In reply to Eric Walker's message of Mon, 25 Mar 2013 22:34:08 -0700: Hi, [snip] >On Sun, Mar 24, 2013 at 10:21 PM, <mix...@bigpond.com> wrote: > >...however, that having been said, the path that the "bouncing" neutrons >> follow >> would be longer because of a "random-walk". Since the path is longer, their >> chances of being captured increases...but maybe this is already included >> in the >> concept of cross-section? >> > >That's kind of what I was thinking -- the more elastic collisions there are >that don't result in an escape, the more there's a chance for a capture. > But I was hoping you would know this one. The cross sections are a little >bit magical.
If you use the absolute cross-section, you get the fraction that leaves immediately without any collisions at all. This is a lower bound. The actual number that escapes will be larger than this, because some of those that bounce will also escape. If you use the absorption cross-section to calculate the transmission fraction, you will get the fraction that doesn't immediately get captured. This is an upper bound. The actual fraction that escapes will be less than this because some of those that bounce will also get captured. In short your initial approach with upper and lower bound was good IMO. Regards, Robin van Spaandonk http://rvanspaa.freehostia.com/project.html
