Intermediate mass fragment production in central collisions of intermediate energy heavy ions

Abstract

We present Z distributions for fragments with 1 < Z < 12 from central collisions of '^^Ar+'^^Sc at incident energies ranging from 35 to 115 MeV/nucleon. We find that the Z distributions can be described by a power law or an exponential and steepen with increasing incident energy. Over the range of incident energies studied, the average number of intermediate mass fragments decreases while the average number of particles increases. When combined with previous results for the charge distributions, a minimum is observed in the extracted power-law parameter. PACS numbers: 25.70.PqThe observation and characterization of a liquid-gas phase transition in nuclear matter will provide valuable information concerning the nuclear equation of state (EOS). Nuclear matter exists in a liquidlike state in its ground state. When nuclear matter is heated to excitations high compared to its binding energy it behaves like a classical gas. So far no unambiguous experimental evidence for the liquid-gas phase transition in nuclear matter exists. In this Letter we report charge distributions, a{Z), for "^^Ar-induced reactions on a "^^Sc target at incident energies of 35 to 115 MeV/nucleon where fragments with 1 < Z < 12 were observed with the Michigan State University (MSU) An array. By selecting central collisions in this nearly symmetric system, we reduce the contribution of spectator matter and create a system with known excitation energy and number of participant particles. We find that the charge distributions from central collisions become steeper as the beam energy is increased from 35 to 115 MeV/nucleon corresponding to excitation energies ranging from 8 to 29 MeV/nucleon assuming that preequilibrium particle emission is not important.The critical point of the liquid-gas phase diagram has been predicted to occur in infinite nuclear matter at an excitation energy as low as 15 MeV/nucleon [1] and as high as 330 MeV/nucleon [2]. Scaling theories [3] suggest that the cluster size distribution near the critical point in the phase diagram follows a scaling function n(Af,p) = Af V(Af(p -pc)) .where p is the order parameter with /(O) = 1 at the critical value of the order parameter {p=pc).

Pre-equilibrium particle emission and critical exponent analysis

“…It was hoped that the problem of impact parameter selection would be less severe and therefore the critical exponents easier to extract for participant fragmentation in symmetric heavy ion collisions. By focussing on very central collisions as a function of beam energy, a minimum of the fit parameter λ, with σ(Z f ) ∝ Z −λ f was observed [9]. This confirmed similar observations obtained from reverse-kinematics reactions [10].…”

A schematic model for fragmentation and phase transition in nuclear collisions

“…Nuclear fragmentation has been intensively studied for the purpose of measuring a possible liquid-gas phase transition in nuclear matter [1,2]. The most accessible quantity in experiments is a measurement of the yield Y (A) against A where A is the number of nucleons in the composite that emerges from the collision [3,4]. A maximum in the fluctuation in the fragment sizes could be an evidence for critical phenomenon.…”

Source size scaling of fragment production in projectile breakup

“…Given a freeze-out density and temperature the model can calculate the properties of the fragments. This freeze-out density ρ/ρ 0 has been chosen to be as close as possible to 0.39, extracted from the analysis of Ar+Sc [24][25][26]. The lattice dimension is 4×5×5 for Cl, giving a 100 sites and a density ratio, ρ/ρ 0 , of 0.35.…”