McMillan co-invented thesynchrotron withVladimir Veksler, and after the war he returned to the Berkeley Radiation Laboratory to build them. He was appointed associate director of the Radiation Laboratory in 1954 and promoted to deputy director in 1958. He became director upon the death of lab founderErnest Lawrence later that year, and remained director until his retirement in 1973.
McMillan was born inRedondo Beach, California, on September 18, 1907, the son of Edwin Harbaugh McMillan and his wife Anna Marie McMillan née Mattison.[1] He had a younger sister, Catherine Helen, whose sonJohn Clauser (that is, McMillan's nephew) won theNobel Prize in Physics in 2022.
McMillan's father was aphysician, as was his father's twin brother, and three of his mother's brothers. On October 18, 1908, the family moved toPasadena, California, where he attended McKinley Elementary School from 1913 to 1918, Grant School from 1918 to 1920, and thenPasadena High School, from which he graduated in 1924.[2]
California Institute of Technology (Caltech) was only a mile from his home, and he attended some public lectures there.[3] He entered Caltech in 1924. He did a research project withLinus Pauling as an undergraduate and received hisBachelor of Sciencedegree in 1928 and hisMaster of Science degree in 1929,[1] writing an unpublished thesis on "An improved method for the determination of the radium content of rocks".[4] He then took hisDoctor of Philosophy fromPrinceton University in 1933, writing his thesis on the "Deflection of a Beam of HCI Molecules in a Non-Homogeneous Electric Field" under the supervision ofEdward Condon.[5][6]
The main focus of the Radiation laboratory at this time was the development of thecyclotron, and McMillan, who was appointed to the faculty at Berkeley as an instructor in 1935, soon became involved in the effort. His skill with instrumentation came to the fore, and he contributed improvements to the cyclotron. In particular, he helped develop the process of "shimming", adjusting the cyclotron to produce a homogeneous magnetic field.[6] Working withM. Stanley Livingston, he discoveredoxygen-15, anisotope of oxygen that emitspositrons. To produce it, they bombardednitrogen gas withdeuterons. This was mixed withhydrogen and oxygen to produce water, which was then collected withhygroscopiccalcium chloride. Radioactivity was found concentrated in it, proving that it was in the oxygen. This was followed by an investigation of the absorption ofgamma rays produced by bombardingfluorine with protons.[8]
In 1935, McMillan, Lawrence and Robert Thornton carried out cyclotron experiments with deuteron beams that produced a series of unexpected results. Deuterons fused with a targetnuclei, transmuting the target to a heavier isotope while ejecting a proton. Their experiments indicated a nuclear interaction at lower energies than would be expected from a simple calculation of theCoulomb barrier between a deuteron and a target nucleus. Berkeley theoretical physicistRobert Oppenheimer and his graduate studentMelba Phillips developed theOppenheimer–Phillips process to explain the phenomenon.[9] McMillan became anassistant professor in 1936, and anassociate professor in 1941.[1] WithSamuel Ruben, he also discovered the isotopeberyllium-10 in 1940.[6] This was both interesting and difficult to isolate due to its extraordinarily longhalf-life, about 1.39 million years.[10]
Following the discovery ofnuclear fission inuranium byOtto Hahn andFritz Strassmann in 1939, McMillan began experimenting with uranium. He bombarded it withneutrons produced in the Radiation Laboratory's 37-inch (94 cm) cyclotron through bombardingberyllium with deuterons. In addition to thenuclear fission products reported by Hahn and Strassmann, they detected two unusual radioactive isotopes, one with a half-life of about 2.3 days, and the other with one of around 23 minutes. McMillan identified the short-lived isotope asuranium-239, which had been reported by Hahn and Strassmann. McMillan suspected that the other was an isotope of a new, undiscovered element, with anatomic number of 93.[11]
At the time it was believed that element 93 would have similar chemistry torhenium, so he began working withEmilio Segrè, an expert on that element from his discovery of itshomologtechnetium. Both scientists began their work using the prevailing theory, but Segrè rapidly determined that McMillan's sample was not at all similar to rhenium. Instead, when he reacted it withhydrogen fluoride (HF) with a strongoxidizing agent present, it behaved like members of therare-earth elements.[12] Since these comprise a large percentage of fission products, Segrè and McMillan decided that the half-life must have been simply another fission product, titling the article "An Unsuccessful Search for Transuranium Elements".[13]
McMillan realized that his 1939 work with Segrè had failed to test the chemical reactions of the radioactive source with sufficient rigor. In a new experiment, McMillan tried subjecting the unknown substance to HF in the presence of areducing agent, something he had not done before. This reaction resulted in the sampleprecipitating with the HF, an action that definitively ruled out the possibility that the unknown substance was a rare earth. In May 1940,Philip Abelson from theCarnegie Institute inWashington, DC, who had independently also attempted to separate the isotope with the 2.3-day half-life, visited Berkeley for a short vacation, and they began to collaborate. Abelson observed that the isotope with the 2.3-day half-life did not have chemistry like any known element, but was more similar to uranium than a rare earth. This allowed the source to be isolated and later, in 1945, led to the classification of theactinide series. As a final step, McMillan and Abelson prepared a much larger sample of bombarded uranium that had a prominent 23-minute half-life from239U and demonstrated conclusively that the unknown 2.3-day half-life increased in strength in concert with a decrease in the 23-minute activity through the following reaction:
This proved that the unknown radioactive source originated from the decay of uranium and, coupled with the previous observation that the source was different chemically from all known elements, proved beyond all doubt that a new element had been discovered. McMillan and Abelson published their results in an article entitledRadioactive Element 93 in thePhysical Review on May 27, 1940.[12][14] They did not propose a name for the element in the article, but they soon decided on "neptunium", since uranium had been named after the planetUranus, andNeptune is the next planet beyond in theSolar System.[15] McMillan suddenly departed for war-related work at this point, leavingGlenn Seaborg to pursue this line of research and discover the second transuranium element,plutonium. In 1951, McMillan shared theNobel Prize in Chemistry with Seaborg "for their discoveries in the chemistry of the transuranium elements".[16]
McMillan joined theNavy Radio and Sound Laboratory nearSan Diego in August 1941. There he worked on a device called a polyscope. The idea, which came from Lawrence, was to usesonar to build up a visual image of the surrounding water. This proved to be far more difficult than doing so with radar, because of objects in the water and variations in water temperature that caused variations in the speed of sound. The polyscope proved to be impractical, and was abandoned. He also, however, developed a sonar training device for submariners, for which he received a patent.[17][21][15]
Oppenheimer recruited McMillan to join theManhattan Project, the wartime effort to createatomic bombs, in September 1942. Initially, he commuted back and forth between San Diego, where his family was, and Berkeley.[17] In November he accompanied Oppenheimer on a trip toNew Mexico on which theLos Alamos Ranch School was selected as the site of the project's weapons research laboratory, which became theLos Alamos Laboratory.[22] With Oppenheimer andJohn H. Manley, he drew up the specifications for the new laboratory's technical buildings.[23] He recruited personnel for the laboratory, includingRichard Feynman andRobert R. Wilson, established the test area known as the Anchor Ranch, and scoured the country for technical equipment from machine tools to a cyclotron.[24]
As the laboratory took shape, McMillan became deputy head of thegun-type nuclear weapon effort under NavyCaptainWilliam S. Parsons, an ordnance expert.[24] The plutonium gun, codenamedThin Man,[25] needed amuzzle velocity of at least 3,000 feet (910 m) per second, which they hoped to achieve with a modified Navy3-inch antiaircraft gun. The alternative was to build animplosion-type nuclear weapon. McMillan took an early interest in this, watching tests of this concept conducted bySeth Neddermeyer. The results were not encouraging. Simple explosions resulted in distorted shapes.[26]John von Neumann looked at the implosion program in September 1943, and proposed a radical solution involvingexplosive lenses. This would require expertise in explosives, and McMillan urged Oppenheimer to bring inGeorge Kistiakowsky.[27] Kistiakowsky joined the laboratory on February 16, 1944, and Parsons's E (Explosives) Division was divided in two, with McMillan as deputy for the gun and Kistiakowsky as deputy for implosion.[28]
McMillan heard disturbing news in April 1944, and drove out to Pajarito Canyon to confer with Segrè. Segrè's group had tested samples of plutonium bred in the Manhattan Project's nuclear reactors and found that it contained quantities ofplutonium-240, an isotope that caused spontaneous fission, making Thin Man impractical.[29] In July 1944, Oppenheimer reorganised the laboratory to make an all-out effort on implosion. McMillan remained in charge of the gun-type weapon,[30] which would now be used only withuranium-235. This being the case, Thin Man was replaced by a new, scaled-back design calledLittle Boy.[31] McMillan was also involved with the implosion as the head of the G-3 Group within the G (Gadget) Division, which was responsible for obtaining measurements and timings on implosion,[32] and served as the laboratory's liaison withProject Camel, the aerial test program being carried out by Caltech. On July 16, 1945, he was present at theTrinity nuclear test, when the first implosion bomb was successfully detonated.[33]
In June 1945, McMillan's thoughts began to return to cyclotrons. Over time they had gotten larger and larger. A 184-inch cyclotron was under construction at the Radiation Laboratory, but he realised that a more efficient use could be made of the energy used to accelerate particles. By varying the magnetic field used, the particles could be made to move in stable orbits, and higher energies achieved with the same energy input. He dubbed this the "phase stability principle", and the new design a "synchrotron".[34][35] Unknown to McMillan, the synchrotron principle had already been invented byVladimir Veksler, who had published his proposal in 1944.[36] McMillan became aware of Veksler's paper in October 1945.[17] The two began corresponding, and eventually became friends. In 1963 they shared theAtoms for Peace Award for the invention of the synchrotron.[37] In 1964, McMillan received the Golden Plate Award of theAmerican Academy of Achievement.[38]
The phase stability principle was tested with the old 37-inch cyclotron at Berkeley after McMillan returned to the Radiation Laboratory in September 1945. When it was found to work, the 184-inch cyclotron was similarly modified.[34][17] He became a full professor in 1946. In 1954 he was appointed associate director of the Radiation Laboratory. He was promoted to deputy director in 1958. On the death of Lawrence that year, he became director, and he stayed in that position until his retirement in 1973. The laboratory was renamed the Lawrence Radiation Laboratory in 1958. In 1970, it split into the Lawrence Berkeley Laboratory and the Lawrence Livermore Laboratory, and McMillan became director of the former.[1][37][39]
McMillan suffered the first of a series of strokes in 1984.[37] He died at his home inEl Cerrito, California, from complications from diabetes on September 7, 1991. He was survived by his wife and three children.[20] His gold Nobel Prize medal is in theNational Museum of American History, a division ofThe Smithsonian, in Washington DC.[44]
Seaborg, Glenn (1993). "Biographical Memoirs: Edwin Mattison McMillan (18 September 1907 – 7 September 1991)".Proceedings of the American Philosophical Society.137 (2):286–291.JSTOR986736.
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