Much research is being conducted on scintillating neutron detectors, as an alternative to helium-3, for which there is a global shortage. This particular application was to design an Ultra-Cold Neutron detector to work in a neutron EDM experiment operating at 0.5 K within superfluid helium. This work was part of my PhD thesis.
Monte Carlo simulations play an essential role in the design and optimisation of a prototype detector. A scintillator is a material which absorbs energetic particles and emits photons of light, which can then be counted using a standard light detector such as a PMT or CCD. To maximise detection efficiency, particularly of low energy particles, the amount collected light must be maximised.
An optical simulation includes the production of scintillation light, the tracking of the photons inside the different volumes and the interaction with the light collection system and conversion into electronic signal. An important question to answer concerns the estimated light yield value (percentage of photons measured versus those created), which is a crucial parameter for optimising detector efficiency and performance. This will depend on choice of scintillator, the geometry, the choice of reflective materials wrapping the detector, the mechanical layout and so on. A Monte Carlo model is an ideal way to iterate through various choices of these parameters and predict the amount of photons of light that will be counted. All of these processes may need to be incorporated in the model to obtain accurate results. This is a sample of the work performed with the Geant4 simulation platform as part of the detector prototyping process.
Geant4 (GEometry ANd Tracking) is a platform for the simulation of the passage of particles through matter using Monte Carlo methods, developed by CERN, and uses object oriented programming in C++. Application areas include high energy physics and nuclear experiments, medical, accelerator and space physics studies. The Geant4 software and source code is freely available from the project web site.
Geant4 includes facilities for handling geometry, tracking, detector response, run management, visualization and user interface. Following is a summary of each of the facilities listed above:
- Geometry is an analysis of the physical layout of the experiment, including detectors, absorbers, etc., and considering how this layout will affect the path of particles in the experiment. PSDetectorConstruction.cc
- Tracking is simulating the passage of a particle through matter. This involves considering possible interactions and decay processes. PSPhysicsList.cc
- Detector response is recording when a particle passes through the volume of a detector, and approximating how a real detector would respond. CathodeSD.cc
- Run management is recording the details of each run (a set of events), as well as setting up the experiment in different configurations between runs. PSRunAction.cc PSPrimaryGeneratorAction.cc
- Geant4 offers a number of options for visualization, including OpenGL, and a familiar user interface, based on Tcsh.
This simulation involves some volumes of sensitive scintillating material, structural and optical materials surrounding it, such as optical fibres, reflectors, and aluminium supports, and a light-sensitive readout volume, simulating a Photomultiplier Tube.
Ionising particles are generated by the simulation, such as alpha particles or neutrons, or photons can be generated directly. Their production, trajectory, absorption and detection in the detector is simulated, and output files produced.
The various classes needed for each section of the simulation are sound in src and the header files in include.
- Define materials, e.g. Scintillating Plastic, Aluminium, Air, Teflon.
- Define Geometry of the various components
- Refer to G4Material PropertiesTable for the physical properties of materials
- Define the properties of surfaces using G4LogicalSurfaces and G4OpticalSurfaces
An sample detector geometry can be seen here:
The pink component is the scintillator, and the turquoise is the light-sensitive PMT.
- Define Particles
- Including electrons, optical photons (G4OpticalPhoton), Ions, neutrons etc.
- Define Processes
- Including scintillation (G4Scintillation), optical absorption (G4OpAbsorption), scattering (G4OpRayleigh) ad boundary processes (G4OpBoundaryProcess)