the 7/2 +, T = 1/2 member.

The extracted spectroscopic factors C2S and the S/$9.87 relative 3lie-spectroscopic strengths are re- ported in table 1, where C 2 is the square of the isospin Clebsch -Gordan coupling coefficients. It is interesting to note that the relative 3He-spectroscopic strengths are very close to unity, so corroborating the suggestion of a similar 3He-cluster structure, with (sd) 3 configura- tion, for all the analyzed levels.

Table 1 3He..spectroscopic strengths from the 14C(6 Li, t) 170 reaction.

Further confirmation of the (sd)3(pl /2) 2 configu- ration of the proposed 170 states can be obtained by comparison of the triton angular distributions meas- ured in the two reactions [1,5]. This is shown in fig. 3 wherein the similarity of both shapes and absolute values is rather well demonstrated. In fact, as expected from the aforementioned isospin coupling and angular momentum mismatch arguments, the 170 T = 1/2 band members have strengths that are roughly 4/5 of those for 160(6Li, t)19Ne. And the 2.8 MeV(9/2 +)

E*(MeV) 7" jrt C2SX 103 a) S/$9.87b)

6.36 1/2 1/2 + 4.9 0.8 ±0.1 7.38 1/2 5/2*" 8.8 1.4 *0.1 8.89 1/2 3/2* 6.3 0.98±(I.07 9.87 1/2 9/2 + 6.4 1

12.27 I/2 (7/2*') 5.1 0.80:0.07 16.3 3/2 9/2 + 5.1 1.6 *0.1

I

a) Estimate absolute uncertainties t25% (due to statistical errors (~ *_ 10%) and absolute v',dues uncertainty (~ ±20%)).

b) The quoted errors come from statistical errors only.

441

Volume 124B, number 6 PHYSICS LETTERS 12 May 1983

The consistency of both the weak coupling analysis and the found spectroscopic strengths offers substan- tial evidence that the 6.36 MeV (1/2+), 7.38 MeV (5/2+), 8.89 MeV (3/2+), 9.87 MeV (9/2 +) and the 12.27 MeV (presently suggested 7]2 +) states represent the L = 0, 2, 4 members of a (sd) 3, T = 1/2 170 rota- tional band. Only a member of the T = 3/2 band is identified at 16.3 MeV (9/24).

In summary, we have at tempted to shed light on the 3 p - 2 h nature of 170 states, building such states by weak coupling of two Pl/2 holes to the members of the (sd) 3 ground state band of 19Ne. By comparison of data from the 14C(6Li, 0 1 7 0 and 160(6Li , t )19Ne

reactions we suggest the existence of two 3 p - 2 h posi- tive parity rotational bands in the 17 O nucleus. This suggestion is also supported by the 3He-spectroscopic strength values extracted by the analysis of the laC(6LI, t )170 data.

Finally, we think that, in order to get better under- standing about such positive parity 17 O states, it would be interesting to test the above results with theoretical calculations, such as those based on the cluster folded potential model [6,11] used to describe three-nucleon states in mass 15 and 19 nuclei.

References

[ 1 ] A. Cunsolo et al., Phys. Rev. C24 (1981) 2127. [2] A. Atima, H. Horiuchi and T. Sebe, Phys. Lett. 24B

(1967) 129. [3] K. Bethge, D.J. Pullen and R. Middleton, Phys. Rev. C2

(1970) 395; V.Z. Gol'dberg, V.V. Davydov, A.A. Ogloblin, S.B. Sakuta, and V.I. Chuev, Yad. Fiz. 12 (1980) 30; Soy. J. Nucl. Phys. 12 (1971) 16.

[4] K. Van der Borg, R.J. De Meijer, A. Van der Woude and H.T. Fortune, Phys. Left. 84B (1979) 51.

[5] A.D. Panagiotou and H.E. Gore, Nucl. Phys. A196 (1972) 145.

[6] B. Buck and A.A. Pilt, Nucl. Phys. A280 (1977) 133. [7] L.M. Martz, Yale University 1978 (thesis). [8] R.K. Bansal and J.B. French, Phys. Lett. 11 (1964) 145;

L. Zamick, Phys. Lett. 19 (1965) 580. [9] F. Ajzenberg-Selove, Nucl. Phys. A375 (1982) 1.

[10] C.C. Lu, M.S. Zisman and B.G. Harvey, Phys. Rev. 186 (1969) 1086.

[ i 1 ] B. Buck, C,B. Dover and J .P. Vary, Phys. Rev. C11 (1975) 1083.

442