Howard Ecker, Ray Hopper, and Alfred O.C. Nier examine a 60 degrees mass spectrometer that was the prototype for the Consolidated-Nier commercial instrument. Photograph courtesy of the University of Minnesota Archives, University of Minnesota - Twin Cities.
Popular descriptions of the Manhattan Project cited a world at war: a nation in a race with the Axis Powers to create the ultimate weapon—a scientific achievement capable of mass destruction and perhaps peace. World War II saw American science at its finest, with the brightest minds working together to solve a worldwide crisis and win a war. The Manhattan Project today is synonymous with “the bomb,” the atomic weapon the United States created and dropped on Japan, ending the war in the Pacific Theater. But the Manhattan Project wasn’t just physics and Robert Oppenheimer: it was chemistry, it was engineering, and, unknown to most, it was mass spectrometry.
Creating an atomic weapon from rare isotopes of uranium—isotopes that were theoretically supposed to undergo fission but in practice hadn’t been shown to do so—required knowing exactly which isotope to use. Only U-235 could be split, releasing the energy required for a massive atomic bomb. And since U-235 was significantly rarer than U-238, which could not undergo nuclear chain fission, identifying isotopes through mass determination became even more important. Detecting the mass difference between these two isotopes required incredibly precise measurement and instrumentation because they were chemically identical and had almost the same mass.
The Manhattan Project united disparate scientific disciplines—physics, engineering, chemistry, and the then niche field of mass spectrometry. The project, begun in 1939 and expanding dramatically after the United States officially entered into World War II, centered on harnessing the exponential potential of fission chain reaction. This potential could provide the means for a powerful weapon. But to make a nuclear weapon a reality, an intensive scientific effort was necessary. Experts from each discipline were essential to complete the project. The mass-spectrometry community was needed as well: physicists were early adopters of mass spectrometry; engineers were necessary in both the adoption and the building of early prototype instruments; and chemists increasingly became users of the technique. As this new group of innovators in mass spectrometry adapted the technology to their needs, disciplinary lines would become blurred, and mass spectrometry would become a more prevalent tool in the scientific community.
A New Instrument and New Possibilities for Uranium
Harold Urey. CHF Collections.
The process and instruments involved in mass spectrometry required knowledgeable scientists who could make sense of both the instrument and the isotopes. For the first few decades of the 20th century groups of physicists used early mass spectrometers in their research. A few chemists too began using the technique. Mildred Cohn, who worked with Harold C. Urey, had used mass spectrometry at Columbia University during the 1930s in research involving the study of isotopes in reaction mechanisms. Urey had built a mass spectrometer before the war (with the assistance of one of Alfred Nier’s students from the University of Minnesota), but by war’s end Cohn had built her own. The work done with mass spectrometers would prove to be an invaluable war resource, one that many physicists and chemists wanted to continue using. But only a small group of individuals was poised to use the technology in meaningful ways.
By the early 1940s mass spectrometers were still a mystery to many chemists, even those who would end up having to work with one during the war era:
As far as I was aware, all I knew about it was that it was a big black box. I had a vague sort of notion of what was inside that box, because I think during an introductory physics course, some years earlier, my textbook had, I think, one paragraph about mass spectrometry. It was something I had encountered before, but I really knew nothing about it. ( Meyerson, 10)
Although mass spectrometry was new, physicists and a growing number of chemists knew that it worked well for difficult isotopic and structural analyses. Through its use in the Manhattan Project mass spectrometry moved from being an unknown, mysterious, and highly technical tool used only by physicists to a widely accepted tool within the scientific community. With the Manhattan Project needing such precise isotopic measurements, it was unsurprising that this new measurement method was the answer.