THE RACE IS ON

The fission effect in uranium was confirmed in laboratories from England to Japan, and its profound implications were well understood by all engaged in nuclear physics. In 1939, nuclear fission was seen as having two possible uses.

discovery, methodically and with patience. By 1938, the team had identified the half — lives of 10 unknown radioactive substances in uranium bombarded by neutrons.

On March 14, the political climate changed for the worse. Austria was annexed by Germany, and Lise Meitner became a German citizen. She had to leave quickly and surreptitiously, before she was arrested for being a Jew. Friends smuggled her across the border to Holland, and she made it out with enough money to buy lunch and with no possessions. She wound up working in a nuclear physics laboratory in Stockholm, Sweden, but she kept up with Hahn back at the laboratory in Germany by mail.

They met clandestinely in Copenhagen in November, planning a new set of neu­tron bombardment experiments, and the letters kept flowing. Hahn steadfastly believed that nuclear fission was impossible, in late December, Meitner convinced him that he had split uranium into two large fragments, one of which was barium, and with that realization history was made. Unfortunately, it was illegal for Otto Hahn to put Lise Meitner’s name on the paper describing the chemical findings as coinvestigator because of her religious affiliation. Meitner and her nephew, the nuclear physicist Otto Robert Frisch (1904-79), published a paper two months later giving the physical explanation of Hahn’s discovery and naming the effect "nuclear fission."

Lise Meitner died in Cambridge, England, in 1968 after a life of dedicated work in nuclear physics. Element 109, meitnerium, was named in her honor in 1997. The most stable known isotope of meitnerium has a half-life of 1.1 seconds.

The first application is a controllable source of constant power. Neu­trons, which are necessary to cause the chain reaction effect, are ejected from a fissioning nucleus at high speed. They can be slowed down to a crawl by repeated collisions with surrounding light nuclei, and at the low speed the free neutrons can be captured by uranium nuclei, causing fur­ther fission. The act of being slowed down imparts energy to the light nuclei, and this material, known as the moderator, is then used as a heat — transfer medium. The process can be controlled easily, by balancing the number of neutrons being produced with the number of neutrons neces­sary to keep the chain reaction stable.

The second possible application is a superbomb. Neutrons produce fis­sion at slow, thermal speed, but there is also fast fission from neutrons at the extreme high end of the speed range. Start a chain reaction in a large enough mass of pure uranium, with no intervening moderator material,

and the fission will run out of control. It happens with such speed and is such a huge explosion that it can destroy an entire city. The military pur­pose of this application was both obvious and terrifying, and it was also seen immediately.

In February 1940, Otto Frisch, Lise Meitner’s nephew, and Rudolph Peierls (1907-95) drafted a memorandum to the British Committee on the Scientific Survey of Air Defense, titled, “On the Construction of a ‘Super­bomb’ Based on a Nuclear Chain Reaction in Uranium.” Similar memo­randa were written in Germany, Japan, and the Soviet Union at about the same time.

Scientists in every country considering the military uses of nuclear fis­sion realized that there was a problem with applying this effect. The useful fission seemed to occur in only one isotope of uranium, U-235, and it was only a small component of naturally occurring uranium. To build a bomb, the 235 isotope had to be nearly pure, and separating U-235 from U-238 was no simple process. In the long run, only the United States had the industrial volume, ability, and materials to achieve this huge task. On top of that, the United States had inherited a large portion of the nuclear phys­ics talent from Europe, as it fled the threatening fascist governments. The United States, which had contributed little to the field of nuclear research, became the world’s center for it during World War II, and science, tech­nology, and international affairs would be changed forever.

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