Initially, Ramsey was in Rabi's magnetron group. When Rabi became a division head, Ramsey became the group leader.[6] The role of the group was to develop the magnetron to permit a reduction inwavelength from 150 centimetres (59 in) to 10 centimetres (3.9 in), and then to 3 centimetres (1.2 in) orX band. Microwave radar promised to be small, lighter and more efficient than older types.[13] Ramsey's group started with the design produced by Oliphant's team in Britain and attempted to improve it. The Radiation Laboratory produced the designs, which were prototyped byRaytheon, and then tested by the laboratory. In June 1941, Ramsey travelled to Britain, where he met with Oliphant, and the two exchanged ideas. He brought back some British components, which were incorporated into the final design. Anight fighter aircraft, theNorthrop P-61 Black Widow, was designed around the new radar. Ramsey returned to Washington in late 1942 as an adviser on the use of the new 3 cm microwave radar sets that were now coming into service,[7] working forEdward L. Bowles in the office of theSecretary of War,Henry L. Stimson.[6]
In 1943, Ramsey was approached byRobert Oppenheimer andRobert Bacher, who asked him to join theManhattan Project. Ramsey agreed to do so, but the intervention of the project director, Brigadier GeneralLeslie R. Groves Jr., was necessary in order to prise him away from the Secretary of War's office. A compromise was agreed to, whereby Ramsey remained on the payroll of the Secretary of War and was merely seconded to the Manhattan Project.[6][14][15] In October 1943, Group E-7 of the Ordnance Division was created at theLos Alamos Laboratory with Ramsey as group leader, with the task of integrating the design and delivery of thenuclear weapons being built by the laboratory.[15]
The first thing he had to do was determine the characteristics of the aircraft that would be used. There were only twoAllied aircraft large enough: the BritishAvro Lancaster and the USBoeing B-29 Superfortress.[15] The United States Army Air Forces (USAAF) wanted to use the B-29 if at all possible, even though it required substantial modification.[16] Ramsey supervised the test drop program, which began atDahlgren, Virginia, in August 1943, before moving toMuroc Dry Lake, California, in March 1944. Mock-ups ofThin Man andFat Man bombs were dropped and tracked by anSCR-584 ground-based radar set of the kind that Ramsey had helped develop at the Radiation laboratory. Numerous problems were discovered with the bombs and the aircraft modifications, and corrected.[17]
Ramsey's Los Alamos badge
Plans for the delivery of the weapons in combat were assigned to the Weapons Committee, which was chaired by Ramsey and answerable toCaptainWilliam S. Parsons.[18] Ramsey drew up tables of organization and equipment for theProject Alberta detachment that would accompany the USAAF's509th Composite Group toTinian. Ramsey briefed the 509th's commander, Lieutenant ColonelPaul W. Tibbets, on the nature of the mission when the latter assumed command of the 509th.[19] Ramsey went to Tinian with the Project Alberta detachment as Parsons's scientific and technical deputy. He was involved in the assembly of the Fat Man bomb and relayed Parsons's message indicating the success of thebombing of Hiroshima to Groves in Washington, D.C.[20]
At the end of the war, Ramsey returned to Columbia as a professor and research scientist.[1] Rabi and Ramsey picked up where they had left off before the war with their molecular-beam experiments. Ramsey and his first graduate student,William Nierenberg, measured various nuclearmagnetic dipole andelectric quadrupole moments. With Rabi, he helped establish theBrookhaven National Laboratory onLong Island. In 1946, he became the first head of the Physics Department there. His time there was brief, for in 1947, he joined the physics faculty atHarvard University, where he would remain for the next 40 years, except for brief visiting professorships atMiddlebury College,Oxford University,Mt. Holyoke College and theUniversity of Virginia. During the 1950s, he was the first science adviser toNATO and initiated a series of fellowships, grants and summer school programs to train European scientists.[1][6][21]
The Harvard cyclotron during construction in 1948. Shown are Ramsey (left) andLee Davenport (right).
Ramsey's research in the immediate post-war years looked at measuring fundamental properties of atoms and molecules by use of molecular beams. On moving to Harvard, his objective was to carry out accurate molecular-beam magnetic-resonance experiments, based on the techniques developed by Rabi. However, the accuracy of the measurements depended on the uniformity of the magnetic field, and Ramsey found that it was difficult to create sufficiently uniform magnetic fields. He developed the separated oscillatory field method in 1949 as a means of achieving the accuracy he wanted.[1]
Ramsey and his PhD studentDaniel Kleppner developed the atomic-hydrogenmaser, looking to increase the accuracy with which thehyperfine separations of atomichydrogen,deuterium andtritium could be measured, as well as to investigate how much the hyperfine structure was affected by external magnetic and electric fields. He also participated in developing an extremely stable clock based on a hydrogen maser. From 1967 until 2019,the second has been defined based on 9,192,631,770 Hz hyperfine transition of acesium-133 atom; theatomic clock which is used to set this standard is an application of Ramsey's work.[22] He was awarded theNobel Prize in Physics in 1989 "for the invention of the separated oscillatory fields method and its use in the hydrogen maser and other atomic clocks".[23] The Prize was shared withHans Georg Dehmelt andWolfgang Paul.[23]
His first wife, Elinor, died in 1983, after which he married Ellie Welch of Brookline, Massachusetts. Ramsey died on November 4, 2011. He was survived by his wife Ellie, his four daughters from his first marriage, and his stepdaughter and stepson from his second marriage.[7][24]
^Kellogg, J. M. B.; Rabi, I. I.; Ramsey, N. F. Jr.; Zacharias, J. R. (October 1939). "The Magnetic Moment of the Proton and the Deuteron. The Radiofrequency Spectrum of2H in Various Magnetic Fields".Physical Review.56 (8):728–743.Bibcode:1939PhRv...56..728K.doi:10.1103/PhysRev.56.728.
