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.