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A modern Cyclotron for radiation therapy
A modern Cyclotron for radiation therapy
For other uses, see Cyclotron (disambiguation).

A cyclotron is a type of particle accelerator. Cyclotrons accelerate charged particles using a high-frequency, alternating voltage (potential difference). In Physics, a charged particle is a particle with an Electric charge. Frequency is a measure of the number of occurrences of a repeating event per unit Time. Electrical tension (or voltage after its SI unit, the Volt) is the difference of electrical potential between two points of an electrical A perpendicular magnetic field causes the particles to spiral almost in a circle so that they re-encounter the accelerating voltage many times. In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges

Ernest Lawrence, of the University of California, Berkeley, is credited with the invention of the cyclotron in 1929. Ernest Orlando Lawrence ( August 8, 1901 – August 27, 1958) was an American physicist and Nobel Laureate best known for his The University of California Berkeley (also referred to as Cal, Berkeley and UC Berkeley) is a major research university located in Berkeley He used it in experiments that required particles with energy of up to 1 MeV.

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How the cyclotron works

Diagram of cyclotron operation from Lawrence's 1934 patent.
Diagram of cyclotron operation from Lawrence's 1934 patent.

The electrodes shown at the right would be in the vacuum chamber, which is flat, in a narrow gap between the two poles of a large magnet. An electrode is an Electrical conductor used to make contact with a nonmetallic part of a circuit (e vacuum chamber is a rigid enclosure from which air and other gases are removed by a Vacuum pump.

In the cyclotron, a high-frequency alternating voltage applied across the "D" electrodes (also called "dees") alternately attracts and repels charged particles. An alternating current ( AC) is an Electric current whose direction reverses cyclically as opposed to Direct current, whose direction remains constant In Physics, a charged particle is a particle with an Electric charge. The particles, injected near the center of the magnetic field, accelerate only when passing through the gap between the electrodes. The perpendicular magnetic field (passing vertically through the "D" electrodes), combined with the increasing energy of the particles forces the particles to travel in a spiral path. In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges

With no change in energy the charged particles in a magnetic field will follow a circular path. In the cyclotron, energy is applied to the particles as they cross the gap between the dees and so they are accelerated (at the typical sub-relativistic speeds used) and will increase in mass as they approach the speed of light. Either of these effects (increased velocity or increased mass) will increase the radius of the circle and so the path will be a spiral.

(The particles move in a spiral, because a current of electrons or ions, flowing perpendicular to a magnetic field, experiences a perpendicular force. Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. For the related yet different principle relating to electromagnetic coils see Right hand grip rule. In Physics, a force is whatever can cause an object with Mass to Accelerate. The charged particles move freely in a vacuum, so the particles follow a spiral path. )

The radius will increase until the particles hit a target at the perimeter of the vacuum chamber. Various materials may be used for the target, and the collisions will create secondary particles which may be guided outside of the cyclotron and into instruments for analysis. The results will enable the calculation 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.

Uses of the cyclotron

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. Nuclear physics is the field of Physics that studies the building blocks and interactions of Atomic nuclei.

Cyclotrons can be used to treat cancer. Cancer (medical term Malignant Neoplasm) is a class of Diseases in which a group of cells display uncontrolled Ion beams from cyclotrons 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. Proton therapy is a type of Particle therapy which utilizes a beam of Protons to irradiate diseased tissue most often in the treatment of cancer Radiation poisoning, also called " radiation sickness " or a " creeping dose " is a form of damage to organ tissue due to excessive exposure to

Cyclotron beams can be used to bombard other atoms to produce short-lived positron-emitting isotopes suitable for PET imaging. The positrons or antielectron is the Antiparticle or the Antimatter counterpart of the Electron. Positron emission tomography ( PET) is a Nuclear medicine imaging technique which produces a three-dimensional image or map of functional processes in the

Problems solved by the cyclotron

60-inch cyclotron, circa 1939, showing a beam of accelerated ions (likely protons or deuterons) escaping the accelerator and ionizing the surrounding air causing a blue glow. This phenomenon of air ionization is analogous to the one responsible for producing the "blue flash" infamously noted by witnesses of criticality accidents. Though the effect is often mistaken for Cherenkov radiation, this is not the case.
60-inch cyclotron, circa 1939, showing a beam of accelerated ions (likely protons or deuterons) escaping the accelerator and ionizing the surrounding air causing a blue glow. This phenomenon of air ionization is analogous to the one responsible for producing the "blue flash" infamously noted by witnesses of criticality accidents. A criticality accident, sometimes referred to as an excursion or a power excursion, occurs when a Nuclear chain reaction accidentally occurs in Fissile Though the effect is often mistaken for Cherenkov radiation, this is not the case. Čerenkov radiation (also spelled Cerenkov or Cherenkov) is Electromagnetic radiation emitted when a charged particle (such as an

The cyclotron was an improvement over the linear accelerators that were available when it was invented. A linear accelerator (also called a linac) accelerates particles in a straight line through an evacuated tube (or series of such tubes placed end to end). A set of electrodes shaped like flat donuts are arranged inside the length of the tube(s). These are driven by high-power radio waves that continuously switch between positive and negative voltage, causing particles traveling along the center of the tube to accelerate. In the 1920's, it was not possible to get high frequency radio waves at high power, so either the accelerating electrodes had to be far apart to accommodate the low frequency or more stages were required to compensate for the low power at each stage. Either way, higher-energy particles required longer accelerators than scientists could afford.

Modern linacs use high power Klystrons and other devices able to impart much more power at higher frequencies. A klystron is a specialized linear-beam Vacuum tube (evacuated electron tube But before these devices existed, cyclotrons were cheaper than linacs.

Cyclotrons accelerate particles in a spiral path. Therefore, a compact accelerator can contain much more distance than a linear accelerator, with more opportunities to accelerate the particles.

Advantages of the cyclotron

Limitations of the cyclotron

The magnet portion of a large cyclotron. The gray object is the upper pole piece, routing the magnetic field in two loops through a similar part below. The white canisters held conductive coils to generate the magnetic field. The D electrodes are contained in a vacuum chamber that was inserted in the central field gap.
The magnet portion of a large cyclotron. The gray object is the upper pole piece, routing the magnetic field in two loops through a similar part below. The white canisters held conductive coils to generate the magnetic field. The D electrodes are contained in a vacuum chamber that was inserted in the central field gap.

The spiral path of the cyclotron beam can only "synch up" with klystron-type (constant frequency) voltage sources if the accelerated particles are approximately obeying Newton's Laws of Motion. In Mathematics, a spiral is a Curve which emanates from a central point getting progressively farther away as it revolves around the point Newton's laws of motion are three Physical laws which provide relationships between the Forces acting on a body and the motion of the If the particles become fast enough that relativistic effects become important, the beam gets out of phase with the oscillating electric field, and cannot receive any additional acceleration. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial The cyclotron is therefore only capable of accelerating particles up to a few percent of the speed of light. To accommodate increased mass the magnetic field may be modified by appropriately shaping the pole pieces as in the isochronous cyclotrons, operating in a pulsed mode and changing the frequency applied to the dees as in the synchrocyclotrons, either of which is limited by the diminishing cost effectiveness of making larger machines. In Particle accelerators an Isochronous cyclotron is a cyclotron that maintains a constant RF driving frequency and compensates for the relativistic mass gain of the accelerated A synchrocyclotron is a Cyclotron in which the frequency of the driving RF electric field is varied to compensate for the mass gain of the accelerated particles as their velocity Cost limitations have been overcome by employing the more complex synchrotron or linear accelerator, both of which have the advantage of scalability, offering more power within an improved cost structure as the machines are made larger. A synchrotron is a particular type of cyclic Particle accelerator in which the magnetic field (to turn the particles so they circulate and the electric field (to accelerate

Mathematics of the cyclotron

The centripetal force is provided by the transverse magnetic field B, and the force on a particle travelling in a magnetic field (which causes it to be angularly displaced, i. The centripetal force is the external force required to make a body follow a curved path e spiral) is equal to Bqv. So,

\frac{mv^2}{r} = Bqv

(Where m is the mass of the particle, q is its charge, v is its velocity and r is the radius of its path. )

The speed at which the particles enter the cyclotron due to a potential difference, V.

v = \sqrt{\frac{2Vq}{m}}

Therefore,

\frac{v}{r} = \frac{Bq}{m}

v/r is equal to angular velocity, ω, so

\omega = \frac{Bq}{m}

And since the angular frequency is

ω = 2πfc

Therefore,

f_c = \frac{Bq}{2\pi m}
A pair of "dee" electrodes with loops of coolant pipes on their surface at the Lawrence Hall of Science. The particle exit point may be seen at the top of the upper dee, where the target would be positioned
A pair of "dee" electrodes with loops of coolant pipes on their surface at the Lawrence Hall of Science. The Lawrence Hall of Science (LHS is a public science center featuring hands-on exhibits and activities The particle exit point may be seen at the top of the upper dee, where the target would be positioned

This shows that for a particle of constant mass, the frequency does not depend upon the radius of the particle's orbit. As the beam spirals out, its frequency does not decrease, and it must continue to accelerate, as it is travelling more distance in the same time. As particles approach the speed of light, they acquire additional mass, requiring modifications to the frequency, or the magnetic field during the acceleration. This is accomplished in the synchrocyclotron. A synchrocyclotron is a Cyclotron in which the frequency of the driving RF electric field is varied to compensate for the mass gain of the accelerated particles as their velocity

The relativistic cyclotron frequency is

f=f_c\sqrt{1-v^2/c^2},

where fc is the classical frequency, given above, of a charged particle with velocity v circling in a magnetic field.

The rest mass of an electron is 511 keV, so the frequency correction is 1% for a magnetic vacuum tube with a 5. 11 kV direct current accelerating voltage. The proton mass is nearly two thousand times the electron mass, so the 1% correction energy is about 9 MeV, which is sufficient to induce nuclear reactions.

An alternative to the synchrocyclotron is the isochronous cyclotron, which has a magnetic field that increases with radius, rather than with time. In Particle accelerators an Isochronous cyclotron is a cyclotron that maintains a constant RF driving frequency and compensates for the relativistic mass gain of the accelerated The de-focusing effect of this radial field gradient is compensated by ridges on the magnet faces which vary the field azimuthally as well. This allows particles to be accelerated continuously, on every period of the radio frequency, rather than in bursts as in most other accelerator types. This principle that alternating field gradients have a net focusing effect is called strong focusing. In Accelerator physics strong focusing or alternating-gradient focusing is the principle that the net effect on a Particle beam of charged particles passing It was obscurely known theoretically long before it was put into practice.

Related technologies

See also

External links

Dictionary

cyclotron

-noun

  1. An early particle accelerator in which charged particles were generated at a central source and accelerated spirally outward through a fixed magnetic and alternating electric fields.
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