Design features

Radiological Assessor Training

DOE-HDBK-1141-2001

Instructor's Guide

The anode is usually encased in

C. Design features

copper, which serves to dissipate the

heat. In many x-ray machines, the

The cathode and anode of the x-ray tube are

anode rotates at a high speed, which

enclosed in an evacuated glass tube or

increases the area of bombardment

envelope. The vacuum is necessary to ensure

and therefore is also useful in

dissipating heat.

that the accelerated electrons will interact in the

target, and not with gas molecules.

The x-ray tube housing is an

insulated metal casing around the

The x-rays are produced in all directions in the

glass envelope that provides both

target. However, only x-rays directed toward the

electrical and radiation shielding.

exit port, or window, will comprise the useful

The housing will intercept most of

the x-rays produced in the target that

beam.

are not part of the useful beam.

Several devices are used to control the size of

the useful x-ray beam. A lead diaphragm is a

sheet of lead with a hole in it. It is placed near

the exit port, and restricts the size of the useful

beam by absorbing x-rays that don't pass

through the hole. The size of the beam is not

adjustable with this type of device unless

another diaphragm with a different-size opening

is used.

For some operations, the size of the useful

beam must be adjusted by the operator. An

adjustable collimator is essentially a set of

movable lead sheets. Two sheets restrict the

width of the beam, and two sheets restrict the

length of the beam. The operator can then

adjust the size of the beam to any desired

combination of length and width.

Often, the lowest energy x-rays are not desired

in the beam. The low energy x-rays can be

filtered out by placing absorbing material (called

filters) in the path of the beam. Aluminum or

copper is commonly used, depending on the

energy of the machine. The addition of filters

increases the average energy of the beam, since

the lower energy x-rays are removed from the

beam when they are absorbed by the filters.

Module 12 6