The Cosmic Ray Detector main components
A short introduction to the main components of Cosmic Ray Detector tiles: Plastic scintillators, WLS fiber and SiPM
Plastic Scintillators
Plastic scintillators convert the energy dissipated by ionizing radiation into photons in the blue to green wavelength region. Basically the energy deposition excites electrons from ground state; when they decay there is light emission (a phenomenon called fluorescence) that happen in the nanoseconds range.
Wavelength Shifters (floors) are added to the base material to match photodetectors sensitivity and to reduce the fraction of absorbed light; higher is the shift less is self-absorption effect. The difference between absorbed and emitted wavelength peaks is called Stokes’ shift.
Plastic scintillators are available in different shapes as shown in the picture below.
WLS Fiber
The tiles used in our detector have a dimension of 10×10 square cm, but photosensor (SiPM) has an area of just 1.2×1.2 square mm; if we would couple directly the photosensor to the scintillator we will loose most of the emitted light.
The scintillator-photodetector coupling issue is well known since the use of PMT (Photo Multiplier Tubes); it arise from the impossibility of using optical coupling schemes relying on reflection or diffraction transmit photons from a large source to a small detector with full efficiency because the flux of photons per unit area and per unit solid angle is constant throughout a given medium.
To satisfy the previous requirement special configurations have been used as shown below; for all geometries the goal was equalize the areas of the scintillator exit point and and the photodetector input.
In our tile the WLS fiber has basically the same function: it collects the light emitted from scintillator and drive it to an exit point area equal to the photodetector area (of course there are inefficiencies; part of light get lost both in the collection and in the fiber SiPM coupling).