The scintillator employed in this experiment is a doped polyvinyltoluene scintillation block. A milled groove running the length of the scintillation block contains a fiber optic cable that is used to direct photons produced by incident energetic particles. The actual process of detecting visible scintillation pulses relies on three absorption-emission stages. Initially, a high-energy particle interacts with the doped plastic releasing photons of a wavelength that is absorbed and reemitted as lower energy blue light by the dye of the scintillation block. This blue light is then absorbed by the fiber optic cable and again emitted as lower energy green light. On the assumption that the photons are emitted evenly in all directions within the fiber optic cable, it follows that a fraction of these photons will become trapped and travel the length of the cable. The direction of travel within the cable is irrelevant as the terminus opposite the SiPM is polished, resulting in the light ultimately being reflected towards the photomultiplier. It should also be noted that green light escaping the fiber optic cable is of an energy too low to cause a secondary scintillation within the block, eliminating the possibility of multiple counts for a single particle detection event.
A MRS (Metal-Resistor-Silicon) based SiPM is used to convert the incident light pulses to an analog voltage. Silicon photomultipliers are Geiger-mode avalanche photodiodes, meaning that they are intended to operate at a reverse bias voltage greater than their reverse breakdown voltage. They are usually constructed of a layer of photo-sensitive pixels atop the standard pn-junction depletion region and silicon substrate of an avalanche photodiode. The pixels of the SiPM operate in parallel, generating a number of charge carriers proportional to the number of pixels fired. When this accumulation of charge carriers reaches an energy great enough to overcome the bandgap of the silicon, an avalanche due to the electric field of the reverse bias potential is initiated through the substrate and avalanche regions of the diode. This ultimately becomes a current pulse proportional the number of pixels fired, preserving an analog response to incident photons despite the binary behavior of individual pixels. This current pulse can then be read as an analog voltage across a bias resistor (Fig. 3) [5, 6, 7, 10, 11].
The discriminator is essential in isolating the individual detection events of the SiPM, and essentially allows output pulses of the SiPM to be tuned for the coincidence counters. While the correct choice of the bias resistor in the detector circuit will minimize the operating noise of the SiPM, it is still necessary to resolve and count individual events. The electronic implementation of the SiPM and discriminator are discussed in detail in the Electronics section.
The scintillation block, SiPM and discriminator circuitry will be assembled on a portable platform to facilitate easy arrangement of the detectors. The detector platforms will employ an intermediate black Delrin fiber optic housing to interface the scintillation block and SiPM. This will serve to eliminate any light interference present in the system while protecting the fiber optic cable from damage. The SiPM and discriminator circuitry will be contained in a shielded box that interfaces with the fiber optic housing. The discriminator is connected via RJ-45 cable to a main circuit board which supplies power and routes discriminator output to coincidence counters (Fig. 4, 5).