 The discovery by Emilio Segrè 's group in April and May 1944 of high levels of plutonium @-@ 240 in reactor @-@ produced plutonium meant that an implosion @-@ type nuclear weapon was required , but studies indicated that this would be extremely difficult to achieve . By August 1944 , the calculations had been made of how an ideal spherical implosion would work ; the problem was how to make it work in the real world where jets and spalling were a problem . Christy worked in Rudolf Peierls 's T @-@ 1 Group , which studied the theory of implosion . He suggested the possibility of using a solid plutonium core that would form a critical mass when compressed . This was an ultra @-@ conservative design that solved the problem of jets by brute force . It became known as the " Christy pit " or " Christy gadget " , " gadget " being the laboratory euphemism for a bomb . However the solid pit was intrinsically less efficient than a hollow pit , and it required a modulated neutron initiator to start the chain reaction . Christy worked with Klaus Fuchs , Paul Stein and Hans Bethe to develop a suitable initiator design , which became known as an " urchin " . The Gadget used in the Trinity nuclear test and the Fat Man used in the atomic bombing of Nagasaki used Christy pits . 
