Cerenkov radiation
through water is only 75% c. This familiar blue glow has a number of very important uses in the world of Physics and Medicine. The Super Kamiokande detector, one example, is used to search for differences in behaviours of neutrinos and antineutrinos, by looking for the Cerenkov radiation produced when neutrinos collide with water molecules and make charged leptons which travel through the water and produce Cerenkov radiation, which is then picked up by ultrasensitive light detectors. Cerenkov radiation isn't just used to determine whether a particle was produced; it can also be used to determine the position from which it was released, and also the exact type of particle produced (provided it's a known particle). Physicists working at CERN devised a method called RICH (Ring-Imaging-Cerenkov- Detector) that capitalizes on the cone shape that Cerenkov radiation travels in. By using a number of very small light detectors, they can determine the angle that the cone forms, and with that knowledge they can calculate the momentum of the particle (and thus know its exact identity by comparing with other known particles). Then the physicists can triangulate its original release point by simply finding the points that the paths with known angles cross over from one detector to the next. This has allowed physicists to determine where a reaction takes place within particle accelerators like the LHC. Another use of Cerenkov radiation is by doctors looking to see where radiation is being absorbed within the body, and so doctors either inject or irradiate the patient, producing Cerenkov radiation, which can then be measured at both the entry and exit of the tissue. Cerenkov detection outperforms traditional PET scanners in many ways: it uses far less expensive equipment, requiring essentially just a low light camera, compared to PET scanners which use a series of gamma detectors to produce a 3D image of the region; it also is far quicker, with CE (Cerenkov emission) scanners taking only a minute, while PET scanners can take 30 to 60 minutes which the patient must be still for. Also, the CE scanners can more effectively pinpoint small tissues which wouldn't be picked up on a PET scanner. In conclusion, particles can indeed travel faster than light, though not in a vacuum. The Cerenkov radiation produced when a charged particle does this is not only a fantastic demonstration of many different physics principles at work, but a tremendously useful phenomenon, which has allowed us to further humanity's understanding of particle physics, medicine, and hopefully many other future applications.
References
Cherenkov radiation. n.d. [Online] Available at: https://www.britannica.com/science/Cherenkov-radiation [Accessed 10/ 9/2021].
Huygens, 1690. Traité de la Lumière. In: Traité de la Lumière. Leyden: Van der Aa, p. 19. Why does light slow down in water? - Fermilab. 2019. Fermilab. [Online] Available at:
https://www.youtube.com/watch?app=desktop&v=CUjt36SD3h8&ab_channel=Fermilab [Accessed 10/9/2021].
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