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 (age 39, Canada)
(October 24, 1997)
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