Re: Dawkins and nuclear physics

"On Mon, 28 Sep 2009 16:22:52 +0100, in article
<qN6uYPFMTNwKFwmW@xxxxxxxxxxxxx>, Ernest Major stated..."

In message <264146985.00003162.050.0001@xxxxxxxxxxxxxxx>, TomS
<TomS_member@xxxxxxxxxxx> writes
In Dawkins' latest book, "The Greatest Show on Earth", he discusses
some of the nuclear physics involved in radioisotope dating.

It seems to me that this could be wrong on some of the details. (The
treatment is on pages 92 and following.)

He says, "There are several kinds of radioactive decay, which offer
opportunities for useful clocks." And, "All these kinds of instability
involve neutrons."

1) A neutron turns into a proton.

2) A proton turns into a neutron.

3) "A stray neutron happens to hit a nucleus and knocks out one proton,
taking its place."

4) "... an atom ejects a so-called alpha particle." "The atom changes
to whichever element is two below it in the periodic table."

My complaints about these:

1) Maybe I'm being nit-picking, but I wouldn't say that a proton
turns into a neutron, but rather that a nucleus emits an electon and
an anti-neutrino with the effect that the number of neutrons goes down
by one and the number of protons increases by one. (If that were all I
had to complain about, I wouldn't say anything.)

If that's all he said then it's oversimplified. But I would quibble with
your description as well. Free neutrons decay to a combination of a
proton, an electron and an anti-neutrino. As neutrons and protons are
discrete particles within the nucleus beta-decay is essentially the
same, but the requirement to account for the difference in energy levels
of the neutron and the proton, and to account for the effects of the
potential well and barrier, means that the half-life is different.
You've sort of glossed over that a neutron is transformed in a proton,
etc.

Good point. I'd say that omitting the point about energy levels
could confuse the reader. Someone might think of this form of decay
as being just neutron -> proton, so why is the half-life different
in different cases? And the fact that the half-lives are different
in different cases is essential to the dating process. Your quibble
accepted.


2) This is really a nucleus emitting an anti-electron (positron) and
neutrino. (I think that's what he means.)

There's two versions - beta+ decay, where a proton is transformed into a
neutron, a positron and an election-neutrino, and K-capture, where an
orbital electron reacts with the nucleus - equivalent to p(e,v)n.

Again, agreed.


Both versions are potentially relevant - for example 40K decays both to
40Ar and 40Ca, and while K-A dating is technically easier to perform I
imagine that it's possible to perform isochron dating on the K-Ca
system.

3) A neutron hitting a nucleus isn't very significant in radioisotope
dating, and I don't think that the effect of knocking out a proton is
characteristic of it. More often, doesn't the neutron initiate fission,
as in a chain reaction?

He may be thinking of the reaction 14N(n,p)14C, which is significant for
C-14 dating (being the primary source of 14C). But I would restrict

Yes, the production of 14C ... I hadn't thought of that.

If that is what he had in mind, I would say that it is, at best,
confusing to the reader who is a newcomer to the subject.

decay to spontaneous reactions not requiring collisions with other
particles. So this would not be decay. Nor would be induced fission.
There's at least one other reaction involved free neutrons (apart from
neutron decay) - neutron absorption (e.g. 14N(n,g)15N (g=gamma).

While confirming that particular neutron absorption occurs via Google, I
also find that there are also neutron transfer reactions (e.g.
14N(14N,13N)15N), but that doesn't involved free neutrons.

4) My only complaint is about "two below it in the periodic table".
If you look at a periodic table, and look two *below* copper, you will
find gold. But an alpha-decay of copper (I don't think that there is
such a natural reaction in such a light nucleus, but if there were) would
take it to cobalt, two *before* (or to the left of) it.

Obviously he means the element with an atomic number two lower, but,
yes, his phrasing could be confusing to those unfamiliar with
radioactive decay.


If I were describing the kinds of radioactive decay important in
dating, I would rather enumerate:

a) Alpha decay.

b) Beta decay.

c) Orbital electron capture.

There are other infelicities in the description that, I think, could
be misleading to the reader, but I won't go on at length.




Thanks for your response.


--
---Tom S.
the failure to nail currant jelly to a wall is not due to the nail; it is due to
the currant jelly.
Theodore Roosevelt, Letter to William Thayer, 1915 July 2

.

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