the power of the atom."
Robert Millikan, Nobel Prize in Physics, 1923
then you've made a measurement. If the result is contrary
to the hypothesis, then you've made a discovery.
When Fermi announced in 1934 that his research group had synthesized new elements beyond number 92, other physicists began pursuing the quest for new elements. In early 1939 two German research scientists, Otto Hahn and Fritz Strassmann, bombarded a chemically pure sample of uranium and noticed after a period of time the sample contained trace amounts of elements from the middle of the periodic table. Initially, they were reluctant to suggest a process for producing these lighter isotopes. That bold step was taken by Lise Meitner (an aunt to Herr Frisch) and Otto Hahn who suggested that the atom had been split into smaller parts in a process they named "fission" borrowing the term from biology. Portraits of these folks appear below.
Otto Frisch Fritz Strassman Otto Hahn Lise Meitner
Let's do the math.
The fission reaction described immediately below is one of about 200 possible reactions..Precisely which fission occurs is determined in part by the speed of the neutron as well as by where it strikes the target nucleus. let's just assume that it is representative of most fissions that occur. A neutron moving at just the right speed strikes an atom of Uranium-235. the atom splits into several pieces. The pieces are: A) two fragments from the middle of the periodic table. These are the so-called nuclear waste materials that no one knows what to do with, (more about that later) B) the electrons are of no consequence; and C) there are neutrons (usually to or three) that came away at high rates of speed. It is the neutrons that make the chain reaction possible.One neutron produces a fission that produces two neutrons, which themselves cause four fissions; you get the picture.
The careful observer will notice that atomic number (the subscripted numbers tell the number of protons in each nucleus) balance left and right; so do the mass numbers (the superscripted numerals which tell the number of protons and neutrons in each atom.) But now things start to get goofy. Lets look at the masses of the particles.
Consider the masses (in am )of these particles U-235 neutron La139 Mo 95 2 neutrons 7 electrons 235.0439 1.0087 138.8061 94.9057 2.0174 .0038 mass before mass after fission 236.0526 235.7330 Lost mass ] .3196 am Matter is destroyed, converted to Energy by E = mc^2 Binding Energy 4.8 x 10^-11 J
The importance of this event in light of world events to follow cannot be over-estimated. If one adds the masses of a U-235 atom and the neutron that split it and then compares that number with the sum of the masses of all of the fission products produced, a startling conclusion appears. The masses are NOT the same. Mass has been lost, annihilated, converted into energy according to the rule predicted by the Einstein equation E = mc^2. The missing mass is small, typically of the order of .1% of the mass of the original atom. The Einstein conversion puts the energy equivalent for this mass at about 300Mev or about 10^-11Joules of energy. If one fission only is to be effected, this is not much energy. But what if we were to fission a mole of U-235 having a mass of 235 g (weighing about 1/2 pound and easily fitting in the palm of your hand). Multiplying 10^-11 Joules by 10^23 atoms yields 10^12 Joules. This is an enormous amount of energy. And there is still 99.9% of the original mass left
In order to cause 10^23 fissions to occur, a chain reaction would probably be necessary. Could such a sequence be orchestrated? Presumably, the Germans were ready to try. On August 2, 1939, thirty days before Germany invaded Poland to start WWII, Albert Einstein, at the behest Leo Szillard, alerted Franklin Roosevelt of this country's need to pursue a path of research that would lead to a chain reaction. (See the letter at http://www.dannen.com/ae-fdr.html Soon thereafter, a joint military-scientific endeavor, code named the Manhattan Engineering District, was launched. Lt. Gen. Leslie R. Groves and J. Robert Openheimer, headed the project which may have been the most comprehensive research project ever conceived by peoplekind. The chain reaction, a peaceful event, came to pass on December 2, 1942, in the squash courts at the University of Chicago. The work there was directed by Enrico Fermi who by now had moved to the west to escape the Nazi oppression of Jews. In time, the Manhattan project collected enough fissionable material for three weapons. The first was detonated in the New Mexico desert in July 1945. Within the month, the others were delivered to Japan.
Strangely enough, the August 2 letter represents the only input that Einstein would have on the bomb project. An avowed pacifist, Einstein spent his last years trying to bring nations closer together.
In order to create a fission bomb, one needs so-called weapons-grade uranium, at least 95% U-235. By contrast, U235 exists in nature at about 1%; nuclear power plants need slightly-enriched uranium at 3-5%. There needs to be critical mass of uranium, enough atoms present to ensure that once the chain reaction begins, there will be sufficient fuel to cause it to continue. Additionally, there needs to be critical array of the fissionable material to ensure that as neutrons are produced, they will readily find other atoms to fission.
For more information about fission in
general, visit these web sites:
Nuclear fission has more peaceful uses today. Nuclear reactors now power most of the larger ships and submarines in the US navy. Power reactors still supply a meaningful fraction of domestic electricity, although no new reactors have been started since the Three Mile Island accident in Pennsylvania in 1978. For a case history of a nuclear project gone wrong, use your favorite search engine to find Chernobyl.
These sites will give you an idea of
how reactors work.
Additionally, there were some applications of nuclear power that never got very far. Checkout "Operation Plowshare" at
and"Atoms for Peace" on the Internet.
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last edited 12/29/05