Effects of geometric constraints on the nuclear multifragmentation process

We include in statistical model calculations the facts that in the nuclear multifragmentation process the fragments are produced within a given volume and have a finite size. The corrections associated with these constraints affect the partition modes and, as a consequence, other observables in the process. In particular, we find that the favored fragmenting modes strongly suppress the collective flow energy, leading to much lower values than those obtained from unconstrained calculations. For a given total excitation energy, this leads to a nontrivial correlation between the breakup temperature and the collective expansion velocity. In particular, we find that under some conditions, the temperature of the fragmenting system may increase as a function of this expansion velocity, contrary to what might be expected.

Figure 1

$E_{A,Z}^{\mathrm{flow}}/{\varepsilon }_{\mathrm{flow}}$ as a function of mass number, for $A_0=168$.

Figure 2

Average flow energy as a function of the average flow velocity. The full curve illustrates the results without inclusion of geometrical constraints whereas the dashed one corresponds to those with their inclusion. For additional details see the text.

Figure 3

Average breakup temperature as a function of average thermal energy. The dash-dotted curve illustrates the results obtained when geometrical constraints are not included. The full and dashed curve are the results when these constraints are included: the full (dashed) curve corresponds to velocity flow values below (above) the one leading to a maximum flow energy, as depicted in Fig. 2. The arrows indicate the direction in which the radial velocity increases.

Figure 4

Average primary multiplicity (upper panel) and average mass number (lower panel) of fragments as a function of the thermal excitation energy. The curves correspond to the same cases illustrated in Fig. 3.

Figure 5

Average fragment kinetic energy as a function of fragment atomic numbers. The lines correspond to a linear fit of the results.