A Cyclotron And Its Place in Physics

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My introduction to a Cyclotron took place  when I was completing my Post Graduation in Physics. The Cyclotron was a gift of Rochester University to the Punjab University through the head of Physics Department Professor HS Hans. In simple terms a Cyclotron is a particle accelerator. It was conceived by Ernest O. Lawrence in 1929 when he was a professor at the University of California in the 1930s. His students and colleagues helped him in this invention.

Dalton enunciated a theory that the Atom was the smallest particle and it could not be subdivided. He talked of indivisibility of the Atom, which he considered to be the fundamental particle. However Neil Bohr and other scientists proved Dalton wrong and propounded the theory that the atom could be broken into smaller particles like electrons, protons and the nucleus. This spawned an entire new branch of physics and Einstein was to use it for his theories of time, space and energy.

The Cyclotron is part of this new development in Physics. It is a machine that accelerates particles along a particular path and is based on the concept that that the atom consists of particles that can be accelerated and photographed as well. Thus Cyclotrons will accelerate electrons and protons which are a component of the atom. When a Cyclotron is used to accelerate electrons it is called a Betatron.

Unlike the line accelerators which existed earlier a Cyclotron will accelerate the particles in a circular path under the influence of a constant magnetic field. The basic principle of a Cyclotron is to move a particle by the application of a strong magnetic field. The Magnetic force so generated results in bending the charged particle into a semi circular path.

The construction of the Cyclotron is an engineering feat. It generally consists of two large D- shaped electromagnets which are separated from each other . Each of these D’s generate a magnetic force which is perpendicular to the plane. It follows that the gap separating the D’s a uniform electric field is generated. When a charge is released from rest in the gap it is accelerated by the electric field and carried into one of the D’s.The magnetic field insures that the charge follows a half-circle. When the charge is in the gap between the D’s the electric field is reversed and the particle is again accelerated across the gap. This continues till the particle can be released from the machine at tremendous velocity.

Cyclotrons are part of Nuclear Physics. They are used in the treatment of Cancer. The principle is to bombard the affected area where the tumor exists with beams from cyclotrons. This is also called proton therapy. The beams penetrate the body and use radiation to kill the tumors.

How large can a Cyclotron be? The world’s largest cyclotron is in Canada at the University of BC. It has 18m diameter Magnets and a main magnet that weighs 4,000 ton. It will be interesting to know that this magnet produces a field of 0.46 T while a 23 MHz 94 kV electric field is used to accelerate the 300 μA beam.


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