Search

Abstract: Using Wang–Landau sampling we study the characteristic behavior of a flexible homopolymer (off-lattice) for chain lengths up to . The Hamiltonian consists of a Lennard-Jones potential between all monomers, and an additional FENE potential between bonded monomers. From the resultant density of states, we calculate thermodynamic properties for a wide range of temperatures, including low temperatures that are inaccessible to traditional Monte Carlo algorithms. Peaks in the specific heat and radius of gyration indicate the coil–globule and solid–liquid transitions. With a careful implementation of the algorithm, we find no evidence of a liquid–liquid transition. [Copyright &y& Elsevier]

Abstract: We use a two-dimensional Wang–Landau sampling algorithm to map out the phase diagram of a Q-state Potts model with in an external field H that couples to one state. Finite-size scaling analyses show that for large Q the first-order phase transition point at is in fact a triple point at which three first-order phase transition lines meet. One such line is restricted to ; another line has . The third line, which starts at the triple point, ends at a critical point which needs to be located in a two-dimensional parameter space. The critical field is positive and decreases with decreasing Q, which is in qualitative agreement with previous predictions. [Copyright &y& Elsevier]

Abstract: We describe a new adaptive sampling method for computing free energies for a wide variety of applications. The method is both simple and accurate. Results for thermodynamic quantities are obtained with this method in the form of continuous functions. [Copyright &y& Elsevier]

Abstract: We present a new version of the semimicroscopic Monte Carlo code “TERS”. The procedure for calculating multiple small angle Coulomb scattering of the residues in the target has been modified. Target-backing and residue charge-reset foils, which are often used in heavy ion-induced complete fusion reactions, are included in the code. New version program summary: Program title: TERS v2.0 Catalogue identifier: AEBD_v2_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEBD_v2_0.html Program obtainable from: CPC Program Library, Queen''s University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 7309 No. of bytes in distributed program, including test data, etc.: 1 219 555 Distribution format: tar.gz Programming language: C Computer: The code has been developed and tested on a PC with Intel Pentium IV processor. Operating system: Linux RAM: About 8 Mbytes Classification: 17.7 External routines: pgplot graphics subroutine library [1] should be installed in the system for generating residue trajectory plots. (The library is included in the CPC distribution file.) Catalogue identifier of previous version: AEBD_v1_0 Journal reference of previous version: Comput. Phys. Comm. 179 (2008) 492 Does the new version supersede the previous version?: Yes Nature of problem: Recoil separators are employed to select and identify nuclei of interest, produced in a nuclear reaction, rejecting unreacted beam and other undesired reaction products. It is important to know what fraction of the selected nuclei, leaving the target, reach the detection system. This information is crucial for determining absolute cross section of the studied reaction. Solution method: Interaction of projectiles with target nuclei is treated event by event, semimicro-scopically. Position and angle (with respect to beam direction), energy and charge state of the reaction products are calculated by Monte Carlo method. Trajectory of each nuclei inside the separator is then calculated by ion optical transfer matrix method. Ratio of the number of trajectories completing their journey up to the detection system to the total number of trajectories is a direct measure of absolute transmission efficiency of the separator. Reasons for new version: The method for calculating mean squared scattering angle (), used earlier [2], was found to be inadequate particularly for low energy heavy residues. Energy loss of beam in the target-backing foil and energy loss of residues in the charge-reset foil (wherever used) needed to be taken into account for better matching of simulated residue parameters with measurements. Summary of revisions: [1.] A new method [3] for calculating multiple small angle Coulomb scattering of residues in the target has been adopted. The change is incorporated in function Weibull() in the program ters_pti2.c. [2.] Isotopically enriched targets are made on a thin backing foil (usually made of carbon) quite often. Energy loss of beam in the backing foil (assuming beam is made to pass through the backing foil first, which is the usual practice) need to be taken into account. This calls for minor changes in the input file ters_pti2.inp. Following is the modified list of input parameters in this file with explanation. [ Zp, Ap, Zt, At –] Atomic no. and mass no. of projectile and target. [ Ep –] Projectile energy [MeV] in laboratory. [ BeamSpot, TarThick –] Dia. [mm] of (circular) beam spot and target thickness [mg/cm2]. [ Zback, Aback, BackThick –] Atomic no. and mass no. of the backing material and thickness [mg/cm2] of the backing foil. [ Qvalue, ILPM –] Q value [MeV] for CN formation and inverse level density parameter. [ AlphaNo, ProtonNo, NeutronNo –] Numbers of evaporated alphas, protons and neutrons. [ Salpha[c] –] Alpha separation energies [MeV], to be left blank if no alpha evaporation. [ Sproton[c] –] Proton separation energies [MeV], to be left blank if no proton evaporation. [ Sneutron[c] –] Neutron separation energies [MeV], to be left blank if no neutron evaporation. [ NEVENT –] Number of events i.e. residues to be considered by the program (maximum ). The parameters in input line number 4 are new in this version. If the target is backed by a carbon foil of thickness 125 μg/cm2, the input line would look like “6 12 0.125”. If the target is self-supporting, i.e. there is no backing, value of the last parameter (thickness) should be zero. However, the first two parameters must not be left blank or have 0 values. The input line would look like “6 12 0.0” in this case. [3.] A new function ThinFoil() has been introduced in the program ters_tra2.c. A thin foil can be inserted anywhere along the path of the residues by calling this function using the following syntax: Status = ThinFoil(argument list); if (Status == 0) continue; The function is particularly useful to place a residue charge-reset foil (usually made of carbon) after the target and is described in Table 1. [4.] There is a minor change in the input file ters_tra2.inp. Following is the modified list of input parameters in this file with explanation. [ Z0, A0 –] Residue atomic number and mass number. [ E0, A00, q0 –] Energy [MeV], mass no. and charge state of the reference particle. [ NEVENT –] Number of events i.e. trajectories to be calculated. [5.] Program/input files which have been modified in this version are suffixed by “2” in their names (before the extension), e.g., ters_pti.c has been renamed ters_pti2.c. The complete list of files included in the distributed code can be viewed in the readme file. Restrictions: The present version of the code is applicable to complete fusion reactions only. Calculation of transmission efficiency has been illustrated with a specific recoil separator, viz. the Heavy Ion Reaction Analyzer (HIRA) [4,5], at IUAC. One has to make necessary changes in the code, while performing calculations for other recoil separators. Also, atomic number of the residual nucleus should not exceed 92, as the method used for calculating stopping power of ions [6] is valid for . The code can perform energy loss calculation only in elemental targets and foils (i.e. compounds or alloys are not supported). Further, number of events (NEVENT) in ters_tra2.inp should not exceed the same in ters_pti2.inp. Running time: From few seconds to several minutes depending on the reaction, number of events and separator layout. References: [1] http://www.astro.caltech.edu/~tjp/pgplot/. [2] G.R. Lynch, O.I. Dahl, Nucl. Instr. Methods B 58 (1991) 6. [3] L. Meyer, Phys. Status Solidi 44 (1971) 253. [4] A.K. Sinha, N. Madhavan, J.J. Das, P. Sugathan, D.O. Kataria, A.P. Patro, G.K. Mehta, Nucl. Instr. Methods A 339 (1994) 543. [5] S. Nath, Nucl. Instr. Methods A 576 (2007) 403. [6] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, vol. I, Pergamon Press, Oxford, 1984. [Copyright &y& Elsevier]