Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

130 CHAPTER 4
A. C. power
Injector power supply
Moculator (pulse supply)
Electron injector (“cun’’)
Microwave power tube
Accelerator waveguide
Iris diaphragm
Vacuum pump
Beam
Figure 4-18. Illustration of the operating principle of a linear accelerator. (From Brobeck WM. Particle Accelerator
Safety Manual, MORP 68-12. Rockville, MD: National Center for Radiological Health; 1968.)
klystron. The high-frequency alternating voltage may be thought of as a voltage wave
that is guided down the accelerating tube (called a waveguide) and carries a bunch
of electrons with each wave. The microwaves energize the waveguide in pulses of
several microseconds each at a pulse repetition rate of about 50 to several thousand
pulses per second. In addition to generating X-ray by causing the electron beam
to strike an internal target, a linear accelerator can be designed to be an electron
irradiator, that is, to bring the electron beam out of the machine and to deliver a
radiation dose with the electron beam. Linear accelerators are widely used to treat
cancers, as well as in research and industrial applications.
Cyclotron
High-energy charged particles, such as protons and deuterons, can be generated for
use in research and for producing radionuclides for medical use. Generally, heavy
charged particles are accelerated by driving them in a circular path, with energy
gained stepwise as the particle completes a revolution in the accelerator. The earliest
machine of this type is the cyclotron, which was invented in 1931 by Ernest Lawrence
and Stanley Livingston. In the cyclotron (Fig. 4-19), the heavy charged particle is injected
into an evacuated chamber containing two opposing hollow electrodes (called
“dees” because of their shape) made of nonmagnetic metal, such as copper. The evacuated
chamber is located between the poles of a strong electromagnet, on the order
of 1.5 T (15,000 gauss) or more. The magnetic field causes the injected ions to travel
in a circular path. The dees, which are separated by a small gap, are connected to
a radiofrequency (RF) voltage source whose frequency is the same as the rotational
frequency of the circulating ions. The injected positive ion is attracted to and enters