Experimental reconstruction of excitation energies of primary hot isotopes in heavy ion collisions near the Fermi energy

Abstract

The excitation energies of the primary hot isotopes in multifragmentation events are experimentally reconstructed in the reaction system 64 Zn + 112 Sn at 40 MeV/nucleon. A kinematical focusing method is employed to evaluate the multiplicities of the evaporated light particles associated with isotopically identified fragments with 3 Z 14. Angular distributions of the velocity spectra of light charged particles and neutrons associated with trigger isotopes are examined. A moving source fit is used to separate the kinematically correlated particles, evaporated from the parents of the detected isotopes, from the uncorrelated particles originating from other sources. The latter are evaluated experimentally relative to those in coincidence with the Li isotopes. A parameter, k, is used to adjust the yield of the uncorrelated particles for different trigger isotopes. For each experimentally detected isotope, the multiplicities, apparent temperatures, and k values for n, p, d, t, and α particles are extracted. Using the extracted values, the excitation energies of the primary hot isotopes are reconstructed employing a Monte Carlo method. The extracted excitation energies are in the range of 1 to 4 MeV/nucleon but show a significant decreasing trend as a function of A for a given Z of the isotopes. The results are compared with those of antisymmetrized molecular dynamics (AMD) and statistical multifragmentation model (SMM) simulations. While some of the experimental characteristics are predicted partially by each model, neither simulation reproduces the overall characteristics of the experimental results.

Internal and kinetic temperatures of fragments in the framework of a nuclear statistical multifragmentation model

“…Before we present the model predictions, we mention that the assumption according to which the system has reached full thermal equilibrium by the time the fragments are formed may be cast into doubt as the excitation energy of the source increases due to the very short lifetime for particle emission. As a matter of fact, some experimental results suggest that the system cools down as it emits particles sequentially [58] or that light fragments are emitted before thermal equilibrium is attained by the remaining system [59]. In the framework of these scenarios, it is difficult to define the emission temperature, and consequently caloric curves, as the system is not in thermal equilibrium.…”

Novel determination of density, temperature, and symmetry energy for nuclear multifragmentation through primary fragment-yield reconstruction

“…In contrast, the transport models do not assume any chemical or thermal equilibration a priori. When fragments are initially formed in the multifragmentation process, many of them are in excited states [19][20][21]. These "primary hot" fragments will deexcite by evaporation processes before they are detected as "secondary cold" fragments.…”

“…To avoid the complication of secondary decay modification of fragment yields, we previously proposed experimental methods for kinematical reconstruction of the primary fragment yields and excitation energies in complex multifragmentation events [19][20][21]. In this Rapid Communication we report on the use of experimental reconstruction of primary fragment yields to characterize the fragmenting source, using the ratio, a sym /T .…”

“…A detailed description of the kinematical focusing analysis is given in Ref. [21]. In Fermi energy heavy-ion collisions, LPs are emitted at different stages of the reaction and from different sources during the evolution of the collision.…”

“…A detailed explanation of the method is given in Ref. [21]. The widths of the multiplicity distributions of LPs associated with all experimentally observed final fragments were evaluated at the primary fragment excitation energies of 2.25 ± 0.25 MeV/nucleon, which was previously found to be the average excitation energy of the primary fragments.…”