Shot Boltzmann was the first nuclear detonation of Operation Plumbbob, fired on 28 May 1957 from a 152-metre tower in Area 7c of the Nevada Test Site. It was a Los Alamos Scientific Laboratory development test of the XW-40 — a lightweight boosted fission warhead weighing 134 kg in total, with a nuclear system of 65 kg. The shot produced 12 kilotons against a predicted 11 kt. Boltzmann was among the Plumbbob shots observed by invited NATO allies — military representatives from member nations who were brought to the Nevada Test Site as part of a deliberate programme to demonstrate American nuclear capability to alliance partners. The United States used these demonstrations to reinforce the credibility of its nuclear guarantee to Western Europe, at a time when NATO defence planning was becoming increasingly dependent on the assumption that tactical nuclear weapons would be used in any land war with the Soviet Union. Named for the Austrian physicist Ludwig Boltzmann (1844–1906), following LASL's convention of naming shots for deceased scientists.