Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne 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



Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory