Saturday, July 20, 2002
F A C T   F I L E


Lise Mietner
By Illa Vij
Atomic explosion

AN Austrian Jewess, who was head of the nuclear physics department at Berlin’s Kaiser Wilhelm Institute, once marginally escaped arrest by Gestapo (the German secret police under Nazi rule) — an arrest which would have landed her in the gas chamber. On the pretext of going to Holland for a vacation, Lise Mietner fled with a small suitcase. She managed a Swedish visa and escaped to Stockholm. Her work helped clarify atomic theories, and also helped the USA beat Germany in the race for the development of the atomic bomb.

Lise dominated a field that was mostly ruled by men. When people learned that Lise had made great contributions in the atomic bomb dropped on Hiroshima, she was compared to Marie Curie. When Lise learnt that her name was being linked with such heavy destruction, she said: "I hope that the construction of the atom bomb will not only help to finish this awful war, but that we will be able to use it for peaceful purposes too."

Lise was born in 1878. Her father was a Viennese lawyer. He inspired her to read books from his library. Science books interested her and of all the world’s scientists, she admired Marie Curie the most.

When Lise grew up, she persuaded her father to allow her to study under Nobel Prize winner Max Planck (known for his quantum theory). Max found her to be extremely intelligent. Under his guidance, she progressed in her work. She carried out experiments along with her colleague Otto Hahn on the natural radioactivity of some atoms. They discovered protactinium which yielded actinium.

 
Lise then studied radium and thorium. She studied the behaviour of beta rays. In 1917, she became head of the physics department in the Kaiser Welhelm Institute of Chemistry. This institute is now called Max Planck Institute of Chemistry.

In the 1930s, scientists were working on uranium. Lise too was busy with the same study. She was in the middle of an important study when she had to flee in order to save her life. When she reached Stockholm, she joined her nephew Otto Frisch. He was working in Copenhagen, with famous Danish physicist Neil Bohr. Her friends Hahn and Strassman, who were in Berlin, continued their work without her, but relayed their findings to her. They claimed that on bombardment of uranium, they obtained two major isotopes, but were unable to get a complete break-up. Lise repeated the experiment and discovered that the split of the uranium atom,into two smaller atoms of different elements such as barium and krypton, was accompanied by the release of an enormous amount of nuclear energy (equal to about two thousand electron volts).

Neil Bohr, the Danish physicist, went to the USA, and discussed the progress with Einstein. Finally the atomic bomb was produced.

Besides working in the Nobel Institute for Physics in Stockholm, Lise was also the visiting professor at the Catholic University of America. In 1966, she shared the Enrico Fermi Award with Hahn and Strassman.

Lise died in 1968, after having given to the world her best. Whether nuclear energy is to be used for peaceful purposes or destructive, it is for us to decide. Such a powerful energy discovered after such pains and agonising years of experiment, must not be used to destroy but to create.

 
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Atomic explosion

THERE are two types of atomic explosions that can be facilitated by Uranium-235: fission and fusion. Fission, simply put, is a nuclear reaction in which an atomic nucleus splits into fragments, usually two fragments of comparable mass, with the evolution of approximately 100 million to several hundred million volts of energy. This energy is expelled explosively and violently in the atomic bomb. A fusion reaction is invariably started with a fission reaction, but unlike the fission reaction, the fusion (Hydrogen) bomb derives its power from the fusing of nuclei of various hydrogen isotopes in the formation of helium nuclei. This article discusses the A-bomb or atomic bomb.

The massive power behind the reaction in an atomic bomb arises from the forces that hold the atom together. These forces are akin to, but not quite the same as, magnetism.

Atoms are comprised of three sub-atomic particles. Protons and neutrons cluster together to form the nucleus (central mass) of the atom while the electrons orbit the nucleus much like planets around a sun. It is these particles that determine the stability of the atom.

Most natural elements have very stable atoms which are impossible to split except by bombardment by particle accelerators. For all practical purposes, the one true element whose atoms can be split comparatively easily is the metal uranium. Uranium's atoms are unusually large, henceforth, it is hard for them to hold together firmly. This makes Uranium-235 an exceptional candidate for nuclear fission.

Uranium is a heavy metal, heavier than gold, and not only does it have the largest atoms of any natural element, the atoms that comprise uranium have far more neutrons than protons. This does not enhance their capacity to split, but it does have an important bearing on their capacity to facilitate an explosion.

There are two isotopes of uranium. Natural uranium consists mostly of isotope U-238, which has 92 protons and 146 neutrons (92+146=238). Mixed with this isotope, one will find a 0.6 percent accumulation of U-235, which has only 143 neutrons. This isotope, unlike U-238, has atoms that can be split, thus it is termed "fissionable" and useful in making atomic bombs. Being that U-238 is neutron-heavy, it reflects neutrons, rather than absorbing them like its brother isotope, U-235.

This feature was published on July 8, 2000