Measurement of Cross Sections and Momentum Distributions of Proton-Rich Radioactive Isotopes Produced from a 345-MeV/Nucleon 78Kr Beam

Abstract We measured the production cross sections and momentum distributions of proton-rich radioactive isotopes (RIs) whose atomic numbers were 18–37. These isotopes were produced by the projectile fragmentation of a 345-MeV/nucleon $^{78}$Kr beam impinged on a 5-mm Be target. The cross sections close to the stability region were reproduced fairly well by the semi-empirical formulas, EPAX3.1a and FRACS1.1. However, these formulas tend to overestimate the cross sections of the RIs near the proton drip line, sometimes by as much as 100-fold. The Abrasion–Ablation model in the LISE$^{++}$ package was employed, using different mass table variations, to describe the experimental results in this region. The best agreement was achieved when the Weizsäcker-Skyrme microscopic-macroscopic mass formula (WS4$_{\mathrm{RBF}}$) and a version of the nonrelativistic Hartree–Fock–Bogoliubov mass model (HFB22) were used. The momentum distribution was represented well by an asymmetric Gaussian distribution. The width of the high-momentum side of the distribution was reproduced fairly well by the Goldhaber model, whereas the width of the low-momentum side was 1.1 times larger than that of the high-momentum side. Moreover, an exponential-shaped low-momentum tail was observed, which began from a height of approximately 1/100–1/1000 of the momentum peak. The momentum means were not reproduced well by Morrissey’s empirical formula: additional velocity loss to the formula was observed. The yield of $^{68}$Br was smaller than the expected yield, as estimated from the yield systematics of its neighboring RIs. Assuming an in-flight decay in the separator, the half-life of $^{68}$Br was estimated to be $105^{+62}_{-25}$ ns.