overall: 5.18 m x 5.24 m x 3.51 m; 17 ft x 17 3/16 ft x 11 1/2 ft
Object Name:
Circular Accelerator, McMillan Synchrotron
circular accelerator, McMillan synchrotron
Description:
Object N-09261.01 consists of an assembly of numerous major subparts or components, most of which were separated during dismantling after the NMAH Atom Smashers exhibition closed.
The major subparts include: coils carrying current to produce magnetic field; laminated steel yoke to provide path for the lines of magnetic field; aluminum clamps and steel tie-bars for clamping magnetic yoke together horizontally; "strong back" for clamping magnet together vertically; toroid-shaped vacuum chamber ("donut") in which the electrons circulate and are accelerated; high vacuum oil diffusion pump for evacuating air from vacuum chamber; assembly for remote positioning of high voltage injector (electron gun); source and transmission line for voltage pulse to injector (electron gun); high voltage transformer (110 kV) of pulse to injector (electron gun); radiation warning light; lead shielding wall to prevent stray radiation from interfering with experiments; magnet to sweep out stray electrons, etc., contaminating the x-ray beam; x-ray beam line; tapered lead plugs for making x-ray beam parallel; electrometer connected to ionization chamber in path of beam (measures beam intensity); crash button; "patch panel" for electronic connections between experiment area and "counting room"; peripheral and/or connecting elements, such as conduit containing cables carrying electric current to upper coil, current-carrying coil for producing magnetic field guide, air blower to cool magnet coils, oscillator to generate 47MHz electric field for accelerating electrons, oil diffusion pump to evacuate acceleration (vacuum) chamber, vacuum-insulated storage vessel ("Dewar") for liquid nitrogen, liquid nitrogen receptacle ("cold trap") to improve vacuum in acceleration chamber by freezing out vapors, target, evacuation port, injector (electron gun), evacuation port, synchrotron light port, RF power input.
Along with the McMillan synchrotron assembly proper N-09261.01, there are 25 related objects in this accession with catalog numbers .02-.26. Mimsy XG catalog records have been created for 13 of the objects in this accession. Additional items: a cross-section of the vacuum chamber and magnet pole pieces are in a separate accession (1978.2302.06) under catalog ID N-10012. There is also a related non-accessioned item N-10022, "Synchrotron Counting Room" sign.
Basic Principles and History
The methods of particle acceleration used before WWII were approaching their limits. The size and cost of cyclotrons and betatrons with ever increasing particle output energy had grown substantially. (See “basic principle of the cyclotron” in Background on Dunning Cyclotron; Object id no. 1978.1074.01 in Modern Physics Collection). For accelerating particles to the highest energies in circular machines, the “synchrotron” was developed in the mid-1940s. In contrast to a cyclotron, particles in a synchrotron are constrained to move in a circle of constant radius by the use of a ring of electromagnets, open in the middle and so much less massive than an equivalent cyclotron magnet. The magnetic field is varied in such a way that the radius of curvature remains constant as the particles gain energy through successive accelerations by a synchronized alternating electric field.
Towards the end of WWII Vladimir Veksler in the USSR and Edwin M. McMillan in the USA independently advanced the following “principle of phase stability”; 1) charged particles forced into a circular path by a magnetic field and accelerated by an oscillating electric field will “bunch” if they lie in the proper phase or “side” of the electric wave; and 2) these particle bunches, if confined in buckets, can be carried to higher energies by gradually increasing the magnetic guide field (as in the electron synchrotron), or by decreasing the oscillation frequency of the electric accelerating field (as in the synchro-cyclotron), or increasing both magnetic field and the electric oscillation frequency (as in the proton synchrotron).
After considerable difficulties and some changes in design, McMillan’s synchrotron began operating in the winter of 1948-49 at its intended energy of about 300 million volts – thought to sufficient to produce subatomic particles called mesons, with a mass between that of the electron and the proton. (In later years mesons would be defined as “hadronic” particles composed of one quark and one antiquark bound together by the strong interaction.) Note: The synchrotron electrons never were brought out of the machine, but were brought into collision with an internal target to produce photons with the bremsstrahlung spectrum; these emerging photon beams (x-rays) would then be used to produce the mesons.
Among the first and most significant experiments performed with this accelerator was production of a new subatomic particle, the “pi-zero” meson. Theoretical physicists had previously predicted the existence of an electrically neutral variety of such particles observed in cosmic rays. In this experiment, J. Steinberger, W.K.H. Panofsky, and J. Steller provided convincing evidence that such charge-neutral mesons were produced by the 330 MeV x-rays emerging from the synchrotron and striking an external target. The experimenters looked for the pair of simultaneous photons into which the unstable meson was expected to decay. They found that the energies of these two photons, and the angle between them, were just what would result from the decay in flight of a particle mass about 150 times that of an electron, moving with a velocity expected for such particles were they created by 300 MeV x-rays. A reproduction of the apparatus for this experiment, made for the NMAH Atom Smashers exhibition, is in the Modern Physics Collection (Object ID no. 1989.3014.01).
Virtually all circular high-energy particle accelerators built since WWII have been based on the principle of phase stability. Although particle energies attainable by pre-War accelerator concepts were sufficient for atom smashing (splitting the nucleus in a target atom), they were not high enough to create the sub-atomic particles that had been discovered in cosmic rays in the 1930’s. With the principle of phase stability, and the ability to build particle accelerators based upon it, the field of “high-energy” or “elementary particle physics” came into existence.
