Page "Effects of nuclear explosions" Paragraph 47

Details of nuclear weapon design also affect neutron emission: the gun-type assembly Hiroshima bomb leaked far more neutrons than the implosion type 21 kt Nagasaki bomb because the light hydrogen nuclei ( protons ) predominating in the exploded TNT molecules ( surrounding the core of the Nagasaki bomb ) slowed down neutrons very efficiently while the heavier iron atoms in the steel nose forging of the Hiroshima bomb scattered neutrons without absorbing much neutron energy.