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Radiopharmaceutical Production. History of Cyclotrons The early years at Berkeley. STOP.
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Radiopharmaceutical Production History of CyclotronsThe early years at Berkeley STOP
In 1929, Dr. Ernest Lawrence came across a German scientific paper, written by R. Wideröe, that described "Kinetic voltage transformation". The apparatus used amounted to a glass tube with an ion source, a small tubular electrode connected to an oscillator, and a deflection plate. This small apparatus proved that RF fields could be used to accelerate particles. Until this time, this was done using a constant high voltage. Lawrence realized that to get acceleration matching that of the current high voltage machines the array of electrodes would be quite long. He theorized that a single electrode could be used if the ion beam was bent into a circular path using a magnetic field. Contents High Voltage Accelerators Ernst Lawrence Linear to Circular Acceleration A little about RF Oscillators Modern Cyclotrons Cyclotron Cartoons History of Cyclotrons STOP
Circular AcceleratorsThe basic mechanics of operation for larger objects
High Voltage Accelerators Rolf Wideröe1929 and the beginnings of high voltage accelerators Wideröe, elaborating a scheme proposed earlier by Gustav Ising in Sweden, sought to use a low potential over and over to accelerate atoms to high energies. In his design a potential of 25,000 volts alternated from positive to negative at radio frequencies. Ions were pulled into a straight cylindrical electrode by a negative potential and then pushed from the other end by a positive potential. One could add more cylinders, each longer than the last to accommodate the increasing speed of the particles, to reach higher energies. R. Wideröe proposed an accelerator by using an alternating voltage across many alternating “gaps.” It was not without a myriad of problems - Focusing of beam - Vacuum leaks - Oscillating high voltages - Again, imagination His professor refused any further work because it was “sure to fail.” - Wideröe still published his idea in Archiv fur Electrotechnic
push pull push + -/+ + + proton + -/+ Accelerator basics Acceleration of Charged Particle using alternating electric fields. Each hollow electrode must be slightly longer than the last to accommodate the increasing velocity.
Ernest Orlando Lawrence In April 1929, UC Berkley’s youngest Physics professor happened across Archiv fur Electrotechnic. Not able to read German very well he just looked at the diagrams and pictures of the journal. Immediately after seeing Wideröes schematic, Ernest fully comprehended it’s implications. He was excited!
Linear to Circular The linear accelerator proved useful for heavy ions like mercury, but lighter projectiles (such as alpha particles) required a vacuum tube many meters long. Lawrence judged that impractical. Instead he thought of bending the particles into a circular path, using a magnetic field, in order to send them through the same electrode repeatedly. A few quick calculations showed that such a device might capitalize on the laws of electrodynamics. The centripetal acceleration of a charged particle in a perpendicular magnetic field B is qvB/c, where q is the charge, v the particle's velocity, and c the velocity of light. The mechanical centrifugal force on the particle is mv2/r, where m is the mass and r the radius of its orbit. Balancing the two forces for a stable orbit yields what is now known as the cyclotron equation: v/r = qB/mc. Lawrence was surprised to find that the frequency of rotation of a particle is independent of the radius of the orbit: f = v/2 r = qB/2 mc, with r disappearing from the equation. The circular method would thus allow an electric field alternating at a constant frequency to kick particles to ever higher energies. As their velocities increased so did the radius of their orbit. Each rotation would take the same amount of time, keeping the particles in step with the alternating field as they spiralled outward.
“R cancels R” Ernest quickly jotted down the following: Fr = mv2/r and FB = qvB thus: r = mv/qB ω= 2πf = v/r substitute: f = qB/2πm R cancels R !
What does this mean ? Ernest Lawrence recognized that the ion’s angular velocity does not depend on the radius. Mother nature was kind to cyclotroneers, for as the particle’s energy (speed) increased, so did it’s orbital path length. For a fixed particle’s q/m and magnetic field, the angular frequency is constant.
Lawrence started to construct a cyclotron, as the machine later was named, in early 1930. A graduate student, M. Stanley Livingston, did much of the work of translating the idea into working hardware. In January 1931 Lawrence and Livingston met their first success. A device about 4.5 inches in diameter used a potential of 1,800 volts to accelerate hydrogen ions up to energies of 80,000 electron volts. Lawrence immediately started planning for a bigger machine. In summer 1931 an eleven-inch cyclotron achieved a million volts. A more complete account of the early history is given on the American Institute of Physics history page Page The First Cyclotron
Making the Cyclotron a Reality It was quickly realized that two hollow, semi-circular electrodes (named DEE’s for their shape) in a strong magnetic field would best serve as the accelerating gap and ion storage.
The 11 inch 1.1 MeVJanuary 1932 Telegram to Lawrence: “Dr. Livingston has asked me to advise you that he has obtained 1,100,000 volt protons. He also suggested that I add ‘Whoopee’!” The 11 MeV cyclotron built by Lawrence and Livingston in 1931 and 1932. It required vacuum seals that could withstand the stresses of the alternating electric field and the magnet. Too poor a vacuum and the circulating particles might be bumped from their paths by air molecules. The particles might also go astray crossing the gap or, what would be the hardest problem, deviate from the horizontal plane of their orbits and crash into inner surfaces of the electrodes.
A Bit About the RF Oscillator An electric field with a frequency of about four million cycles per second lay in the realm of short radio waves. Lawrence's experience with these waves would come in handy, and recent advances in high-power vacuum-tube oscillators would be indispensable. Combined with a reasonable magnetic field, a potential on the electrodes of only ten thousand volts could accelerate an alpha particle to one million electron volts. Bigger magnets promised higher energies. We know that: f = qB/2m The DEE has capacitance C so L is chosen: fr=1/2(LC) For 1p1 & 1 Tesla B-field the fr ~ 15 MHz (RF)
Pace of Development was Unprecedented February 1932 September 1932 Even before the 11-inch was completed, the 27 inch was being designed. Left Photo is of Ernest Lawrence and M.S. Livingston (L to R)
60 inch and 88 inch cyclotrons at Berkeley Livingston Standing Technician working
A Lesson to be Learned (1934) A critical experiment carried out by Irene Curie and Frederic Joliot in their basement lab at the Radium Institute led them to a very significant conclusion in mid-January 1934. By bombarding stable elements with nuclear projectiles, they were the first to discover artificial radioactivity--a normal element was changed to a radioactive one through human intervention. Said Lawrence after the announcement from France, "We have been kicking ourselves that we haven't had the sense to notice that the radiations given off do not stop immediately after turning off the bombarding beam." Although swamped in radioactivity for months, the Berkley Cyclotron “Rad-lab” missed the discovery: “…the Laboratory missed the discovery because the same switch operated the cyclotron and the Geiger counter.” – “We felt like kicking our butts.” [Thornton]
Dee Ion Source Magnet Return Yoke Vacuum Chamber RF Cavity Vacuum Pump Modern Cyclotrons The basic components are still the same The technology has been improved so the cyclotron is much more reliable
