The concept is that a trace amount of material with an extremely high fast neutron absorption cross section can be alloyed with the fissile material, so that you can assemble a much larger mass of the fissile material via implosion before a chain reaction starts. Then once the reaction starts, the neutron poison burns away quickly, causing the device to rapidly become far more supercritical. Could this work to boost the efficiency of a nuclear weapon?
Are there any such isotopes with a significantly higher fast neutron absorption cross section than the fissile isotopes' fast neutron fission cross sections? I'm looking for at least 10 barns at 1MeV, and absorbing the neutron without releasing any other neutrons. I've looked and haven't been able to find any isotope with that property.