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

From: mexicodoug <mexicodoug_at_meteoritecentral.com>
Date: Sat, 29 Dec 2007 19:54:51 -0600
Message-ID: <00fa01c84a86$f8416ed0$4001a8c0_at_MICASA>

Hi Rob,

Egads Rob, you keep working on that mechanism of where the neutrons come
from in the first place.

But now that it is potentially testable, I'll stop calling it an
assertation, and call it the EPR hypothesis.

But your new marker sounds slick!

Deuterated water might come in with comets...and there is plenty of
semiheavy water in the ice caps :-)

What about 10-Be or Uranium ratios (238/235) I had mentioned. Any hope for
them IYO?

Best wishes,
Doug



----- 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 Sat 29 Dec 2007 08:54:51 PM PST