Limitation to complete fusion during a collision between two complex nuclei

Abstract

Experimental results on the limits observed for complete fusion between heavy nucl. ei are discussed. It is shown that there is no critical angul. ar momentum independent of the bombarding energy and of the entrance channel. Excitation energy and total. angular momentum of the compound system do not appear as good parameters for determining the complete fusion cross section. A'll the available results can be presented in terms of a critical distance of approach R~. Such a distance is obtained when the kinetic energy of incoming nucleons equals the interaction potential and i.s related to the nuclear-matter density of both nuclei.For very heavy partners, a fusion barrier occurs, in addition to the interaction barrier. NUCLEAR REACTIONS Heavy-ion complete fusion; calculated critical distance of approach, 8 =(1.0+0.07)(A(~3+&2~~3). COMPLETE FUSION AND CRITICAL ANGULAR MOMENTUMWhen nuclear reactions are induced by heavy ions, an important question is which part of the total reaction cross section goes into complete fusion and then leads to a compound nucleus. It has important consequences for the yield for the production of new isotopes and in itself it might give information on the dynamical properties of nuclear matter.A number of experimental results' ' have been obtained during the last decade, mainly on the ratio (o«/8") of the complete-fusion cross section as compared to the total reaction cross section. The first conclusion was that the completefusion cross section did not always represent the largest fraction of the total cross section and a reasonable explanation was given by considering the large angular momenta brought into the compound system by heavy projectiles of large momenta. The concept of a critical value of angular momentum was stated by several authors. " However, more recent data have shown that such a critical limit is certainly not characterized by only the properties of the compound system. In particular, experiments by Zebelman et al. '0' " have very clearly demonstrated that the entrance channel had to be considered since, for the same compound nucleus at the same excitation energies (",00Yb) four different critical LK were deduced from o cF measurements, when four different projectiles ("B,~C, "0, ' Ne) were used. Similar results have been obtained at Orsay' with a completely different experimental method of determination of l;, S. For the formation of ' VTe at the same excitation of 107 MeV energy, l";, was found equal to 70 when the projectile was 40Ar, and only 50 with '4¹ A second set of results has also very clearly shown that for a given system, l";, does not stay at a constant value whatever the bombarding energy and, therefore, the excitation energy of the compound system. If a limiting of lk is found at a given ener gy and if such a critical lh is a constant, then by increasing the energy, oc"/os should decrease at the same rate as (L";,/L ")'.Although the first experiments' could give the feeling that this was the case, new data from Natowitz, Chuliek, and Namboodiri' with C, N, ...

Extension of the Liège intranuclear-cascade model to reactions induced by light nuclei

“…In the spirit of critical-distance fusion models [31,32], we define an interaction radius R int and we prescribe that only nucleons whose collective trajectory intersects the sphere of radius R int shall fuse with the target nucleus. The interaction radius is defined as…”

Towards a unified description of light ion fusion cross section excitation functions

“…Since the first sturlies of the limiting conditions of nuclear fusion [1,2], the identification of specific entrance-channel [3][4][5] and compound nucleus [6][7][8][9][10] characteristics, which determine the salient features of the fusion cross sections in various energy regimes, has been a topic of considerable interest. Ignoring usually the spins ofthe colliding nuclei, the sharp cut-off approximation has been invoked.…”

Macroscopic dynamics of the fusion process

“…As a matter of fact the fusion cross section is smaller than the one predicted by a static model using the fusion barrier. This is known in static approaches and was the reason to introduce the critical distance concept [23]. We see however here that there is no need to introduce the notion of critical distance to explain the data and that the decrease of (1F at medium bombarding energies appears naturally as a result of friction forces.…”