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.
Alfred Nier was at the forefront of mass spectrometry as it was used in the Manhattan Project. A gifted young physicist who earned his Ph.D. in 1936, Nier had already been the source for dozens of scientists in need of a precise mass-measuring device. Nier’s instrument expertise and groundbreaking work extended to the study and measurement of isotopes; by the 1930s Nier had been involved with measuring the mass of uranium isotopes:
There are actually two uranium series. There’s the U-238 that decays to U-206, and the U-235 that decays to U-207, each at a different rate. So you have like two hourglasses, running at the same time, see? So, if you measured the isotopes of lead accurately, and you knew the isotopic composition of uranium accurately, you could then determine and compare the ages by the two methods. But one didn’t know the relative abundances of the isotopes of uranium accurately. [Francis W.] Aston had observed the isotopes on his photographic plates, and showed that for U-235 there was a little smudge on the plate. But that’s as far as he got. So people had guessed at the relative abundances of the uranium isotopes, but I think they were off by a factor of three or some amount like that. It was realized then that we could now accurately measure the uranium isotopes. (Nier, 60)
Nier’s work in isotopes did not go unnoticed by the scientific community. With rumors swirling about what nuclear fission could do—and how devastating the results could be if that power were to fall into the wrong hands—accurate data about isotopes was necessary. The buzz surrounded fission and not Nier’s work specifically, but it led to a serendipitous meeting that would prove necessary for the Manhattan Project to begin. In 1939 Nier met physicist Enrico Fermi at an American Physical Society meeting:
Hear Alfred Nier:
NIER: But when my paper came on, everybody walked out, except the chairman, my wife, and somebody else. And I remember the chairman of the session was Ed Condon, who I knew already, as I said, the man who was later going to be the Director of the Bureau of Standards. So I had this small courtesy group who listened to me tell about the isotopes of iron and nickel at the American Physical Society meeting. So that was the crazy thing that happened there. However, that was the meeting right after nuclear fission was discovered and where I met [Enrico] Fermi. I knew John Dunning already, who was the man in charge of the Columbia cyclotron, and was interested in nuclear physics, and through him, I met Fermi at that meeting.
GRAYSON: At that meeting?
NIER: At that meeting. That was April of 1939, and fission had just been discovered a few months before. It was one of the things that was talked about a lot at the meeting. And that’s when I got acquainted with Fermi. So that was a positive thing that came out of the meeting. [...] Dunning had figured out that, if I just souped up the spectrometer a little bit, I could collect enough separated isotopes of uranium to make possible a determination of the fissionable isotope. He knew how much uranium it would take to detect fission if they bombarded my samples with neutrons. I don’t remember the exact conversation, but he pointed out that if I could collect some fraction of uranium-235, they ought to be able to verify it was fissionable nuclide. (Nier, 74)
Consequently, in 1940 Fermi asked Nier and his lab to provide a small sample of U-235 for John R. Dunning at Columbia University. Fermi and Dunning thought that U-235 would be the best bet for fission, but neither could be certain until a pure sample could be procured. Nier was able to produce such a sample by using a mass spectrometer he had created earlier, which allowed Dunning to demonstrate that the isotope U-235 could undergo fission. With that discovery Nier was put to the task of separating uranium using mass spectrometry.
Alfred O.C. Nier with a 180 degrees Mass Spectrometer Tube constructed during his fellowship at Harvard University, 1937-38. Photo taken during his 1989 oral history interview, CHF Collections.
The U.S. government and the Manhattan Project leaders knew they needed massive quantities of the isotope to fuel a bomb, but they were as yet unsure what method could enrich uranium quickly. There were several possibilities—by centrifuge, gaseous diffusion, gas centrifuge, electromagnetic isotope separation (a hybrid instrument consisting of a mass spectrometer and a cyclotron), or thermal diffusion. The centrifuge method was abandoned in November 1942, but other methods showed promise. Nier himself worked on ways to capitalize on the functionality of the mass spectrometer for such a task, but Dunning and his group, ironically using data from Nier, proved that the gaseous-diffusion method worked best. With Nier’s help the United States was able to achieve uranium enrichment on a large scale.
Nier’s pioneering efforts in mass spectrometry meant that his lab was uniquely qualified for Manhattan Project work. Nier himself noted that his combination of experience and knowledge made him one of the only people in the entire nation who would be up to the task:
You know, nobody’s indispensable, but there wasn’t anybody else in the world who had the experience I had. It was certainly the right place to be, for me to be, because I had the background. Keep in mind, there weren’t many mass spectrometers in the world at that time. We were the only people who could even make measurements of these kinds. True, there were people coming along. Consolidated was manufacturing instruments, 180-degree instruments, which were used in the oil industry. They sold them to big oil companies, where they could do routine analyses of hydrocarbon mixtures in their plants in 1 percent of the time needed by the old methods of analysis. So there were spectrometers available. But the companies that made them . . . you were supposed to use them in a certain way. It’s just like when you buy an instrument now. Unless you use it the way it’s made for, it isn’t too useful. There weren’t many people who had the flexibility that we had, in that if a new problem came up, I said, “Sure, we’ll go home and try it,” and next week, we’d probably have an answer. And that’s the way we lived during that time. There certainly were many clever people who could have done the same thing, but they didn’t have the mass-spectrometry experience. That was really the unique thing that we had that other people didn’t have, was the combination of experience and ability to develop new instrumentation. (Nier, 121–122)
Several Nier mass-spectrometry instruments were employed at various Manhattan Project labs across the United States. In fact, most of the mass spectrographs used for monitoring the separation of uranium during the Manhattan Project were Nier models.
Until the middle of 1942 we made all of their isotope analyses. I wish I still had the telegram which I got from Gene Booth after they had sent us some critical samples. They never told us which samples were critical so as not to prejudice us. I got this wonderful telegram from Gene saying that either I could read minds or we did a good job. It was to tell me that everything was as it was supposed to be. The measurement confirmed that the diffusion method was performing as hoped for and could be developed further. We had built four machines. I sent two to Columbia and two to Virginia. (Nier, 98)