Many-particle correlations and Coulomb effects on temperatures from fragment momentum fluctuations

We investigate correlations in the fragment momentum distribution owing to the propagation of fragments under the influence of their mutual Coulomb field after the breakup of an excited nuclear source. The magnitude of the effects on the nuclear temperatures obtained from such distributions is estimated with the help of a simple approach in which a charged fragment interacts with a homogeneous charged sphere. The results are used to correct the temperatures obtained from the asymptotic momentum distributions of fragments produced by a Monte Carlo simulation in which the system's configuration at breakup is provided by the canonical version of the statistical multifragmentation model. In a separate calculation, the dynamics of this many-particle charged system is followed in a molecular dynamics calculation until the fragments are far away from the breakup volume. The results suggest that, although the magnitude of the corrections is similar in both models, many-particle correlations present in the second approach are non-negligible and should be taken into account to minimize ambiguities in such studies.

Figure 1

Density of particles with momentum between $p$ and $p+\mathrm{\Delta }p$, for $T=5$ MeV. In panel (a) the Monte Carlo distribution at breakup is displayed, whereas panel (b) exhibits the asymptotic distributions for both treatments presented in this work. For details, see the text.

Figure 2

Distribution of $q$ for $T=5\text{MeV}$. (a) Results predicted by the Monte Carlo model at the breakup stage (solid line) and after the propagation in the Coulomb field (dashed line). The selected species is the ${}^4\mathrm{He}$ nucleus. (b) Asymptotic $q$ distributions for proton and ${}^4\mathrm{He}$. The solid lines correspond to the results obtained with the Monte Carlo model, whereas the dashed lines represent the predictions made with the deterministic model. For details, see the text.

Figure 3

Caloric curves constructed from temperatures associated with different species. The predictions of the Monte Carlo model are displayed in panel (a), whereas those from the deterministic model are exhibited in panel (b). The dashed lines represent the input temperatures used in the SMM calculations, which also provide both the average excitation energy and its width. The open symbols represent the calculations at breakup, and the solid ones correspond to the asymptotic values. For details, see the text.

Figure 4

Caloric curves constructed using temperatures given by the Monte Carlo treatment, corrected by the deterministic model. The dashed lines represent the input temperatures used in the SMM calculations. For details, see the text.