Signals for a Transition from Surface to Bulk Emission in Thermal Multifragmentation

Abstract

Excitation-energy-gated two-fragment correlation functions have been studied between E(*)/A = (2-9)A MeV for equilibriumlike sources formed in 8-10 GeV/c pi(-) and p+197Au reactions. Comparison with an N-body Coulomb-trajectory code shows an order of magnitude decrease in the fragment emission time in the interval E(*)/A = (2-5)A MeV, followed by a nearly constant breakup time at higher excitation energy. The decrease in emission time is strongly correlated with the onset of multifragmentation and thermally induced radial expansion, consistent with a transition from surface-dominated to bulk emission expected for spinodal decomposition.

The decay time scale for highly excited nuclei as seen from asymmetrical emission of particles

“…For illustration, we show the evaporation times calculated according to the widths found in the model for 29 Si and 27 Al. A rapid lifetime drop with the excitation is a general behavior of evaporation lifetimes (see also, e.g., [13]), and it reflects a gradual transition to simultaneous multifragment decay, which has been discussed in the literature [14]. One can see a qualitative agreement for the extracted and the calculated times.…”

Liquid-Gas Phase Transition in Nuclear Multifragmentation

“…Recent analysis of the IMF correlation functions from high energy hadron induced multifragmentation suggests the saturation time occurs at much shorter time scale (<100 fm/c) as shown in the right panel of fig. 7 [48]. Considering the space-time ambiguity and model dependence in extracting the time information, the correlation analysis is probably not reliable in extracting a time scale less than 50 fm/c.…”

“…Considering the space-time ambiguity and model dependence in extracting the time information, the correlation analysis is probably not reliable in extracting a time scale less than 50 fm/c. [48].…”

“…Cascade and transport calculations can be used to estimate the number of particles emitted and the energy lost in the "prompt" or early stage of the reaction. Such calculations may suggest some optimum ways of estimating the number such as imposing energy thresholds on the data to minimize the "pre-equilibrium" contributions [48,78]. However, the prompt contributions cannot be completely eliminated from the data using the energy threshold gates.…”

“…Neutrons do not interact with matter as much as the charged particles so they are more difficult to detect. Very often, E n is estimated using the average number of neutrons emitted and the mean neutron energy, E n = N n < E n > [48,10,78]. Conservation of particles imposes some constraints on the value of neutron multiplicity N n .…”