Advancements in Thermal Neutron Scattering Law Generation


The design of neutron systems, for example nuclear reactors and neutron sources, requires the usage of Monte-Carlo simulation software to predict the distributions of neutrons throughout the system. At low energies, the interactions of neutrons with a material is driven by interactions on the atomic and molecular scale and thus a proper description of this behavior is critical for accurate simulations of the low-energy distributions within the system. When using modern Monte-Carlo software, this information is stored in nuclear data libraries, called the thermal scattering law (TSL), and are used as input during the simulation. Most of the TSL libraries that are packaged with modern Monte-Carlo simulation software, such as the MCNP family of codes and PHITS, have been developed using NJOY code system, which is limited to a few selected materials, and can also only support the creation of TSL libraries with limited physics options. Several projects at the European Spallation Source (ESS) have been launched in order to expand these capabilities, which also have application to the simulation of neutron scattering experiments at instruments at neutron scattering sources.

Figure 1: Workflow diagram showing the coupling between NCrystal and OpenMC.


The developments are centered around the use and expansion of the NCrystal tool [1,2]. The tool contains a detailed treatment of thermal scattering cross-sections, in particular supporting coherent and incoherent elastic scattering, and inelastic scattering in polycrystalline and single crystal materials, however it was not previously possible to use the tool to create TSL libraries for Monte-Carlo simulation software. Thus we created a new tool, called NJOY+NCrystal [3], which couples the above mentioned NJOY code to NCrystal and greatly extends the capabilities of NJOY to a wide range of polycrystalline materials. In addition, to support more advanced physics possibilities, we developed a plugin feature for NCrystal that allows a user to include custom physics processes, such as small-angle neutron scattering and magnetic scattering. These processes can be accessed in a Monte-Carlo simulation by coupling NCrystal with a respective Monte-Carlo code. We have done this for OpenMC, as shown in Figure 1, thus making it possible to simulate neutron interactions in novel moderator/reflector materials, such as diamond nano-particles and the clathrate hydrates, in a Monte-Carlo simulation.

What´s next?

The tools are currently being used to investigate the performance of novel moderator and reflector materials which could be used in the design of a second neutron source at ESS as a part of the HighNESS project [4]. This source would focus on high-intensity applications of cold, very-cold and ultra-cold neutrons and be complementary to the currently existing high-brightness source. Work is also underway to extend these methods to support additional physics, for example the simulation of neutrons in textured materials, which can also be of interest for neutron scattering instruments.


The initial developments of NCrystal were carried out by scientists in the ESS detector group. Later developments were carried out by an international group of scientists from the ESS, the University of Milano-Bicocca in Milano, Italy and DTU Physics, Technical University of Denmark.

Parts of the work described here were funded by the HighNESS project at the European Spallation Source. HighNESS is funded by the European Framework for Research and Innovation Horizon 2020, under grant agreement 951782.


Jose Ignacio Marquez Damian

Monte Carlo Simulation Scientist

European Spallation Source ERIC, Lund, Sweden


Douglas Di Julio

Radiation Physicist

European Spallation Source ERIC, Lund, Sweden


Thomas Kittelmann

Researcher in Neutron Detector Technologies

European Spallation Source ERIC, Lund, Sweden



[1] NCrystal: A library for thermal neutron transport - Comput. Phys. Comm., 246 (2020), Article 106851

Link: 10.1016/j.cpc.2019.07.015


[2] Rejection-based sampling of inelastic neutron scattering - J. Comput. Phys., 380 (2019), pp. 400-407

Link: 10.1016/


[3]:  NJOY+NCrystal: An open-source tool for creating thermal neutron scattering libraries with mixed elastic support - Nuclear Inst. and Methods in Physics Research, A 1027 (2022) 166227




THE EUROPEAN SPALLATION SOURCE: THE HIGHNESS PROJECT- 14th International Topical Meeting on Nuclear Applications of Accelerators, November 30 to December 4, 2021, Washington, DC