[meteorite-list] Neutron production in hyper-velocity impacts

From: Göran Axelsson <axelsson_at_meteoritecentral.com>
Date: Sun, 30 Dec 2007 06:10:58 +0100
Message-ID: <47772862.4080605_at_acc.umu.se>

Rob Matson wrote:

... snip ...
> 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
>
>

As the temperature that is required to get kinetic fusion between atoms
is way too high to be reached in an impact that way to generate neutrons
is closed. The only remaining way that I see is by photo spallation of
atoms by high energy photons.
Typically photons begin to produce neutrons on interaction with normal
matter at energies of about 7 to 40 MeV

For you who don't feel at home with electron volts as energy unit, MeV
is huge, visible light lies in 1.5-3 eV, x-rays between 124 eV to 124
keV, and gamma rays above that.

What needs to be calculated is the number of photons that is emitted by
the plasma surrounding the impactor during it's ascent. A good
approximation should be black body radiation and with the knowledge of
area and typical plasma temperature a number of high energetic photons
could be calculated.

Basically I see the "possible" process to be...
 1. An impactor enters the atmosphere and creates a plasma.
 2. The plasma acts as a black body, radiating energy in a continuous
spectra from IR, visible, UV, x-rays and up to gamma rays.
3. Photons with a sufficient energy has the possibility to eject
neutrons from ordinary atoms.
4. After cooling down by repeated impacts with atoms the neutrons are
finally absorbed by nitrogen atoms creating carbon 14.

To get a number we need to
 1. Get the plasma temperature (literature sources, maybe measurements
from Shoemaker-Levy 9?)
 2. Calculate the amount of photons, this is just a matter of using
Stefan-Boltzmann law or Plancks law. We also have to get an
approximation of the size of the fireball.... or is it a plasma ball?
3. Divide the number above by half. Half of the neutrons will hit the
impactor and be absorbed in other atoms.
4. Find the elusive constant that describes how many photons actually
creates a neutron and not nuclear fission. This one is for a nuclear
physicist to calculate.
5. Compare the number of atoms created with the amount of C14 already in
the atmosphere.

Am I totally wrong or not? Anyone wants to try to do all or part of the
calculation needed to finally let this assertion die?

My gut feeling is that there is at best a minute amount of C14 created
by a hyper velocity impact. My best argument against it is if a large
impact would create nuclear reactions then every meteor striking the
atmosphere would also create C14 and with all the incoming material in
form of micrometeorites it would add a large portion to the C14 in the
atmosphere. I'm sure that some scientists would have noticed the
difference between C14 produced by cosmic radiation and the amount added
to the atmosphere each year.

G?ran
Received on Sun 30 Dec 2007 12:10:58 AM PST