arXiv subjects

S. Das Gupta

Publications and source records attributed to S. Das Gupta.

At least 37 records · Page 2Linked to original sources

arXiv2009

Symmetry energy from fragment observables in the canonical thermodynamic model

Different formulas relying measurable fragment isotopic observables to the symmetry energy of excited nuclei have been proposed and applied to the analysis of heavy ion collision data in the recent literature. In this paper we examine the quality of the different expressions in the framework of the McGill Canonical Thermodynamic Model. We show that even in the idealized situation of canonical equilibrium and in the absence of secondary decay, these formulas do not give a precise reconstruction of the symmetry energy of the fragmenting source. However, both isotopic widths and isoscaling appear very well correlated to the physical symmetry energy.

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arXiv2009

Phase diagram for the asymmetric nuclear matter in the multifragmentation model

We assume that, in equilibrium, nuclear matter at reduced density and moderate finite temperature, breaks up into many fragments. A strong support to this assumption is provided by date accumulated from intermediate energy heavy ion collisions. The break-up of hot and expanded nuclear matter according to rules of equilibrium statistical mechanics is the multifragmentation model. The model gives a first order phase transition. This is studied in detail here. Phase-equilibrium lines for different degrees of asymmetry are computed.

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arXiv2008

Extending the canonical thermodynamic model: inclusion of hypernuclei

The canonical thermodynamic model has been used frequently to describe the disassembly of hot nuclear matter consisting of neutrons and protons. Such matter is formed in intermediate energy heavy ion collisions. Here we extend the method to include, in addition to neutrons and protons, lambda particles. This allows us to include productions of hypernuclei in intermediate energy heavy ion collisions. We can easily predict average mass number of hypernuclei produced and values of relative cross-sections of different hypernuclei. Computation of absolute cross-section was not attempted at this stage and will require much more detailed work.

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arXiv2008

Bimodality and Coulomb effects with a canonical thermodynamic model

The effect of the Coulomb interaction on the phase diagram of finite nuclei is studied within the Canonical Thermodynamic Model. If Coulomb effects are artificially switched off, this model shows a phenomenology consistent with the liquid-gas phase transition. The inclusion of Coulomb does not significantly affect the phase diagram but it drastically modifies the nature and order parameter of the transition. A clear understanding of the phenomenon can be achieved looking at the distribution of the largest fragment produced in each fragmentation event. Possible connections with experimental observations are outlined.

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arXiv2008

Exploring the Lattice Gas Model for isoscaling

Isotopic spin dependent lattice gas model is used to examine if it produces the isoscaling behaviour seen in intermediate energy heavy ion collisions. Qualitative features are reproduced but quantitative agreement with experiments is lacking.

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arXiv2008

Isoscaling, Symmetry Energy and Thermodynamic Models

The isoscaling parameter usually denoted by $α$ depends upon both the symmetry energy coefficient and the isotopic contents of the dissociating systems. We compute $α$ in theoretical models: first in a simple mean field model and then in thermodynamic models using both grand canonical and canonical ensembles. For finite systems the canonical ensemble is much more appropriate. The model values of $α$ are compared with a much used standard formula. Next we turn to cases where in experiments, there are significant deviations from isoscaling. We show that in such cases, although the grand canonical model fails, the canonical model is capable of explaining the data.

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arXiv2008

Comparison of Statistical Multifragmentation Model simulations with Canonical Thermodynamical Model results: a few representative cases

The statistical multifragmentation model (SMM) has been widely used to explain experimental data of intermediate energy heavy ion collisions. A later entrant in the field is the canonical thermodynamic model (CTM) which is also being used to fit experimental data. The basic physics of both the models is the same, namely that fragments are produced according to their statistical weights in the available phase space. However, they are based on different statistical ensembles, and the methods of calculation are different: while the SMM uses Monte-Carlo simulations, the CTM solves recursion relations. In this paper we compare the predictions of the two models for a few representative cases.

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arXiv2007

Specific heat and bimodality in canonical and grand canonical versions of the thermodynamic model

We address two issues in the thermodynamic model for nuclear disassembly. Surprisingly large differences in results for specific heat were seen in predictions from the canonical and grand canonical ensembles when the nuclear system passes from liquid-gas co-existence to the pure gas phase. We are able to pinpoint and understand the reasons for such and other discrepancies when they appear. There is a subtle but important difference in the physics addressed in the two models. In particular if we reformulate the parameters in the canonical model to better approximate the physics addressed in the grand canonical model, calculations for observables converge. Next we turn to the issue of bimodality in the probability distribution of the largest fragment in both canonical and grand canonical ensembles. We demonstrate that this distribution is very closely related to average multiplicities. The relationship of the bimodal distribution to phase transition is discussed.

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arXiv2007

Cross-sections of neutron rich nuclei from projectile fragmentation: canonical thermodynamic model estimates

A remarkably simple dependence of fragmentation cross-section on average binding energy has been established in experimental data. This dependence was empirically parametrised leading to a very useful formula for extrapolation. We find that the canonical thermodynamic model, which has been used in the past for successful computations of many observables resulting from multifragmentation, reproduces the salient fetures of fragmentation cross-sections of very neutron rich nuclei very well. This helps towards a theoretical understanding of the observed data.

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arXiv2007

Properties of the largest fragment in multifragmentation: a canonical thermodynamic calculation

Many calculations for the production of light and intermediate particles resulting from heavy ion collisions at intermediate energies exist. Calculations of properties of the largest fragment resulting from multifragmentation are rare. In this paper we compute these properties and compare them with the data for the case of gold on carbon. We use the canonical thermodynamic model. The model also gives a bimodal distribution for the largest fragment in a narrow energy range.

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arXiv2006

A Model for Phase Transition based on Statistical Disassembly of Nuclei at Intermediate Energies

Consider a model of particles (nucleons) which has a two-body interaction which leads to bound composites with saturation properties. These properties are : all composites have the same density and the ground state energies of composites with k nucleons are given by -kW+σk^{2/3} where W and σare positive constants. W represents a volume term and σa surface tension term. These values are taken from nuclear physics. We show that in the large N limit where N is the number of particles such an assembly in a large enclosure at finite temperature shows properties of liquid-gas phase transition. We do not use the two-body interaction but the gross properties of the composites only. We show that (a) the p-ρisotherms show a region where pressure does not change as $ρ$ changes just as in Maxwell construction of a Van der Waals gas, (b) in this region the chemical potential does not change and (c) the model obeys the celebrated Clausius-Clapeyron relations. A scaling law for the yields of composites emerges. For a finite number of particles N (upto some thousands) the problem can be easily solved on a computer. This allows us to study finite particle number effects which modify phase transition effects. The model is calculationally simple. Monte-Carlo simulations are not needed.

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arXiv2005

On a variational principle model for the Nuclear Caloric curve

Following the lead of a recent work we perform a variational principle model calculation for the nuclear caloric curve. A Skyrme type interaction with and without momentum dependence is used. The calculation is done for a large nucleus, i.e, in the nuclear matter limit. Thus we address the issue of volume fragmentation only. Nonetheless the results are similar to the previous, largely phenomenological calculation for a finite nucleus. We find the onset of fragmentation can be sudden as a function of temperature/excitation energy.

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arXiv2004

Mononuclear caloric curve in a mean field model

A mean field model is used to investigate if a plateau in caloric curve can be reached in mononuclear configuration. In the model the configuration will break up into many pieces as the plateau is approached.

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arXiv2004

The Thermodynamic Model for Nuclear Multifragmentation

A great many observables seen in intermediate energy heavy ion collisions can be explained on the basis of statistical equilibrium. Calculations based on statistical equilibrium can be implemented in microcanonical ensemble (energy and number of particles in the system are kept fixed), canonical ensemble (temperature and number of particles are kept fixed) or grand canonical ensemble (fixed temperature and a variable number of particles but with an assigned average). This paper deals with calculations with canonical ensembles. A recursive relation developed recently allows calculations with arbitrary precision for many nuclear problems. Calculations are done to study the nature of phase transition in intermediate energy heavy ion collision, to study the caloric curves for nuclei and to explore the possibility of negative specific heat because of the finiteness of nuclear systems. The model can also be used for detailed calculations of other observables not connected with phase transitions, such as populations of selected isotopes in a heavy ion collision. The model also serves a pedagogical purpose. For the problems at hand, both the canonical and grand canonical solutions are obtainable with arbitrary accuracy hence we can compare the values of observables obtained from the canonical calculations with those from the grand canonical. Sometimes, very interesting discrepancies are found. To illustrate the predictive power of the model, calculated observables are com$data from the central collisions of Sn isotopes.

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arXiv2004

Radial flow has little effect on clusterization at intermediate energies in the framework of the Lattice Gas Model

The Lattice Gas Model was extended to incorporate the effect of radial flow. Contrary to popular belief, radial flow has little effect on the clusterization process in intermediate energy heavy-ion collisions except adding an ordered motion to the particles in the fragmentation source. We compared the results from the lattice gas model with and without radial flow to experimental data. We found that charge yields from central collisions are not significantly affected by inclusion of any reasonable radial flow.

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arXiv2004

Multiplicity Distributions of Intermediate Mass Fragments in the Thermodynamic Model

Multiplicity distributions of intermediate mass fragments (IMF) seen in intermediate energy heavy ion collisions have been of great interest in the last ten years. The distributions of single intermediate sized element are close to Poissonian and have been subjected to widely different physical interpretations. We calculate the multiplicity distributions in a thermodynamic model whose physical basis is equal population in phase space which implies equilibrium statistics. We found distributions of single element are to a good approximation Poissonian, and such result is attributed to the small emission probability of a given IMF among all the possible channels. We also found significant deviation from Poisson distribution for a range of IMFs, and suggest strong correlations as an explanation. Systematics of the multiplicity distributions are explored in both one component and two component models. In one component model, sudden changes for the multiplicity distributions are found at the phase transition region. In two component model, the multiplicity distributions have more subtleties. Results from the current model are compared with available data, and good agreements are found. New signals for the multi-particle correlations are also suggested.

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arXiv2004

Grand Canonical Model Predictions For Nuclear Fragmentation

The grand canonical ensemble has been used to make predictions for composite yields using simple models for nuclear fragmentation. While this gives correct model prediction for high energy collisions, it can give very inaccurate results at intermediate energy.

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arXiv2003

Specific heat at constant volume in the thermodynamic model

A thermodynamic model for multifragmentation which is frequently used appears to give very different values for specific heat at constant volume depending upon whether canonical or grand canonical ensemble is used. The cause for this discrepancy is analysed.

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