* Radioactivity directly implied the breakdown of atomic elements. This led to the obvious question of whether atomic nuclei could be built up as well.
In 1919, following up a 1915 set of experiments conducted by his student Ernst Marsden, Rutherford publicly announced the results of an exacting set of experiments in radioactive phenomena. He placed a bit of radioactive material at one end of a sealed, evacuated cylinder; the cylinder was coated with zinc sulfide at the other end. The radioactive material produced alpha particles that traveled down the tube, striking the zinc sulfide and emitting "scintillations" of light. The zinc sulfide acted as a primitive type of "scintillation detector". If the cylinder was filled with hydrogen gas, the light scintillations were particularly bright. This was due to simple considerations of momentum: if an alpha particle struck a hydrogen nucleus -- a single proton, effectively four times lighter than an alpha particle -- then the collision would transfer momentum to the proton, causing it to have a velocity up to four times greater than the alpha particle. If the cylinder was filled with oxygen or carbon dioxide, both of which were substantially heavier than alpha particles, the scintillations would be damped. However, if the cylinder was filled with nitrogen, the same bright scintillations would occur. Rutherford guessed that the alpha particles were knocking protons out of the nitrogen nucleus. The collisions resulted in the absorption of an alpha particle into the atomic nucleus and the emission of a proton, resulting in the synthesis of an oxygen-17 atom (atomic number 8) from a nitrogen-14 atom (atomic number 7).
This was confirmed in 1925, when the British physicist Patrick Maynard Stuart Blackett (1897:1974) decided to follow up on Rutherford's experiment to see what was going on in more detail. He used a cloud chamber to observe interactions between alpha particles and nitrogen, taking 20,000 pictures, observing 400,000 alpha particle tracks, and observing a grand total of eight collisions between alpha particles and nitrogen atoms. He would win the Nobel prize in 1948 for this work.
* Rutherford was the first to observe transmutation of the elements through radioactive decay, and also the first to perform transmutation of the elements through "nucleosynthesis". This gave him a double justification for being labeled an alchemist. Many others wanted to get in on the act in his wake. Nucleosynthesis an intriguing notion in itself; as the Latvian-American chemist Aristid V. Grosse suggested in 1932, it also presented the possibility of creating isotopes that weren't actually found in nature.
In 1934, the Joliot-Curies were following up the experiments that had led to the discovery of the neutron, bombarding an aluminum foil target with alpha particles generated from a radioactive source, and moving the source back from the target to investigate the effect of lowering the energy of the particles on neutron emission from the target.
The experiment was fairly cut-and-dried as far as that went, but after the alpha particle source was removed, the target kept on emitting radiation, with the level falling off over a few minutes, as if some short-lived radioactive isotope had been produced by the bombardment. After double-checking their work, they found out that there was indeed a radioactive material in the aluminum foil, with a half-life of 2.6 minutes.
Their analysis suggested that the alpha particles had converted aluminum-27 (atomic number 13) into phosphorus-30 (atomic number 15), ejecting a neutron in the process. In search of proof of this notion, a helpful chemist then showed them how to dissolve the irradiated aluminum in acid, with the phosphorus combining with hydrogen to be drawn off as a gas. The gas was radioactive while the aluminum in acid was not, and the gas turned out to contain phosphorus-30. Phosphorus-30 was radioactive, decaying with the half-life of 2.6 minutes as observed by the Joliot-Curies. Phosphorus-30 was not found in nature. This was because of its short half-life and because it wasn't part of any persistent radioactive decay series: any phosphorus-30 that was created by one process or another would quickly beta-decay into silicon-30 (atomic number 14) and effectively disappear. The experiment was very important, very much making up for the couple's embarrassment at missing the neutron, and they were awarded the Nobel Prize in 1935 for this discovery.
* Other short-lived radioactive isotopes not found in nature were then quickly synthesized. There was a problem with using alpha particles for nucleosynthesis, however: they were positively-charged, and so they were repelled by the positively-charged nucleus of the target atoms. As atomic number increased, the number of positive charges in the nucleus increased. Alpha particle penetration became increasingly difficult, until above a certain atomic number it wouldn't take place at all. Following the discovery of the neutron, Enrico Fermi got to thinking it might be a better tool for nucleosynthesis, since it was neutral and could in principle penetrate any nucleus, no matter how large. The fact that the neutron was uncharged also made producing a stream of them a bit tricky, but Fermi found that he could bombard a block of paraffin with protons and produce the desired stream of neutrons through collisions.
Fermi and his colleagues in Corbino's lab started by bombarding hydrogen with neutrons and gradually worked his way up the periodic table. He got nowhere until 21 March 1934, when he bombarded fluorine with neutrons and ended up with radioactive fluorine. He soon began to synthesize other radioactive isotopes. Initially, Fermi and his research team thought high-energy fast neutrons would be best for their studies, but then they found out that they got better results if they placed the neutron source on a wooden table instead of a marble one. Fermi had a hunch that the wooden table was slowing down the neutrons. He tried using water or paraffin to slow down the neutron stream and found, somewhat to his surprise, that slow neutrons had a greater probability of interacting with nuclei. It was pointed out later that in this particular case, being Italian had paid benefits all by itself: marble tables are relatively common in Italy, a land noted for its marble quarries, fairly rare elsewhere, and in another country the connection might not have been noticed for some time. Of course, since neutrons could penetrate heavy nuclei while alpha particles couldn't, Fermi and his people went on to bombard uranium (atomic number 92) with neutrons. He did seem to get an element with atomic number 93, but the results of the experiment were very confusing, and it would take a while to straighten them out.
* The Italian physicist Emilio Segre (1905:1989), a colleague of Fermi's in Corbino's lab, decided to investigate another angle on the matter. One of the mysteries of science at the time was that there were a number of vacancies in the periodic table of the elements, at atomic numbers of 43, 61, 85, and 87, where no element could actually be found in nature. In 1937, Segre went to the United States where he bombarded molybdenum (atomic number 42) with neutrons, and managed to synthesize element 43. He wasn't quite sure it was a proper element, but after World War II the scientific community came to a consensus that it was, and so Segre gave his element the name "technetium". The reason it wasn't found in nature was obvious in hindsight: it had no stable isotopes. It did have three radioactive isotopes, the longest-lived being technetium-97, with a half-life of 2,600,000 years. That's a long time in human terms but not long by cosmic standards, and any technetium formed by natural processes quickly disappears below the noise level. The other three vacancies in the periodic table had also been discovered by that time. Element 87 was discovered in 1939, to be named "francium" in honor of the nation in was discovered; element 85 was discovered the next year, 1940, to be named "astatine"; and element 61 was discovered in 1947, to be named "promethium". To no surprise, none of these elements had stable isotopes either, and even their most stable isotopes didn't have very long half-lives. Francium-223 has a half-life of 21.8 minutes, astatine-210 has a half-life of 8.1 hours, and promethium-145 has a half-life of 17.7 years,
* That filled in the vacancies in the periodic table, but what about the unfilled entries above the end of the table at uranium, element 92? As mentioned, Fermi had thought he had created element 93 in 1934 but hadn't been sure enough to make the claim that he had. In 1940, two American physicists, Edwin Mattison McMillan (1907:1991) and Philip Hauge Abelson (1913:2004), repeated Fermi's experiment, bombarding uranium with neutrons, and produced element 93. Since the planet Uranus was followed by the planet Neptune, they logically named the next element beyond uranium "neptunium". Neptunium had no stable isotopes -- no element above uranium does -- though its longest-lived isotope, neptunium-237, has a respectable half-life of 2,140,000 years. Incidentally, it also parents a decay series along the lines of uranium and thorium. The half-life of neptunium-237 is still too short for it to persist in nature.
The discovery of neptunium led directly to the next element. McMillan and another American physicist, Glenn T. Seaborg (1912:1999), found that neptunium could perform beta decay, releasing an electron to convert a neutron into a proton and creating element 94. Using the same naming convention of following the order of the planets as they were known at the time, they named the element "plutonium". Its longest-lived isotope is plutonium-244, with a half-life of 82,000,000 years. Seaborg had to keep his discovery secret for the time being; the British had discreetly protested the publication of the discovery of neptunium, since it clearly gave important clues to Nazi scientists, and atomic scientists in the US had agreed to perform self-censorship on their discoveries. This would prove wise, since plutonium would have, to put it mildly, serious military applications.
* Since that time, the list of transuranium elements has been brought up to element 116. The following table lists all transuranium elements known as last notice, along with their dates of discovery. One of the interesting speculations about nuclear physics is that there may be even heavier elements that are relatively stable. Nuclear researchers long ago noticed that nuclei with certain "magic numbers" of protons or neutrons are relatively stable. Such magic numbers correspond to filled nuclear energy shells, analagous to the way that elements with filled electron energy shells are inert. The numbers 2, 8, 20, 28, 50, and 82 are known to be "magic" for both protons and neutrons, and nuclei that have magic numbers for both protons and neutrons are unusually stable, or "doubly magic". The highest known magic number for neutrons is 126. The normal lead atom is doubly magic, with 82 protons and 126 neutrons, making it more stable than any known element higher in the periodic table. The next higher magic proton number may be 114, 120, or 126; the next higher magic neutron number is agreed to be 184. That implies that if nuclear scientists could get over the barriers that block construction of heavier nuclei, they might obtain atoms that are surprisingly stable. For now, however, they remain the stuff of science fiction.
* The discovery of nuclear fission had immediate and dramatic consequences. Leo Szilard (1898:1964) was a Jewish Hungarian physicist who had left Berlin for Britain in 1933, after the Nazis had passed a decree effectively throwing all academics out of their positions. He went on to the US in 1938, signing on with Columbia University in New York City and eventually becoming an American citizen.
In 1933, Ernest Rutherford had described the idea of atomic power as "moonshine". He knew perfectly well that there were enormous energies locked up in the atom, but he couldn't see that there was any way to release those energies except through radioactive decay, which was a stingy process at best. It was an unfortunate comment. Rutherford was a man of enormous accomplishments; his deceptively simple experiments were implemented with a level of care and craftsmanship compared by some physicists to that needed to build a Stradavarius violin, and his lab produced almost a dozen Nobel prize winners. However, the times were beginning to pass him by. Rutherford would die in 1937, of an infection following relatively minor surgery, not living long enough to find out how spectacularly off target he had been in his assessment of the potential of atomic energy.
In 1934, annoyed at a newspaper article that cited Rutherford's "moonshine" remark, Szilard had realized that nuclear fission might be the key to building a super-powerful "atomic bomb" through a "chain reaction": the fission of one atom by neutrons releasing more neutrons to perform fission on other atoms, leading to a cascade that released a vast amount of energy. Chain reactions were not particularly part of physics -- but they were known in chemistry. Szilard contacted the British chemist Chaim Weizmann (1874:1952), a prominent Zionist and later first president of Israel, to see if it could be done, but the experiments went nowhere. Szilard had originally focused on producing a chain reaction in the relatively light element beryllium, which seemed a promising at the time but to no surprise in hindsight wasn't a very good candidate. Lise Meitner's paper on nuclear fission in uranium opened the door. Weizmann's experiments had used energetic fast neutrons to perform fission, but Fermi's work established that slow neutrons worked better, and if the fission released slow neutrons a chain reaction might well be possible. Further research indicated that was the case. Szilard managed to convince his colleagues to keep quiet about research into fission processes, and in 1939 he contacted two other Hungarian physicists in the US, Eugene Paul Wigner (1902:1995) and Edward Teller (1908:2003), to collaborate on the development of the atomic bomb: it might be a terrible weapon, but it would be even more terrible if Hitler got it first. Szilard was a friend of Albert Einstein's, the two having collaborated on the invention of a commercially unsuccessful refrigerator of all things, and Szilard knew that Einstein's prestige would help get a warning on the matter to the highest levels. The three Hungarians visited Einstein and persuaded him to write a letter to US President Franklin Delano Roosevelt to propose that the United States begin a high-priority program to build an atomic bomb. The letter, dated 2 August 1939, was personally delivered to the president by economist Alexander Sachs, who had close access to the president. In response, the US government set up a committee and provided a grant of the princely sum of $6,000 USD for research. The grant would be delayed until early the next year, 1940. By that time, World War II was in full swing, and many European physicists who had fled the Nazis were becoming concerned about the atomic bomb and the idle way the US government was investigating it. Some prominent physicists had remained in Germany, most prominently Werner Heisenberg, and they might well be working on the Bomb for Hitler.
* Enrico Fermi was also at Columbia, having fled Fascist Italy in 1938 when Mussolini began to pick up antisemitism from his new ally Hitler. Fermi's wife was Jewish and he brought her and the children out with him; they all left when he went to Stockholm to be awarded his Nobel prize, and they didn't go back. He and Szilard decided to collaborate on building an "atomic pile" using uranium oxide in which controlled chain reactions could be performed for research purposes. To have a sustained chain reaction, the average number of neutrons ejected from the fission of a uranium atom to cause the fission of another uranium atom had to be greater than 1. In more formal terms, the "reproduction factor k" had to be at least one. Studies had shown that on average, uranium releases 2.5 neutrons per fission event, which put the maximum value of k as 2.5. Although that was much more than needed for a self-sustaining chain reaction, neutrons were invariably lost during the fission process, and so obtaining a chain reaction was not simple. To complicate matters considerably, although it might be hard to start a chain reaction, nuclear processes occur very quickly, and even a reproduction factor slightly greater than one would cause a chain reaction increasing exponentially, cascading out of control in a hurry. Fortunately, the researchers discovered that a small fraction, less than a percent, of the neutrons emitted by the fission process were "delayed" neutrons. These delayed neutrons were emitted by the fission fragments after a certain time delay, instead of being emitted by the fission action itself. The time delay involved was surprisingly long, on the order of ten seconds. This means that if the value of k was between 1 and 1.01, the delayed neutrons decided the balance of the chain reaction, and the reaction was slow enough to be controlled.
The pile was built out of blocks of graphite, since it would act as a "moderator", slowing down neutrons to allow them to more easily produce fission reactions, while not absorbing neutrons, ensuring that the neutron flux was not damped. The graphite had to be very pure, since even small traces of neutron-absorbing materials in the graphite could damp the chain reaction. Prototype piles were assembled at Columbia to test out concepts, Funds remained thin for much of the year, but by October 1941 the US government had increased support, and after the Japanese bombed Pearl Harbor on 7 December 1941 the project began its rise the top of the priority list. Arthur Holly Compton, then at the University of Chicago, was put in charge of the project, with his organization known as the "Metallurgical Laboratory" as a cover. Szilard and Fermi went to Chicago in the spring of 1942 to work in the lab. The Metallurgical Lab built a further series of test piles, with work going on around the clock and the k factor steadily increasing. Finally, late in 1942, the group assembled a pile that would be able to perform a controlled chain reaction. On 2 December 1942, workers carefully pulled out cadmium control rods that had been inserted into the pile to dampen a chain reaction, That afternoon, Fermi announced: "The reaction is self-sustaining." The pile ran for 11 minutes and was shut down. They were now "moonshiners".
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