X - Ray Imaging Intensity, Quality and Attenuation

The effects of x – rays on matter depend on

1. The intensity of the x – rays, that is the energy per square metre of the target.

2. The quality of the x – ray beam, that is the spread of wavelengths in the beam.

If the energy of the beam is absorbed by the target, the beam is said to be attenuated. The dominant attenuation processes are simple scattering by electrons and absorption and reemission of x – rays by atomic electrons in the inner shells of atoms. Simple scattering affects photons that have energies in the range 0 – 30 keV, while absorption and reemission by atomic electrons in the inner shells affects those electrons with energies in the range 0 – 100 keV.

Both processes result in essentially exponential decay of the x – ray intensity of the beam as it passes through the material. For a given intensity of x – rays and a given material, there will be a certain thickness of material that reduces the x – ray intensity by 50%. This thickness is known as the half value thickness,in the diagram below.

The attenuation constantis a constant that allows us to calculate the intensity of x – rays at any depth. The decay of x – ray intensity is exponential, sowhereis the intensity of x – rays at the surface of the material andis the attenuation constant. To find the half value thickness putand solve for

Hence