A STUDY OF THE 12C(12C, α)20Ne REACTION AT HIGH ENERGIES

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A STUDY OF THE 12C(12C, α)20Ne REACTION AT HIGH ENERGIES

A. Panagiotou, H. Gove

To cite this version:

A. Panagiotou, H. Gove. A STUDY OF THE 12C(12C,α)20Ne REACTION AT HIGH ENERGIES.

Journal de Physique Colloques, 1971, 32 (C6), pp.C6-237-C6-239. �10.1051/jphyscol:1971653�. �jpa- 00214871�

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JOURNAL DE PHYSIQUE Colloque C6, suppldment au no 11-12, Tome 32, Novembre-Dkcembre 1971, page C6-237

A STUDY OF THE 12C(12C, a)'ONe REACTION AT HIGH ENERGIES

A. D. PANAGIOTOU (*) and H. E. GOVE Nuclear structure Research Laboratory University of Rochester, Rochester, N. Y., U. S. A.

R6surn6. - La reaction 12C(12C, ol)zoNe a Btt Ctudite entre Radl'C) ⁼45 MeV et Elab(12C) = 52 MeV. Les fonctions d'excitation ont tte obtenues pour l'ttat fondamental, les niveaux 2+ (1,63 MeV) et 4+ (4,25 MeV) du 20Ne. Les distributions angulaires des niveaux & basse tnergie d'excitation du 2oNe ont kt15 obtenues a Bab(12C) = 50 MeV et compartes a [Pt(cos 6)12.

La section efficace de la reaction depend trBs fortement de l'energie incidente et des spins des niveaux du 2oNe.

Abstract. - The reaction 12C(12C, a)zONe was studied at 45

<

E,,c < 52 MeV. Excitation functions were obtained to the ground, 2+ (1.63 MeV) and 4+ (4.25 MeV) levels of 2oNe. Angular distributions to the low-lying levels of 20Ne were obtained at E12c = 50 MeV, and compared with [PL(cos 6)]2. The cross section for the reaction depends very strongly on the incident energy and the spin of the 2oNe levels.

Systematic studies of the '2C('2C, a)'ONe reaction have been carried out at relatively low incident energies (9 < El,, < 15.6 MeV in the Centre of Mass., by Borggreen et al. and Almquist et al. El]. They have found strong structure in the excitation curves, with average widths of 100 5

< r >

⁵200 keV. High spin resonances in the compound nucleus 24Mg were reported, at excitation 23 < ~ 2< 29.6 MeV, with 4 ~ ~

seemingly single spin ( J = 8, 10 mainly). The angular distributions to the ground state of "Ne were, in some cases, well fitted with almost pure [P,(cos 0)12 curves. Level densities, for J = 8 states a t 25 MeV excitation, of approximately IO/MeV were calculated.

The motivation for our work was the study of the reaction mechanism at higher incident energies of 45

<

El,,

<

55 MeV. 12Cf5 ions were accelerated by the MP Tandem accelerator of the University of Rochester, and allowed to impinge on self-supporting carbon foils of various thicknesses (10-100 pg/cm2).

The outgoing a-particles were detected on the focal plane of the split-pole Enge spectrograph by a proportional counter. A silicon surface-barrier detector, positioned a t 30° to the beam, monitored the target.

Figure 1 shows the excitation functions obtained for the ground, 2+ (1.63 MeV) and 4' (4.25 MeV) states of 'ONe. A 100 yg/cm2 carbon foil was bombarded with l Z C beam, 48 6 El,,

<

^{52 MeV}

in the laboratory. The energy increment was 300 key.

The average energy loss was about 280 keV. The target thickness remained constant to about 5

%

throughout the run.

The yield curve for the ground state is dominated by two resonances, a; and at, centered a t 38.73 and 39.28 MeV excitation and having widths

rub

^E400 keV,

r,,z -

250 keV respectively. From the overall intensity and shape of the yield curves t o the three

ON^

levels two observations can be made : a) the cross section is proportional to the spin of the 'ONe level, favoring higher spins, and b) there seems to be a correlation between the excitation functions t o the three levels. Namely, the resonance a: appears in all three yield curves. Although the excitation functions were obtained in a rather small energy region, such correlations are not expected from the statistical model. Michaud and Vogt [2] have suggested that

<( doorway >> states in the

+

12C system populate

strongly, via a-particle emission, states in 'ONe having a-cluster structure. Such states in the compound system could lead to correlations in the excitation functions to levels with a-particle structure. A further test of this would be the observation of correlations in the yield curves to other states in 'ONe at higher excitation, with different a-cluster configuration.

The angular distributions were obtained with a 10 pg/cm2 carbon target and 50 MeV incident beam.

Figure 2 shows a sample of the distributions to the low spin levels. Those to higher spin states exhibit much less structure. In figure 3 a n attempt is made to fit the distributions for the ground, 1- (5.78 MeV) and 2+(7.43 MeV) levels with [P,(cos 0)12, assuming a single 1. The rather good fit of the distribution t o the ground state with l = 10 suggests that the resonances

(*) Present address : University of Oxford, Nuclear Physics in the compound system, probably the entire are Laboratory, Keble Rd., Oxford, England. single-spin resonances with spin 10. The angular

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971653

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A.. D. PANAGIOTOU AND H. E. GOVE

EXCITATION FUNCTIONS ' 2 ~ ( ' 2 ~ , a ) 2 0 ~ e

BLAB

= 10"

I t I 1

24 25 26

El,, C.M. (MeV)

FIG. 1 . - Excitation functions to the ground, 2+ (1.63 MeV) and 4+ (4.25 MeV) levels of 2oNe.

distribution to the 1 ^-level is well fitted (for 0 cm > 200) with 1 = 9, the minimum angular momentum permitted by the triangle rule. The distribution to the 2' (7.43 MeV) level, on the other hand, follows the [P,,(cos 0)l2 curve (I = 8, 10 and 12 are permitted).

"C ("c, a ) ON^

E,2c= 50.0 MeV

FIG. 2. - Angular distributions for the 12C(l2C, a)20Ne reaction.

FIG. 3. - Comparison of the angular distributions with [PZ(COS 6')]2.

The angular distribution to the 2' (1.63 MeV) level decreases sharply and monotonically with angle.

Also, a t O,,

<

200 the distribution to the 1- level is

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A STUDY OF THE 12C(12C, a)20Ne REACTION AT HIGH ENERGIES C6-239

forward peaked. The integrated cross sections for incident energies, but that either there is an appre- the 2' (1.63 MeV), 4' (4.25 MeV), and 3- (7.17 MeV) ciable direct component or the compound system are much larger than those for the 2 - and 4- states, is highly deformed, i. e., there possibly exist quasi- which are populated only via compound nucleus for- molecular states, a-clusters, in which nucleon clusters mation. All these observations suggest that the reac- predominate to a greater extent than the statistical tion mechanism is not entirely compound at these expectation for formation of sucha cluster.

References

[I] BORGGREEN (J.) et al., and ALMQUIST (E.) et al., Pro- Press, 1963, A. Ghiorso, R. M. Diamond and ceedings of the Third Conference on ^ctReactions H. E. Confett, ed.

Between Complex Nuclei D, U. of California [2] MICHAUD (G.) and VOGT (E. W.), Phys. Letters, 1969, 30B, 85.