Half value layer is often applied in radiography, this is because it is easier to remember or recall the values and simple calculation are readily performed. There are often a multiple factors the affect half value layer. These factors often brings the difference in value of HVL in different material. For the case of aluminum the following factors always affect HVL. Thickness of the aluminum attenuator, purity of the aluminum, detector material of dosimeter used. Fluctuation of x-ray output, detector location in x-ray field. Ultra high purity of aluminum always produce more accurate measures of half value layer (Edenman and Sidney). This is often different from the aluminum alloys since they have got a lot of impurity available. Although the half value of different materials was one of the important parameters for the Quality assurance and Quality control, a constant monitoring had not been performed because of measurement using the process of ionization chamber. The formulae for calculating HVL was given by A = Ao e,
Different material have got different half value layer. This brings the difference in capability in shielding of the same material. As can be seen from the table above wood have the highest half value than aluminum. The value of wood was 6.00mm while that of aluminum was 1.6mm. Half value layer was the most used factor for describing both the penetrating ability and the penetration through specific objects or material. Different shielding strength of different objects also differ when different material are used. This is evident from the first table, the Sr is different for both the aluminum and wood.
There are often a multiple factors the affect half value layer. These factors often brings the difference in value of HVL in different material. For the case of aluminum the following factors always affect HVL. Thickness of the aluminum attenuator, purity of the aluminum. These factors are