The light emitted by a scintillator is extremely faint — far too dim to see with the naked eye. A photodetector amplifies this faint light into a measurable electrical signal.
Types of Photodetectors #
There are two broad approaches to converting and amplifying light into an electrical signal: vacuum-tube based and semiconductor based.
Photomultiplier Tubes (PMTs) #
A Photo Multiplier Tube (PMT) is a vacuum tube that converts and amplifies light into an electrical signal.
- Incoming light knocks an electron (a photoelectron) loose from a photocathode (the photoelectric effect)
- That electron strikes a series of electrodes (dynodes), each collision releasing more electrons in a cascade
- The result is a current amplified by a factor of millions
PMTs are highly sensitive and well established, but need high voltage (hundreds to thousands of volts) and are bulky and fragile.
Semiconductor Photodetectors (SiPM/MPPC) #
A SiPM (Silicon Photomultiplier) achieves the same single-photon sensitivity as a PMT using semiconductor technology instead. It consists of a large array of tiny photodiodes wired in parallel; only the cells struck by light respond, and their combined output gives the signal strength.
Hamamatsu Photonics’ MPPC (Multi-Pixel Photon Counter) is a well-known type of SiPM. Being compact, low-voltage, and highly sensitive makes it easy to use in educational detectors.
Comparing PMT and SiPM/MPPC #
| PMT | SiPM/MPPC | |
|---|---|---|
| Drive voltage | Hundreds to thousands of volts | Tens of volts |
| Size | Large | Compact (a few mm across) |
| Durability | Fragile | Rugged |
| Cost | Relatively expensive | Relatively affordable |
In recent years, compact detectors have increasingly moved from PMTs to SiPM/MPPCs.
Role in Cosmic Ray Detection #
Converting the scintillator’s faint light into an electrical signal is what makes it possible to record a muon’s passage as numerical data. The signal’s strength (pulse height) also gives a rough measure of the energy the particle deposited in the scintillator. OSECHI uses a Hamamatsu Photonics MPPC as its photodetector.
Pulse Height and Energy #
The longer the distance a particle travels through the scintillator, and the more energy it loses in the material, the more light is produced. Examining the signal’s pulse height therefore offers a clue to the type and energy of the particle that passed through.