The work of the Neutronics Group at the ISIS Neutron and Muon Source can be split into two main areas
Firstly the group is responsible for simulating the target, reflector and moderator assemblies of the two target stations in order to maximise the 'useful neutrons' available to instruments.
The ISIS proton beam impacts the target with an energy of 800MeV but the majority of instruments need neutrons at meV which means that simulations need to span at least 9 orders of magnitude in energy.
The second area that the group is responsible is on-going operational support work such as shielding and activation calculations. This includes support to the instruments that ISIS is developing for the ESS (LOKI and FREIA).
The simulations are typically performed using high performance computing facilities in order to model the complex geometry and physics processes going on in the spallation and moderation process.
As well as performing the simulations the neutronics group is also involved in work to benchmark and validate the simulations and the underlying nuclear data. In order to do this we compare with real data from ISIS and use the instruments on ISIS to probe the structure and properties of materials.
The majority of the simulations performed by the neutronics group are Monte Carlo radiation transport simulations. In these simulations the problem geometry, materials and a source term are defined. The source term could be the proton beam, a neutron spectrum, a decay gamma ray or any other distribution of particles to represent the problem. In the neutronics group we typically use MCNP or FLUKA as the main codes.
The majority of instruments at ISIS use neutron guides, which are not included in the typical radiation transport codes therefore we produce data that can be used by instrument simulation tools such as McStas and produce the moderator component used to read that data.
Monte Carlo simulations are considered 'embarrassingly parallel' meaning that it scales well on high performance computing using many CPU cores. However large scale complex models still take a significant amount of time to complete without advanced variance reduction techniques.
The Main research areas of the group are:
- Moderator materials and thermal scattering kernel development
- Moderator optimisation
- Benchmarking and validation of simulation tools
- Development of simulation tools for spallation sources