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www.GOALias.blogspot.comincreases, the radius of the circular path increases. So the path is aspiral one.The whole assembly is evacuated to minimise collisions between theions and the air molecules. A high frequency alternating voltage is appliedto the dees. In the sketch shown in Fig. 4.8, positive ions or positivelycharged particles (e.g., protons) are released at the centre P. They movein a semi-circular path in one of the dees and arrive in the gap betweenthe dees in a time interval T/2; where T, the period of revolution, is givenby Eq. (4.6),1 2πmT = =ν qBcqBor νc= (4.8)2 π mThis frequency is called the cyclotron frequency for obvious reasonsand is denoted by ν c.The frequency ν aof the applied voltage is adjusted so that the polarityof the dees is reversed in the same time that it takes the ions to completeone half of the revolution. The requirement ν a= ν cis called the resonancecondition. The phase of the supply is adjusted so that when the positiveions arrive at the edge of D 1, D 2is at a lowerpotential and the ions are accelerated across thegap. Inside the dees the particles travel in a regionfree of the electric field. The increase in theirkinetic energy is qV each time they cross fromone dee to another (V refers to the voltage acrossthe dees at that time). From Eq. (4.5), it is clearthat the radius of their path goes on increasingeach time their kinetic energy increases. The ionsare repeatedly accelerated across the dees untilthey have the required energy to have a radiusapproximately that of the dees. They are thendeflected by a magnetic field and leave the systemvia an exit slit. From Eq. (4.5) we have,qBRv = (4.9)mwhere R is the radius of the trajectory at exit, andequals the radius of a dee.Hence, the kinetic energy of the ions is,1 q B Rm2 2m2 2 22v =(4.10)The operation of the cyclotron is based on thefact that the time for one revolution of an ion isindependent of its speed or radius of its orbit.The cyclotron is used to bombard nuclei withenergetic particles, so accelerated by it, and studyMoving Charges andMagnetismFIGURE 4.8 A schematic sketch of thecyclotron. There is a source of chargedparticles or ions at P which move in acircular fashion in the dees, D 1and D 2, onaccount of a uniform perpendicularmagnetic field B. An alternating voltagesource accelerates these ions to highspeeds. The ions are eventually ‘extracted’at the exit port.141