by Wikibooks, open books for an open world
Available in 104 free installments
Owner:
Let us start exploring the magnitude of ?I by placing different absorbers in turn in the radiation beam. What we would find is that the magnitude of ?I is highly dependent on the atomic number of the absorbing material. For example we would find that ?I would be quite low in the case of an absorber made from carbon (Z=6) and very large in the case of lead (Z=82).
We can gain an appreciation of why this is so from the following figure:
The figure illustrates a high atomic number absorber by the large circles which represent individual atoms and a low atomic number material by smaller circles. The incident radiation beam is represented by the arrows entering each absorber from the left. Notice that the atoms of the high atomic number absorber present larger targets for the radiation to strike and hence the chances for interactions via the Photoelectric and Compton Effects is relatively high. The attenuation should therefore be relatively large.
In the case of the low atomic number absorber however the individual atoms are smaller and hence the chances of interactions are reduced. In other words the radiation has a greater probability of being transmitted through the absorber and the attenuation is consequently lower than in the high atomic number case.
With respect to our spaceship analogy used in the previous chapter the atomic number can be thought of as the size of individual meteors in the meteor cloud.
If we were to precisely control our experimental set-up and carefully analyse our results we would find that:
Therefore if we were to double the atomic number of our absorber we would increase the attenuation by a factor of two cubed, that is 8, if we were to triple the atomic number we would increase the attenuation by a factor of 27, that is three cubed, and so on.
It is for this reason that high atomic number materials (e.g. Pb) are used for radiation protection.
