PhySics ProJect Topic: Cyclotron Teacher: Mr. S.P. Dixit Sir

PhySics ProJectTopic: Cyclotron

Teacher: Mr. S.P. Dixit Sir

BY: Rajesh BansalClass: XII C3

CYCLOTRON

CyclotronThe Cyclotron is a machine to accelerate Charged Particles or Ions to high energies.It was convented by E.O. Lawerence and M.S. Livingston in 1932 to investigate nuclear structure.The Cyclotron uses both Electric and Magnetic fields in combination to increase the energy charged particles.As the fields are perpendicular to each other they are also called Crossed fields.Cyclotron uses the fact that the frequency of revolution of the charged particles in a magnetic field is independent of its energy.

The Particles move most of the time inside two Semicircular Disc-like metal containers, D1 and D2.Which are called dees as they look like the letter D.Inside the Metal boxes the particle is shielded and is not acted on by the Electric field.The magnetic field, however acts on the particle and make it go round in a circular path inside dee.Every time particle moves from one dee to another it is acted upon by the electric field.The sign of electric field is changed alternately in tune with the circular motion of the particle. This ensures that the particles is always accelerated by the electric field. Each time the acceleration increases the energy of the particle.

This Frequency is called the cyclotron frequency for obvious reasons and is denoted by VC.The Frequency Va of the applied voltage is adjusted so that the polarity of the dees is reversed in the same time that it takes the ions to complete one half of the revolution. The requirement Va = Vc is called the Resonance condition.The Phase of the supply is adjusted so that when the positive ions arrive at the edge of D1 , D2 is at a lower potential and the ions are accelerated across the gap.Inside the dees the particles travel in a region free of the electric field .The increase in their kinetic energy is qv each time they cross from one dee to another (V refers to the voltage across the dees at that time).

HistoryThe Cyclotron was invented and patented by Ernst Lawrence of the University of California, Berkeley, Where it was first operated in 1932.A graduate student M. Stanley Livingston, did much of the work of translating the idea into working hardware.Lawrence read an article about the concept of a drift tube linac by Rolf Wideroe, who had also been working along similar lines with the Betatron concept.The first European Cyclotron was construct in Leningrad in the Physics Department of the Radium Institute headed by Vitaly khlopin (ru).

Cyclotron PatentAt the Radiation Laboratoryof theUniversity of Californiaat Berkeley Lawrence constructed a series of cyclotrons which were the most powerful accelerators in the world at the time; a 27 inch (68cm) 4.8 MeV machine (1932), a 37 inch (94cm) 8 MeV machine (1937), and a 60 inch (1.5 m) 16 MeV machine (1939). He also developed an 184 inch (4.7 m)synchrocyclotron(1945).Leningrad instrument was first proposed in 1932 by George Gamow and Lev Mysovskii (ru) and was installed and became operative by 1937.In Nazi Germany a Cyclotron was built in Heidelberg under supervision of Walther Bothe and Wolfgang Gentner, with support from the Heereswaffenamt, and became operative in 1943.

Cyclotron WorkingPrinciple Of OperationCyclotrons accelerated charged particles beam using a high frequency alternating voltage which is applied between two D-shaped electrodes (also called dees)An additional Magnetic field (B) is applied in perpendicular direction to the electrode plane, enabling particles to re-encounter the accelerating voltage many times at the same phase, to achieve this the voltage frequency must match the particles cyclotron resonance frequency with the relativistic mass m and its charge q.

The frequency is given by equality of Centripetal force and Magnetic Corentz force. The particles, injected near the centre of the Magnetic field, increase their kinetic energy only when recirculating through the gap between the electrodes thus they travel outwards along the spiral path .Their radius will increase until the particles hit a target at the perimeter of the vacuum chamber or leave the cyclotron using a Beam tube, enabling their use e.g. for particle therapy.Various materials may be used for a target, and the collisions will create secondary particles which may be guided outside of the cyclotron and into instruments for analysis.

Particle AccelerationParticle EnergySince the particles are accelerated by the voltage many times, the final energy of the particles is not dependent on the accelerating voltage but the diameter of the accelerating chamber, the dees.Cyclotrons can only accelerate particles to speeds much slower than thespeed of light,nonrelativisticspeeds. For nonrelativistic particles, the centripetal force Fcrequired to keep them in their curved path isWhere m is Particles mass, v is the Velocity and r is the Diameter of the path.This force was provided by theLorentz forceFBof the magnetic fieldB

where qis the Particle's charge. The particles reach their maximum energy at the periphery of the dees, where the radius of their path is r = Rthe Diameter of the dees .Equating the above two forces FB = qvBSo we put the output energy of the particles is

Therefore, the limit to the cyclotron's output energy for a given type of particle was the strength of the magnetic fieldB, which was limited to about 2 T forferromagnetic electromagnets, and the diameter of the dees R, which was determined by the diameter of the magnet's pole pieces. So very large magnets were constructed for cyclotrons, culminating in Lawrence's 1946 synchrocyclotron, which had pole pieces 184 inches (15 feet or 4.6 m) in diameter.

Radiation TherapyRelativistic considerationsInthe nonrelativistic approximation, the frequency does not depend upon the radius of the particle's orbit, since the particle's mass is constant.As the beam spirals out, its frequency does not decrease, and it must continue to accelerate, as it is travelling a greater distance in the same time period.In contrast to this approximation, as particles approach thespeed of light, theirrelativistic massincreases, requiring either modifications to the frequency, leading to thesynchrocyclotron, or modifications to the magnetic field during the acceleration, which leads to theisochronous cyclotron.

Types Of Cyclotron

Isochronous 1. CyclotronAn alternative to the Synchrocyclotron is the Isochronous cyclotron, which has a magnetic field that increase with radius, rather then with time Isochronous cyclotrons are capable of producing much greater than beam current than synchrocyclotrons, but require azimuthal variations in the field strength to provide a strong focusing effect and keep the particles captured in their spiral trajectory , for this reason , an Isochronous Cyclotron is also called an AVF (azimuthal varying field) Cyclotron

This allows particles to be accelerated continuously, on every period of theradio frequency(RF), rather than in bursts as in most other accelerator types.This principle that alternating field gradients have a net focusing effect is calledstrong focusing. It was obscurely known theoretically long before it was put into practice.Examples of isochronous cyclotrons abound; in fact almost all modern cyclotrons use azimuthally-varying fields. The TRIUMF cyclotron mentioned below is the largest with an outer orbit radius of 7.9metres, extracting protons at up to 510MeV, which is 3/4 of the speed of light. The PSI cyclotron reaches higher energy but is smaller because of using a higher magnetic field.

2. SynchrocyclotronA synchrocyclotron is a cyclotron in which the frequency of the driving RF electric field is varied to compensate for relativistic effects as the particles' velocity begins to approach the speed of light.This is in contrast to the classical cyclotron, where the frequency was held constant, thus leading to the synchrocyclotron operation frequency being

UsageFor several decades, cyclotrons were the best source of high-energy beams fornuclear physicsexperiments; several cyclotrons are still in use for this type of research.The results 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 intonuclear transmutationof the elements used in the target.

Cyclotrons can be used inparticle therapyto treatcancer.Ion beams from cyclotrons can be used, as inproton therapy, to penetrate the body and kill tumors byradiation damage, while minimizing damage to healthy tissue along their path.Cyclotron beams can be used to bombard other atoms to produce short-livedpositron-emitting isotopes suitable forPET imaging.More recently cyclotrons currently installed at hospitals for particle therapy have been retrofitted to enable them to producetechnetium-99m.Technetium-99m is a diagnostic isotope in short supply due to difficulties at Canada'sChalk Riverfacility.

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Related TechnologiesThe spiraling of electrons in a cylindrical vacuum chamber within a transverse magnetic field is also employed in themagnetron, a device for producing high frequency radio waves (microwaves).Thesynchrotronmoves the particles through a path of constant radius, allowing it to be made as a pipe and so of much larger radius than is practical with the cyclotron andsynchrocyclotron.The larger radius allows the use of numerous magnets, each of which imparts angular momentum and so allows particles of higher velocity (mass) to be kept within the bounds of the evacuated pipe.

The magnetic field strength of each of the bending magnets is increased as the particles gain energy in order to keep the bending angle constant.

(1) One of the world's largest cyclotrons is at theRIKENlaboratory in Japan. Called the SRC, for Superconducting Ring Cyclotron, it has 6 separated superconducting sectors, and is 19m in diameter and 8m high.Built to accelerate heavy ions, its maximum magnetic field is 3.8tesla, yielding a bending ability of 8tesla-metres. The total weight of the cyclotron is 8,300 tonnes.

Notable Examples

It has accelerated uranium ions to 345MeV per atomic mass unit.(2) TRIUMF , Canada's national laboratory for nuclear and particle physics, houses the world's largest cyclotron.[17]The 18m diameter, 4,000 tonne main magnet produces a field of 0.46Twhile a 23MHz 94kVelectric field is used to accelerate the 300A beam.The TRIUMF field goes from 0 to about 320 inches radius with the maximum beam radius of 310 inches. This is because it requires a lower magnetic field to reduce EM stripping of the loosely bound electrons.Its large size is partly a result of using negative hydrogen ions rather than protons. The advantage is that extraction is simpler; multi-energy, multi-beams can be extracted by inserting thin carbon stripping foils at appropriate radii.TRIUMF is run by a consortium of eighteen Canadian universities and is located at theUniversity of British Columbia, Vancouver, Canada.

Advantages & LimitationsThe cyclotron was an improvement over thelinear accelerators(linacs) that were available when it was invented, being more cost- and space-effective due to the iterated interaction of the particles with the accelerating field.In the 1920s, it was not possible to generate the high power, high-frequency radio waves which are used in modern linacs (generated byklystrons).As such, impractically long linac structures were required for higher-energy particles. The compactness of the cyclotron reduces other costs as well, such as foundations, radiation shielding, and the enclosing building.Cyclotrons have a single electrical driver, which saves both money and power. Furthermore, cyclotrons are able to produce a continuous stream of particles at the target, so the average power passed from a particle beam into a target is relatively high.Thespiralpath of the cyclotron beam can only "sync up" with klystron-type (constant frequency) voltage sources if the accelerated particles are approximately obeyingNewton's Laws of Motion.If the particles become fast enough thatrelativistic effects become important, the beam becomes out of phase with the oscillating electric field, and cannot receive any additional acceleration.The classical 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.Cost limitations have been overcome by employing the more complexsynchrotronor modern,klystron-drivenlinear accelerators, both of which have the advantage of scalability, offering more power within an improved cost structure as the machines are made larger.