Fragmentation of exotic oxygen isotopes

Abstract

Abrasion-ablation models and the empirical EPAX parametrization of projectile fragmentation are described. Their cross section predictions are compared to recent data of the fragmentation of secondary beams of neutronrich, unstable 19,20,21 O isotopes at beam energies near 600 MeV/nucleon as well as data for stable 17,18 O beams. I IntroductionFragmentation of energetic heavy-ion beams is widely used to produce secondary beams of exotic nuclei far from stability [1]. In order to assess the feasibility of experiments utilizing secondary beams, a precise knowledge of the relevant production cross sections is necessary. Usually, production cross sections are deduced from empirical parameterizations of measured cross sections, e.g., the frequently used EPAX parametrization [2]. Alternatively, fragmentation models with a microscopic background have been applied, such as abrasion-ablation models [3][4][5] or intranuclear cascade simulations [6].The validity of both the empirical parameterization and the physical models has been verified mainly for medium-to heavy-mass fragments (see, e.g., Refs. [2,7-10]). In particular, it has been shown that in the few cases in which fragmentation cross sections of projectiles with different neutron-toproton ratios were studied the observed shift of the centroids of the isotope distributions was rather well reproduced by EPAX (see Fig. 11 in Ref. [2]). The data are much too scarce, however, to investigate in detail how the shapes of the distributions, in addition to their centroids, vary with the neutron-or proton-excess of the projectiles.Recently, two-step fragmentation processes were discussed in the context of an efficient production of very neutron-rich isotopes. This process involves an unstable neutron-rich fragment as an intermediate product which undergoes fragmentation again, yielding the final nucleus of interest. On the basis of the EPAX parameterization, considerable gain factors for the production of specific very neutron-rich isotopes in two-step fragmentation processes in comparison to one-step fragmentation were deduced. These findings, however, are in contrast to results obtained on the basis of an abrasion-ablation model [11].Here, we briefly describe various abrasion-ablation models as well as the EPAX parametrization and compare their predictions with experimental data of the fragmentation of the unstable neutron-rich nuclei 19,20,21 O, together with those of the stable 17,18 O isotopes [12]. The results can serve to illustrate the effect of isospin on the fragmentation process and thus help to clarify the above discrepancies between various predictions. II Fragmentation ModelsThe EPAX parameterization follows earlier approaches by Rudstam or Silberberg, Tsao, and coworkers (see references