The initial information for calculation of multifragmentation stage consists from the atomic mass number , charge of excited nucleus and its excitation energy . At high excitation energies MeV the multifragmentation mechanism, when nuclear system can eventually breaks down into fragments, becomes the dominant. Later on the excited primary fragments propagate independently in the mutual Coulomb field and undergo de-excitation. Detailed description of multifragmentation mechanism and model can be found in review .
The probability of a breakup channel is given by the
expression (in the so-called microcanonical approach ,
The channel temperature is determined by the equation constraining
the average energy associated with partition :
According to the conventional thermodynamical formulae the average energy
of a partitition is expressed through the system free energy
Calculation of the free energy is based on the use of the liquid-drop
description of individual fragments . The free energy
of a partition can be splitted into several terms:
Parameters MeV, MeV, MeV
are the coefficients of the Bethe-Weizsacker mass formula at .
is a spin and isospin degeneracy
factor for fragment ( fragments with are treated as the
thermal wavelength, is the nucleon mass, fm,
MeV is the critical temperature, which corresponds to the
liquid-gas phase transition.
is the inverse level density of the mass fragment and
MeV is considered as a variable model parameter, whose
value depends on the fraction of energy transferred to the internal
degrees of freedom of fragments . The free volume
available to the translational
motion of fragment, where
and its dependence on the
multiplicity of fragments was taken from :
The light fragments with , which have no excited states, are considered as elementary particles characterized by the empirical masses , radii , binding energies , spin degeneracy factors of ground states. They contribute to the translation free energy and Coulomb energy.
At comparatively low excitation energy (temperature) system will disintegrate into a small number of fragments and number of channel is not huge. For such situation a direct (microcanonical) sorting of all decay channels can be performed. Then, using Eq. (), the average multiplicity value can be found. To check that we really have the situation with the low excitation energy, the obtained value of is examined to obey the inequality , where and for and for , respectively . If the discussed inequality is fulfilled, then the set of channels under consideration is belived to be able for a correct description of the break up. Then using calculated according Eq. () probabilities we can randomly select a specific channel with given values of and .
The individual fragment multiplicities in
the so-called macrocanonical ensemble  are distributed
according to the Poisson distribution:
Fragment atomic numbers are also distributed
according to the Poisson distribution  (see
Eq. ()) with mean value defined as
At given mass of fragment the charge
distribution of fragments are described by Gaussian
It is assumed  that at the instant of the
nucleus break-up the kinetic energy of the fragment in the
rest of nucleus obeys the Boltzmann distribution at given temperature
The initial conditions for the divergence of the fragment system are determined by random selection of fragment coordinates distributed with equal probabilities over the break-up volume . It can be a sphere or prolongated ellipsoid. Then Newton's equations of motion are solved for all fragments in the self-consistent time-dependent Coulomb field . Thus the asymptotic energies of fragments determined as result of this procedure differ from the initial values by the Coulomb repulsion energy.
The temparature determines the average excitation
energy of each fragment: