Origins of intermediate velocity particle production in heavy ion reactions

Abstract

Investigation of intermediate-velocity particle production is performed on entrance channel mass asymmetric collisions of 58 Ni+C and 58 Ni+Au at 34.5 MeV/nucleon. Distinctions between prompt pre-equilibrium ejections, multiple neck ruptures and an alternative phenomenon of delayed aligned asymmetric breakup is achieved using source reconstructed correlation observables and time-based cluster recognition in molecular dynamics simulations. PACS number(s): 25. 25.70.Lm, 25.70.Mn, 25.70.Pq During the last decade, reaction mechanisms studies of heavy ion collisions in the Fermi energy domain have unraveled new and interesting phenomena that can have major consequences on our understanding of both nuclear dynamics and excited nuclear matter properties. It was first established that binary dissipative collisions dominate the reaction cross section in this energy range for a large domain of entrance channel masses and asymmetries [1][2][3][4][5]. Early, it has been noticed that unlike the lower energy case, where exclusively two primary fragments (or one fusion fragment) are formed [6], the intermediate energy heavy ion collisions produce exotic structures that populate the intermediate-velocity (IV) zone between the two main collision partners [8][9][10][11][12][13][14][15][16]. This production of particles and fragments at mid-velocity is usually explained as the onset of the participant-spectator model of relativistic energies [17]. However, in the Fermi energy domain, the interplay between two-body interactions and collective motions from the nuclear mean field modifies this simple picture and can potentially give rise to peculiar mechanisms leading to such an important observed IV particle production. Of those mechanisms, fast light particles and coalescence clusters ejected by nucleon-nucleon collisions and multiple neck ruptures are usually expected to be responsible for the main IV particle production. [18] and references therein for more information on detectors and energy calibration. The main trigger for event recording was a charged particle multiplicity of at least 3 particles. The present work is restricted to events selected in the off-line analysis by a total detected charge of at least 31 units. This ensures good reconstruction of the quasi-projectile and IV particles characteristics.In order to evaluate properties of the IV material, it needs to be isolated from its surrounding. This is done by reconstructing on an event-by-event basis the quasiprojectile (QP) and quasi-target (QT) emitters using a probabilistic reconstruction algorithm for the first nucleus and angular and velocity cuts for emissions of the second one. The statistical quasi-projectile reconstruction algorithm proceeds by two steps consisting in building probability tables for the attribution to the QP of a final detected particle and an application of these probabilities on an event-by-event basis. The main hypotheses on which the algorithm relies on are: (1) the event 1

Correlations with projectile-like fragments and emission order of light charged particles

“…Experimental and theoretical studies have indicated that the midrapidity particles are produced from a combination of complex mechanisms. Gingras et al used a molecular dynamics simulations to show that the formation of the midrapidity particles could be attributed to both a prompt emission, due to nucleon-nucleon collisions, and a later emission from the tails of the QP and QT [19]. Similarly, three-body Coulomb trajectory calculations showed that the production of midrapidity IMFs could be connected to both a fast emission from the neck region and a later emission from the surface of the QP or QT [20].…”

New “3D calorimetry” of hot nuclei

“…The process of deeply inelastic diffusion becomes the dominant phenomenon [4][5][6][7][8]. It is accompanied by an important emission of light particles, thermally unbalanced [8][9][10]. But we also observe an important production of Intermediate Mass Fragments (IMFs) at the interface of the two colliding nuclei.…”

“…The collisions present mostly a strong binary character preserving a very strong memory of the entrance channel [3]. The process of deeply inelastic diffusion becomes the dominant phenomenon [4][5][6][7][8]. It is accompanied by an important emission of light particles, thermally unbalanced [8][9][10].…”

“…But we also observe an important production of Intermediate Mass Fragments (IMFs) at the interface of the two colliding nuclei. This latter is usually called Neck Emission [8,[11][12][13][14][15]. Fusion is also observed but its cross section becomes low for symmetric collisions above 30 A.MeV [3,[16][17][18][19].…”

Quasiprojectile and intermediate velocity isotopic ratios for light fragments emitted in theAr36+Ni58andGauthier1, Roy2, St-Onge3 et al. 2014Phys. Rev. CSelf Cite0000View full textAdd to dashboardCite

“…Instead of simply calculating integrals and subtracting distributions, probability tables are constructed [21], based on the forward QP emission distributions for each Z < 5 isotope. These tables allow us to allocate particles to the QP and the MR emission sources while keeping available complete information about these particles.…”