Thank you, Robin. On Sat, Mar 23, 2013 at 3:49 PM, <[email protected]> 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. 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? 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. Eric
