We are all aware of incredible images in science. The DNA helix, the universe, neurons, and lightning are images that come to mind when pondering the most interesting aspects of science. The first time we see these videos, our jaws drop, but after the third or fourth time seeing them, they become familiar and no longer invoke the same wonder. After noticing this phenomenon, I went on the prowl for something that would make my jaw drop at the sheer aesthetic or beauty of a scientific process or contraption. After a bit of searching, I found just the thing.
If you have never seen a neutrino detector, stop reading (at least for a second) and look it up. After a quick google image search, you will find images of gigantic underground caverns that have been filled with water. The walls of the caverns are covered with thousands of glowing golden orbs. In some images, scientists paddle inside detectors in row boats to reveal the sheer size of the apparatus. In one image, Neil Degrasse Tyson gives a knowing smile as he proudly perches on an inflatable raft inside a detector that could be mistaken for an ancient cave. After seeing these images, many questions immediately come to mind. Why are the detectors so gigantic? What are those orbs on the walls? And how does the water help to show the nature of neutrinos?
To understand the fundamental concept of these detectors, it is important to know a bit about what a neutrino is. Neutrinos are a class of subatomic particles that are produced during radioactive decay or during a nuclear reaction. Both nuclear fission in stars and nuclear reactors produce “showers” of neutrinos. Neutrinos have yet to be understood in great depth because they have proven to be quite elusive to scientists. They do not carry a charge, and were once thought to have no mass. Today, scientists have determined their mass to be close to a million times less than that of an electron. What is known
is that they are very plentiful. An estimated 65 billion neutrinos pass through every square centimeter on Earth every second. So, in order to study these neutral and extremely small particles, detectors must be huge. Infact, one detector contains 50,000 tons of water.
But why water? Scientists utilize water in these detectors because they take advantage of a phenomenon that occurs when particles travel through liquids. When particles travel through a liquid at rates faster than the speed of light for that liquid, light called Cherenkov Radiation is produced. Now, you may have searched for a typo in the previous sentence. Nothing can ever travel faster than the speed of light, right? Yes, that is correct. But, the speed of light refers to the speed of photons traveling in a vacuum. In vacuums, photons do not collide with particles or become defracted which slows them down. Due to the density of water, the speed of light in water is a fraction of the universal “speed limit” c, and therefore the neutrinos do not violate the laws of physics by traveling faster than the encumbered photons through water.
Now that we understand that water allows neutrons race past photons and produce rings of light, the purpose of the orbs on the walls becomes evident. The obs are light-sensitive photomultiplier tubes that can pinpoint the origin and type of neutrino by recording the intensity, location, and time interval the light was produced for.
photo courtesy: Wikipedia