Photomultiplier Tubes

When detecting particles, scientists aren’t able to simply look at with their eyes and see a particle, they are way too small. In order to detect a given particle then, there needs to be some other way to look at it moving and interacting. One such way is to look for a flash of light. Sometimes, when a small particle like a neutrino or muon is passing through a material in a detector, it can bump into and accelerate molecules or other atoms. This acceleration causes a small blip of light to be created and flash into existence. If the detector is able to capture and record this light flash, then it would be known there was a particle interaction that occurred and the data recorded.

However, this is light created from only one small subatomic particle. Given the size of the interactions happening with the particles and the energies of such small particles, it is no where near sensitive enough to be picked up by our eyes, or even individual cameras or detectors to look at. There needs to be some way to take that small flash of light and amplify the signal to a high enough amount to be detected and collected in the data. To achieve this, there is a technology called a photomultiplier tube (or PMT for short) to observe the interactions and light flash! The prefix photo means light, so this is a light multiplying device, exactly what is needed to catch particles in their travels.

The PMT is a sensitive device that takes an individual incoming photon and directs it towards a metal plate. Due to the photoelectric effect, when the photon enters the device and strikes the metal, an electron is released further into the PMT. The device can then take that single electron and accelerate it with electric fields before causing it to hit another metal plate. This collision process causes about ten more electrons to be emitted and travel further into the device. By repeating this procedure for each of these new electrons multiple times, the tube can multiply continually and therefore creating more and more electrons with that same energy. Doing this numerous times can amplify that original signal with one single photon about a billion times. All these electrons that are generated at the end of the device are then observed by a final detector at the end of the PMT and the signal can now be determined. The whole device needs to be kept free of any air so that as few particles as possible are causing the flashes of light other than the desired particles being searched for. Each PMT is thus kept under vacuum seal, eliminating all the air inside the glass tube.

By surrounding experiments involving dark matter particle searches with PMTs, even small signals emitted by a rare occurrence of dark matter interactions can be highlighted and detected! Other particles such as neutrinos, muons, or alpha particles can also get through to the detector and cause flashes of light, and these need to be distinguished from that of a dark matter particle candidate. By surrounding a detector with many PMTs, the experiment can capture light emitted from different angles and ensure maximum light measurement. These are used in many different experiments, some of which can be seen here: neutrino observations, and here: dark matter searches.


Teacher Resource - Curriculum Connections

Ontario Grade 10 Science (SNC2D)

Technology as application of light :
E1.1 – analyse a technological device that uses the properties of light (e.g., microscope, retroreflector, solar oven, camera), and explain how it has enhanced society

Ontario Grade 12 Physics (SPH4U)

Technology as application of quantum mechanics :
F1.2 – assess the importance of relativity and quantum mechanics to the development of various technologies

Photoelectric Effect :
F2.1 – use appropriate terminology related to quantum mechanics and special relativity, including, but not limited to: quantum theory, photoelectric effect, matter waves, time dilation, and mass–energy transformation

Inquiry of photoelectric effect :
F2.4 – conduct a laboratory inquiry or computer simulation to analyse data (e.g., on emission spectra, the photoelectric effect, relativistic momentum in accelerators) that support a scientific theory related to relativity or quantum mechanics

Light as a particle :
F3.1 – describe the experimental evidence that supports a particle model of light

Ontario Grade 12 Earth and Space Science (SES4U)

Technology application in astronomy :
B1.2 – analyse why and how a particular technology related to astronomical research was developed and how it has been improved over time

Electromagnetic radiation for astronomical observations :
B3.3 – describe the characteristics of electromagnetic radiation and the ways in which each region of the electromagnetic spectrum is used in making astronomical observations