MCViNE – An object oriented Monte Carlo neutron ray tracing simulation package

Abstract

MCViNE (Monte-Carlo VIrtual Neutron Experiment)is an open-source Monte Carlo (MC) neutron ray-tracing software for performing computer modeling and simulations that mirror real neutron scattering experiments. We exploited the close similarity between how instrument components are designed and operated and how such components can be modeled in software. For example we used object oriented programming concepts for representing neutron scatterers and detector systems, and recursive algorithms for implementing multiple scattering. Combining these features together in MCViNE allows one to handle sophisticated neutron scattering problems in modern instruments, including, for example, neutron detection by complex detector systems, and single and multiple scattering events in a variety of samples and sample environments. In addition, MCViNE can use simulation components from linear-chain-based MC ray tracing packages which facilitates porting instrument models from those codes. Furthermore it allows for components written solely in Python, which expedites prototyping of new components. These developments have enabled detailed simulations of neutron scattering experiments, with non-trivial samples, for time-of-flight inelastic instruments at the Spallation Neutron Source. Examples of such simulations for powder and single-crystal samples with various scattering kernels, including kernels for phonon and magnon scattering, are presented. With simulations that closely reproduce experimental results, scattering mechanisms can be turned on and off to determine how they contribute to the measured scattering intensities, improving our understanding of the underlying physics.

Monte Carlo simulation of the resolution volume for the SEQUOIA spectrometer

“…These simulations were verified against initial measurements of the spectrometer showing excellent agreement [22]. More recent simulations were performed using McVine [10] for a sample of uranium nitride (UN) measured on SEQUOIA. The input to McVine was the same McStas simulation used in the instrument design phase.…”

“…This is one of the standard methods of analysis for triple axis spectrometers with the Restrax package [6,7], but becomes more difficult on pulsed spallation source direct geometry spectrometers due to the asymmetric pulse shape and the large numbers of pixels. The advent of generalized Monte Carlo simulation packages such as McStas [8,9] McVine [10], Vitess [11][12][13] and NISP [14] make this case easier to realize and they have been particularly fruitful in several cases [15][16][17][18]. However sample kernels are hard to develop and the simulations can be very time intensive; especially if one wants to map out volumes of momentum transfer (Q) and energy transfer (ω) space.…”

Using Monte Carlo ray tracing simulations to model the quantum harmonic oscillator modes observed in uranium nitride

“…In order to gain a better understanding of the interplay of various contributing factors to the observed scattering intensity for uranium nitride, the MC ray tracing program MCViNE [4] developed in the DANSE [11] software development project (Distributed Data Analysis for Neutron Scattering Experiments) was used to simulate the experiments. All simulations in this work consist of four steps: (1) simulation of the neutron beam incident on the sample, which depends on the instrument of choice (ARCS or SEQUOIA) and the experimental settings, including the T 0 chopper frequency and the Fermi chopper choice, phase, and frequency; (2) simulation of neutron scattering by the "sample assembly"; (3) simulation of the detector intersection of neutron rays scattered from the sample, and generation of event-mode NeXus data files that are in the same format as the measurement data files, but include extra information of event probability; (4) reduction of the simulated NeXus file to I (Q,E).…”

“…For neutron scattering applications, the use of these simulations has generally been restricted to instrument design. However, with the availability of large parallel computing clusters and software packages tuned to MC ray tracing [4][5][6], it is now possible to carry out this type of simulation and provide detailed understanding of direct-geometry inelastic neutron scattering experiments. The neutron trajectories are modeled from the time that the neutrons are emitted from the moderator, through a sample, until they are finally captured by 3 He detector tubes.…”

Energy resolution and neutron flux of the 4SEASONS spectrometer revisited

“…Symbols denote the observed values, while solid lines denote the values calculated by using Eqs. (1),(6), and(7). The observed and calculated values are normalized so that the values at 100 Hz are unities.…”