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RE: Radioactive decay



Tom McCarthy writes:

I was reading in Leon Lederman's book, From Quarks to the Cosmos, that
Fermi's theory of the weak interaction is correct in describing the
radioactive decay of all particles. Does this mean the emission of alpha
particles is the result of a weak interaction?

--

Well, I haven't read the book, but you are correct - alpha decay
is not the result of a weak interaction (nor are gamma decays).
This would refer to beta decays, either by electron or positron
emission, as well as electron capture (where the nucleus captures
an electron from an inner orbit, changing a proton to a neutron
and emitting a neutrino in the process).

Alpha decays are a "classical" example of quantum tunneling -
the total energy of the system is less after the decay than
before, but there is an energy barrier which cannot be surmounted
classically. However, eventually (and how long this takes depends
exponentially on how high the barrier is - this is a standard
example in nuclear physics courses) the alpha particle will
tunnel through. The energy barrier is due to the sum of the
strong and electromagnetic forces - when bound in the nucleus
the alpha particle's energy is less than if it just starts to
separate; at longer distances the strong force is no longer a
factor and the Coulomb repulsion takes over. The enormous
variation in alpha-decay lifetimes - from fractions of a
nanosecond to longer than the age of the universe - is due to
this exponential energy factor.

Sue Willis

Suzanne Willis, Professor, Physics Department
Northern Illinois University, DeKalb, IL 60115 USA
http://niuhep.physics.niu.edu/~willis/ swillis@niu.edu
phone: 815-753-0667 fax: 815-753-8565