How can a neutrino be detected if it has no mass and does not interact with normal matter?

Neutrinos were first theorized in the 1930's because some nuclear decay reactions didn't produce enough energy or particles to conserve energy, which is required by the First Law of Thermodynamics. In a reaction, the mass and energy going in must equal the mass and energy coming out. (Mass and energy are related by Einstein's famous equation E=mc2.) There had to be a particle that didn't interact normally with matter and therefore couldn't be detected. This particle was named the neutrino, which means "little neutral one". We can't detect neutrinos directly but we have detected them by other means.

Because the neutrino only interacts via the weak nuclear force (which acts over a very small distance), it could travel through a light-year of lead before striking an atom and being detected. Scientists used nuclear reactors as sources for many neutrinos so that the chances of an interaction would be higher. When a neutrino strikes a proton, it creates a positron and a neutron. The positron then combines with its antiparticle, an electron, to create two photons. These photons are what scientists can detect.

Submitted by Marc (Canada)
(October 24, 1997)

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