Background on Nier Mass Spectrograph; object id no. 1990.0446.01; catalog no. N-09567
This object consists of the following three components: ion source with oven and acceleration electrode; semicircular glass vacuum chamber; ion collector with two plates. The original device included an electromagnet, which is not part of this accession.
In 1939, as political tensions in Europe increased, American physicists learned of an astonishing discovery: the nucleus of the uranium atom can be split, causing the release of an immense amount of energy. Given the prospects of war, the discovery was just as worrying as it was intellectually exciting. Could the Germans use it to develop an atomic bomb?
The Americans realized that they had to determine whether a bomb was physically possible. Uranium consists mostly of the isotope U-238, with less than 1% of U-235. Theoreticians predicted that it was the nuclei of the rare U-235 isotope that undergo fission, the U-238 being inactive. To test this prediction, it was necessary to separate the two isotopes, but it would be difficult to do this since they are chemically identical.
Alfred Nier, a young physicist at the University of Minnesota, was one of the few people in the world with the expertise to carry out the separation. He used a physical technique that took advantage of the small difference in mass of the two isotopes. To separate and collect small quantities of them, he employed a mass spectrometer technique that he first developed starting in about 1937 for measurement of relative abundance of isotopes throughout the periodic table. (The basic principles of the mass spectrometer are described below.)
As a measure of the great importance of his work, in October 1939, Nier received a letter from eminent physicist Enrico Fermi, then at Columbia University, expressing great interest in whether, and how, the separation was progressing. Motivated by such urging, by late February 1940, Nier was able to produce two tiny samples of separated U-235 and U-238, which he provided to his collaborators at Columbia University, a team headed by John R. Dunning of Columbia. The Dunning team was using the cyclotron at the University in numerous studies to follow up on the news from Europe the year before on the fission of the uranium atom. In March 1940, with the samples provided by Nier, the team used neutrons produced by a proton beam from the cyclotron to show that it was the comparatively rare uranium-235 isotope that was the most readily fissile component, and not the abundant uranium-238.
The fission prediction was verified. The Nier-Dunning group remarked, "These experiments emphasize the importance of uranium isotope separation on a larger scale for the investigation of chain reaction possibilities in uranium" (reference: A.O. Nier et. al., Phys. Rev. 57, 546 (1940)). This proof that U-235 was the fissile uranium isotope opened the way to the intense U.S. efforts under the Manhattan Project to develop an atomic bomb. (For details, see Nier’s reminiscences of mass spectrometry and The Manhattan Project at: http://pubs.acs.org/doi/pdf/10.1021/ed066p385).
The Dunning cyclotron is also in the Modern Physics Collection (object id no. 1978.1074.01; catalog no. N-09130), and it will be presented on the SI collections website in 2015. (Search for “Dunning Cyclotron” at http://collections.si.edu/search/)
The Nier mass spectrometer used to collect samples of U-235 and U-238 (object id no. 1990.0446.01)
Nier designed an apparatus based on the principle of the mass spectrometer, an instrument that he had been using to measure isotopic abundance ratios throughout the entire periodic table. As in most mass spectrometers of the time, his apparatus produced positive ions by the controlled bombardment of a gas (UBr˅4, generated in a tiny oven) by an electron beam. The ions were drawn from the ionizing region and moved into an analyzer, which used an electromagnet for the separation of the various masses. Usually, the ion currents of the separated masses were measured by means of an electrometer tube amplifier, but in this case the ions simply accumulated on two small metal plates set at the appropriate positions. Nier’s mass spectrometer required that the ions move in a semicircular path in a uniform magnetic field. The mass analyzer tube was accordingly mounted between the poles of an electromagnet that weighed two tons, and required a 5 kW generator with a stabilized output voltage to power it. (The magnet and generator were not collected by the Smithsonian.) The ion source oven, 180-degree analyzer tube, and isotope collection plates are seen in the photos of the Nier apparatus (see accompanying media file images for this object).
Basic principles of the mass spectrometer
When a charged particle, such as an ion, moves in a plane perpendicular to a magnetic field, it follows a circular path. The radius of the particle’s path is proportional to the product of its mass and velocity, and is inversely proportional to the product of its electrical charge and the magnetic field strength. A mass spectrometer consists of three components: an ion source, a mass analyzer, and a detector. The ion source converts a portion of the sample into ions. There is a wide variety of ionization techniques, depending on the phase (solid, liquid, gas) of the sample and the efficiency of various ionization mechanisms for the unknown species. An extraction system removes ions from the sample and gives them a selected velocity. They then pass through the magnetic field (created by an electromagnet) of the mass analyzer. For a given magnetic field strength, the differences in mass-to-charge ratio of the ions result in corresponding differences in the curvature of their circular paths through the mass analyzer. This results in a spatial sorting of the ions exiting the analyzer. The detector records either the charge induced or the current produced when an ion passes by or hits a surface, thus providing data for calculating the abundance and mass of each isotope present in the sample. For a full description with a schematic diagram of a typical mass spectrometer, go to: http://www.chemguide.co.uk/analysis/masspec/howitworks.html
The Nier sector magnet mass spectrometer (not in Smithsonian Modern Physics Collection)
In 1940, during the time that Nier separated the uranium isotopes, he developed a mass spectrometer for routine isotope and gas analysis. An instrument was needed that did not use a 2-ton magnet, or required a 5 kW voltage-stabilized generator for providing the current in the magnet coils. Nier therefore developed the sector magnet spectrometer, in which a 60-degree sector magnet took the place of the much larger one needed to give a 180-degree deflection. The result was that a magnet weighing a few hundred pounds, and powered by several automobile storage batteries, took the place of the significantly larger and heavier magnet which required a multi-kW generator. Quoting Nier, “The analyzer makes use of the well-known theorem that if ions are sent into a homogeneous magnetic field between two V-shaped poles there is a focusing action, provided the source, apex of the V, and the collector lie along a straight line” (reference: A.O. Nier, Rev. Sci. Instr., 11, 212, (1940)). This design was to become the prototype for all subsequent magnetic deflection instruments, including hundreds used in the Manhattan Project.
associated person: Nier, Alfred O.
maker: Nier, Alfred O.
ID Number: 1990.0446.01
accession number: 1990.0446
catalog number: 1990.0446.01
Date made: ca 1940-02
Physical Description: glass; metal (overall material)
Measurements: semicircular vacuum chamber: 26 7/16 in x 14 in; 67.15125 cm x 35.56 cm
Measurements: ion source: 12 1/4 in x 1 5/8 in; 31.115 cm x 4.1275 cm
Measurements: ion collector: 7 1/4 in x 1 1/8 in; 18.415 cm x 2.8575 cm
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Credit Line: Board of Regents, University of Minnesota
place made: United States: Minnesota, Minneapolis