0.9 Cyclotron  (Page 2/5)

We shall, therefore, refer cyclotron with acceleration of charged particles such as proton, ionized deuteron, alpha particle and similar other ions.

Construction of cyclotron

It consists of two hollow semicircular Dees so named because of their D-shape. The plane of Dees is the plane of revolution of charged particle, preferably a plane midway in the Dees. The Dees are constructed of conducting material like copper in order (i) to function as electrodes for applying alternating electrical potential using electrical source known as “electrical oscillator” and (ii) to shield moving charged particle from electric field within the Dees. The Dees are kept face to face diametrically opposite at a small distance known as the “gap”. Electric field operates only in the gap to change speed of the charged particle. We should note that electric field does not accelerate charged particle when it is moving along semicircular path within the Dees as it is shielded from electrical field.

Cyclotron

There is an exit channel at the perimeter of one of Dees which finally guides the accelerated charged particle towards a target. The whole set up of Dees is placed between two poles of a powerful magnetic such that its field is perpendicular to the plane of Dees and hence perpendicular to the plane of motion.

This system of Dees is placed in evacuated confinement so that the charged particle moves unhindered.

Working principle

The charged particle (say a positively charged proton) is released near mid point of the face of one of the Dees. Being in the electric field from one Dee to another, it is accelerated by the electric force in the direction of electric field. As the particle enters the adjoining Dee, the magnetic force, being perpendicular to it, renders the charged particle to move along a semicircular path within the Dee. By the time, it emerges again in the narrow gap separating the two Dees, the electrical polarity of Dees changes so that the particle is again accelerated again with an increase in speed.

Working of cyclotron

But as the speed of the particle has increased, the radius of curvature of the semicircular path increases in accordance with the formula :

$$r=\frac{mv}{qB}$$

For given charge, mass and magnetic field, the radius is proportional to the speed. Clearly, the charged particle begins to move in a larger semicircular path after every passage through the gap. By the time particle reaches the gap successively, electric polarity of Dees keeps changing ensuring that the charged particle is accelerated with an increase in speed. This process continues till the charged particle reaches the periphery and exits through the guide with high energy and bombards a given target being investigated. The description of different segments of the path of accelerated particle is given here :

1: Path is a straight line. Particle is accelerated due to electric force. Speed and kinetic energy of the particle increase.

2: Path is a semicircular curve. Particle is accelerated due to magnetic force. This acceleration is centripetal acceleration without any change in speed and kinetic energy of the particle.