Complex sentence: Reacting to alpha particle bombardment, the beryllium atoms emit neutrons in a rate of about 1 neutron each 5--10 nanoseconds; these neutrons then trigger the chain reaction in the compressed supercritical plutonium.
Split into simpler sentences: Reacting to alpha particle bombardment, the beryllium atoms emit neutrons in a rate of about 1 neutron each 5--10 nanoseconds. These neutrons trigger the chain reaction in the compressed supercritical plutonium.