THE (Li6, He6) SPIN-ISO SPIN-FLIP REACTION

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Submitted on 1 Jan 1971

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THE (Li6, He6) SPIN-ISO SPIN-FLIP REACTION

V. Chuev, V. Davidov, B. Novatskii, A. Ogloblin, S. Sakuta, D. Stepanov

To cite this version:

V. Chuev, V. Davidov, B. Novatskii, A. Ogloblin, S. Sakuta, et al.. THE (Li6, He6) SPIN- ISO SPIN-FLIP REACTION. Journal de Physique Colloques, 1971, 32 (C6), pp.C6-167-C6-168.

�10.1051/jphyscol:1971630�. �jpa-00214848�

J O U R N A L DE P H Y S I Q U E Colloque C6, supplkment all no 11-12, Tome 32, Novembre-De'cembre 1971, page C6-167

THE (Li6, He6) SPIN-I SO SPIN-FLIP REACTION

V. I. CHUEV, V. V. DAVIDOV, B. G. NOVATSKII, A. A. OGLOBLIN, S. B. SAKUTA and D. N. STEPANOV

Kurchatov Institute of Atomic Energy, Moscou, U. S. S. R.

Rbum6. - La rCaction (Li6, He6) a ete etudiee a E n i s ) = 31,s MeV sur les noyaux cibles suivants : Li7, Be9 et CIA. La selectivlte observee dans cette rCaction peut &tre expliquee dans certains cas par un mkcanisrne simple d'interaction de ⁽⁽ simple particule 1).

Abstract. - The (Li6, He6) reaction was investigated on Li7, Be9 and Cl3 target nuclei at EL^ = 31.8 MeV. The observed selectivity of the reaction can be accounted in some cases for a sirnple single-particle interaction mechanism.

The (Li6, He6) [ I ] and (Li6, Li6*) (3.56 MeV,

T = 1 ) [2] reactions are characterized by simultaneous

spin- and isospin-flip. Taking into account that the incident and out-going nuclei have the a-particle core, one can assume that the reaction is similar t o the transition from the triplet to singlet state in the two nucleon system, i. e. the (d, n2) or (d, d*) reactions, where n2 is the dineutron and d * is the singlet deuteron.

Under the assumption of the single-particle nature of the process this implies that the reaction takes place due to the term (a,, a,) ^(T,, 2,) in the scalar part of the nucleon-nucleon potential. Therefore the investiga- tion of the ( ~ i ~ , He6) reaction may yield, in principle, information about the nature of the Majorana term in the nuclear interactions.

The work described here concerns the further inves- tigation of the (Li6, He6) reaction [I]. The spectra (Fig. I ) and angular distributions (Fig. 2) were measur- ed in the (Li6, ~ ereaction on the p-shell target ~ ) nuclei Li7, Be9 and C13 at ELiS = 31.8 MeV. The attempts t o study the mirror (Li6, ~ i ~ * ) reactions were

~~nsuccessful because of carbon and oxygen impurities in the targets.

The distinctive feature of the reaction, clearly seen from the spectra, is its strong selectivity. Of all the levels of the residual nuclei only the ground states (in all the cases they are the mirror states) and some of the lowest excited states are seen. No positive-parity states of B~ and N t 3 were observed. This allows to suggest that in the unresolved doublet a t the excitation energy of 3.5 MeV in N13 the 3.51 MeV (312-) state is excited.

Since the spins for Li6 and He6 are 1 ⁺ and 0+, respectively, thc selection rule for the (Li6, He6) reaction has the form

kinematics of the ( ~ i ~ , ~ ereaction at a n incident ~ ) energy of about 30 MeV favours the transfer of small values of the momenta - zero or unity, since AL

-

and q

-

Since only the negative-parity states are populated in the reaction, the momentum transfer AL is equal t o

where AL is the orbital momentum transfer. The FIG. 1. - Energy spectra from the (Lis, He6) reactions.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971630

C6- 168 V. I. CHUEV, ^{V. V.} DAVIDOV, B. G. NOVATSKII

- -

the interacting neutron of the target nucleus does not \

change its orbital quantum number in the course of the

reaction. In this case [ l ] transitions from a neutron

L " ' ~

state I, j toproton states 1 , j a n d I, j' are allowed, where I

IO@ f

zero in all the cases. Then the spin values of the final states excited must be If = Ii, Ii zk I. This means that in ~ e ' a n d B~ nuclei 112-, 312-, 512- states can be excited, and in N t 3 nuclei-112- a n d 312- states. In fact, the levels observed in the experiment have these spin values (Fig. I). T h e nearest levels which have the same spins a n d are not excited in the (Li6, He6) reaction lie a t a rather high excitation energy : Be7-6.51 MeV (512-), N13-8.92 MeV (112-), B9 _no lower than 8.9 MeV.

The lack of transitions with A L # 0 suggests that

. o l t\ E1*=31,8 ^Mev

4' \ 71i(61~,b~~rb44

0 I

References a t j = l - 4

j - 1 a t j = l + J i ' j1 = ( j +

'

The fulfillment of this selection rule implies that a s a

result of the reaction the odd neutron either charge- ^\ _0- *"0-

exchanges into proton without its quantum ^-0--Q

numbers being changed (ground state transition) o r charge-exchanges with the spin-flip and with the for- mation of the excited state (112- in Be7, 312- in N13).

However, with the selection rules being fiilfilled, the excitation of the 512- state in the BS nucleus can not be accounted for the odd neutron interaction. The mecha-

nism of the reaction appears t o be more complicated _\ (for example, the excitation of the B9 core). ^\

[ I ] CHUEV (V. I.), DAVIDOV (V. V.), MANKO (V. I.), [2] NAGATANI (K.), BOYD (D. P.), DONOVAN (P. F.), NOVATSKII (B. G.), SAKUTA (S. B.), S.I.EPA- BEAKDSWORTH (E.), ASSIMAKOPOULOS (P. A.),

NOV (D. N.), Phys. Letters, 1970, 31B, 624. Phys. Rev. Letters, 1970, 24, 675.

FIG. 2. - Angular distributions from the (Lio, He6) reactions.

The curves are driven through experimental points. Solid curves

.0,04 b

'\

oC(sIi,%eYJNj.st

,402 ^\ _\

O.,o, '0

0

present the ground state transitions ; dashed curves excited

state transitions. 0 20 40 60 80

ole,,,