[meteorite-list] What to look for if large impacts liberate neutrons - part 2 of 2

From: Rob Matson <mojave_meteorites_at_meteoritecentral.com>
Date: Sat, 29 Dec 2007 17:34:10 -0800
Message-ID: <GOEDJOCBMMEHLEFDHGMMGEFLDHAA.mojave_meteorites_at_cox.net>

Part 2

I left off on the subject of better elements for a ground-based
record of a large neutron producing event (whatever its source).
Looking for carbon-14 isn't the best approach since nitrogen is
not a large constituent in the earth's crust, and worse -- it has
a poor neutron cross section compared to other more common elements
in the crust.

Silicon seems like a natural choice since it makes up a whopping
28% of the crust (compared to nitrogen's paltry .0019 %); however,
silicon's neutron cross section is even worse than nitrogen's:
only 0.43 barns. Still, earth-crust Si is 1300 times better than
nitrogen as a neutron detector.

But iron is better still -- while it's only 5.63% of the crust,
its neutron cross section is 2.56 barns, which makes it three times
better than silicon as a neutron event signaler.

But there is one element I found that is even better than iron;
it's rare, but its neutron cross section is 48,800 barns (!) which
more than makes up for its rarity relative to iron. It's gadolinium
(Gd). It's about 9000 times rarer than iron, but its huge neutron
affinity more than makes up for it. For a given kilo of earth,
gadolinium ends up being a little more than twice as good as iron
as a neutron getter.

Here are the five most common isotopes of Gd, along with their
isotopic percentages:

Gd-155: 14.80%
Gd-156: 20.47%
Gd-157: 15.65%
Gd-158: 24.84%
Gd-160: 21.86%

All of these are stable isotopes with the exception of Gd-160,
and even Gd-160 has a half-life more than 100 billion times
greater than the age of the universe. The two isotopes we care
about are Gd-155 and Gd-157. Gd-155 has a neutron cross section
of 60700 barns, while Gd-157's is 254000 barns. When Gd-155
absorbs a neutron, it becomes Gd-156; likewise, Gd-157 gets
transmuted to Gd-158.

>From the above table, you can see that the natural ratio of
Gd-156/155 is 1.38; for Gd-158/157 it's 1.59. For gadolinium
that has been exposed to neutrons, you would expect these ratios
to go up. In fact, the closer to the neutron source, the greater
the neutron flux, and the higher the isotopic anomaly should be.
So if you wanted to pinpoint the location of a neutron event,
just look for the locations with the highest Gd 156/155 and
158/157 ratios.

One thing that could foul up this test is if isotopic abundances
of Gd are quite different in an iron meteorite (for example) than
in terrestrial Gd. Fortunately, this is not the case. Murthy
and Schmitt (1963) reported that meteoritic and terrestrial Gd
had the same isotopic abundances to within 1%.

One calculation that remains is to determine how high a neutron
dose is required to have a good chance of detecting its signature
in Gd.

I still have a big problem coming up with the mechanism by which
E.P.'s large impact is supposed to generate these neutrons. Since
the temperature is too low to achieve a nuclear reaction thermally,
and the impact velocity is far too low to do it kinetically, the
only thing left I can think of is some sort of fusor-like plasma
reaction -- alas, without the benefit of deuterium. --Rob
Received on Sat 29 Dec 2007 08:34:10 PM PST

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