X-ray energy spectrum - DOE-HDBK-1141-2001

Radiological Assessor Training

DOE-HDBK-1141-2001

Student's Guide

When electrons strike and excite target atoms,

the kinetic energy of the electrons is deposited in

the target as heat. When electrons ionize target

atoms, characteristic x-rays will be emitted as

electrons from outer shells fill vacancies created

by ejected electrons.

B. X-ray energy spectrum

The energy of the x-ray photons coming out of

the x-ray machine is of interest to the users of

the machine. The typical energy spectrum from

an x-ray machine consists of the characteristic x-

rays from the target, which have discrete

energies, and the bremsstrahlung photons which

have a whole range of energies, the maximum

energy depending on the potential difference

across the tube. For a typical x-ray machine, the

bremsstrahlung photons far outnumber the

characteristic x-rays.

C. Design features

The cathode and anode of the x-ray tube are

enclosed in an evacuated glass tube or

envelope. The vacuum is necessary to ensure

that the accelerated electrons will interact in the

target, and not with gas molecules.

The x-rays are produced in all directions in the

target. However, only x-rays directed toward the

exit port, or window, will comprise 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.

Module 12 - 3