Neutrinos

Neutrinos: an introduction

In the time that you have been reading this article, about 10 000 000 000 000 000 neutrinos have passed through you without your noticing. Neutrons are tiny, yet with the power to confirm or overthrow a number of scientific theories. Neutrinos, meaning ‘little neutral ones’, are everywhere, all around us. These tiny elementary particles travel through space at close to the speed of light and have no charge. They were once thought to have no mass either, but scientists now suggest that they do have a mass; it is estimated to be less than a billionth of the mass of a hydrogen atom, but the research continues.

Neutrinos come in three types, according to the standard model of particle physics: the electron neutrino, muon neutrino, and tau neutrino, which have all been confirmed experimentally. A fourth, ‘sterile’ type has been proposed, which is immune to the weak force of the standard model. Were sterile neutrinos to be found, a new realm of physics beyond the standard model would open up.

Unlike most other particles, neutrinos are able to escape from dense regions such as the core of the sun or the milky way, and they can travel long distances from far-away galaxies without being absorbed, carrying information about these areas. In this sense, neutrinos are cosmic messengers, and neutrino astronomy is becoming increasingly important. So far, only two sources of extraterrestrial neutrinos have been observed: the sun and supernovae. On earth, both natural and artificial neutrino sources exist: radioactive materials from inside earth can undergo beta decay, producing geo-neutrinos. In addition, nuclear fission reactors produce neutrinos, and particle accelerators are being used as neutrino sources for research. Finally, when cosmic rays hit Earth’s atmosphere, atmospheric neutrinos are emitted as decay products of pions and muons.

Scientists now propose that cubic-metre antineutrino detectors could be used to non-intrusively monitor and safeguard nuclear reactors. Neutrino detectors would provide real-time information about the reactor core power and possibly even its isotopic composition. An array of about 500 such detectors worldwide would be able to calculate the power output of individual reactors, allowing the detection of clandestine nuclear weapons testing. Geo-neutrinos produced during natural radioactive dacay of uranium, thorium and potassium in earth’s crust may help answer the question of crust composition.

Neutrinos are very useful for studying astronomical and cosmological phenomena, and neutrino detectors are being built worldwide, deep underground to filter out the ‘noise’ of other particles.