ASNT

used to contain the electrons for many thousand revolutions. The doughnut-shaped tube is usually made of vacuum-compatible glass or ceramic and is coated on the inside with a conductive layer of palladium connected to a ground. The doughnut is placed between the poles of an electromagnet that produces a pulsating field (Figure 6b). Electrons injected into the tube as the magnetic field increases will be accelerated in a circular path. The force acting on the particles is proportional to the rate of change of f lux and the magnitude of the field. Because the electrons circle the orbit many times before striking the target, there is a

BETATRONS The first successful betatron was built at the University of Illinois, reaching energies of 2.3 MeV and an X-ray dose output equivalent to that of 1 g of radium. Later, machines capable of X-ray energies up to 300 MeV were constructed, although today the most common betatrons used for radiographic purposes operate between 2.5 MeV and 7.5 MeV (Figure 5). Betatrons meet the needs of applica- tions that require the penetration associated with MeV X-rays, along with some degree of portability, but can also tolerate the longer exposure times associated with the low output dose of betatrons relative to linear accelerators. To accelerate electrons to high speed, the betatron (Figure 6) uses the magnetic induction effect of a transformer. The primary winding in a transformer is connected to an alternating current (AC) voltage source that establishes a varying f lux in an iron core. The secondary winding on this core has induced in it a voltage equal to the product of the number of turns in the secondary winding and the f lux time rate of change. The resulting electric current is made up of the free electrons present in the wire. The betatron is essentially such a transformer except that, instead of wire, the secondary is a hollow circular tube (Figure 6a). This tube, which can be called a doughnut, is

Earth ground

Equilibrium orbit

Expanded orbit

Target structure

Injector

Palladium coated interior

X-ray beam

(a)

Upper pole

Doughnut

H r

Steel water Steel water

H r

Electron orbit

H

Lower pole

H

H

Electrons moving toward reader at this point

(b)

Figure 5 German 6MeV betatron from 1942. Developments in Germany in the late 1930s led to successful betatrons at the University of Illinois in the early 1940s.

Figure 6 Diagram of betatron generator: (a) top view; (b) cross section.

CHAPTER 3

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Part 2