How the Cosmic Ray Telescopes Work
A scintillator is a special type of plastic that emits a photon when a charged particle passes through it. Scintillators can be made of plastic or liquid, even laundry detergent has been used. When the charged particle hits the plastic, it excited the electrons in the plastic to a higher energy level. When the electrons relax, they emit a photon, usually in the violet or ultra-violet range. If you look at the edges of a piece of scintillator, it will often look purple. This is due to the scintillation from background radiation in the atmosphere.
So what happens once this photon is created? It probably won’t be going in the direction we want it to go (toward the detector), so it bounces around inside the scintillator for a while. Eventually, it makes it into the plastic light guides and into the detector, which in our case is a photomultiplier tube. Both the scintillator and the light guides must be highly polished so that the photon is easily reflected by total internal reflection and kept inside the plastic. Of course some photons may escape. To try and keep the largest number of photons that we can, we wrap the scintillator assembly in white paper. White is very reflective, so the photons will be reflected back inside the scintillator and continue on toward the detector. We want as many photons as possible to make it to the detector because each photon indicates the presence of a muon (or other charged particle). The scintiallator and photomultiplier tube are wrapped in black paper and tape to ensure that no photons enter the system from the outside. We want only scintillated photons to be detected.
Photmultiplier Tube (PMT)
The PMT works using the principle of the photoelectric effect. The photoelectric effect was Einstein’s famous discovery that won him the Nobel Prize. As you may, or may not, recall, the photoelectric effect simply says that if a photon with high enough energy is incident on a piece of metal it will eject an electron. The top layer of the PMT is a photocathode, which ejects an electron when a photon from the scintillator hits it. That electron then hits what is known as a dynode. This is a material from which many electrons are emitted when one hits it. The number of electrons emitted depends on the material and the energy of the incident electron. Those electrons go on to hit a second dynode, which causes even more electrons to be ejected. The chain continues for each dynode. The PMT shown has 10 dynodes. At the end of this process there are enough electrons to produce an electrical current. The current is sent through a cable and detected by the computer as a signal. The base provides power to the PMT and also connects the PMT to the DAQ board.
For more detail on the workings of the PMT see the QuarkNet Tutorial.
Teacher notes on the above QuarkNet Tutorial.
Data Acquisition Board
The board is what determines if the paddle (we refer to the scintillator and PMT combination as a “paddle”) has detected a charged particle and then tells the computer to register a “hit”. The circuit board has space for five input cables, so can detect signals from up to five paddles at a time. You can watch the counter on the board record signals. It records in hexadecimal, so you may see letters as well as numbers on the digital display.
The laptop computers are equipped with a Linux operating system and data acquisition software specifically designed for use with the paddles. The DAQ allows you to record data such as all hits, coincidences between paddles, and the time of flight of a muon between two paddles. See the computer operating commands page for more information on using Linux.
Why do we need two paddles?
Two paddles are necessary to determine that what we are detecting really are muons. Background electrons are continuously being created and annihilated and are often detected by the paddles. With two paddles, we can look for coincidence hits. This means that two paddles both record a hit at the same time (or within a very small interval). Because muons have a higher energy than the average background electron, they are traveling almost at the speed of light in a straight path and will pass through two detectors almost simultaneously. Electrons are just wandering around and it is unlikely, although possible, that two would hit two detectors at the same time. Having two detectors also allows us to do interesting research. We can place absorbers between the two detectors and look at how the rate is affected. We could change their separation and orientation to see how muon rate is affected.
Need more information for getting started with your experiment? Visit the Getting Started page.