Search

Monte Carlo-transport codes are designed to simulate the complex neutron transport physics associated with nuclear systems. These codes are tasked with simulating phenomena such as temperature effects on cross-sections, thermo-physical effects, reaction rates, and kinematics. It is not computationally possible to simulate the physics of a system exactly. However, many of the approximations made by modern simulation codes have been well validated. This article investigates an impactful simulation error caused by an approximation made in many Monte Carlo-transport codes. The approximation that target-at-rest is valid for neutrons at energies 400 times that of the thermal energy of the target particle is found to be inaccurate in certain scenarios. This paper identifies such cases, notably TRISO [1] fuel and instances where fuel infiltrates the pores of graphite in Molten Salt Reactors. The breakdown of this approximation occurs particularly when there exists a small length scale between fuel, a material with absorption resonances, and moderator, a scattering material. When threshold values are too small, resonance escape probabilities can deviate by as much as 1% per resonance, forming a baseline defect. Furthermore, two distinct anomalies were observed upon temperature variation, directly attributed to the transition between target-at-rest and target-in-motion physics. Equations provided in this study offer predictions for the temperature ranges within which these anomalies occur, based on system temperature and threshold value. The recommendations put forth in this paper advocate for incorporating the threshold value as a user-defined variable in transport Monte Carlo codes employing this approximation. Additionally, users are advised to conduct convergence studies to ensure that the chosen threshold value is sufficiently high to mitigate the influence of baseline defects and anomalies., Comment: 27 pages, 33 figures

Previous results indicated that high $p_T$ particle suppression in Au+Au interactions is a final state effect, since R$_{dA}$ ratios were compatible with unity, albeit within large experimental errors. It is important to test this conclusion to higher precision since the modification of structure functions may be involved. Recent d+Au data taken in 2008 improve the integrated luminosity by about a factor of thirty compared to the 2003 data. A more precise measurement of both $\pi^0$ and $\gamma$ at higher $p_T$ will shed new light on whether the initial state in the heavy nuclei is modified., Comment: 2 pages, 1 figure, Quark Matter 2009 conference proceedings (cosmetic changes following suggestions of reviewers)

It is demonstrated that many aspects of nuclear collisions, as for instance the evolution of $\pi^{+}/\pi^{-}$ and $K/\pi$ ratios with $x_F$ and $\sqrt{s}$, are influenced by isospin effects already present in elementary nucleon--nucleon collisions., Comment: 3 pages, 5 figures. International Europhysics Conference on High Energy Physics, Aachen, Germany, July 2003, submitted to JHEP