In reply to Eric Walker's message of Wed, 9 Sep 2015 08:24:27 -0500: Hi, [snip] >Hi, > >This question is for Robin or anyone else who might know. Suppose you know >that the cross section for a neutron to tunnel from 7Li to 61Ni is 1 barn >at 10 MeV.
I assume this means that the Li7 has an energy of 10 MeV? >Now simplify the problem to that of holding the 7Li stationary >for a duration of time near the 61Ni so that the cross section remains 1 >barn, after which you take the 7Li away. Given the separation distance and >time that were needed to achieve this (perhaps the separation distance >would be very close to the distance that the 10 MeV brought the 7Li in the >original problem statement), is there a way to determine how long it would >be necessary to hold the 7Li at twice a distance from the nucleus and still >have a neutron tunneling cross section of 1 barn? If I understand it correctly, the concept of cross section is based on the notion that an impinging particle only has one go at causing a reaction, based upon it's velocity as it approaches. The sort of situation you are actually talking about is different. The Li7 is essentially bound to the Ni, so the low energy neutron gets trillions of chances every second to tunnel across the gap (because of vibrations). Because of the bound state of the molecule and the Ni nucleus, the total tunneling chance approaches unity in the long run. The real question is, how long does it take on average, or IOW what is the half-life of the reaction, which will indeed be a separation dependant function. Answer - I wish I knew. ;) However reasoning backwards, it must be a reasonably short time, because the Hot-cat works. ;) I have a formula for charged particles, where the initial charged particle is free, however this situation is quite different, because the barrier comprises the nuclear binding energy of the neutron to the Li7 nucleus, and to make matters more complicated, nuclear force goes as the sixth power of distance rather than the square of distance that is appropriate for electric forces. The formula could probably be adapted, if I knew the appropriate function for the nuclear force. IOW what I know is only qualitative, not quantitative. > >Eric Regards, Robin van Spaandonk http://rvanspaa.freehostia.com/project.html
