ON THE INTERMEDIATE STRUCTURE IN THE 12C-12C-SCATTERING

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ON THE INTERMEDIATE STRUCTURE IN THE 12C-12C-SCATTERING

H. Fink, W. Scheid, W. Greiner

To cite this version:

H. Fink, W. Scheid, W. Greiner. ON THE INTERMEDIATE STRUCTURE IN THE 12C-12C-SCATTERING. Journal de Physique Colloques, 1971, 32 (C6), pp.C6-193-C6-195.

�10.1051/jphyscol:1971638�. �jpa-00214856�

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JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 11-12, Tome 32, Novembre-Dgcembre 1971, page C6-193

ON THE INTERMEDIATE STRUCTURE IN THE C- C- SCATTERING

(*)

H. J. FINK, W. SCHEID and W. GREINER

Institut fiir Theoretische Physik der Universitat FrankfurtIM., Germany

R6sum6. - On suggkre que des &tats quasi liks du systeme ion-ion sont formks dans la diffusion 12C- 12C. L'excitation d'ktats quasi molkculaires donne naissance une structure intermediaire importante dans les sections efficaces pour les knergies superieures a la barrihre coulombienne.

Abstract. -It is suggested that in the 12C-12C-scattering quasibound molecular states of the ion-ion-system are formed. The excitation of quasimolecular states leads to a large intermediate structure in the cross sections for energies above the Coulomb barrier.

The cross section for the elastic scattering of "C on 12C shows intermediate structures with an average width of about 0.3 MeV if the bombarding energy exceeds the Coulomb barrier [I]. Comparing the elastic cross sections for the scattering of 12C on 12C and 1 6 0 on 160 it is suggested that the stronger intermediate structures in the 12C

+

12C case arise from the lower stability of the 12C shells against deformations. During the collision the nuclear deformation changes so that collective states are mainly excited. Therefore we assume that the elastic channel is strongly coupled on inelastic excitations of collective states in the individual nuclei. The excitation of collective states is enhanced if simultaneously the relative motion between the 12C nuclei is in a quasibound state in which the 12C-12C system forms a nuclear molecule. It is shown, that the excitation of quasibound states leads to intermediate structures.

The existence of nuclear molecules in the 12c-12C system was already suggested from the much smaller intermediate structure near the Coulomb barrier by Almquist et al. [2] and Imanishi [3].

We start with a molecular-type potential shown in figure 1 which is derived on the basis of a Thomas- Fermi model [4]. The potential exhibits quasibound states up to a certain angular momentum (I = 8).

In these states the nucleus-nucleus system forms a quasi-molecule. For higher angular momentum the potential has virtual states which are responsible for the gross structure. To excite quasibound states we propose the following double resonance mechanism [4].

(*) This work has been supported by the Bundesministerium

fiir Bildung und Wissenschaft and by GSI

POTENTIAL

C.70. RW=0.3

FIG. 1. - Real potential for the 12C-l2C-scattering. The centri- fugal potentials are added for different angular momenta and the positions of the virtual and quasibound states are shown.

First an ingoing partial wave resonates with the cor- responding virtual state. Then by nucleus-nucleus interaction collective excitations in one or both

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971638

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C6-194 H. J. FINK, W. SCHEID AND W. GREINER

nuclei occur. Due to that the relative motion loses kinetic energy and angular momentum, and drops into the quasibound part of the potential. If the coupled low-energy excitation accounts for the diffe- rence in energy and angular momentum between the virtual state and one of the quasibound states, this quasibound state is strongly excited. Then intermediate structure occurs.

In figure 2 the elastic 90° cross section is calculated with an optical potential and compared with experimental data. The real part of the potential is shown in figure 1, the absorptive potential is assumed to be proportional to the statistical level density of the compound nucleus depending on energy and angular momentum.

lrni \

ELASTIC CROSS SECTION

15 20 25 30 35

crn. ENERGY CMeVl

FIG. 2. -The 90° differential elastic cross section without coupling of inelastic excitations.

We determined the elastic and inelastic cross sections by a coupled channel calculation [4]. As low-energy excitations of the I2C nuclei we took into account the excitation of one or both I2C nuclei into the first 2+-state (4.43 MeV). Using the experimental B(E 2)-transition probability from the 2+-level into the ground state the transition potentials are fixed by the real potential. The results are shown in figure 3-5.

'-re

ELASTIC CROSS SECTION

5 10 15 20 25 30 35

c.m. ENERGY CMeVl

FIG. 3. - The 90° differential elastic cross section. It is assumed that one or both of the 12C nuclei can be excited into the first

2+ level of 1% (4.43 MeV).

c.m. ENERGY [MeV]

'

^INELASTIC ^c12[ c ~ T c ~ ~ * ( ~ + ) ] c12 g o 0

I

7 1.0

FIG. 4. - The 90° differential inelastic cross section for the single excitation 1 2 ~ ( 1 2 ~ , 12~*(2+)) 12C.

.

1fl

D

E

U

z 0

!- 0 w c n .

m cn

0 .

LL

U .

U - 01:

4

W

f -

.01

!

^INELASTIC ^C" [Id2, c12*(2*)] C1'*(2+) 9 0 °

*---•

coupled 2+, ( 2 : 2 + )

z

0.01

10 15 2 0 25 3 0 35

c.m. ENERGY [ M ~ v ]

L

- - - ' 7

FIG. 5. ^-The 90° differential inelastic cross section for mutual excitation 12C(12C, 12~*(2+)) 12C*(2+).

The imaginary potential in the inelastic channels has been set equal to zero, because of the low compound level densities expected for such high I-states at rela-

'!

+ - - - + E x p .

- c o u p l e d : 2+, (2'. 2 ' )

' ~ m L L ' 7 ~ k 1 ' 1 L 1 ~ I L ' m a I c L ' L I

- -

10 15 2 0 2 5 3 0 3 5

f

-

r

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ON THE INTERMEDIATE STRUCTURE IN THE IZC-12C-SCATTERING C6- 1 95

tively low energies (Yrast level) 151. The comparison of Since the inelastic cross sections are sensitive t o the figure 2 and figure 3 demonstrates that the proposed positions of the quasibound potential states, the mechanism is able to produce intermediate structure quantitative analysis of the intermediate structure with the correct width. improves the determination of the real potential.

References

[I] GOBBI (A.), Contribution to the Symposium on Heavy- [4] SCHEID (W.), GREINER (W.) and LEMMER (R.), Phys.

Ion Scattering. Argonne Nat. Lab.. March 1971. -,

-

Rev. Letters. 1970. 25. 176. ^, ^,

[2] ALMQUIST (E.), BROMLEY (D. A.) and

KUEHNER

^(J.^A.), _{[ 5 ]}_=LING ( G . ) , sCHEID (w.) and GREINER (W.), Phys.

Phys. Rev. Letters, 1960, 4, 515. Letters, 1971, 36B, 64.

[3] IMANISHI (B.), Phys. Letters, 1968, 27B, 267.