A sphere of plutonium surrounded by neutron-reflecting blocks of tungsten carbide. A re-creation of the August 21, 1945 criticality accident at Los Alamos to measure the radiation produced when an extra block was added, making the mass supercritical.
Los Alamos National Laboratory
Attribution
From LANL's report on criticality accidents, 2000, <a rel="nofollow" class="external free" href="http://www.csirc.net/docs/reports/la-13638.pdf">http://www.csirc.net/docs/reports/la-13638.pdf</a> PDF  at <a rel="nofollow" class="external free" href="http://www.csirc.net/library/la_13638.shtml">http://www.csirc.net/library/la_13638.shtml</a>