Monte Carlo simulation of the resolution volume for the SEQUOIA spectrometer

Abstract

Abstract. Monte Carlo ray tracing simulations, of direct geometry spectrometers, have been particularly useful in instrument design and characterization. However, these tools can also be useful for experiment planning and analysis. To this end, the McStas Monte Carlo ray tracing model of SEQUOIA, the fine resolution fermi chopper spectrometer at the Spallation Neutron Source (SNS) of Oak Ridge National Laboratory (ORNL), has been modified to include the time of flight resolution sample and detector components. With these components, the resolution ellipsoid can be calculated for any detector pixel and energy bin of the instrument. The simulation is split in two pieces. First, the incident beamline up to the sample is simulated for 1 × 10 11 neutron packets (4 days on 30 cores). This provides a virtual source for the backend that includes the resolution sample and monitor components. Next, a series of detector and energy pixels are computed in parallel. It takes on the order of 30 s to calculate a single resolution ellipsoid on a single core. Python scripts have been written to transform the ellipsoid into the space of an oriented single crystal, and to characterize the ellipsoid in various ways. Though this tool is under development as a planning tool, we have successfully used it to provide the resolution function for convolution with theoretical models. Specifically, theoretical calculations of the spin waves in YFeO 3 were compared to measurements taken on SEQUOIA. Though the overall features of the spectra can be explained while neglecting resolution effects, the variation in intensity of the modes is well described once the resolution is included. As this was a single sharp mode, the simulated half intensity value of the resolution ellipsoid was used to provide the resolution width. A description of the simulation, its use, and paths forward for this technique will be discussed.

Energy resolution and neutron flux of the 4SEASONS spectrometer revisited

“…The broken lines in the figures indicate the calculated values using Eqs. (1) and (6), and the designed values of the geometrical parameters of the Fermi chopper, i.e., D = 20 mm and w = 0.4 mm. The calculated integrated intensities were normalized to coincide with the observed value at E i = 19 meV and f = 300 Hz.…”

“…Therefore, it is important to understand the relationship between the resolution and flux for the instrument quantitatively and prepare a convenient tool to estimate their values as a function of the incident energy (E i ) and the rotation speed ( f ) of the monochromatizing chopper. Monte Carlo simulation can easily model the whole instrument including advanced optics, and would be the best method to precisely reproduce the resolution and flux [1][2][3][4][5][6][7][8]. However, Monte Carlo simulation takes too long time to use to decide the experimental condition before or during the experiment, though it is useful for data analysis after the experiment.…”

“…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.…”

Instrumental resolution of the chopper spectrometer 4SEASONS evaluated by Monte Carlo simulation

“…However, precise calculations using analytical methods are complicated in the case of spectrometers with multiple optical components, advanced neutron guide shapes and multiple choppers. In this case, Monte-Carlo based numerical methods based can be used to evaluate the resolution function [4][5][6][7]. Stimulated by preceding works, to estimate the resolution function of the 4SEASONS spectrometer, which is operating at J-PARC, we performed an McStas [4] simulation, based on the model described in Ref.…”