Re: Dawkins and nuclear physics

"On Mon, 28 Sep 2009 20:08:09 +0200, in article
<1j6rqok.yhx2hcromkhcN@xxxxxxxxxxxxxxxxx>, J. J. Lodder stated..."

TomS <TomS_member@xxxxxxxxxxx> wrote:

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

(Oops. I mistyped here. I should have written "I wouldn't say that a
neutron turns into a proton ...". Other infelicities of wording here
can't be excused as typos. :))

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.)

The full reaction is
n -> p + e^- + nu_anti
This effectively is neutro turing into proton.
(with some other junk)

But I can imagine a reader being confused as to why some neutrons
turn into protons in a very short time, some in a very long time,
and some never turn into protons, and how, given that kind of
variable rate of change can be the basis for a clock.


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

Idem.

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?

Cosmic ray neutrons produce instable elements.
(that can be used for dating)
Cosmic ray induced fission is irrelevant.

Agreed.


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.

There is no such thing as a canonical representation
of the periodic table.
But yes, two places below it in the element list
might be more appropriate.

Yes, I recognize that there is no "canonical representation". But
I think that everybody in the intended audience would picture the
periodic table in such a way as I suggested. And "two places below
it in the element list" seems to me to be no better - my image of
an "element list" starts with hydrogen at the top. (I suspect that
you have a typo here.)


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.

You are forgetting spontaneous fission,
(dating U-containing glasses)

You are right, I was forgetting that. Thanks.


Jan



--
---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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