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Re: Nuclear topics in physics?



A model for barrier penetration is FTIR (frustrated total internal
reflection). The model is exact in that the equations are of the same
form (obvious, they're both described by the same wave equation). For
visual frequencies measurements are very difficult! However, rather
easy at microwave frequencies. I have done this for the advanced lab at
NPS quantitatively. {1/T= a[b + Sinh^2( c*D)]} Where a, b, and c
depend on the indices of refraction and the incident angle, D is the
separation. If enuff sign up, etc. I'll demonstrate this, among
others, at the SF NCNAAPT metng.

bc


"John S. Denker" wrote:

Regarding the barrier-penetration model of nuclear decay,

Ludwik Kowalski wrote:

Nowhere did I bring the alpha decay under consideration;
the ratio of alpha over fission probabilities was not even
mentioned in my outline.

Why not??!?!!?!?

Not checking the alpha-decay prediction seems analogous
to writing down the model
all odd numbers are prime
and saying the model is OK because we didn't bother
to check it for numbers greater than 8.

Normally when one writes down a model, one takes
responsibility for _everything_ the model predicts.

If the model is valid only on a restricted domain,
there ought to be a scientific reason for restricting
the domain; otherwise
-- it's not a _scientific_ model,
-- it's not a prediction but rather a postdiction,
-- it's what we call a just-so story, like the one
about how the camel got his hump.
http://www.boop.org/jan/justso/camel.htm

Knowing that alpha decay is a possibility worth considering
seems like a prerequisite for any discussion of nuclear decay.

Is it not true that the model predicts a rapid increase of the
fission probability per unit time (lambda) when Z goes up
from 80 to 100? That was the main point of my crude
argumentation based on drawn diagrams of V(r).

Any one-coordinate barrier-penetration model that makes
such a prediction will also predict that you'll never see
fission, because such nuclei will succumb to alpha decay
long before they fission.

As far as I recall, the calculations of that kind, done by
theoretical physicists, often disagree with experimentally
measure lambdas by very large factors.

Let's not make a virtue of wrong physics.

I suspect this is still true today.

a) I'm sure it is still possible to get wrong answers
by using bogus hand-wavy arguments... but I hope that's
not the point.
b) More to the point, it is nowadays possible to make very
accurate nuclear-structure calculations.

========================

We've had this conversation before. At some point, when
a model makes spectacularly wrong predictions, you have
to decide that the model is wrong.

(There _are_ ways of modelling fission in terms of barrier
penetration, but the physical radius r is not the right
reaction coordinate, and finding the proper reaction
coordinate would be a tour de force, not suitable for
the elementary context Ludwik proposed.)