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

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sun, 30 Dec 2007 15:11:22 -0600
Message-ID: <0bb201c84b28$88321de0$b64fe146_at_ATARIENGINE>

Hi, Rob, Doug, List,

    I got only one thing to say:
    Ye Gad!(olinium), I think he's got it!

    The crustal abundances of gadolinium are variously
given as 5.2 to 7.7 ppm. The levels of 158Gd would be
1.5 ppm, and 157Gd would be about 1 ppm. To detect
a meaningful shift in isotope ratios, one would need an
accuracy of better than 50 parts per billion. I have no
idea what the state of the art is. Chemically separate all
the gadolinium and run it through a mass spectrometer?

    There is an interesting Wikipedia piece on gadolinium:
http://en.wikipedia.org/wiki/Gadolinium

    Whether gadolinium would be found in the rocks or soil
at all the locations you would want to test is another crucial
point. Gadolinium does not seem to be freely distributed in
the crust:
    "Gadolinium is never found in nature as the free element,
but is contained in many rare minerals such as monazite and
bastn?site. It occurs only in trace amounts in the mineral
gadolinite, which was also named after Johan Gadolin.
Today, gadolinium is prepared by ion exchange and solvent
extraction techniques, or by the reduction of its anhydrous
fluoride with metallic calcium."

    What if the area you want to test doesn't have any monazite
or bastn?site or gadolinite? Of course, they're talking about
ore concentrations, but the high variation in the reported
natural abundance of gadolinium (+/- 20%) suggests that
you might get stuck trying to test a low-gadolinium area.

    Another problem is that the change in the isotope levels
is cumulative, representative of all the neutron flux in the area
over all geological time periods. There is no way to spot an
isolated neutron event. The test is, as Rob pointed out, a
differential test. You would have to test numerous samples
from over a wide area to locate a "hot spot." Then, you
would have to ascertain that the hot spot area did not also
contain an increased concentration of neutron-emitting
materials: you would find a gadolinium hot spot over a
thorium deposit, for example.

    Practically, this involves a large number of gadolinium
isotope assays. No problem if they're cheap and easy at the
level of accuracy required, but somehow I doubt it's that
easy.

    Because gadolinium is so efficient at soaking up neutrons,
it has a number of applications in places where we really want
to soak up neutrons, like nuclear reactor control rods. I found
(Googling my fingers off) that Lawrence-Livermore developed
a technique to separate gadolinium isotopes (some isotopes are
better neutron absorbers than others) with lasers and to do it
in bulk. There are a lot of references on ways to use gadolinium
but little on natural assays. On the plus side, there are obviously
a lot of people with experience kicking gadolinium isotopes
around...

    I'm not quibbling with the idea, BTW, just looking for a
handle on the problems of practical application.

    It's a great idea.


Sterling K. Webb
--------------------------------------------------------------
----- Original Message -----
From: "Rob Matson" <mojave_meteorites at cox.net>
To: "mexicodoug" <mexicodoug at aol.com>; <meteorite-list at meteoritecentral.com>
Sent: Saturday, December 29, 2007 7:34 PM
Subject: [meteorite-list] What to look for if large impacts
liberateneutrons - part 2 of 2


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


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Received on Sun 30 Dec 2007 04:11:22 PM PST