From the wikipedia entry on Leo Szilard –
During his time in Berlin he was working on numerous technical inventions. For example, in 1928 he submitted a patent application for the linear accelerator and, in 1929, he applied for a patent for the cyclotron. During the 1926-1930 period, he worked with Einstein to develop a refrigerator, notable because it had no moving parts. Szilard’s 1929 paper, Über die Entropieverminderung in einem thermodynamischen System bei Eingriffen intelligenter Wesen” (On the reduction of entropy in a thermodynamic system by the interference of an intelligent being) Z. Physik 53, 840-856, introduced the thought experiment now called Szilard’s engine and was important in the history of attempts to understand Maxwell’s demon.
Developing the idea of the nuclear chain reaction
Szilárd went to London in 1933 where he read an article in The Times summarizing a speech given by Ernest Rutherford in which he rejected the possibility of using atomic energy for practical purposes. Rutherford’s speech remarked specifically on the recent 1932 work of his students John Cockcroft and Ernest Walton in “splitting” lithium into alpha particles, by bombardment with protons from a particle accelerator they had constructed:
- We might in these processes obtain very much more energy than the proton supplied, but on the average we could not expect to obtain energy in this way. It was a very poor and inefficient way of producing energy, and anyone who looked for a source of power in the transformation of the atoms was talking moonshine. But the subject was scientifically interesting because it gave insight into the atoms.
Although the atom had been split and energy released, nuclear fission had not yet been discovered. However Szilárd was reportedly so annoyed at Rutherford’s dismissal that on the same day the article about the Rutherford speech was printed in the morning paper, Szilard conceived of the idea of nuclear chain reaction (analogous to a chemical chain reaction), using recently-discovered neutrons. The idea did not use the mechanism of nuclear fission, which was not then known, but Szilárd realized that if neutrons could initiate any sort of energy-producting nuclear reaction, such as the one that had occurred in lithium, and could be produced themselves by the same reaction, energy might be obtained with little input, since the reaction would be self-sustaining. The following year he filed for a patent on the concept of the neutron-induced nuclear chain reaction. Richard Rhodes described Szilárd’s moment of inspiration:
- In London, where Southampton Row passes Russell Square, across from the British Museum in Bloomsbury, Leo Szilard waited irritably one gray Depression morning for the stoplight to change. A trace of rain had fallen during the night; Tuesday, September 12, 1933, dawned cool, humid and dull. Drizzling rain would begin again in early afternoon. When Szilard told the story later he never mentioned his destination that morning. He may have had none; he often walked to think. In any case another destination intervened. The stoplight changed to green. Szilard stepped off the curb. As he crossed the street time cracked open before him and he saw a way to the future, death into the world and all our woes, the shape of things to come.
Szilárd first attempted to create a nuclear chain reaction using beryllium and indium, but these elements did not produce a chain reaction. During 1936, he assigned the chain-reaction patent to the British Admiralty to ensure its secrecy (GB 630726 ). Szilárd also was the co-holder, with Nobel Laureate Enrico Fermi, of the patent on the nuclear reactor (U.S. Patent 2,708,656).
During 1938 Szilárd accepted an offer to conduct research at Columbia University in Manhattan, and moved to New York, and was soon joined by Fermi. After learning about the successful nuclear fission experiment conducted during 1939 in Germany by Otto Hahn, Fritz Strassmann, Lise Meitner, and Otto Robert Frisch, Szilárd and Fermi concluded that uranium would be the element capable of sustaining a chain reaction. Szilárd and Fermi conducted a simple experiment at Columbia and discovered significant neutron multiplication in uranium, proving that the chain reaction was possible and enabling nuclear weapons. Szilárd later described the event: “We turned the switch and saw the flashes. We watched them for a little while and then we switched everything off and went home.” He understood the implications and consequences of this discovery, though. “That night, there was very little doubt in my mind that the world was headed for grief.”
At around that time the Germans and others were in a race to produce a nuclear chain reaction. German attempts to control the chain reaction sought to do so using graphite, but these attempts proved unsuccessful. Szilárd realized graphite was indeed perfect for controlling chain reactions, just as the Germans had determined, but that the German method of producing graphite used boron carbide rods, and the minute amount of boron impurities in the manufactured graphite was enough to stop the chain reaction. Szilárd had graphite manufacturers produce boron-free graphite. As a result, the first human-controlled chain reaction occurred on December 2, 1942.
The Manhattan Project
(etc. – see link below for rest of entry including some information about the Manhattan Program efforts)
1911 – Heike Kamerlingh Onnes discloses his research on metallic low-temperature phenomenon characterised by no electrical resistance, calling it superconductivity.
(from a timeline of low-temperature discoveries on wikipedia)
APS has put the entire Physical Review archive online, back to 1893. Focus Landmarks feature important papers from the archive.
A long series of discoveries beginning with Einstein’s relativity in 1905 led up to Bethe’s discovery of the correct nuclear reactions. Hydrogen fusion seemed like a good candidate because according to E = mc2, the small mass difference between the fusing hydrogen and the resulting helium would liberate an enormous amount of energy. Also, spectral analysis in the 1920s revealed that most stars, including the sun, are mostly hydrogen. Another important piece of the puzzle was the development of quantum mechanics, including essential concepts like tunneling, which allows some classically forbidden reactions to occur with a small probability.
In his papers, Bethe worked out two nuclear reaction mechanisms whereby fusion would occur. In one mechanism, two protons fuse together as one transforms into a neutron to form a so-called deuteron. The deuteron captures a proton, making a helium-3 nucleus, which further reacts to produce helium-4. The other mechanism, called the C-N-O cycle, assumes a small amount of carbon is present to catalyze a reaction chain that involves nitrogen and oxygen intermediaries and ultimately produces helium nuclei from protons.
While Bethe correctly identified the two possible mechanisms of solar fusion, he incorrectly judged which was responsible for energy production in our sun. “The most important source of energy in ordinary stars,” he wrote in the first sentence of the second paper, “is the reaction of carbon and nitrogen with protons.”
Bethe concluded that proton-proton fusion dominates energy production only in stars that are about 1000 times fainter than the sun. The problem was that in the 1930s, the sun’s core temperature was thought to be about 20 million degrees. The true temperature is closer to 14 million degrees. We now know that proton-proton fusion is the energy source of our sun. Only in more massive stars is the C-N-O cycle relevant.
23 January 2008
by –Jason Socrates Bardi
Jason Socrates Bardi is a senior science writer at the American Institute of Physics.
Physical Review Focus
American Physical Society
|Energy Production in Stars
H. A. Bethe
Phys. Rev. 55, 103
(issue of January 1939)
|Energy Production in Stars
H. A. Bethe
Phys. Rev. 55, 434
(issue of March 1939)
Definitely worth seeing –
Hydrogen Core Convection in a 15 Solar Mass Main Sequence Star
Especially this one –
- images: (radial velocity)
AND This – which is amazing – there is a strange moment in it about the first quarter to one third of the way into it. (my note).
Convection_Ra10.mpg (Warning 32 MB)This is a movie of the entropy in 2d convection with periodic side boundaris and isentropic top and bottom boundaries. The Rayleigh number in this calculation is 1.d10 and the prandtl number is 0.1, clearly in the turbulent regime. In this calculation the small scale structure cascades to the large scale “roll” as expected in 2d.
Fusion Chemistry – A Closer Look
The composition of the sun can be described in several ways. By modern estimates, the composition by mass is: 71% H, 27% He, and 2% other heavier elements. By number of atoms of a given type, the sun’s composition is: 91% H, 9% He, and 0.1% other heavier elements. Hydrogen can mean either H atoms or H2 molecules and context is needed to make the meaning of the word clear.
In the sun’s core neither hydrogen molecules nor neutral hydrogen atoms with one proton in the nucleus and one orbital electron are present. The violent, hot environment of the sun’s center rips atoms apart into their constituent pieces: protons, electrons, and other bare atomic nuclei. Hydrogen in the sun’s core is ionized, a bare proton, represented by the symbol p+. It is these protons that fuse together with the release of energy.
The scenario outlined above is called the proton-proton chain. It is the most important process for producing the sun’s energy, although it is not the only set of reactions that occur.
Other fusion reactions
At even higher temperatures inside the sun and other aging stars, other nuclei undergo fusion reactions. These reactions occur in layers, with the higher temperature layers closer to the center. Some examples are given in the table below.
(see chart and full explanation on the link below – amazing explanation of the Standard Solar Model)