Space-time characteristics of fragment emission in theE/A=30 MeVXe129+

Abstract

Intermediate-mass-fragment emission has been studied in central E/A = 30 MeV Xe + " 'Cu reactions. The measured fragment multiplicities, reduced-velocity correlation functions, and emission velocities have been compared with schematic three-body trajectory calculations and with three statistical models with input based upon a dynamical BNV code. The statistical models which include expansion either explicitly or implicitly are able to generate a sufBcient number of fragments. The three-body trajectory calculations indicate a mean emission time of = 200 fm/c, consistent with sequential decay. Dynamical expanding-emitting source calculations predict a similar time scale for fragment emission and give satisfactory agreement with the experimental correlation functions if the experimental angular distributions are incorporated into the model. The Berlin multifragmentation model gives good agreement with the experimental charge distributions, and, depending upon the choice of radius parameter, can provide agreement with either the correlation functions or the fragment emission velocities, but not with both simultaneously. Although an overall good agreement is obtained in the statistical model comparisons, even in the most violent collisions the angular distributions and fragment emission velocities are incompatible with completely equilibrated decay from a single source. PACS uumber(s): 25.70.Pq I. INTR.C)DU CTI(3N Studies of IMF (intermediate mass fragment, 3 & Z ( 20) emission [1] have provided a wealth of information about the space-time extent of excited nuclear systems. At high bombarding energy, E/A ) 100 MeV, the fragments appear to be emitted on the very short time scale implied by a prompt multi&agment disassembly of heated and expanded nuclear systems [2,3]. At low bombarding energy, E/A & 20 MeV, standard compound nucleus decay accounts for much of the fragment yield following either complete or incomplete fusion reactions [4 -6]. To gain a systematic understanding of the interaction between complex nuclei it is important to characterize the intermediate bombarding energy regime, where the fragment emission time scale changes from sequential to simultaneous [7,8], and nuclear expansion begins to occur [9,10].Observables which have been employed to infer the spatial and temporal extensions of fragmentation sources are &agment yields, &agment-&agment reduced-velocity correlation functions, and emission velocities of emitted fragments.The IMF yields calculated with statistical emission models are strongly dependent on the density of the emitting system [11 -14]. Comparisons of experimental data with model predictions have shown that nuclear expansion is needed to reproduce the observed fragment multiplicities and charge distributions, even at bombarding energies as low as E/A = 35 MeV [13]. The fragment emission time scale, as determined from fragmentfragment correlations, is a direct measure of the spacetime extent of the source [2,3,7,8, 15 -26]. The fragment emission velocities are sensitive to an expansion ...

Nuclear multifragmentation and phase transition for hot nuclei

“…In fig. 19 the fragment charge distribution measured for central 129 Xe+ nat Cu collisions at 30 MeV/nucleon incident energy [139] is compared to the Berlin Multifragmentation Model (also called MMMC-Microcanonical Metropolis Monte Carlo) [105] and to the sequential statistical model GEMINI [140]. The BMM calculations give good qualitative agreement with the experimental charge distribution over the entire Z range measured.…”

“…The agreement between experimental and simulated velocity spectra for fragments of given charges (Z=6, 11, 18 and 27), for the different beam energies, is quite remarkable (see [5]). Finally relative velocities between fragment pairs were also compared through reduced relative velocity correlation functions [14,15,16,17] (see fig. 3).…”

Properties of the initial participant matter interaction zone in near-Fermi-energy heavy-ion collisions

“…To put this on a somewhat more quantitative basis we have used various experimental results from the literature to evaluate the magnitude of secondary decay contributions to systems such as those we investigate [13,[24][25][26][27][28][29][30]. Briefly, for the more central collisions, these results indicate that ∼10% of the total observed light charged particle emission comes from secondary decay.…”

“…This TLF source may consist of one or more excited intermediate mass fragments (IMFs) that are undergoing secondary decay. In systems such as ours, in addition to a heavy composite nucleus, an average IMF multiplicity of ∼1 to 1.5 is expected [13,24,[27][28][29]. The IMFs are typically excited to ∼3 MeV/nucleon and average multiplicities of emission from such IMFs have been determined experimentally [13,25,26].…”