Date: Around 1993
Please explain how beta particles can be positive.
Technically, beta particles are electrons produced by radioactive
decay, when a neutron changes into a proton, an electron which is ejected from
the nucleus, and a neutrino (also ejected, but very hard to detect), are
negatively charged. However, some radioactive processes result in the
emission of a positron, a particle of the same mass as an electron but
positively charged. Both processes are referred to as "beta decay". Beta
particles were discovered by Madame Curie and Becquerel in the late 1890's.
Positrons were not detected until 1933 (and this was in cosmic rays, not
radioactive decay); in fact, their existence was predicted on theoretical
grounds by Dirac, three years before they were detected! A positron has only
a fleeting existence, at least in our part of the universe, since when it
meets up with an electron the two annihilate, producing gamma rays. (This
process is called pair annihilation.) That is probably why positrons were not
detected in radioactive decay until much later; they annihilate long before
they can show themselves in the sort of detector used by Curie & Becquerel.
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Update: June 2012