Task: Question Generation
Input (English): Passage: This was followed up by a group of scientists at the Collège de France in Paris: Frédéric Joliot-Curie, Hans von Halban, Lew Kowarski, and Francis Perrin. In February 1939, the Paris Group showed that when fission occurs in uranium, two or three extra neutrons are given off. This important observation suggested that a self-sustaining nuclear chain reaction might be possible. The term "atomic bomb" was already familiar to the British public through the writings of H. G. Wells, in his 1913 novel The World Set Free. It was immediately apparent to many scientists that, in theory at least, an extremely powerful explosive could be created, although most still considered an atomic bomb was an impossibility. Perrin defined a critical mass of uranium to be the smallest amount that could sustain a chain reaction. The neutrons used to cause fission in uranium are considered slow neutrons, but when neutrons are released during a fission reaction they are released as fast neutrons which have much more speed and energy. Thus, in order to create a sustained chain reaction, there existed a need for a neutron moderator to contain and slow the fast neutrons until they reached a usable energy level. The College de France found that both water and graphite could be used as acceptable moderators.Early in 1940, the Paris Group decided on theoretical grounds that heavy water would be an ideal moderator for how they intended to use it. They asked the French Minister of Armaments to obtain as much heavy water as possible from the only source, the large Norsk Hydro hydroelectric station at Vemork in Norway. The French then discovered that Germany had already offered to purchase the entire stock of Norwegian heavy water, indicating that Germany might also be researching an atomic bomb. The French told the Norwegian government of the possible military significance of heavy water. Norway gave the entire stock of 187 litres (41 imp gal; 49 US gal) to a Deuxième Bureau agent, who secretly brought it to France just before Germany invaded Norway in April 1940. On 19 June 1940, following the German invasion of France, it was shipped to England by the Earl of Suffolk and Major Ardale Golding, aboard the steamer Broompark. The heavy water, valued at £22,000, was initially kept at HM Prison Wormwood Scrubs, and was later secretly stored in the library at Windsor Castle. The Paris Group moved to Cambridge, with the exception of Joliot-Curie, who remained in France and became active in the French Resistance.
Output (English): What is the full name of the person who defined a critical mass of uranium to be the smallest amount that could sustain a chain reaction?