Ask A Scientist General Science Archive

Question:  How do they know that radioactive decay is constant?
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Because the probability that any radioactive nucleus will decay
is constant in time until it does decay.  That's for an undisturbed
nucleus.  The probability is enhance if the nucleus encounters
another particle such as an electron, neutrino, neutron, etc.
 
--- Jade
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Scientists have been studying radioactive decay for about a century, and
measuring it very precisely.   When it is studied, the probability of decay
(from an unstable nucleus) turns out to be constant and does not change with
time.  If it changed, even a slight amount, someone would have noticed.   It
is also possible to get information from studying old rocks and also
studying stars and galaxies.  Every thing looks pretty constant.

Obviously, if you hit a nucleus with another particle,  you can change the
probability of decay.  Also, the decay rate can be slightly altered by the
local chemical environment.  For example, when radioactive beryllium-7 is
put inside a carbon-60 "cage" the decay rate increases by 1%.  When Be-7
decays, it absorbs an electron into the nucleus.  By putting the Be-7 atom
inside the soccer-ball shaped C-60 molecule, it makes more electrons
available.

Bob Erck
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