This event is a satellite workshop of ICNS 2025 https://icns2025.dk, taking place in Copenhagen and Lund on July 7-10, 2025. The purpose is to share knowledge on simulation tools for simulation of neutron scattering instrumentation. The event will be held on the day after ICNS, 11th of July and take place in Lyngby north of Copenhagen. While it is a satellite event to ICNS, attendance of the main conference is not a requirement to participate in this event.
The event will consist of talks, sandwich lunch and a mingle / poster session in the late afternoon.
The theme will be on simulation of neutron scattering instrumentation and contributions can include anything relevant to this endeavour.
Registration and abstract submission have been closed.
Welcome and reports from McStas, Vitess and SIMRES
The McStas[1,2] neutron ray-trace simulation package is a versatile tool for simulating neutron instruments and experiments alike. McStas was first released from RISØ in 1998[3] and has since grown to become a widely accepted open source software, backed by an international collaboration and a large user community.
McStas applies Monte Carlo and ray-tracing methods for transport of neutrons in the cold and thermal energy range, as applied at large scale facilities, both at research reactors and spallation sources such as those available at ESS, ILL, PSI, ISIS, HIFR, SNS and J-PARC.
The contribution presents current status and recent developments of the software framework and will include key examples from recent and planned, future development.
References
[1] P. Willendrup, and K. Lefmann, Journal of Neutron Research, vol. 22, no. 1, pp. 1-16, (2020)
[2] P. Willendrup, and K. Lefmann, Journal of Neutron Research, vol. 23, no. 1, pp. 7-27, (2021)
[3] K. Lefmann and K. Nielsen, Neutron News 10, 20, (1999)
VITESS is an open-source software package for the simulation of neutron instruments and virtual neutron experiments [1]. It allows simulating practically all components used in current neutron scattering instruments and thus simulating all kinds of instruments at existing neutron sources. Instruments are usually first designed using a Graphical User Interface and then optimized using scripts exported from the GUI.
VITESS is under active development. The code has been migrated to a GIT server at Forschungszentrum Jülich (FZJ), where automatic deployment and testing of the program has been enabled to facilitate releases. Testing is realized by dedicated test instruments for each of the main options of each module. This will automatically be executed after each update of the de-veloper branch to keep the program error-free.
The latest version 3.7 contains two new source modules for a better interworking with neutronic simulation software in order to facilitate a combined optimization of source and instruments: KDSource, which increases the number of neutron trajectories using the kernel density estimator method, and SourceAI; which creates a function to define the moderator characteristic using AI. There is also a new module to simulate prisms. The monochromator module allows simulating different monochromator rotations and oscillations now. From this version on, NeXus output is included, handling of the modules from the GUI is improved and the graphics tool GR from FZJ is the standard tool to visualize the monitor output.
The concept and the main features of the VITESS program and the new features of version 3.7 are presented. Finally, an outlook is given to future versions of the package bringing new methodologies especially for digital twins.
References
[1] https://vitess.fz-juelich.de/
The neutron ray-tracing code SIMRES has been developed since 1990th from the program Re-sTrax for three-axis spectrometer [1] into a general instrument simulation program [2]. Its ar-chitecture is different from the versatile simulation package McStas [3]. SIMRES is a desktop application designed for interactive work with a virtual instrument constructed from a given list of beamline components. While this concept limits the program's versatility, it enables features such as reverse tracing, adaptive variance reduction and event history logging to be integrated into the core of the ray tracing code. The reverse tracing with variance reduction can speed up simulations by orders of magnitude in the case of small samples. The loggers monitor the histories of successful events, open possibilities not available in real experi-ments, such as mapping of guide reflection angles, or exporting event lists which represent sampling distributions or resolution functions.
The program development has been driven by several instrumentation projects including dif-fractometers equipped with focusing monochromators and the time-of-flight engineering diffractometer BEER@ESS. Therefore, SIMRES is best suited to simulating similar instruments and provides advanced components for this purpose. They include, focusing arrays of mosaic and elastically bent perfect crystals, realistic neutron guide models accounting for waviness and misalignment, and a stack of supermirror-coated Si blades acting as a bi-spectral switch for the BEER and DREAM instruments at ESS. SIMRES can call a McStas simulation for a part of the components sequence by sharing MCPL data [4]. This largely extends the capability of the program, for ex-ample by employing advanced sample models or the Union compo-nents available in McStas.
These advanced features will be demonstrated on two examples. The first one deals with the modelling of neutron beam attenuation in the mentioned bi-spectral switch component, which may be severely affected by diffraction in the Si blades enhanced by residual elastic strain. SIMRES provides the bent crystal component XTAL, which allows to assess this effect and predict realistic instrument spectra. The other example focuses on the problem of false strains, which affect residual stress measurements near material boundaries. Such measure-ments are often performed using a very small gauge volume of several mm³, which makes simulations of the experiment very slow. By employing the reverse tracing for higher speed and calling a McStas union model for a composite sample, SIMRES can yield the distribution of false strains to be used for subsequent treatment of experimental data.
References
[1] J. Saroun, J. Kulda, Physica B 234–236 (1997) 1102
[2] SIMRES repository, https://github.com/saroun/simres
[3] P. Willendrup, and K. Lefmann, J. Neutron Research 23 (2021) 7-27
[4] T. Kittelmann, et al., Comp. Phys. Comm. 218 (2017) 17-42
Updates from MCPL, NCrystal and Union, Virtual Experiments I
This contribution aims to give users an update on the MCPL and NCrystal projects. These two open source projects, both maintained by the Modelling group at the ESS DMSC, aim to facilitate integration and collaboration between users of various Monte Carlo transportation codes. MCPL enables the interchange of particle states between codes, while NCrystal provides an ambitious backend for shared material models. The presentation will discuss the two projects, how they are used in e.g. McStas, as well as update on recent developments.
References:
The McStas Union components provide a framework to simulate complex systems, both in terms of scattering physics and geometry. Materials are defined using a combination of available scattering processes and an absorption cross-section. Geometry is described using a combination of basic shapes, including spheres, cones and boxes. These can be combined to form complex nested geometries. Multiple scattering is simulated and all scattering processes in relevant material sampled according to their individual cross-sections. The Union components can be used to describe samples, sample environments, filters, detectors and more.
Such simulated systems can provide surprisingly complex results, and the Union components include tools to visualize and investigate the scattering and absorption that occurred in the system.
Recently surface physics was added to the system, allowing the scattering length density to be provided as part of the material definition. This allows refraction to be simulated whenever the ray traverses an interface. In addition, it is possible to define surface processes such as a reflectivity curve or thin absorbing layer. These processes can then be attached to any of the faces of the individual geometries. This additional feature will expand the use case of Union components to reflectometry samples as well as reflective and refractive optics.
The applicability of inelastic neutron scattering is limited by inherently weak signals.
Mapping significant portions of (Q, ω)-space can take days, making parametric studies as a
function of temperature, pressure, and magnetic field essentially impossible. The triple-axis
community has addressed this limitation by improving the efficiency of triple-axis
spectrometers through the construction of multiplexing analyzer arrays, which cover large
intervals of scattering angles in the horizontal plane. Examples include CAMEA (PSI) and
MACS (NIST). In contrast, direct spectrometers provide large coverage but suffer from even
lower flux.
BIFROST, currently under construction at ESS, employs a CAMEA-like multiplexing
backend on an indirect geometry time-of-flight (ToF) front end. The primary spectrometer
enables an unprecedented polychromatic sample flux of 6·109 n/s/cm2 at 2 MW accelerator
power, with a bandwidth of 1.7 Å, whilst retaining a primary spectrometer resolution ΔEi/Ei
of 4 %, common in cold neutron spectroscopy. The multiplexing backend consists of 9
Q-channels, each covering 5.2° and containing 5 fixed analyzers probing a scattered neutron
energy range of 2.7 to 5.0 meV. The analyzers utilize the graphite crystal mosaicity combined
with position sensitive neutron detectors, resulting in a back-end resolution considerably
better than on a classical TAS setup.
We here present extensive McStas simulations of BIFROST and its energy resolution in different
operation modes. We show that it is possible to obtain excellent energy resolution even
in deep inelastic scattering where we at 10 meV energy transfer find a resolution of approx.
30 μeV and we illustrate this by a complete virtual phonon experiment.
Over the last decades, the neutron user community has steadily grown more diverse. While initially experts in instrument and scattering techniques performed the experiments, now users come from various scientific backgrounds and use a a combination of many different methods in order to answer scientific questions. It is therefore more important than ever to provide them with excellent tools for teaching how a neutron scattering experiment is conducted at a large-scale facility, and understanding their results in the context of the specific instrumental settings used.
Monte-Carlo instrument simulation software packages can accurately and effectively model neutron trajectories, whether it be used for optimization of properties for new instruments, or for the prediction of results from real or at least prototypical experiments given an existing instrument. However, their use is usually restricted to experts, and the developer and instrument scientist community cannot realistically provide support to a wide audience of fresh users.
In our approach, we combine the simulation backend with NICOS [1], our experiment control software, in order to present the user with the same interface that they would face at the real instrument, while the detector results are obtained from Monte-Carlo simulations running in the background. The resulting “digital twin” can then be used for teaching and experiment preparation, i.e. getting to know the instrument and NICOS, checking expected results and time needed, but also assessing influence of instrument parameters, e.g. on Q resolution.
We will present the current state of our digital twins for the MLZ instruments, how the two backends for Vitess and McStas are integrated into NICOS, and how the resulting virtual instruments are foreseen to be run and provisioned for multiple users in conjunction with our “Data Analysis as a Service” platform.
References
[1] https://nicos-controls.org/
To increase neutron flux on small samples, we are developing a nested mirror optic (NMO) array for the PUMA thermal triple-axis spectrometer at the MLZ. This device is intended to reduce the beam size to 5 mm x 5 mm while preserving 50% of the incoming neutrons, re-sulting in an 8-fold increase in the flux available for small samples. However, the complex neutron flight paths generated from novel optics creates a new challenge in analyzing beam characteristics, such as the shape and the resolution function. We have integrated the McStas neutron simulation package with the McStasScript Python API to create a user-friendly GUI for simulating the PUMA instrument, including the new NMO optics and other potential components. This combined program enables virtual neutron scattering experi-ments on PUMA, focusing on ease of use for users unfamiliar with McStas or neutron scatter-ing. For staff, it facilitates testing optics, particularly NMO arrays. For users, it allows experi-ment simulations to optimize instrument parameters and acquire resolution functions. For students, it serves as a platform for learning neutron scattering techniques, offering practice in alignment and measurements without needing physical access to the instrument. We will discuss the progress of the NMO setup for PUMA and the McStasScript-PUMA integration, including planned features and capabilities. We will particularly show our efforts to facilitate outside usage, including recent student demonstrations.
Virtual Experiments II, Verification, Instrument Design
Inelastic neutron scattering is uniquely powerful to probe excitations in condensed matter, enabling extensive mapping of S(Q,E) in single-crystals. Effectively utilizing the time-of-flight neutron spectrometers and efficiently rationalizing large datasets resulting from these experiments can drastically benefit from recent developments in data science and computer simulations of both instruments and the physics of the material of interest, embedded in model scattering kernels.
This presentation will discuss examples from our group, combining first-principles atomistic simulations, Monte Carlo instrument simulations, and machine learning algorithms to facilitate the efficient utilization of tof INS [1-5]. We will highlight examples on retrieving extensive excitation isosurfaces in 4D Q-E space, quantifying phonon linewidths including corrections for tof resolution, and modeling the effect of disorder in complex crystals.
Our conclusions emphasize the need for accurate resolution simulations, and machine learning workflows enhancing the efficient use of the full 4D data volumes in (Q,E) space, merging sample models with instrument digital twins.
Figure 1 (left) Monte Carlo simulation of tof INS spectrometer point-spread-function, and convolution of S(Q,E) model from DFT compared with measurement. (right) Automated exploration, filtering, and symmetry folding of 4D data from INS.
[1] J. Ding et al., Nature Physics 21, 118–125 (2025).
[2] X. He et al, Proceedings of the National Academy of Sciences 122, e2419159122 (2025)
[3] X. He et al., PRX Energy 3, 013014 (2024).
[4] Q. Ren et al., Nature Materials 22, 999–1006 (2023)
[5] T. Lanigan-Atkins, Nature Materials 20, 977–983 (2021)
Very cold neutrons (VCN) cover a wide spectral range within the long-wavelength tail of typ-ical sources for cold neutrons, with energies below 1 meV down to few hundreds of neV, the domain of ultra-cold neutrons (UCN). Clathrate hydrates exhibit intrinsic properties that make them suited as moderators in novel, more intense sources of VCN. Such sources hold potential to significantly enhance existing neutron scattering techniques, including small-angle neutron scattering (SANS) for improved spatial resolution and time-of-flight (TOF) or neutron spin-echo (NSE) spectroscopy for better energy resolution. In particle physics, higher VCN intensities would increase the sensitivity of experiments employing slow neutron beams, such as searches for neutron-antineutron oscillations or static neutron electric dipole mo-ments (EDM) [1].
The efficacy of clathrate hydrates as moderators arises from localized low-energy modes of entrapped guest molecules, enabling efficient down-scattering of neutrons without being constrained by a dispersion relation. These facilitate down-scattering of neutrons, which is not restricted by any dispersion relation, allowing an efficient thermalization even at lowest neutron temperatures. Of particular interest are hydrates hosting dioxygen (O2) and tetrahy-drofuran (THF) as guest molecules. THF provides a broad excitation spectrum, while dioxy-gen provides an additional path for neutron slowdown by exploiting the zero-field splitting of its magnetic triplet ground state [2].
Here, we report results from a comprehensive experimental investigation of low-energy dy-namics and total cross section of different clathrate hydrate compounds using neutron scat-tering and transmission techniques. This campaign is part of a collaborative effort to develop a high-intensity cold neutron source at the European Spallation Source (ESS), including the development of novel scattering kernels to inform future design studies [3]. Our measure-ments serve as a baseline for NCrystal [4] scattering kernels, which are available to the com-munity [5] to explore this novel moderator material for emerging neutron facilities. First studies were made as part of the HighNESS project for the ESS second moderator [3].
References
[1] V. Czamler, PhD Thesis, Univ. Grenoble Alpes (2024)
[2] O. Zimmer, Phys. Rev. C 93, 035503 (2016)
[3] V. Santoro et al., arXiv:2309.17333 (2023)
[4] X.-X. Cai, T. Kittelmann, Comput. Phys. Commun., 246, 106851 (2020)
[5] S. Xu, ncmat-clathrates, GitHub (2024)
This contribution provides an overview of the various Monte Carlo Ray tracing efforts at the neutron scattering facilities at Oak Ridge National Laboratory (ORNL). There are basically three different aspects to the work at ORNL, namely: Instrument optimization, integration with instrument engineering, and modeling of complex sample scattering. For instrument optimization both the Spallation Neutron Source Second Target Station project and the HFIR Beryllium Replacement project (HBRR) are demonstrating innovative optics designs to over-come unique challenges. New tools like the tally components have been developed to facili-tate and expedite optimization of these designs. These codes and tools are run on a variety of both CPU and GPU computing resources allowing comparison of these platforms. These two projects have also integrated the engineering modeling with the ray tracing efforts to better coordinate design changes and performance effects. This also involves coordination and integration with particle transport codes. Finally, from a data analysis perspective, the excellent neutron instrument models, provided by McStas and MCViNE, allow simulation of instrument features and characterizing instrument resolution aspects to a high degree. Web based interfaces have also been developed to help users use these complex simulations.
This work has been carried out at the SNS and HFIR neutron scattering facilities sponsored by Basic Energy Sciences of the US Department of Energy. Parts of this work was sponsored by ORNL’s Lab Directors’ Research and Development funds.
The South African Neutron Radiography (SANRAD) facility, decommissioned in 2013, is undergoing a strategic upgrade as part of Necsa’s Beam Line Centre programme aligned with the new Multi-Purpose Reactor (MPR). The upgrade includes the redevelopment of the Neutron Radiography (NRAD) beamline at SAFARI-1, with emphasis on optimizing neutron optics to enhance radiographic quality and flux efficiency.
This work aims to determine an optimal collimator configuration on a radial beamline - particularly filter design, pinhole geometry, and positioning - to achieve a high-flux, low-divergence thermal neutron beam with a flat intensity profile suitable for digital radiography, while mitigating gamma and fast neutron contamination.
Monte Carlo simulations using McStas and MCNP were employed to model the full NRAD beamline system. Anisotropic neutron source data from MCNP was converted to MCPL format for McStas-based ray tracing. To enhance statistics, the MCPL source neutron spectrum and divergence was captured and converted to a virtual source with logarithmic wavelength sampling which enables the correct representation of the entire applicable spectrum.
Neutron and gamma filter performance was assessed using a hybrid modelling approach: thermal neutron interactions via NCrystal and gamma attenuation via empirical MCNP data. Multiple pinhole diameters (5 - 30 mm) and positions (L = 4.4 - 6.1 m) were simulated to evaluate flux, L/d ratios, spatial uniformity, and thermal-to-total neutron ratio.
An optimal configuration using a 2 mm long pinhole positioned 1.9 m from the core box (L = 6.13 m) and diameters of 8, 15, and 25 mm satisfies all design targets: flat beam profile (28.28 cm), L/d ratios (800, 400, 250), and a thermal neutron flux of 1.94×10⁷ n/cm²/s at the detector. A composite filter of 100 mm single-crystal Al₂O₃ and 50 mm Bi achieves 98.1% thermal purity, with fast neutron and gamma reductions of 97.3% and 99.6%, respectively.
The study establishes a validated neutron optical design for the new NRAD beamline, offering superior spatial uniformity and reduced radiation background. These results provide a high-fidelity foundation for mechanical design, thermal analysis, and eventual commissioning of a world-class neutron radiography capability at the SAFARI-1 reactor, aligned with the broader objectives of Necsa’s MPR programme.
ABSTRACT:
The NCNR is currently designing a new cold neutron crystal spectrometer, QMS, to replace the SPINS cold triple axis on the NG-5 beamline. The primary spectrometer before the sample consists of an elliptical guide and double focusing monochromator that hosts a velocity selector, V-cavity supermirror polarizer, resolution masks, and fast neutron filter within the incident beam. The guide and monochromator instrument parameters were globally optimized via McStas to maximize the flux at the sample position. Additional McStas studies were then employed to optimize the design and placement of the remaining devices. In all cases, this analysis successfully leveraged the complicated flux profile to improve the performance of each device. This talk will outline the key design elements of each device operating within the incident beam, the strategy employed to optimize these elements, and their expected performance on the QMS instrument.
Instrument Design II, Component Contributions, AI/ML
The ICONE (Innovative COmpact NEutrons facility) is a project of a French High-Intensity Compact Accelerator-driven Neutron Source (HiCANS) aimed at delivering an instrument suite for the French scientific community at the 2035 horizon. ICONE will produce neutrons by accelerating protons to an energy 25 MeV and impacting them on a Beryllium target. The resulting neutrons will then be moderated to a useful energy range of 2–100 meV, making them suitable for use on neutron scattering instruments.
One of the key challenges on HiCANS is the design and optimization of inelastic neutron scattering instruments. Indeed, the inherent need for beam filtering in direct and indirect geometries in order to analyze the energy transfer introduces a loss in neutron flux. To make full use of the produced neutrons on ICONE, combination of simulation tools is employed to develop and optimize a virtual model of an inelastic instrument, which will then enable the calculation of instrument performance by the realization of virtual experiments using McStas, and the maximization of the signal/noise ratio using the OpenMC software packages.
The data generated will then be reduced using the SCIPP software package developed at ESS and analyzed allowing direct comparison and benchmarking with previous experimental results obtained at LLB on Orphée reactor. This conception will present the entire simulation chain and the first results obtained.
Neutron lenses can greatly benefit various experimental techniques, such as small-angle neutron scattering (SANS) and neutron imaging. In the past, diffractive neutron optics have been demonstrated, but to date, they have been underutilized1. A Fresnel zone plate (FZP) has the intrinsic advantage compared to a compound refractive lens (CRL) that it has negligible absorption, lower chromatic aberration, lower incoherent scattering, and scalability to large diameters inaccessible by the CRL. In this talk, we will present how we use the McStas package to aid our experimental preparation and post-experiment validation for our neutron FZP developments. The McStas package plays a crucial role as an indispensable tool with remarkable flexibility, enabling us to simulate the performance of the focusing SANS and imaging instruments with diffractive lenses. We improved the existing FZP_simple.comp to achieve a better modelling of the diffraction efficiency of neutron FZPs and allowing for the introduction of imperfections, providing a better understanding of realistic lens performance and aiding design decisions for future lenses.
We model the imaging performance of a full-field neutron microscope instrument with an FZP objective lens to validate our experimental results, as shown in Figure 1(a)(top). To overcome strong chromatic blurring, we are developing an achromatic lens formed by an FZP with a defocusing CRL2. Simulations help benchmark the expected performance of this hybrid lens, as shown in Figure 2(a)(bottom). Gravity and chromatic blurring effects of the FZP in a focusing SANS instrument can also be estimated, as shown in Figure 1(b).
McStas plays an important part in the development and optimization of neutron instrumentation. In the McStas package there is a specific need for a component that simulates bent single crystals. These crystals grant a high resolution and high peak reflectivity in specific focusing conditions, at the cost of lower integrated reflectivity, and a heavy dependence on the diffraction geometry. These crystals therefore give great power to the experiment, but also great responsibility to the instrument setup. These crystals are therefore often used
for engineering diffractometers and triple-axis spectrometers.
The current crystal components in McStas either implement a very simplified model of a crystal (infinitely thin), or a very detailed model of a crystal (NCrystal), neither of which supports the case of a bent single crystal of finite thickness. This work therefore implements a new component in the McStas library called Monochromator Bent. The component is based upon the crystal component in the SIMRES software package. The component simulates neutron diffraction in a crystal of finite thickness, which can be bent or mosaic, and any combination of these.
We then define showcase configurations, that highlights the capabilities of the component. In these configurations, we compare the component to its sister in the SIMRES software, and the NCrystal component in McStas. Finally we also simulate the SALSA instrument at the ILL, and compare the results to data taken from SALSA. The results from these comparisons show that the component is able to simulate the effects of a bending mosaic/non-mosaic crystal with a finite thickness. The added features to McStas from this component is then both diffraction off of bent single crystals as well as a more realistic crystal simulation than the current monochromators, whilst still being faster than the NCrystal component.
A new class of McStas components records the details of each neutron interaction with guide surfaces. The resulting data can be refined using large-scale data analysis tools (e.g. python’s pandas package). This enables a detailed study of e.g. the reflectivity of the guide coating required on each surface of each segment for ideal neutron transport of guide designs, especially of those following non-trivial geometries. Other applications include the investigation of outliers in phase space, correlation between certain guide surfaces, and the significance of individual guide surfaces.
In this talk, we will discuss inner workings of the components themselves and provide examples of how they were used to optimize several beam line designs at ORNL.
Current software for Monte Carlo particle tracing allow to read Monte Carlo Particle Lists (MCPL) generated by other software, usually a neutronics simulator. Currently, it is possible to perform statistical estimates of the multivariate distribution of the phase-space variables of neutrons from a MCPL file. In this work, we propose an alternative that learns the multivariate probability distribution by means of current generative models developed by the machine learning community. We present ways of sampling these models both for Vitess and McStas, and discuss the advantages and disadvantages of the proposed method.
Posters, Snacks and Drinks
Neutron scattering methods are essential for exploring the structure and dynamics of matter, yet tools for manipulating neutron beams remain limited compared to those available for X-rays. This project bridges that gap by leveraging expertise in X-ray optics from the Center for High-Energy X-ray Systems (CHEXS) and the Technical University of Denmark to advance neutron optics technologies. Inspired by the demonstrated potential of Wolter mirror configurations for neutron beam focusing, the study focuses on simulation and design efforts to pave the way for future neutron optics focusing units.
The project comprises two main components. First, a Python program was developed to simulate the reflectance of multilayer thin films for neutron mirrors, validated against the Gen-X software. Second, a python code was developed for optimizations of multilayer coatings (m=3) for high reflectance at a neutron wavelength of λ = 0.3 nm. For broader angular ranges, Ni/Ti supermirrors are recommended to maintain continuous high reflectance across a range of angles and wavelengths.
To complement these efforts, the structural, mechanical, and optical properties of nickel- and titanium-based thin films were experimentally studied to enhance their performance in neutron and X-ray optics. These findings contribute to advancing state-of-the-art neutron and X-ray technologies, with implications for scientific and industrial applications. By addressing both simulation and material design challenges, this project lays the groundwork for innovative neutron optics solutions.
Neutron scattering facilities rely on efficient moderators to optimise the performance of experiments for fundamental and applied physics. Cold and water moderators are crucial for tailoring neutron energy spectra and pulse profiles to meet diverse experimental requirements. Achieving optimal moderator design requires advanced computational tools to simulate neutron pulse evolution within the TRAM (Target Reflector and Moderator assembly) and integrate the resulting leakage from the moderators as a source into the instrument's optical model downstream to the sample location.
This study benchmarks the FLUKA-CERN simulation model of the ISIS Target Station 1 (TS1) against experimental neutron fluence rate measurements at instruments fed by either water or cold moderators. By coupling the TRAM FLUKA-CERN model with the McStas one of representative beamlines, a detailed analysis of neutron fluxes, energy spectra, and spatial distributions at sample positions was performed. The Monte Carlo predictions were compared with experimental data, providing critical validation and development insights for future advancements.
The integration of FLUKA-CERN and McStas required a specialised interface to optimise the coupling process. This interface ensured precise data transfer, enabling accurate and realistic simulations of neutron beams as they propagate through the instruments.
The findings confirm the robustness of FLUKA-CERN in simulating neutron transport and thermalisation processes for both cold and water moderators. This work is particularly significant for the development of next-generation pulsed neutron sources, such as ISIS-II, offering actionable recommendations for enhancing moderator performance and mitigating the background at the samples.
Neutron scattering techniques are typically performed at large facilities such as ISIS-RAL in the UK, the Institut Laue-Langevin ILL in France or the future European Spallation Source ESS in Sweden. However the European neutron landscape is shrinking, making access to instruments more scarce [1]. In this context, Compact Accelerator-based Neutron Sources (CANS) represent a perfect complement to provide additional beamtime. This opens up new opportunities for industrial research and education, as well as promotes iteration and planning prior to scheduled measurements at large facilities [2,3]. This work relies on the combined workflow of Monte Carlo simulations and analysis methods developed at ESS [4-6] to assess the feasibility of neutron scattering techniques using CANS. The effective implementation of such techniques in compact sources is expected to increase their popularity in the near future.
[1] C. Carlile and C. Petrillo, Neutron scattering facilities in Europe, ESFRI Scripta Vol. 1 (2016).
[2] I. S. Anderson et al., Physics Reports 654, 1–58 (2016).
[3] Y. Otake et al., Nuclear Physics News 33, 17–21 (2023).
[4] T. Kittelmann et al., Computer Physics Communications 218, 17–42 (2017).
[5] P. K. Willendrup and K. Lefmann, Journal of Neutron Research 22, 1–16 (2020).
[6] P. K. Willendrup and K. Lefmann, Journal of Neutron Research 23, 7–27 (2021).
The European Spallation Source (ESS) Test Beamline (TBL) will serve as a multi-purpose in-strument designed for early-stage performance evaluation of the accelerator and neutron source, as well as for testing, integration, and calibration of neutron detection instrumenta-tion. Among the detector technologies employed at TBL, position-sensitive He-3 tubes de-tector (LPSD) play a crucial role in characterizing the direct and transmitted neutron beams by imaging their spatial distribution and intensity. In this work, we present Geant4-based simulations of LPSD modules operated at two different gas pressures: 10 bar and 1 bar. Each module consists of four He-3 tubes. The simulation aims to evaluate the detector response and optimize beam reconstruction methods to mitigate the effects of tube separation on spatial resolution. At TBL, an accurate characterization of the incident and transmitted beam in terms of time structure, spatial resolution, and flux is essential for neutron scattering ex-periments. To achieve this, precise calibration and a deep understanding of the performance of He-3 detectors are required. Our simulations provide insights into beam reconstruction techniques, helping to enhance the reliability of neutron imaging at TBL.
In recent years, Monte Carlo particle transport simulations have become available that make it possible to simulate neutron interaction in samples and surrounding environments in neutron scattering instruments. Compared to previous raytracing approaches, neutrons that enter the sample environment area are followed, collision by collision, and all interactions are simulated. This allows for computation of components of the signal that were previously considered “background” such as inelastic scattering and multiple scattering in diffraction experiments.
In this work we present an analysis routine developed at the European Spallation Source ERIC which combines the neutron transport code OpenMC with the NCrystal toolkit. This routine incorporates a detailed simulation of measurements and a post-processing step to account for effects not included in the neutron transport simulations, such as instrument resolution. The combination of OpenMC and NCrystal provides access to built-in neutron-nucleus scattering physics for an extensive range of materials over a wide range of energies (10
See PDF attached.
NeuMATIX (NEUtron & MATerials Integrated eXpertise) bridges the gap between advanced material modeling and Monte Carlo ray tracing (MCRT) simulations. Traditionally, MCRT software requires a scattering kernel as input, often relying on simplified material representations that capture only a limited number of atomistic sites and symmetry rules from crystallographic data. While effective for some materials, this approach is insufficient for complex, disordered materials that exhibit diffuse scattering—a signature of structural disorder. To accurately model such materials, larger atomistic representations are required, incorporating off-lattice site atoms, rotational and translational dynamics of framework occupants, and temperature effects. By integrating software like Paradyse, which produces these detailed scattering kernels, with MCRT tools such as McVine, we can significantly enhance our understanding of disordered materials. This work demonstrates the potential of this approach using the total scattering instrument NOMAD. We explore how the scattering kernel generated by Paradyse, when input into a simulated NOMAD beamline, transforms the understanding of material behavior. In this talk, we will present our workflow for this integration and discuss the new insights it unlocks in the study of complex materials.
This abstract was already submitted for the ICNS conference
C3DP is a software solution designed to create and optimize complex, non-traditional scattered beam collimators. Scattered beam collimators are critical for enhancing the sample signal-to-background ratio, particularly in experiments involving specialized sample environments like pressure cells or diamond anvil cells. These environments often generate significant background noise, making it challenging to identify the sample signal accurately.An optimized scattered beam collimator is essential to mitigate such background contributions. However, when the optimized collimator size exceeds the capabilities of additive manufacturing, C3DP provides an innovative approach by tailoring the collimator's shape to meet printing constraints while maintaining optimal performance.C3DP enables the simulation and optimization of uniquely shaped collimators for specific sample environments. It integrates instrument simulations with the special sample setup, extracting geometries that transition from neutron-simulated models to engineering CAD designs, ready for 3D printing. In this talk, we will demonstrate the features and capabilities of C3DP through practical examples, showcasing its role in advancing neutron scattering experiments for challenging sample environments.
Molecular dynamics (MD) is a powerful tool for neutron science, as it enables direct computation of the dynamic structure factor—proportional to scattering intensity—used to interpret and plan experiments. Recent advances in machine-learned interatomic potentials (MLIPs) have made accurate MD simulations more accessible. Here, we present a workflow that combines MLIPs with MD to predict instrument-specific inelastic neutron scattering (INS) spectra by incorporating each instrument's kinematic constraint and resolution function. We demonstrate the approach on elemental silicon, crystalline benzene, and hydrogenated scandium-doped barium titanate, validating results against data from ARCS (SNS), TOSCA and MAPS (ISIS), and IN1 Lagrange (ILL). While focused on INS, the method extends to diffraction and quasi-elastic scattering with neutrons, X-rays, or electrons. The good agreement between simulated and experimental results highlights the potential of this approach for guiding and interpreting experiments, while also pointing out areas for further improvement.
Neutron polariser and analyser based on polarising supermirror technology [1-3] have enabled the widespread use of polarised neutrons in the past decades. They operate on polarised neutron reflectometry principles and are highly sensitive to incident neutron’s wavelength and incident angle. Modern beamline often shapes the neutron beam using sophisticated optical elements such as curved guides and elliptical guides. The complex beam profile makes it necessary to use Monte Carlo ray-tracing simulation to design polarisation equipment. The leading simulation software are McStas [4,5] and Vitess [6] which have incorporated many polarisation devices.
At present, simulation of polarising supermirror in virtually all publicly available software is simplified by considering only the reflection and transmission at the supermirror coating. In transmission polarising device simulation, the simplification has led to unrealistically high polarisation and transmission in device performance. To address this issue, we have developed a new supermirror module that also includes absorption in substrate, refraction at the substrate interface, and multiple internal reflection in double-side coated supermirror. At the device level, multiple reflection between supermirrors across different channels in a v-cavity polariser has also been included. The results revealed that internal reflections in a double-side coated supermirror can degrade the polarisation of a polariser by almost 10%. Also, without dividers to separate the channels in a multi-channel v-cavity polariser, the crosstalk would make a v-cavity non-functional as a polariser.
Consequently, mitigations have been incorporated in our designs of a v-cavity polariser. They include using single-side coated supermirrors in double-v cavity and using dividers with reflective coating on top of absorption coating. Both are technologically viable. Simulations show that the changes allow us to reach 96% polarisation and 42% transmission at 2 Å in a v-cavity for BIFROST instrument. The results also point to the importance of having high-reflectivity polarising supermirror in contrast to the present focus on reaching higher m-value.
[1] P. Böni, Physica B 234-236, 1038 (1997).
[2] T. Krist, C. Lartigue, F. Mezei, Physica B 180-181, 1005 (1992).
[3] T. Bigault, et. al., J. Phys.: Conf. Ser. 528, 012017 (2014).
[4] P. Willendrup and K. Lefmann, J. Neutron Res. 22, 1-16 (2020).
[5] P. Willendrup and K. Lefmann, J. Neutron Res. 23, 7-27 (2021).
[6] K. Lieutenant, et. al., Proc. SPIE Int. Soc. Opt. Eng. 5536, ed. M. Sanchez del Rio 134 – 145 (2004).
We present some examples of preparing CAD-geometries for use by particle transport codes. To be precise we have prepared geometries for OpenMC[1], but the underlying libraries are able to use the same geometry in both MCNP[2] and fluka[3].
Our vehicle for preparing geometries is the newly developed tool CAD_to_OpenMC[4], which is able to ingest a geometry defined in a step-file (or a set set thereof), create a surface-discretized version of it, and assemble it in a binary HDF5-based format which is readable by particle transport codes - if compiled with support for the DAGMC[5] extension.
Below is one of the biggest and most advanced geometry models we have worked on, namely the Molten Salt Reactor Experiment[6].
References.
[1] P. K. Romano and B. Forget,
“The OpenMC Monte Carlo particle transport code,
” Ann. Nucl. Energy, vol.
51, pp. 274–281, Jan. 2013, doi: 10.1016/j.anucene.2012.06.040.
[2] J. A. Kulesza et al.,
“MCNP Code Version 6.3.0 Theory & User Manual,
” Los Alamos National Laboratory,
Los Alamos, NM, USA, LA-UR-22-30006, Rev. 1, Sep. 2022. doi: 10.2172/1889957.
[3] T. T. Böhlen et al.,
“The FLUKA Code: Developments and Challenges for High Energy and Medical
Applications,
” Nucl. Data Sheets, vol. 120, pp. 211–214, Jun. 2014, doi: 10.1016/j.nds.2014.07.049.
[4] E. B. Knudsen and L. Chierici,
“CAD_to_OpenMC: from CAD design to particle transport,
” J. Open Source
Softw., p. to appear.
[5] P. P. H. Wilson, T. J. Tautges, J. A. Kraftcheck, B. M. Smith, and D. L. Henderson,
“Acceleration techniques
for the direct use of CAD-based geometry in fusion neutronics analysis,
” Fusion Eng. Des., vol. 85, no.
10–12, pp. 1759–1765, Dec. 2010, doi: 10.1016/j.fusengdes.2010.05.030.
[6] D. Shen, G. Ilas, J. J. Powers, and M. Fratoni,
“Reactor Physics Benchmark of the First Criticality in the
Molten Salt Reactor Experiment,
” Nucl. Sci. Eng., vol. 195, no. 8, pp. 825–837, Aug. 2021, doi:
10.1080/00295639.2021.1880850.
The Brazilian Multipurpose Reactor (RMB) project is going through a new phase of instru-mentation design. We are studying and designing instruments based on Monte Carlo simu-lations and experimental analysis and testing. The very first thermal neutron instrument to be designed is a neutron powder diffractometer. There are two steps to develop the final in-strument version according to the project description, since the reactor construction is in course, viz., the study and commissioning of the reactor at the IEA-R1 reactor in IPEN/Brazil and posteriorly its transfer and adaptation to the RMB reactor. As an outcome of the first step, we intend to improve the current knowledge that researchers have about the reactor by developing Monte Carlo models of the core, simulation instruments, etc. On that matter, there is already a diffractometer at the IEA-R1 reactor, namely, Aurora [1].
Designing and simulating Aurora on McStas allows us to investigate the correspondence between the experimental results and the simulations, with a particular emphasis on neutron flux behavior before and after interaction with the monochromator. The neutron flux spectra were analyzed by means of activation foils technique and the instrument detector’s re-sponse. Therefore, the present work demonstrates a previous study for benchmarking the McStas simulations by means of simulations and measurements as described in literature [2]. Once the McStas simulation is validated, we are able to perform accurate simulations and better estimations of the new IEA-R1/RMB diffractometer and its components.
References
[1] C.B.R. Parente, V.L. Mazzocchi, J. Mestnik-Filho, Y.P. Mascarenhas, R. Berliner, Aurora—A high-resolution powder diffractometer installed on the IEA-R1 research reactor at IPEN-CNEN/SP, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Volume 622, 2010.
[2] D. Potashnikov, A. Pesach, O. Rivin, O. Ozeri, Z. Yungrais, M. Bertelsen, E.N. Caspi, Verification of the McStas code using two double axis neutron diffractometers, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Volume 1063, 2024.
In this presentation, I will introduce Vitess-AI, an initiative to enhance the VITESS neutron simulation application by integrating machine learning (ML) and large language model (LLM) technologies.
Vitess-AI aims to streamline two critical simulation workflows: (1) the optimization of complex, multi-parameter instrument configurations during the design phase, and (2) the preparation and planning of experiments on operational instruments through digital twin technologies.
By coupling VITESS with instrument control systems like NICOS and data sources such as NCrystal, the integration of LLMs will support data-driven optimization and exploratory scenario analysis.
In this first phase of the project, I will outline:
1. Key challenges related to domain knowledge representation, simulation validation, and model interpretability.
2. Points for discussion with the community, including the need for physics-informed ML, uncertainty quantification, and explainability tools to ensure transparency and trust in AI-assisted simulations.
3. An initial assessment of additional requirements for expanding the approach to other simulation platforms.
The Monte-Carlo simulation software package VITESS offers only one way to create and modify virtual instruments: the TCL/Tk graphical user interface. This old GUI is not only due for a visual overhaul, but it also lacks some features that are currently required by users. While it is possible to modify command line parameters directly in a script, there is no tool or API to make semantically correct changes.
To address these limitations, a new Python package has been developed. This new software aims to provide an API for VITESS-related software, the base logic for a new GUI, and a general tool for use in Python and Jupyter Notebooks.