An application of Fleck effective scattering to the difference formulation for photon transport

We introduce a new treatment of the difference formulation[1] for photon radiation transport without scattering in 1d slab geometry that is closely analogous to that of Fleck and Cummings[2] for the traditional formulation. The resulting form is free of implicit source terms and has the familiar effective scattering of the field of transport

Accurate and efficient radiation transport in optically thick media -- by means of the Symbolic Implicit Monte Carlo method in the difference formulation

The equations of radiation transport for thermal photons are notoriously difficult to solve in thick media without resorting to asymptotic approximations such as the diffusion limit. One source of this difficulty is that in thick, absorbing media thermal emission is almost completely balanced by strong absorption. In a previous publication [SB03], the photon transport equation was written in terms of the deviation of the specific intensity from the local equilibrium field. We called the new form of the equations the difference formulation. The difference formulation is rigorously equivalent to the original transport equation. It is particularly advantageous in thick media, where the radiation field approaches local equilibrium and the deviations from the Planck distribution are small. The difference formulation for photon transport also clarifies the diffusion limit. In this paper, the transport equation is solved by the Symbolic Implicit Monte Carlo (SIMC) method and a comparison is made between the standard formulation and the difference formulation. The SIMC method is easily adapted to the derivative source terms of the difference formulation, and a remarkable reduction in noise is obtained when the difference formulation is applied to problems involving thick media

Isospin effects on the energy of vanishing flow in heavy-ion collisions

Using the isospin-dependent quantum molecular dynamics model we study the isospin effects on the disappearance of flow for the reactions of $^{58}Ni$ + $^{58}Ni$ and $^{58}Fe$ +$^{58}Fe$ as a function of impact parameter. We found good agreement between our calculations and experimentally measured energy of vanishing flow at all colliding geometries. Our calculations reproduce the experimental data within 5%(10%) at central (peripheral) geometries

Nuclear Flow in Consistent Boltzmann Algorithm Models

We investigate the stochastic Direct Simulation Monte Carlo method (DSMC) for numerically solving the collision-term in heavy-ion transport theories of the Boltzmann-Uehling-Uhlenbeck (BUU) type. The first major modification we consider is changes in the collision rates due to excluded volume and shadowing/screening effects (Enskog theory). The second effect studied by us is the inclusion of an additional advection term. These modifications ensure a non-vanishing second virial and change the equation of state for the scattering process from that of an ideal gas to that of a hard-sphere gas. We analyse the effect of these modifications on the calculated value of directed nuclear collective flow in heavy ion collisions, and find that the flow slightly increases.Comment: 12 pages, REVTeX, figures available in PostScript from the authors upon reques

Fission-Fusion Neutron Source Progress Report Sept 30, 2009

In this report the authors describe the progress made in FY09 in evaluating the feasibility of a new concept for using the DT fusion reaction to produce intense pulses of 14 MeV neutrons. In this new scheme the heating of the DT is accomplished using fission fragments rather than ion beams as in conventional magnet confinement fusion schemes or lasers in inertial confinement schemes. As a source of fission fragments they propose using a dust reactor concept introduced some time ago by one of us (RC). An attractive feature of this approach is that there is no need for a large auxiliary power source to heat the DT plasma to the point where self-sustaining fusion become possible. Their scheme does require pulsed magnetic fields, but generating these fields requires only a modest power source. The dust reactor that they propose using for their neutron source would use micron-sized UC pellets suspended in a vacuum as the reactor fuel. Surrounding the fuel with a moderator such as heavy water (D{sub 2}O) would allow the reactor to operate as a thermal reactor and require only modest amounts of HEU. The scheme for using fission fragments to generate intense pulses of 14 MeV neutrons is based on the fission fragment rocket idea. In the fission fragment rocket scheme it was contemplated that the fission fragments produced in a low density reactor core could be guided out of the reactor by large magnetic fields used to form a 'rocket exhaust'. Their adaptation of this idea for the purposes of making a neutron source involves using the fission fragments escaping from one side of a tandem magnet mirror to heat DT gas confined in the adjacent magnetic trap

Two-proton Correlation Functions for 36Ar + 45Sc at E/A=80 MeV

Impact-parameter filtered longitudinal and transverse two-proton correlation functions measured for 36Ar+ 45Sc collisions at E/A=80 MeV are compared to predictions of the BUU transport model. For a cut on large transverse energies, the overall trends of the measured correlated functions are rather well reproduced by calculations for central collisions. Systematic discrepancies become visible, however, for calculations with larger impact parameters

Mass Dependence of Directed Collective Flow

Sidewards directed fragment flow has been extracted for 84Kr+197Au collisions at E/A=200MeV, using techniques that are free of reaction plane dispersion. The fragment flow per nucleon increases with mass, following a thermal or coalescencelike behavior, and attains roughly constant limiting values at 4≤A≤12. Comparisons of the impact parameter dependences of the measured coalescence-invariant proton flow to Boltzmann-Uehling-Uhlenbeck calculations clearly favor a momentum dependent nuclear mean field