Good morning boys and girls. In keeping with Th’ Gausslings weakness (sickness??) for odd and specialized information, a quantity known mainly to nuclear reactor operators and other nukkenvolk is trotted out.
Neutron lethargy, or logarithmic energy decrement, u, is a dimensionless logarithm of the ratio of the energy of source neutrons to the energy of neutrons after a collision: u = ln(Eo/E), or, u2-u1 = ln(E1/E2). So, if you plot a curve of E vs u (E = Eo*exp(-u)), you see an exponential decay of energy per unit collision showing that the greatest delta E’s of energy result from the early collisions.
Basically, it shows that in order to obtain thermal neutrons from fission decay neutrons, you have to contain them so that they can rattle around and dump energy before they fly out of the area of interest. As to the number of collisions that are needed? Well, that is a different issue.
Source- Glasstone & Edlund, The Elements of Nuclear Reactor Theory, Van Nostrand, 1952, p 146.
[Note: It happens that of the 1300 or so posts I have written, this is the most popular. Who knew? But there are at present much more informative links out there.
>>> Obsolete links deleted. Sorry. <<<
Neutron Fluence Measurements, http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/34/065/34065175.pdf
Lamentations on Chemistry 
Sorry, the vodka is kicking in. You might do a piece on Fission one day. I think we’re still investing a pretty penny in it. I thought that was supposed to be the cure for all our energy ills.
Hi Rad, there are a number of new nuclear plants in the licensing phase right now. Construction will begin soon, if not already. The nuclear genie was let out of the bottle in the early 1940’s. Nuclear is coming back. What remains to be seen is how radwaste will be delt with. The current projects are beset with problems.
No nuclear reactors if they will have cleaning ladies.
what are you trying to say uncle Al? Cleanliness is next Godliness
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Just got that book in my hand, hope I pass Nuclear Rector Theory this time.
Good luck!!
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