Part 4: Conclusions and Future Directions 100
A.2 Basic concepts in MCViNE
tailed information from experiments. This requires a fundamental understanding of the measured scattering and an accurate and reproducible approach to data reduction. The simulation of neutron experiments offers an approach to decouple aspects of the measured signal and parse the individual contributions described above.
The Monte Carlo VIrtual Neutron Experiment (MCViNE) package is a Monte Carlo neutron ray tracing package developed during the commissioning of the ARCS instrument. It is used to simulate experimental results from SNS inelastic scattering instruments [129,3,12]. Monte Carlo ray tracing simulations have played an important role in neutron instrument design and optimization for the last several generations of instruments. These simulations typically treat a neutron instrument as a linear chain of optical components, providing computational efficiency and simplifying the coding.
MCViNE differs from previous packages in its focus on making the simulations useful for interpre- tation of neutron scattering spectra, and as a result several key features of its design are different.
MCViNE provides easy setup for running of simulations by inexperienced users. Its construction al- lows for the flexible re-arrangement of components, which is important for constructing a simulation that closely mimics experiment conditions. In addition, the inherent handling of multiple scattering makes it possible to turn on and off this scattering component to see its effect on the scattering spectra.
This section describes the basic concepts of MCViNE, and compares experiments and simulations of samples typically used for instrument calibration, aluminum and vanadium. Next, a template for introducing a high-temperature furnace into the simulation is constructed and tested with empty furnace measurements. Finally, experimental data and simulations are compared for a powder sample of chromium.
Figure A.1: A neutron incident on a scattered can be scattered multiple times. Scattering events are represented in different colors corresponding to the scattering kernel used for this event. Red arrows are paths of neutron propagation, and at each scattering event, the original neutron is also propagated out of the scatterer.
kernels for each scatterer. A powder-diffraction kernel allows coherent and incoherent elastic scat- tering. A single-phonon kernel allows coherent and incoherent inelastic single-phonon scattering. A multi-phonon kernel allows multi-phonon scattering using an incoherent approximation. Multiple scattering processes, allowing a single neutron to undergo more than one scattering process, can be allowed or turned off. FigureA.1shows how a neutron incident on the scatterer, can be scattered by the various scattering kernels defined for the scatterer. The neutron follows the red path and can be scattered (i) elastically by the powder diffraction kernel, following the dashed line out of the scat- terer in the same direction, (ii) inelastically by the single-phonon kernel, or (iii) in a multi-phonon process.
A ‘sample assembly’ is constructed from several scatterers, and arranged in a specific geometrical configuration to simulate the exact physical layout of all components in the path of the beam. For the most simple case, the sample assembly may only consist of a single scatterer in the form of a sample with a plate or rod geometry. This will be placed at the center of the sample position. A more complex sample assembly may simulate a powder sample inside of a niobium foil sachet. This sample assembly would consist of a powder sample, simulated as a plate, with a thin Nb plate on the front and back of the sample. The choice of positioning is at the discretion of the user, but is
Step 2: Sample Scattering
Step 3: Detector Interception
Step 4: Data Reduction Step 1: Beam
Simulation
Figure A.2: The simulation proceeds in four steps, as shown for a schematic of the ARCS instrument.
First, the neutrons travel from the moderator to the sample (yellow path). Second, the neutrons are incident on the sample and scatter from the sample (pink path). Third, the neutrons are intercepted by the detector array (green path). Fourth, the event-mode NeXus file is reduced using Mantid.
usually most easily defined by having the sample at the center of the beam position, and the Nb plates each offset from the beam center by one-half the thickness of the sample.
The orientation of the scatterers are individually defined inx, the direction of the incident beam, y, 90-degrees to the incident beam in the horizontal plane of the beam, andθ, the angle relative to
the incident beam. Thez variable defines the third axis, normal to thex−y plane. The coordinate system follows the right-hand rule with positivexfollowing the incident neutrons, positivey in the direction the finger curl, and positivezin the direction of the thumb. An example of the user-modifed input file to construct the sample assembly is given in SectionA.4.
Construction of the sample assembly is the primary task of the user before beginning the simu- lation. The instrument configuration of the various guides and choppers down the flight path before the sample has been previously developed. The detector configuration to intercept the scattered neutron is also static and does not require user modification.
The simulation sequence follows four steps. A schematic in Fig. A.2 shows neutrons passing
component in the chain until the neutron reach the sample. This beam simulation is static, as long as the instrument components remain unchanged. As a result, this aspect of the simulation only needs to be run once for a given incident energy and number of neutrons.
Second, the neutron are scattered from the sample assembly. The sample assembly, as described above, is the collection of scatterers used to describe the sample and additional components including the sample container and sample environment.
Third, the scattered neutrons are intercepted by the detector array. This assigns a detector pixel ID and time-of-flight to each neutron that reaches the detector. The collected neutrons are then processed into an event-mode NeXus file. This approach to data collection is the same as is currently in use at the SNS. That is, instead of relying on histograms of detector events for finite bins of energy and time, neutrons are tagged individually.
The final step in the simulation sequence is the reduction of the NeXus file using Mantid. This reduction step uses the routines and procedures that are used for reduction of experimentally- collected data.
It is also possible to introduce a radial collimator between the second and third steps, allowing the neutrons to pass through the collimator after scattering from the sample and before entering the detectors.