so successful that the military continued to use it for many years after the war, and it was still in use in some countries in the 1980s. Alvarez was awarded the National Aeronautic Association's Collier Trophy in 1945 "for his conspicuous and outstanding initiative in the concept and development of the Ground Control Approach system for safe landing of aircraft under all weather and traffic conditions". Alvarez spent the summer of 1943 in England testing GCA, landing planes returning from battle in bad weather, and also training the British in the use of the system. While there he encountered the young Arthur C. Clarke, who was an RAF radar technician. Clarke subsequently used his experiences at the radar research station as the basis for his novel Glide Path, which contains a thinly disguised version of Alvarez. Clarke and Alvarez developed a long-term friendship. Manhattan Project In the fall of 1943, Alvarez returned to the United States with an offer from Robert Oppenheimer to work at Los Alamos on the Manhattan Project. However, Oppenheimer suggested that he first spend a few months at the University of Chicago working with Enrico Fermi before coming to Los Alamos. During these months, General Leslie Groves asked Alvarez to think of a way that the US could find out if the Germans were operating any nuclear reactors, and, if so, where they were. Alvarez suggested that an airplane could carry a system to detect the radioactive gases that a reactor produces, particularly xenon-133. The equipment did fly over Germany, but detected no radioactive xenon because the Germans had not built a reactor capable of a chain reaction. This was the first idea of monitoring fission products for intelligence gathering. It would become extremely important after the war. As a result of his radar work and the few months spent with Fermi, Alvarez arrived at Los Alamos in the spring of 1944, later than many of his contemporaries. The work on the "Little Boy" (a uranium bomb) was far along so Alvarez became involved in the design of the "Fat Man" (a plutonium bomb). The technique used for uranium, that of forcing the two sub-critical masses together using a type of gun, would not work with plutonium because the high level of background spontaneous neutrons would cause fissions as soon as the two parts approached each other, so heat and expansion would force the system apart