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Nuclear Decay Cause
Name: Bill M.
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 3/22/2004
Question:
What actually causes and unstable nucleus to actually
decay. Is it because of neutrino interaction?
Replies:
There are probably a lot of ways to look at this. Here is one:
The root cause is in the tension between the attractive, short-range
nuclear force, and the repulsive, long-range electrostatic force. The
nuclear force is hugely attractive when nucleons are very near each other,
but drops off rapidly as the distance between nucleons increases. You can
think of the nuclear force as being active only between nearest neighbors,
and it does not make much difference whether those neighbors are neutrons
or protons. The electrostatic force is much weaker, but also much less
diminished by distance, and acts mainly between protons, pushing them apart.
If you do not have enough neutrons around, the repulsive, electrostatic
force wins out, because every proton feels repulsion from every other
proton, but only feels attraction from its nearest neighbor. On the
other hand, if you have too many neutrons, the Pauli exclusion principle
(no two particles of the same kind can have exactly the same quantum
numbers) forbids them to occupy states that protons would be allowed to
occupy, and so you also do not get as much nearest-neighbor attraction as
you would get with a more optimal ratio of protons to neutrons.
An unstable nucleus is one in which the ratio is off far enough that
the nucleus really cannot hold itself together, but it might not decay
immediately, because relatively stable decay products must form and
separate. This might take a long time, because the nucleus is continually
rearranging itself, and most arrangements do not correspond to the required
relatively stable decay products.
Tim Mooney
The "real" cause of radioactive decay has two meanings. The first is a
statistical definition. If a light nucleus has too many neutrons compared to
the number of protons in the nucleus, the nucleus will be radioactive. What
constitutes "too many" depends upon the element. So for example 3H is
radioactive and so is14C. For heavy nucleii if there are "not enough"
neutrons, that isotope will be radioactive. Thus
235U is much more radioactive than 238U. The second definition of the
"actual cause" means if I sit and look at a single nucleus, will there be
any indication that it is "about to decay" the answer is no one knows at
least at the present time. Radioactive decay appears to be a statistical
event.
Vince Calder
It is true that a neutrino comes out, and the decay is caused by "weak-nuclear-force"
interactions. So some physicists would say that means the answer to your question is
simply "yes". Probably a snide answer.
It occurs to me you might be asking whether a passing neutrino, from outside the nucleus,
triggers the decay.
Occasionally a neutrino does trigger a nuclear decay, but most radioactive decays happen
with no visible external provocation.
If a ball in a very small dimple on a hill suddenly rolled out and down, we would probably
ascribe it to provocation by molecular ("Brownian") motion. Or a breath of wind. Or
_something_ moving.
In the quantum mechanical world we do not have anything analogously visibly moving. An
unstable particle simply has a finite, indeterminate lifetime describable only by
probabilities. Maybe there is nothing moving, or maybe there are things moving so
small that we can never see them and pin them down.
Some tentative nature-of-space theories imagine empty vacuum to be a seething sea of
almost-there particles which occasionally pop through the "surface" into existence.
They very promptly disappear too, conserving energy and particle counts in the long run,
if not in the very short run. But you can see that this implies that no particle ever
sits in a truly "quiet" place. Particles with permanent existence sometimes temporarily
don't exist for a prescribed length of time, and when they reappear they are
outside the tiny "box" they were in. This is called "tunneling". Some describe nuclear
decay as an example of tunneling. Then your question is: what determines the average
time-delay probability of tunneling, and what underlying events happen at the
moment it occurs? We only have the average; cannot do anything to see the underlying.
So far this is just philosophy. As yet we have no way to distinguish between silent
probabilistic uncertainties, and a roiling sea of virtual particles. So far they are
a consistently smeared-together picture with known average numbers but no underlying
explanation. So far this issue is on the same level as " Why is an electron?"
in slightly over my head-
Jim Swenson
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Update: June 2012
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