nucleon

Volume 141B, number 1,2

PHYSICS LETTERS

21 June 1984

INTERMEDIATE BEHAVIOUR OF REACTION MECHANISMS IN 27A1 + 63Cu COLLISIONS AT 13.4 MeV/NUCLEON

J.L. LAVILLE, G. BIZARD, R. BOUGAULT, J.F. LECOLLEY, M. LOUVEL, M. L'HARIDON, R. REGIMBART Laboratoire de Physique Corpusculaire LA34, 14032 Caen, France

C. STEPHAN, L. TASSAN-GOT lnstitut de Physique Nuel&ire, B.P. No. 1, 91406 Orsay, France

and L. KOWALSKI Department o f Physics and Geoscience, Montclair State College, Uppermontclair, N Y 0743, USA

Received 22 November 1983

Projectile-like fragment production at the gazing angle has been studied in 27AI + 63Cu collisions at 13.4 MeV/nucleon. Isotopes are emitted with a roughly constant velocity which is significantly lower than the beam one. A specific dissociation mechanism of partially damped projectile-like nucleus has been observed in fragment-fragment coincidence measurements.

It is actually well known from various experiments [ 1 - 3 ] that heavy ion interactions endeavour a rapid transition which threshold is situated about 10 MeV/ nucleon. Specific features occur above that energy, i.e.: (i) The onset of preequilibrium emission for fast light particles [4]. (ii) The fall of fusion and incomplete fusion which set place to fragmentation processes [2,5]. (iii) The saturation of linear momentum transfer from projectile to target [6] Using the ALICE accelerator facility we studied the production of fragments emitted at the grazing angle (0 = 9 o5) in collisions of 27 A1 with a 1 mg/cm 2 63Cu target, at a beam energy of 362 MeV, i.e. 7.5 MeV/nucleon above the Coulomb barrier. The main purpose of this experiment was to check the possible onset of fragmentation expected around 15 MeV/nucleon [1,2,5]. The 27A1 nucleus was chosen because the distribution of projectile-like fragments should be less influenced by preexisting a-substructures than in 0.370-2693/84/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

the case of reactions induced by 12C, 160 or 20Ne ions. Emitted fragments were detected both inclusively and in coincidence by a magnetic spectrometer and a ZkE-E telescope. The magnetic spectrometer - exten. sively studied in a previous paper [7] - provided isotope separation for heavy ions from Z1 = 6 up to Z1 = 20 in an energy range between 5 and 15 MeV/nucleon. The 2 t E - E telescope, consisting of an ionization chamber as a ~tE and a solid state position Sensitive detector as an E identified elements from Z2 = 5 up to Z2 = 12. Both detectors were set on the same side of the beam at the same ffrxed in-plane angle 0 = 9°5, the out-of-plane angles being respectively • = 0 ° and • = 8 °. The angular aperture was z~01 = -+2° for the first detector, A02 = +2°5 for the second one. This geometrical configuration was used in order to detect coincident events arising from projectile fragmentation which are expected to occur at the grazing angle. Apart for the Mg, A1 and Si isotopes, which are dominated by a large quasi-elastic peak, the energy 45

Volume 141B, number 1,2

PHYSICS LETTERS

21 June 1984

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Fig. 1. (a) Energy per nucleon spectra of oxygen isotopes. The labelled arrows correspond to the following assumptions: (1) pure two-body projectile fragmentation; (2) multi-fragmentation process where the projectile is split into the detected fragment and nucleons; (3) final two-body deep inelastic scattering, the detected fragment being emitted by Coulomb repulsion.in the final state. The dashed lines are drawn to guide the eye. (b) Isotopes central velocities versus their mass numbers. #~xp) are the experimental values; #(1) are the values corresponding to a pure two-body projectile fragmentation [arrow (1) in (a)].

spectra of isotopes emitted at the grazing angle exhibit a large bump centered around 9 MeV/nucleon, say to a value noticeably lower than the projectile energy (13.4 MeV]nucleon). As an example, oxygen energy spectra are presented in fig. la: this emission obeys an intermediate mechanism which is neither pure projectile fragmentation (arrows 1 and 2) nor deep inelastic scattering (arrow 3). Moreover, we did not observe a 46

noticeable element production above the projectile value (Z 1 = 13) and this roles out a classical fully relaxed process. The velocities corresponding to the maxima of the energy spectra are plotted in fig. 1 b versus the fragment mass number: we can see that the isotopes are emitted with rather close velocities which exhibit a roughly constant downshift from the values 13(1) expected under the assumption of a pure two-

Volume 141B, number 1,2

PHYSICS LETTERS

21 June 1984

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Fig. 2. Differential cross sections for isotope production at the grazing angle. The dashed curve is the Lukianov model prediction with an effective temperature parameter T = 7 MeV; this calculation is normalized to the 19F cross section. body projectile fragmentation. Such a constant shift could be explained by a slowing down of the projectile itself in a primary stage of the reaction. Inclusive differential cross sections for isotope production are shown in fig. 2; they are obtained by integration over the relaxed energy bump. For the Mg, A1 and Si elements the taft of the quasi-elastic peak has been roughly subtracted. Following the idea of an intermediate mechanism between relaxed and direct phenomena and in order to check the main trend of our results, we compared these isotope cross sections to the prediction of the Lukianov model which has been developed to account for projectile fragmentation phenomena at relativistic energies but was found to be valid at 20 MeV/A [5]. This systematics, which assumes that the reaction proceeds via the projectile excitation and its sequential decay, describes qualitatively the main features of the experimental isotope distributions as we can see in fig. 2. Fragment-fragment coincidence measurements give further insight on a possible process which appears to be more complicated than a simple projectile fragmentation: fig. 3 shows the Z sum distribution of correlated fragments 1 and 2 emitted in an angular domain close to the grazing angle; of course, due to efficiency limitations in the Z1, Z2 detection, this experimental histogram is far from representing the real primary Z distribution. Nevertheless it reveals clearly in

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a qualitative way an important amount of Z contributions above the projectile value. The coincident fragments are emitted with mean velocities ~1, 2 = O. 13 which are close to the value/~M = 0.142 (see fig. lb) evaluated from the inclusive spectra: this indicates that inclusive and coincident events originate from a similar primary process where the projectile is slowed down, even if these coincident events do not represent the whole part of the inclusive cross section. The observation of Z-values higher than the projectile one and of large energy relaxation shows that pure projectile fragmentation is not the relevant process. From 47

Volume 141B, number 1,2

PHYSICS LETTERS

these remarks, we propose for the observed correlated events the following mechanism that we assume to occur in two stages: (i) the incident nucleus is slowed down and excited in the interaction; it picks up a few nucleons to the target nucleus; (ii) it dissociates. Such a projectile splitting has been reported in partially damped 86Kr + 166Er collisions [8] at 12 MeV/ nucleon, which corresponds to the same value of 7.5 MeV/nucleon above the Coulomb barrier. We report here a similar result observed with a much lighter projectile. So, the dissociation process appears to be a specific feature of the reaction mechanism at that intermediate energy. In conclusion, projectile-like fragments are produced at the grazing angle with similar velocities which are significantly less than the beam velocity. The specific mechanism of isotope production at the grazing angle is not clearly established. Some features, like the velocity shifts may account for incomplete fusion [9,10], but other channels could also contribute: we deal here with a situation where the Wilczynski prescription [2] about the limitations due to critical angular momenta applies; the entrance channel maximum angular momentum/max corresponding to a hard grazing collision between the 27 A1 and 63 Cu

48

21 June 1984

nuclei is/max = 100 h at this energy; this value overcomes the maximum one for every incomplete fusion channel until a-capture by the target (94 h), allowing the onset of more direct channels. Following this idea, the production of a given isotope should occur either through a break-up fusion mechanism, or through a more direct process. Our data may account for such an alternative since they are consistent with the formation of a primary excited nucleus followed either by target capture of a projectile residue, or by direct splitting.

References [1] D.K. Scott, MSUCL 337 (1980); D.K. Scott et al., LBL 7729 (1978). [2] K. Siwek-Wilczynska et al., Phys. Rev. Lett. 42 (1979) 1599. [3] T.C. Awes et aL, Phys. Lett. 103B (1981) 417. [4] D.K. Scott, MSUCL 355 (1981). [5] C.K. Gelbke et al., Phys. Rep. 42 (1978) 311. [6] J. Galin et al., Phys. Rev. Lett. 48 (1982) 1787; V. Viola et al., Phys. Rev. C26 (1982) 178. [7] L. Tassan-Got et al., Nucl. Instrum. Methods 200 (1982) 271. [8] A. Olmi et al., Phys. Rev. Lett. 44 (1980) 383. [9] C. Gerschel et al., NucL Phys. A387 (1982) 297C. [10] C. Egelhaaf et al., Nucl. Phys. A405 (1983) 397.