  The Christy Gadget is the informal name for the plutonium device detonated in
the Trinity test on July 16, 1945. In September 1944, Robert Christy, working
in the theoretical implosion group, proposed a novel concept that altered the
design of the nuclear core in Fat Man. While scientists originally intended to
use a hollow sphere of plutonium, this design entailed substantial risk, due to
the likelihood of asymmetries resulting from implosion. Christy proposed
changing the design to a solid sphere of plutonium with a modulated neutron
source, and the design was eventually adopted, tested at Trinity, and used in
the attack on Nagasaki. While there is no question regarding the important role
that Christy played in demonstrating its feasibility as a reliable design,
there is a debate as to who initially proposed the idea; though most sources
have attributed this invention to Christy, some historical sources have
attributed credit to Christy's group leader, Rudolf Peierls, or indeed other
scientists. This paper seeks to outline and resolve this dispute. We present
new unclassified evidence extracted from previously unavailable sources (to
unclassified audiences) from the National Security Research Center archives at
Los Alamos National Laboratory. This evidence consists of 1945-1946 patent
documentation, oral history interview tapes of Christy and Peierls, and monthly
1944 progress reports from the Theoretical Division. Though Christy and Peierls
share joint credit on the patent, both Christy's and Peierls' words and
writings, together with sources from Hans Bethe and Edward Teller, support the
traditional view that Christy was indeed the originator of the idea. While
Christy does deserve the majority of the credit for the invention and design,
we acknowledge the important role Peierls and von Neumann played in its
development.
