Re: What is meant by "forbiddenness of the decay path?"

[Bernard L Cohen <blc+@pitt.edu>]

	In a beta decay, a neutron changes into a proton (or vice versa).
If  the proton has the same orbital motion in the nucleus as the neutron
had, this transformation can go relatively rapidly and is called
"allowed". However, if the orbital angular momentum of the proton is
different from that of the neutron, the transition is impeded and goes at
a much slower rate -- it is then called "forbidden". If the angular
momentum change is by one unit, it is called "first forbidden", if by two
units, it is called "second forbidden", etc. A first forbidden transition
is about 100 times slower than an allowed transition, a second forbidden
transition is about 10,000 time slower than a first forbidden transition,
a third forbidden transition is about 10,000 times slower than a second
forbidden, and a fourth forbidden is about 100,000 times slower than a
third forbidden. This is because the beta decay interaction has great
difficulty in changing the angular momentum of a neutron or proton.
	The half life iis determined by the transition rate, and is hence
much longer for forbidden transitions than for allowed. Transition rates
are also influenced by the amount of energy released, the electric charge
of the nucleus (atomic number), the wave functions for the initial and
final states, etc.
	If you want more details or examples, don't hesitate to ask.

Bernard L. Cohen
Physics Dept.
University of Pittsburgh
Pittsburgh, PA 15260
Tel: (412)624-9245
Fax: (412)624-9163
e-mail: blc+@pitt.edu


On Wed, 10 Nov 1999, Jacobus, John (OD) wrote:

> I am reviewing a manual that discusses beta decay, which indicates that some
> paths are forbidden?  Can anyone tell me that refers to?  Are there
> probabilistic values associated with this?
> 
> -- John 
> 
> "It isn't necessary to imagine the world ending in fire or ice-there are two
> other possibilities: one is paperwork, and the other is nostalgia." 
> -Frank Zap 
> 
> 
> 
> John Jacobus, MS
> Health Physicist
> National Institutes of Health
> Radiation Safety Branch, Building 21
> 21 Wilson Drive, MSC 6780
> Bethesda, MD  20892-6780
> Phone: 301-496-5774      Fax: 301-496-3544
> jjacobus@exchange.nih.gov (W)
> jenday@ix.netcom.com (H)
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