Editing Little Boy (section)

===Counter-intuitive design===
The material was split almost in half, with at one end a group of rings of highly enriched uranium with 40% of the supercritical mass, and at the other end another group of slightly larger rings with 60% of the supercritical mass, which was fired onto the smaller group, with four polonium-beryllium [[modulated neutron initiator|neutron initiator]]s to make the supercritical mass explode.{{sfn|Monk|2012|pp=409–410}}{{sfn|Coster-Mullen|2012|p=28}}

A hole in the center of the larger piece dispersed the mass and increased the surface area, allowing more fission neutrons to escape, thus preventing a premature chain reaction.{{sfn|Coster-Mullen|2012|pp=23–24}} But, for this larger, hollow piece to have minimal contact with the tungsten carbide [[Tamper (nuclear weapon)|tamper]], it must be the projectile, since only the projectile's back end was in contact with the tamper prior to detonation. The rest of the tungsten carbide tamper surrounded the sub-critical mass target cylinder (called the "insert" by the designers) with air space between it and the insert. This arrangement packs the maximum amount of fissile material into a gun-assembly design.{{sfn|Coster-Mullen|2012|pp=23–24}}

For the first fifty years after 1945, every published description and drawing of the Little Boy mechanism assumed that a small, solid projectile was fired into the center of a larger, stationary target.{{sfn|Samuels|2008}} However, critical mass considerations dictated that in Little Boy the more extensive, hollow piece would be the projectile. Hollow cylinders have higher critical masses than solid pieces of fissile material, because any neutrons encountered by or generated by the material are more likely to get scattered in the air than to continue a chain reaction. The larger piece would also avoid the effects of neutron reflection from the tungsten carbide tamper until it was fully joined with the rest of the fuel. Once joined and with its neutrons reflected, the assembled fissile core would comprise more than two [[critical mass]]es of uranium-235.<ref>The [[critical mass]] of any given nuclear system is not simply a matter of mass — it is a more complex function of the mass, its geometry, and properties like neutron reflection, among other things. As an illustrative example, the "bare sphere" critical mass of 70%-enriched uranium is {{convert|87.2|kg}}, but with a {{convert|5|cm}} beryllium neutron reflector, it drops to {{convert|36.5|kg}}, and with a {{convert|10|cm}} beryllium reflector, it drops to {{convert|23.7|kg}}. {{cite journal|last=Glaser|first=Alexander|title=On the Proliferation Potential of Uranium Fuel for Research Reactors at Various Enrichment Levels|journal=Science and Global Security|volume=14|pages=1–24|year=2006|issue=1 |doi=10.1080/08929880600620542|bibcode=2006S&GS...14....1G }} So while the {{convert|38.53|kg}}, 80%-enriched, cylindrical "projectile" was an insufficient amount of enriched uranium to be a "bare sphere" critical mass, inside of a neutron-reflecting system it could potentially be dangerously close to criticality even prior to weapon assembly, or just prior to full assembly. After weapon assembly, the {{convert|64.2|kg}} 80%-enriched material, in a solid cylinder and encased in a neutron-reflecting tungsten tamper, would have composed more than one critical mass.</ref> In 2004, [[John Coster-Mullen]], a truck driver and model maker from Illinois who had studied every photograph and document on the Hiroshima bomb to make an accurate model, corrected earlier published accounts.{{Sfn|Monk|2012|pp=409–410}}