The properties of isotopes far from the stability valley have been one of the hot research fields in the nuclear physics and astrophysics. As an important tool for promoting this research, radioactive-ion-beam facilities based on projectile fragmentation have been established around the world, and the knowledge of reaction mechanisms of projectile fragmentation is very important for using these facilities effectively.

Researchers at Institute of Modern Physics, Chinese Academy of Sciences (IMP) studied 57 MeV/nucleon ⁴⁰Ar beam bombarding on ⁹Be and ¹⁸¹Ta targets by using the Radioactive Ion Beam Line in Lanzhou (RIBLL) at Heavy Ion Research Facility in Lanzhou (HIRFL). With the magnetic rigidity scan method, the longitudinal momentum distributions and cross sections for about 100 projectile-like fragments with Z>8 have been measured and their production mechanism has been studied.

The measurement results show that the higher energy part of these fragments is dominated by the projectile fragmentation, and in the lower part, there exists competition among projectile fragmentation, nucleon transfer and others. By comparing the EPAX2 and AA model calculations, the measured cross sections can be well reproduced for the fragments near the stability line. But for the fragments far from the stability, especially for the proton-rich isotopes, the cross sections are over-estimated in the calculations. In addition, the target dependence of the production cross section has also been studied. Compared to the ⁹Be target, there exists an enhancement for the proton-rich and neutron-rich fragments with ¹⁸¹Ta target. This phenomenon decreases rapidly with the increasing projectile energy. The enhancement for the proton-rich and neutron-rich fragments is mostly duo the de-excitation of the pre-fragments and the nucleon-transfer process.

The results have been published in Physics Review C 85, 024621 (2012).

This article can be linked as follows：http://prc.aps.org/abstract/PRC/v85/i2/e024621

Fig.1 Particle identification spectra for the ⁴⁰Ar + ⁹Be reaction at Bρ=2.5Tm（Image by IMP）

Fig.2 Production cross sections of projectile-like fragments for ⁴⁰Ar at 57 MeV/nucleon on the ⁹Be and ¹⁸¹Ta targets（Image by IMP）