Editing Critical mass (section)

===Use of a tamper===
In a bomb, a dense shell of material surrounding the fissile core will contain, via inertia, the expanding fissioning material, which increases the efficiency. This is known as a [[tamper (nuclear weapons)|tamper]]. A tamper also tends to act as a neutron reflector.  Because a bomb relies on fast neutrons (not ones moderated by reflection with light elements, as in a reactor), the neutrons reflected by a tamper are slowed by their collisions with the tamper nuclei, and because it takes time for the reflected neutrons to return to the fissile core, they take rather longer to be absorbed by a fissile nucleus.  But they do contribute to the reaction, and can decrease the critical mass by a factor of four.<ref>Serber, Robert, ''The Los Alamos Primer: The First Lectures on How to Build an Atomic Bomb'', (University of California Press, 1992) {{ISBN|0-520-07576-5}} Original 1943  "LA-1", declassified in 1965, plus commentary and historical introduction</ref>  Also, if the tamper is (e.g. depleted) uranium, it can fission due to the high energy neutrons generated by the primary explosion.  This can greatly increase yield, especially if even more neutrons are generated by fusing hydrogen isotopes, in a so-called [[Boosted fission weapon|boosted configuration]].