In reply to Eric Walker's message of Sat, 23 Mar 2013 16:47:17 -0700: Hi, [snip] >Thank you, Robin. > >On Sat, Mar 23, 2013 at 3:49 PM, <mix...@bigpond.com> wrote: > >>So I >> >think you would take the weighted average of these to get an upper bound >> on >> >the absorption cross section of a block of normal nickel; e.g., 100 * .68 >> + >> >50 * .26 = 81 barns. >> > >My earlier calculation was flawed. I neglected to include data for the >other isotopes of nickel found in nature, so the weighted average was taken >over parts that added up to less than 100 percent. If you included the >other isotopes, especially the trace one with the much higher cross >section, I think the cross sections would have gone up.
Then you should be able to follow the same procedure, but include all the natural isotopes, no? > >I see that the page you link to is for 58Ni. Is there a straightforward >way to to get the total cross section for nickel in its natural isotopic >abundances? I don't know. If I were in your shoes, I would just do what you did, but include all the isotopes. > >Note however that the absorption cross section increases as the speed of the >> neutrons decreases, hence WL's emphasis on "ultra cold". >> >> (See e.g. >> http://atom.kaeri.re.kr/cgi-bin/endfplot.pl?j=f&d=mcnp&f=mcnp/Ni-58) >> > >Nickel seems to have a high total absorption cross section. With W-L there >is an implicit (or perhaps explicit?) assumption that the "ultra cold" >neutrons being generated will be absorbed in sufficient numbers to avoid >thermalizing, spilling out, spreading out into the environment and sending >a neutron counter sky high (not necessarily a GM counter). Suppose 1 W is >being generated by way of neutron capture and we are sure that it is >neutron capture that is involved. I'm curious whether you think that some >configuration of nickel in an unshielded cell could be found to absorb all >of the neutrons without setting off a neutron detector, or whether >new physics would be needed to explain the lack of neutrons leaking out. I am no supporter of WL theory as they proclaim it. This is one of the reasons. Regards, Robin van Spaandonk http://rvanspaa.freehostia.com/project.html
