🚀 go-pugleaf

In article <1992Aug5.220941.22969@hsv3.lsil.com>, mvp@hsv3.lsil.com (Mike Van Pelt) writes:
|> In article <132161@lll-winken.LLNL.GOV> shaw@s119.es.llnl.gov (Henry Shaw 510 423-4645) writes:
|> >|>      3) What is the mechanism that makes fallout radioactive?
|> >Neutron capture on stable nuclei of the target material is what is
|> >responsible for most of the radioactivity.
|> 
|> It's my understanding that most of the radioactivity is from fission
|> products.  The reason a low-altitude blast produces more fallout is
|> that when all the vaporized trees, buildings, cats, etc. recondense and
|> drop out of the sky, they take a lot of those fission products with
|> them.  In a high altitude blast, the fission products are finely
|> divided at high altitude, and tend to stay in the upper atmosphere long
|> enough to get diluted and for the worst of it to decay.
|> 
|> -- 
|> Mike Van Pelt                     Will your long-winded speeches never
|> Headland Technology/Video 7       end?  What ails you that you keep on 
|> sun!indetech!hsv3!mvp             arguing?    --    Job 16:3
|> mvp@hsv3.lsil.com

It depends on the fraction of the yield that is due to fission versus the fraction 
of the yield that is due to fusion, as well as the location of the explosion (air vs. ground
blast, and if a ground blast, the chemical composition of the materials being irradiated).   
It also depends on whether one is talking about the makeup of "early" fallout, or "late" fallout.  For 
long times post-event, you (and Paul Deitz, who contacted me via e-mail) are correct, the primary 
activities are fission products such as Sr-90 and Cs-137.  Nevertheless, the isotope that will result
in the largest effective dose equivalent commitment to the global population is C-14, which is
not a fission product (except in minor amounts via ternary fission), but the result of (n, alpha)
reactions on N-14 in the air. (see Ionizing Radiation: Sources and Biological Effects, 1982, 
United Nations Scientific COmmittee on the Effects of Atomic Radiation, pp. 19-20, and p. 211ff)

Explosions in which the fireball touches the ground produce mostly short-lived activation products like
Na-24 (T1/2 = 15 hr), K-42 (T1/2 = 12 hr) Mn-56 (T1/2 = 2.6hr).  As far as I know, for short times,
these isotopes dominate the activity in fallout from groundbursts (but I could be wrong).  Consider the 
mass of material involved in fission that can give rise to fission products versus the amount of 
"target" material that is exposed to a significant neutron flux.  

-- 
Henry Shaw, Earth Sciences Dept., LLNL |    All opinions are my own.
Internet: shaw4@llnl.gov               |  
CIS: 76347,2523                        |
Fidonet: Henry Shaw at 1:161/55        |