History of Proton Therapy - Report Example

When the beam leaves the electric field region, it enters the magnetic field region. It is bent 180 degrees, then renters the electric field region at the correct time to be accelerated in the opposite direction. As the charged particle energy increases, so does the radius of the path of the particle. When the power is sufficient, the particle is extracted from the cyclotron. If a cyclotron whose beam extraction energy is constant, the beam is then directed to an energy degrader and selection system.

The electric field goes through 1 cycle (or multiple) for each particle revolution - this is the "cyclotron frequency" of the RF accelerating voltage. The oscillating electric field's angular frequency is given by ω = qBIm, where q is proton charge, B is magnetic field strength, and m is proton mass. The accelerating RF voltage and the protons will get out of synchronization (and limit the maximum energy) unless the frequency of the RF field decreases or the magnetic field increases.

The "synchrocyclotron" and the "isochronous cyclotron" were developed to address this problem. The synchrocyclotron varies the frequency of the accelerating voltage to track the relativistic effects. The accelerator at the Harvard Cyclotron Laboratory was a synchrocyclotron, which used a rotating condenser to vary the RF angular frequency. One result of this scheme is that only photons that arrive at the accelerating gaps at the right time are accelerated - this creates a pulsed time structure in the extracted proton beam.

Because the accelerating frequency is variable in a synchrocyclotron, it is no longer necessary to use high-accelerating voltages used in cyclotrons. In the isochronous cyclotron, the magnetic field increases to compensate for the relativistic mass effects. The extracted proton beam is constant and not pulsed or bunched. Synchrotrons: Synchrotrons are circular ring devices where protons travel in vacuum pipes under the influence of magnets positioned around the circle's circumference. Acceleration is achieved by the application of RF fields in an accelerating cavity located in the ring.

The magnetic fields must be increased synchronously with the acceleration to keep the particles constrained in the constant radius path. A variety of beam focusing schemes can constrain the beam size, which will affect the size and expense of the magnetic elements controlling the beam.