[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 |
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