What are neutrinos and how are they detected?

Often in the study of astronomy, the focus of many research programs and independent study revolves around the hulking celestial bodies that have captured humanity’s imagination for thousands of years. Oft forgotten are the small particles of matter that are the basic essentials for physical existence. The study of these tiny particles has evolved from studying atoms to studying subatomic particles and finally to studying elementary particles. Elementary particles are particles that are not know to have any substructure, which means, if this remains true under scientific scrutiny, that elementary particles are the basic building blocks of which all matter in the universe in composed. One such elementary particle is called the neutrino.

Image of a Neutrino

Neutrinos are elementary particles that are similar to the more commonly known electron. The difference, however, between the electron and the neutrino is in the particle’s charge. This difference is crucial in determining the behavior of the particle. Electrons are affected greatly by electromagnetic forces due to the particle’s negative charge. Neutrinos on the other hand carry no charge making them mostly impervious to strong force electromagnetism. Instead, these particles are only affected by weak nuclear force and gravitation although the effect from gravity has been shown to be negligible in laboratory study. The name neutrino literally means “small neutral one.”

The vast majority of the neutrinos that pass through Earth come from the Sun. Since the particles are electrically neutral and are not affected by strong force, the neutrinos pass through the Earth relatively unaffected. It can be said that during the daytime, solar neutrinos shine down on humanity, but during the night, these neutrinos shine up from underneath! More than fifty trillion solar neutrinos pass through the human body every second. There are three types, or “flavors,” of neutrinos. These are electron neutrinos, muon neutrinos, and tauon neutrinos, which each have a unique antimatter particle called an antineutrino. Electron neutrinos and antineutrinos are created when a proton becomes a neutron or vice versa through the process of beta decay, which is a type of radioactive decay in which a beta particle, an electron or positron, is emitted.

The proposed existence of the neutrino was first theorized in 1930 by Wolfgang Pauli. This idea was formulated to preserve the laws of conservation of energy, conservation of momentum, and the conservation of angular momentum in regards to beta decay. Pauli stated that an undetected particle was carrying away the observed difference in regards to the energy, momentum, and angular momentum of the initial and final particle. Upon first theorizing the existence of the neutrino, Pauli named the unknown particle the neutron, but a more massive sub atomic particle with no charge was found two years later and was also named the neutron. The naming confusion was remedied by Enrico Fermi as he changed the name to neutrino when proposing the theory of beta decay.

Observation of Neutrinos

The neutrino particle remained only a proposed idea until 1956 when a group of researchers published the article “Detection of the Free Neutrino: a Confirmation” in Science. This discovery was awarded the Nobel Prize in physics almost 40 years later in 1995. The experiment, now known as the neutrino experiment, took neutrinos created in a nuclear reactor by beta decay and shot them into protons which produced neutrinos and positrons of which both were able to be detected. Further, in 1962 a team of researchers found that there were different types of neutrinos when they identified the already hypothesized muon neutrino. A third lepton, the tau, was discovered at Stanford in 1975. It was theorized that there would be an associated neutrino with this particle as well, but this was not confirmed until 2000 by the DONUT.

SNO neutrino detector at work

Neutrinos are detected in several ways. As previously stated, the first detection of neutrinos was accomplished in the neutrino experiment in 1956 using induced beta decay to observe the particles. In the modern scientific era, a neutrino detector is used to identify and study neutrinos. These apparatus must be very large in order to detect neutrinos because of the weak-reacting nature that neutrinos exhibit. They are often built underground in order to differentiate cosmic rays from background radiation. These apparatus take on several different incarnations from large volumes of water that are watched by phototubes for the Cherenkov radiation, which occurs when electromagnetic radiation is emitted as a charged particle passes through an insulator at a constant speed greater than the speed of light in that medium, to detectors made of large volumes of chlorine or gallium which are then checked for argon or germanium, respectively.

The study of the massive celestial bodies will continue to captivate the mind and imagination of humanity for many generations to come. Discovering new stars and planetary bodies will continue to drive the study of astronomy outward, but let science not forget that the inward study of the very particles that make up every physical body in the universe will garner us much insight into the great celestial bodies that surround us.

References

http://www.ps.uci.edu/~superk/neutrino.html

http://hyperphysics.phy-astr.gsu.edu/Hbase/particles/neutrino.html