Control console for Berkeley 60-inch cyclotron; object ID no. N-10006
This object is the second control console for the Berkeley 60-inch cyclotron. Built in late summer or early fall of 1944, it consists of a blond wood desk unit with numerous controls (lights, switches, buttons, and gauges). Its estimated weight is 600 lbs. The object has 6 separate components: 1) the console proper; 2) a rectangular black instrument cover, marked "D2" and "Arc Sup"; 3) a black cylindrical variable resistor ("Model A Helipot", 30K ohms); 4) a shiny metal support for microphone ("Shure Dynamic Microphone Model 508B"), similar to the one in place on the console; 5) an electrical component, perhaps a high current resistor, gray-green with white porcelain core and metal band with Westinghouse symbol; 6) miscellaneous screws, nuts and washers, in a paper bag. Components 3) through 6) are stored in the lower right-hand drawer of the console 1).
N-10006 is one of five of the whole parts in accession no. 313045, objects from the Berkeley 60" cyclotron. The other four parts are: N-10001, cyclotron beam deflector; N-10002, ion source gas inlet anode; N-10003, two ion source hot cathode probes; N-10004, set of ion source cones (~35 employed with N-10002 and N-10003).
A cyclotron is a machine used to accelerate charged particles to high energies. The first cyclotron was built by Ernest Orlando Lawrence and his graduate student, M. Stanley Livingston, at the University of California, Berkeley, in the early 1930's. Charged particles accelerate outwards from the center of a circular chamber along a spiral path. The particles are held to a spiral trajectory by a static magnetic field and accelerated by a rapidly varying (radio frequency) electric field. In order to produce beams with larger particle energies, Lawrence built a series of cyclotrons of ever increasing size, denoted by the diameter of the circular-shaped accelerating chamber. The 60-inch cyclotron, with its 220 ton magnet, was the fifth in that series, which culminated in 1946 with the Berkeley 184-inch cyclotron and its 4000 ton magnet. For a brief history of Lawrence and the development of the cyclotron see:
The first control console for the Berkeley 60-inch cyclotron was built in 1939, when the machine began operations; the second was built for use after a general renovation of the machine, with operations dating from 1945 dedicated exclusively to biomedical research. Although both control consoles reflect “a well-engineered machine” – in contrast to its predecessor, the 27-inch cyclotron – they differ significantly in style. The original was severely industrial, whereas the second resembles a finished piece of furniture in the blond-wood, ultra-modern style of that period.
Professor John R. Dunning (1907-1975) and his team constructed a similar cyclotron starting in 1936 in the basement of the Pupin Physics Laboratories building at Columbia University in New York City. In March 1940, the team used 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 core component of Dunning’s “Columbia University Cyclotron” is also in the Modern Physics Collection of the National Museum of American History (see object ID no. 1978.10874.01).
Basic principles of the cyclotron
The cyclotron is the simplest of circular particle accelerators. (Go to https://www.physics.rutgers.edu/cyclotron/theory_of_oper.shtml to see a diagram of a typical cyclotron.) At its center is a vacuum chamber which is placed between the pole pieces of a large electromagnet. Within the chamber is a pair “dees” - two flat D-shaped hollow metallic shells - positioned back-to-back forming a cylindrical space, with a uniform gap between the straight sides of the two dees. The plane of the dees is parallel to the faces of the magnet pole pieces. An alternating voltage is applied across the gap between the dees, creating an associated time-varying electric field in that space.
Electrically charged particles, such as protons or alpha particles, are introduced into the chamber from an ion source at the center. The charged particles are constrained to travel in a circular path inside the dees in a plane perpendicular to the direction of the static uniform magnetic field produced by the electromagnet. The electric field accelerates the particles across the gap between the dees. The electric field is made to alternate with the “cyclotron period” of the particle (determined by magnetic field strength and the particle’s mass and charge). Thus, when the particles complete a semi-circle and arrive at the gap again, the electric field has reversed, so that the particles are again accelerated across the gap. Due to their increased speed in the constant magnetic field, the particles now move in a larger circle.
The increasing speed of the particles causes them to move in a larger radius with each half-rotation, resulting in a spiral path outward from the center to the outer rim of the dees. When they reach the rim the particles are pulled out by a deflecting electrode, and hit a target located at the exit point at the rim of the chamber, or leave the cyclotron through an evacuated beam tube to hit a remote target. Nuclear reactions due to the collisions of the particle beam and the target atoms will create secondary particles which may be guided outside of the cyclotron and into instruments for analysis.
Although the radius of the particle’s radius increases with its speed (energy), it can be decreased with a higher magnetic field strength. In a given cyclotron, the limit to the energy for a given type of particle is set by the strength of the magnetic field and the diameter of the dees, which is in turn determined by the diameter of the magnet pole pieces. Thus very large powerful magnets were constructed for cyclotrons. However, due to relativistic effects, as particles approach the speed of light, their relativistic mass increases. Thus, the classical cyclotron is capable of accelerating particles up to only a few percent of the speed of light. To achieve higher particle energies, later generation cyclotrons used either modifications to the frequency of the electric field (as in the “synchrocyclotron”), or modifications to the magnetic field during acceleration (as in the “isochronous cyclotron”).
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. For an example of the powerful electromagnets used in modern synchrotrons for high energy physics research, see object id no. 2012.0186.01, Sections of Magnets for Superconducting Super Collider, presented on the SI collections website. (Search for “Sections of Magnets for Superconducting Super Collider” at http://collections.si.edu/search/)
Uses of cyclotrons.
For several decades, cyclotrons were the best source of high-energy beams for nuclear physics experiments; several cyclotrons are still in use for this type of research. They enable the determination of various properties, such as the mean spacing between atoms, and the creation of various collision products. Subsequent chemical and particle analysis of the target material may give insight into nuclear transmutation of the elements used in the target.
In medicine, cyclotrons and synchrotrons can be used in particle therapy to treat cancer. Ion beams from these accelerators can be used, as in proton therapy, to penetrate the body and kill tumors by radiation damage, while minimizing damage to healthy tissue along their path. The particle beams can be used to bombard other atoms to produce short-lived positron-emitting isotopes suitable for PET imaging.
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