Evaporation-cost dependence in heavy-ion fragmentation

Abstract

Inclusive multineutron and multiproton removal cross sections from 112 Sn and 104 Sn at relativistic energies have been measured. The data show two distinct regimes of the reaction process that depend on the nucleon evaporation cost of the final nucleus. This behavior is universal by regarding the mass or asymmetry of the initial system or target composition. A state-of-the-art cascade and deexcitation model reproduces the observed trend but systematically fails in reproducing cross sections for the removal of the more bound nucleon species. The fragmentation of a many-body bound system from the fast collision with an extra particle is a generic problem in areas as different as atomic physics through electron-induced ionization [1], nuclear physics through the nuclear fragmentation in spallation targets [2], or astrophysics through the ejection of rocks from gravitational rings after asteroid collisions [3]. This complex process depends a priori on the two-body interaction cross section, the geometry of the system, and the binding of its individual constituents. This multiparticle removal probability is challenging to predict to a high precision since it also depends on processes, such as the re-interaction of scattered components and the release of dissipation energy by statistical emission of particles, e.g., the ionization of atoms by energetic electrons is impacted by the Auger effect. Models for nuclear fragmentation have been numerous [4]. At kinetic energies larger than 100 MeV/nucleon, it is possible to accurately reproduce experimental fragmentation cross sections by modeling the reaction in two steps: intranuclear cascade (INC) followed by statistical deexcitation of the remnant nucleus [2]. Fragmentation results from the interplay of both processes [5]. In the case of one-nucleon removal, the proton-neutron asymmetry has recently demonstrated important limits of our treatment of direct reactions [6,7], and the role of evaporation in weakly bound nuclei has been questioned for deeply bound nucleon removal [8]. In this Rapid Communication, we present new fragmentation data from stable and unstable Sn isotopes at incident energies of ∼165 MeV/nucleon. We characterize these data by the difference in emission cost between the removed species and the other one, C = C removed − C other , where C n = S n is the neutron-evaporation cost, C p = S p + V c is the proton-evaporation cost, S n(p) is the neutron (proton) separation energy, and V c is the Coulomb barrier. We show that the ejection of identical nucleons presents two universal regimes that depend on the sign of C.Fast 104 Sn and 112 Sn beams at 155 and 173 MeV/nucleon, respectively, have been produced at the RIBF facility, operated conjointly by the RIKEN Nishina Center and the CNS of the University of Tokyo, by fragmentation of a 124 Xe primary beam of 0.5 e μA onto a 0.555 g/cm 2 9 Be production target. The secondary cocktail beams were composed of 104 Sn ( 112 Sn) at 25% (77%) purity. The achieved intensity of 104 Sn was 350 pps. Secondary targets w...

Improving the description of proton-induced one-nucleon removal in intranuclear-cascade models

“…Because large excitation energies can be realized in numerous ways, some averaging takes place and the predictions become less sensitive to the details of the initial conditions. At the same time, it should be stressed that discrepancies do seem to increase for the removal of, e.g., several protons from stable nuclei [4]. This is consistent, inasmuch as the constraints on the excitation energy in that case are even stricter than for one-proton removal, as evidenced by the smallness of the associated cross sections.…”

“…However, it was realized some time ago that these models systematically fail to describe inclusive cross sections for the removals of few nucleons, for example, [3,4]. This is especially surprising in view of the fact that these observables are associated with peripheral reactions and mostly involve collisions between quasifree nucleons; one would therefore expect the intranuclear cascade to provide an accurate description of this particular dynamics.…”

“…The original equation [13] [Eq. (4)] is an implicit definition formulated in terms of the nucleon momentum p:…”

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

“…[9]); see also the work described in Ref. [10], which is particularly relevant to our subject because it was performed with a preliminary version of the model described in the present paper.…”

Experimental study of the knockout reaction mechanism usingO14at 60 MeV/nucleon

“…Some invalidity of INC approach for the one-and multi-nucleon removal reactions was discussed in Ref. [48,49]. On the other hand, significant core excitation in low-lying excited states has been described within the Faddeev-AGS quantum scattering framework [50].…”