DIRECT TRANSPER REACTIONS INDUCED BY 16O AT INCIDENT ENERGIES ABOVE THE COULOMB BARRIER

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DIRECT TRANSPER REACTIONS INDUCED BY 16O AT INCIDENT ENERGIES ABOVE THE

COULOMB BARRIER

A. Cunsolo, H. Faraggi, M. Lemaire, J. Loiseaux, M. Mermaz, A. Papineau, J.

Quebert

To cite this version:

A. Cunsolo, H. Faraggi, M. Lemaire, J. Loiseaux, M. Mermaz, et al.. DIRECT TRANSPER RE- ACTIONS INDUCED BY 16O AT INCIDENT ENERGIES ABOVE THE COULOMB BARRIER.

Journal de Physique Colloques, 1971, 32 (C6), pp.C6-171-C6-173. �10.1051/jphyscol:1971632�. �jpa-

00214850�

J O U R N A L DB P H Y S I Q U E Colloque C6, supplkment au no 1 1-12, Tome 32, Novembre-Decembre 1971, page C6- 17 1

DIRECT TRANSFER REACTIONS INDUCED BY

1 6 0

AT INCIDENT ENERGIES ABOVE THE COULOMB BARRIER

A. CUNSOLO (*), H. FARAGGI, M. C. LEMAIRE, J. M. LOISEAUX, M. C. MERMAZ, A. PAPINEAU and J. L. QUEBERT (**)

Dtpartement de Physique Nucliaire C. E. N. Saclay, France

RburnC.

-

Les reactions de transfert induites par des ions ^{' 6 0} sur differents noyaux cibles de la couche 2p-lf presentent les caractkristiques gknerales d'une reaction directe de surface, pouvant &re dkrite comme un processus quasi Clastique dans le cadre du modkle de diffraction de Frahn et Venter. La grande selectivite de ces reactions directes de transfert peut &tre appliquke a des etudes de spectroscopic.

Abstract.

-

The transfer reactions induced by 1 6 0 ions on various lf-2p shell target nuclei, exhibit the general features of a surface direct reaction which can be described as a quasielastic process in the framework of the Frahn and Venter diffraction model. The great selectivity of these direct transfer reactions can be applied to spectroscopic studies.

Quartet structures have been largely investigated in the If-2p shell nuclei by means of the (160, 12C) reaction. An accurate description of the multi-nucleon transfer including nuclear structure effects is very difficult, nevertheless interesting spectroscopic results have been obtained assuming that the reaction proceeds via a direct and selective process [I]. Thus various experiments have been devoted to the study of the reaction mechanism occurring in reactions induced by 160 ions at incident energy above the Coulomb barrier. We have measured angular distributions for the ¹⁶⁰ elastic scattering (160, 15N) one-proton trans- fer reaction, (160, 14C) two-proton transfer reaction and (160, lZC) four-nucleon transfer reaction.

The general feature arising from the experimental data is the great selectivity of the transfer reactions for a definite number of excited states. The (160, "N) one-proton transfer reaction has been performed on 44,48Ca, 4 8 p 5 0 ~ i , 5 6 ~ e and 62.64,68Ni target isotopes at incident energies above the Coulomb barrier, 48 and 56 MeV depending on reaction Q-values. The comparison of the various relative yields of 15N spectra with the (3He, d) one-proton spectroscopic factors shows that (160, 15N) reactions populate the well-known single particle states, except for the : ; ~ i target case for which the 1.06 MeV, 1112- level of

::v

is also strongly excited. The 1112- state wave-

functionc an possibly result in the coupling of the 2' :;~i,, core excitation with the If,,, proton indepen- dent particle state.

The difference in selectivity between the (160, 14C) two-proton transfer and (160, 12C) four-nucleon transfer reaching the same residual nuclei is well observed as expected. Figure 1 shows a typical comparison between the two 54Fe(160, 12C)58Ni and 56Fe(160, 14C)''Ni spectra obtained respectively at 48 and 56 MeV incident energies. The (4p-2h) structures appearing between 4 and 10 MeV excitation

(*) Visiting Scientist according to the contract BT-11/12 766

between the CEN of Saclay and the CSFNISM-Catania. On leave ^from Istituto di Fisica Nucleare and INFN-Universita di

Catania (Italy).

(*:') Permanent address : Laboratoire de Physique Nucleaire,

UniversitC de Bordeaux (France).

600 700 B M

CHANNEL NUMBER

FIG. 1.

-

Comparison between the sdFe(l60, lzC)s$Ni 56Fe(16O, 14C)SaNi spcctta obtained respectively ^at 48

56 MeV ^{1 6 0} incident energies.

and and

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971632

C6- 172 A. CUNSOLO, H. FAR energy is anticorrelated with the two-proton strength which shows up only on the ground state, the first 2' excited levcl, the 3.52 and 4.5 MeV levels.

The 160 elastic angular distribution on 54Fe target has been measured at 48 and 52 MeV incident energics.

The two similar elastic cross-sections obey the Rutherford scattering law up to thc grazing angles which are respectively 500 and 450 C. M. After the grazing angles, a rise above the Rutherford cross- section is observed, followed by a fast exponential decrease without any oscillation. This behavior is the signature in the heavy ion case of a Fresnel diffraction scattering due to strong nuclear absorption in presence of very high Coulomb field 221. A phase shift analysis with the parametrization of Frahn and Venter [3]

gave for the best fits respectively at 48 and 52 MeV the following three parameters : the radius R = 1.55 (54'13

+

l6'I3) fermi, the diffuseness para- meter d = 0.32 and 0.45 fermi and the absorption term p/4 A = 0.50 and 0.31. It is well known in a phase shift analysis that there is an ambiguity in the determination of the two last parameters d and p/4 A.

In the framework of this strong absorption model which describes successfully the elastic scattering, Frahn and Venter treat a nucleon or a cluster transfer as a quasi-elastic process 141. This surface direct reaction cross-section has its maximum predicted at the grazing angle where the apsidal distance

is equal to the nuclear interaction radius R = ro(A:l3

+

of the two heavy fragments. At distances D larger than the interaction radius R, the cross-section obviously decreases with the angle. For distances D smaller than R (backward angles), the cross-section is damped by strong absorption phenomena. Figure 2 presents such an example of one nucleon transfer reaction, namely the 50Ti(160, 1 5 1 V ) 5 1 ~ reaction, at 48 MeV incident energy.

The angular distributions are successfully fitted with only two parameters : the interaction radius r, ⁼ 1.90 fermi and the diffuseness parameter d = 0.60 fermi. This value of r, corresponding to the grazing angle is, as expected, much larger than the value given by an elastic scattering phase shift analysis known as the ro quarter point recipe.

The angularsdistributions(Fig. 3) obtained at 46 MeV incident energy for ("0, 12C) alpha-transfer reaction on 5 4 ~ e target-isotope indicate clearly that this reaction occurs also through the same kind of direct surface interaction. The corresponding two free parameters of the fits are : r, = 2.10 fermis and d = 0.5 fermi.

In this quasi-elastic model kinematic effects and Q-value dependences involved in the entrance and

AGGI, M. C. LEMAIRE

I ^{I I 1}

FIG. 2. - One-proton transfer angular distributions of SOTi(160, 1SN)SIV reaction performed at 48 MeV 1 6 0 incident

energies.

FIG. 3. - Alpha-transfer angular distributions of 54Fe(160, 12C)S*Ni

reaction performed at 46 MeV 1 6 0 incident energies.

DIRECT TRANSFER REACTIONS INDUCED BY 1 6 0 C6-173 outgoing channels are partially taken into account.

The spectroscopic reduced amplitude of the transfer cluster is only proportional to the derivative of the reflection coefficient of the phase shift distribution which is approximated by a Woods-Saxon form.

At present the nuclear structure correlation effects and the final binding energy dependence are not disentangled.

A zero-range DWBA calculation was performed in order to obtain a crude estimate of the variation of the cross-section due to the final state binding energy difference, the so-called Q-value dependence. The optical model parameters used are those which fit the 48 MeV ¹⁶⁰ elastic data. The calculation pre- sented in figure 4 corresponds to an L = 2 transition assuming that the 4 nucleons are transferred in the

FIG. 4. -- Cross-section intensities at the grazing angle versus binding energy and reaction Q-value.

54Fe(16O, 12C)ssNi, E l 6 ,

-

^{46 MeV, L = 2 .}

the alpha-particle binding energy on the projectile).

The full curve shows the real case in which the final binding energy and Q-value vary together. We see that the variation of the cross-section with Q is, at most, a factor 2 in 2 MeV Q value change.

On figure 5 are plotted, versus the Q-value, the first 2+ cross-sections for various residual nuclei measured at the same angle 40 deg. and at 48 MeV 160 incident energy. For a given series of isotopes, the reaction occurs under the same kinematic conditions, including the Coulomb barrier. The cross-section then depends only on the spectroscopic factor and of the reaction Q-value. For all the isotopes a much sharper variation is observed than that given by the maximum Q-value dependence (line marked maximum DWBA slope in Fig. 5).

2p

3

orbit. According to the ~ ~ ~ l ~ ~~ FIG. ih 5. - i First ~ 2' ~cross-sections of various If-2p shell nuclei k ~ transformation the corresponding a-particle bound ^{at 40° lab and 48 MeV 1 6 0} incident energy, plotted versus

Q value reaction.

state wave function is then described in a first approxi- mation by a 6 D wave function. The lower dashed

curve corresponds to a variation of Q keeping the Thus we can conclude that in spite of the impor- final binding energy constant. The upper dash-dot tance of the Q-value dependence, the abrupt variation curve corresponds to the variation of the final binding of the cross-section values establishes definitively the energy keeping the Q-value constant (both of these neutron blocking effect consistent with the quartet curves are unphysical since they imply a variation of structures.

References

[ I ] FAKAGGI (H.), JAFIKIN (A.), LEMAIRE (M. C.), MER- [2] FRAHN (W. E.), Phj).~. Rev. Lelters, 1971, 26, 568.

MAZ (M. C.), FAIVRE (J. C.), GASTEBOIS (J.), [3] FRAHN (W. E.) and VENTER (R. H.), Ann. Phys., 1963, LOISEAUX (3. M.) and PAPIKEAU (A.), Ann. Phys. 24, 243.

(de-Shalit Memorial), t o be published. Phys. Rev. [4] FRAHN (W. E.) and VENTER (R. H.), NLICI. Phys., 1964, Letters, 1970, 24, 1188 and 1971, 15, 900. 59, 651.