Re: decay of radioactive isotopes

From: Bjoern Feuerbacher (feuerbac_at_thphys.uni-heidelberg.de)
Date: 07/01/04 

Date: Thu, 01 Jul 2004 18:09:19 +0200

Frederik wrote:
> Bjoern Feuerbacher <feuerbac@thphys.uni-heidelberg.de> wrote in message news:<cbrj5p$e3k$1@news.urz.uni-heidelberg.de>...
>
>>Frederik wrote:
>>
>>>I read somewhere that only 11% of the K-40 elements confined in some
>>>type of earth layers decays to Ar-40.
>>
>>Well, that agrees nicely with the branching ratio of 10.72% given
>>at the web site mentioned above for the EC + B+ - decay.
>>
>>
>>
>>>The remainder would then decay to stable isotopes of potassium.
>>
>>No. Why do you think so? It decays to Ca-40.
>>
>
>
> I didn't think so, I doubted that. At the time I read about the 11% of
> K-40 decaying to Ar-40, as a novice at radioactivity, I was not aware
> of the possibility for radioactive isotopes to decay to different
> atoms.

No offense intended, but didn't you learn that at school?

The most common decay possibilities are:
1) Alpha decay: an alpha particle (= nucleus of an helium atom
= 2 protons + 2 neutrons) is emitted. That obviously changes the
element. This happens mostly for heavy elements.
2) Beta minus decay: a nucleus with N neutrons and Z protons decays into
one with N-1 neutrons, Z+1 protons, an electron and an
electron-antineutrino. Again, that obviously changes the element.
This happens for isotopes which have considerably more neutrons than
protons.
3) Beta plus decay: a nucleus with N neutrons and Z protons decays into
one with N+1 neutrons, Z-1 protons, a positron and an electron neutrino.
Again, that obviously changes the element. This happens for isotopes
which have considerably more protons than neutrons.
4) Gamma decay: a nucleus goes from from a higher energetic state to
a lower energetic state (e.g. it goes from oscillating wildly to
oscillating only a bit, or from fast rotation to slow rotation, etc.),
and a photon is emitted. Neither Z nor N changes. This is about the
only decay mode I'm aware of in which the element does not change.
5) Spontaenous fission: the nucleus breaks up into smaller nuclei
(one could consider alpha decay as a special case of this). See the
remarks at (1).

Then there are also more exotic decay modes like electron capture
or the ones which are triggered by capturing other particles (e.g.
nuclear reactors depend on a chain reaction, which is caused by
neutrons hitting the uranium nuclei). But in most of them, the element
indeed is changed - with the exception of (3), there is almost no
radioactive decay in which the element stays the same!

> So I contacted the author of the article to verify this. It was
> the author that claimed that the remainder would decay into stable K
> isotopes.

What article are you talking about, specifically? Who was the author?
He seems to be quite ignorant about quite elementary nuclear physics.

> Since the only two stable K isotopes are K-39 and K-41, and
> I didn't find a compatible decay type to reach this result, I decided
> to post a question about this.
>
> So now you know why :-)

Hope it is more clear now for you...

Bye,
Bjoern

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