Neutrino

Neutrinos are fundamental particles that are part of the matter that makes up the universe. A fundamental particle, also known as an elementary particle, is a bit of matter smaller than an atom. The three main fundamental particles are neutrons, protons, and electrons. Neutrinos are similar to electrons but lack an electrical charge. They are high-energy particles produced when radioactive elements decay. However, because they lack an electrical charge, neutrinos barely interact with other matter and can pass through other types of matter—even the human body—without leaving any noticeable trace.

Notoriously difficult to detect, neutrinos have been called "ghost" particles. However, because they travel at the speed of light and are not absorbed, interfered with, or altered by encounters with other matter, they provide a good source of astronomical information, such as distances between stellar bodies.

Sources and Discovery

Researchers think most neutrinos in existence today were created about fifteen billion years ago, just after the universe formed. However, new neutrinos are produced when stars collide, are born, or die. Additional neutrinos are generated by man-made nuclear sources, such as power plants, particle accelerators, and nuclear explosions.

Although neutrinos have existed almost since the beginning of the universe, physicists were unaware of them until 1934. That year, research into the process of beta decay—the radioactive decay process by which a proton inside the nucleus of an atom changes into a neutron, or vice versa—led Italian physicist Enrico Fermi (1901–1954) to theorize that an unknown particle that interacted very weakly with other forms of matter had to exist. Researchers Frederick Reines (1918–1998) and Clyde L. Cowan (1919–1974) confirmed the existence of neutrinos in 1956, and Reines received a Nobel Prize for the discovery in 1995.

Types

Three types, or flavors, of neutrinos are known to exist. They are named for the charged particles with which they are associated: electron neutrinos, muon neutrinos, and tau neutrinos.

In the late twentieth century, Japanese physicist Takaaki Kajita (1959– ) was investigating why the number of detected neutrinos did not match the number of expected neutrinos in the interaction between cosmic rays and Earth's atmosphere. Similar problems had confounded researchers since the 1960s when they began theoretical computations into the numbers of neutrinos generated from various sources. Kajita discovered that neutrinos can change from one type to another. For example, an electron neutrino can shift to become a muon neutrino.

About the same time, a research team led by Canadian physicist Arthur B. McDonald (1943–), working at the Sudbury Neutrino Observatory in Ontario, Canada, was studying neutrinos generated by the Sun. McDonald's group developed the same theory, called neutrino oscillation or neutrino flavor oscillation. The group determined that neutrinos do have slight mass and are capable of changing from one type to another. Kajita and McDonald shared the 2015 Nobel Prize in physics for their discovery.

Detection

The inherent difficulty of detecting neutrinos means that scientists must resort to extreme measures to uncover their presence. They use underground laboratories to eliminate sources of background radiation, such as cosmic rays, or build enormous devices to detect them. Several types of detectors are used in the search.

In their research, Reines and Cowan used cadmium chloride targets immersed in water with scintillation detectors nearby. Scintillation detectors are able to sense the tiny pulses of light produced when certain materials absorb radiation. They picked up trace gamma rays, which provided evidence of neutrinos.

Radiochemical detection methods use a tank filled with a chemical, such as chlorine or gallium, to create chemical chain reactions that allow researchers to count neutrinos. However, this method does not allow scientists to determine neutrino direction or type.

Cherenkov detectors, or ring-imaging detectors, capture and measure light generated by charged particles as they move through a substance, such as water, ice, heavy water, or mineral oil, at a speed faster than the speed at which light moves through that substance. Faint blue light, known as Cherenkov light, is released when neutrinos collide with atomic nuclei or protons and unleash charged muons or electrons. Using this method, researchers are able to determine the type of neutrino by the shape and amount of Cherenkov light detected.

Since neutrinos are found in space and can help astronomers and physicists understand more about distant stellar bodies and the composition of space itself, a number of telescopes are used to detect neutrinos. To provide shielding from extraneous radiation sources and facilitate detection of the faint light signals that indicate the presence of neutrinos, these telescopes are located underground, underwater, or under ice. They are also very large. Because neutrinos are difficult to detect, it is necessary to search large areas to increase the odds of finding and identifying them.

Continuing Research

Research to unlock more information about neutrinos is ongoing. KM3NeT is a research infrastructure created to house the next generation neutrino telescopes. KM3NeT is located 3.5 kilometers (just over 2 miles) below the surface of the Mediterranean Sea. When completed, the telescopes would use detectors, which are light sensor modules that can find faint light in the deep sea from charged particles created by neutrinos colliding with Earth. Astronomers would use the telescopes to search for neutrinos from sources such as supernovae and colliding stars. Additional strings of detectors were planned along the coasts of Greece, Italy, and France within five years, though the detector will be operational before the entire array is connected.

Researchers at Daya Bay in China also have been investigating some unexpected neutrino behavior, which might indicate the existence of an additional type or flavor of neutrino. This detector, located near a six-reactor nuclear facility, finds more neutrinos than some others because of the steady stream of neutrinos created by the power plant and, therefore, has increased the odds of noticing such anomalies. The Daya Bay researchers and their colleagues at other facilities continually seek new and better ways to learn more about neutrinos.

Bibliography

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"The Chameleons of Space." The Nobel Prize in Physics 2015. The Royal Swedish Academy of Science. Web. 19 Feb. 2016. http://www.nobelprize.org/nobel‗prizes/physics/laureates/2015/popular-physicsprize2015.pdf

Francis, Matthew R. "Casting a Net for Neutrinos." Symmetry (18 Feb. 2016).Web. 19 Feb. 2016. http://www.symmetrymagazine.org/article/casting-a-net-for-neutrinos

"Neutrinos." Fermilab. Fermi Research Alliance LLC. Web. 19 Feb. 2016. http://www.fnal.gov/pub/science/particle-physics/experiments/neutrinos.html

"Neutrinos Offer a New Way to Investigate the Building Blocks of Matter." US Dept. of Energy, 9 May 2024, www.energy.gov/science/articles/neutrinos-offer-new-way-investigate-building-blocks-matter. Accessed 25 Nov. 2024.

Timmer, Josh. "Neutrinos Continue Run of Odd Behavior at Daya Bay." ARS Technica.Condé Nast, 16 Feb. 2016. Web. 19 Feb. 2016. http://arstechnica.com/science/2016/02/neutrinos-continue-run-of-odd-behavior-at-daya-bay/

"What's a Neutrino?" University of California Irvine School of Physical Sciences. University of California Irvine. Web. 19 Feb. 2016. http://www.ps.uci.edu/~superk/neutrino.html