where [[sigma].sub.pp], [[sigma].sub.nn], and [[sigma].sub.np] is the proton- proton, neutron-neutron, and neutron-proton interaction cross section and it is expressed in millibarn
(mb), [beta] = v/c, [E.sub.lab] is the incident kinetic energy of the nucleon in MeV in the laboratory frame of reference, and [rho] is the nuclear matter density in unit of [fm.sup.-3].
The prefix yocto- means 10 exp -24, and yoctosecond (unlike anything below a millibarn
) has a good chance of being used.
It turns out that even a barn is rather large for expressing the area over which certain nuclear reactions occur, and physicists have felt it necessary to define an even smaller area, the millibarn
: A unit of nuclear cross section equal to 1/1000 barn