Evaporation residue cross sections in complete fusion reactions between52Cr-56Fe,63Cu-Ni and63Cu-Ag

Abstract

Evaporation residue cross sections have been measured by the technique of the telescope AE. E counter at very small angles. The angular distribution has been followed down to 2~ ', and rather precise values were obtained for that fraction of the compound nucleus decay after complete fusion for the systems 63Cu + Ni, 63Cu + Ag and 52Cr + 56Fe at two bombarding energies. A comparison has been done with evaporation calculations using the code ALICE, and the rotating liquid drop concept. It is shown that a large discrepancy occurs at low energy for these heavy projectiles. The origin of this difference is discussed.Evaporation residue cross sections have been measured recently with the telescope technique for various combinations of targets and projectiles [1][2][3][4], and a great interest is actually focused on the complete fusion reaction mechanism at energies in the range 5-10 MeV/amu. The telescope counter method has been described by Gutbrod et al. [1] and up to now, most of the results have been obtained with projectiles of masses lower than 40. The main difficulty in the measurements is due to the fact that evaporation residues (E.R) are peaked forward in the beam direction, and the width of the angular distribution decreases when the mass of the projectile and consequently the momentum increases. Then it becomes quite a challenge to detect evaporation residues with projectiles heavier than argon ions since the counter should be placed at very small angles. One of the main interest for heavy projectile induced reactions is to compare the E.R cross sections with the total inelastic cross section, particularly well above the Coulomb barrier, where it is known that many other phenomena occur, like compound nucleus fission decay, fission like processes and deep inelastic processes. In the present work, we have used new beams of copper ions and chromium ions produced by the accelerator ALICE in Orsay. A special device along the beam line has been set up in order to obtain a well focused beam spot on the targets (about 2 mm of diameter). Two bombarding energies (207 and 264 MeV) were choosen for 52Cr and one (347 MeV) for 63Cu ions. The targets were made by evaporation on a backing of 40 ~tg per cm 2 of carbone. The thickness was 150 p.g per cm 2 for natural nickel and 100 ~tg per cm 2 for 56Fe. The silver target was a self supporting foil of 200 ~tg per cm 2. The telescope counter consisted of an ionisation chamber for the AE measurement and a solid state surface barrier detector for the E measurement. The angular acceptance of the system being A 0 = 0.55 ~ was defined by the distance of 105 cm between the target and the entrance window of the telescope (1 cm of diameter). A typical AE. E spectrum is shown on Figure l for the system 52Cr+56Fe, with 207MeV chromium ions and the telescope located at 0= 7~ '. It can be seen that evaporation residues are well separated from all the other events. It is of course necessary to measure the angular distribution down to very small angles. This has been done down to 2~ ' wi...

The influence of the entrance channel mass asymmetry on the reaction mechanism

“…The beam axis was accurately determined by counting evaporation residues and elastically scattered particles on both sides of the beam direction. As already mentioned elsewhere [7], evaporation residues (ER) are well separated from all the other events. The total ER cross sections were deduced from the integration of the angular distributions.…”

Comparison between experimental decay of the compound nuclei150Gd and evaporation calculations

“…The shapes and magnitudes of the experimental excitation functions are rather well reproduced by the calculations, but a systematic shift to lower energies of some 10-15 MeV occurs for the calculated curves. However, most of the comparisons were made in various laboratories between experimental and calculated data only on the total evaporation residue cross sections [5][6][7][8]. Then, the relative importance of the different exit channels was not tested since the goal was to reproduce the sum of all those exit channels.…”