What Is It Called When A Primary Photon Changes Direction But Not Energy

In some situations it is more desirable to express the attenuation rate in terms of the mass of the cloth encountered past the photons rather than in terms of distance. The quantity that affects attenuation rate is not the total mass of an object but rather the expanse mass. Area mass is the amount of material backside a 1-unit area, as shown below. The area mass is the product of textile thickness and density:

Expanse Mass (g/cm ² ) = Thickness (cm) 10 Density (g/cm ³ ).

The mass attenuation coefficient is the charge per unit of photon interactions per ane-unit of measurement (g/cm ⁱⁱ ) area mass.

Mass Attenuation Coefficient

Mass Attenuation Coefficient

The effigy compares 2 pieces of material with unlike thicknesses and densities just the same area mass. Since both attenuate the same fraction of photons, the mass attenuation coefficient is the same for the two materials. They do non have the same linear attenuation coefficient values.

The relationship between the mass and linear attenuation coefficients is

Mass Attenuation Coefficient (�/ r ) = Linear Attenuation Coefficient (�) / Density ( r ).

Notice that the symbol for mass attenuation coefficient (�/ r ) is derived from the symbols for the linear attenuation coefficient (�) and the symbol for density ( r ). Nosotros must be careful non to be misled by the relationship stated in this style. Confusion often arises as to the upshot of material density on attenuation coefficient values. Mass attenuation coefficient values are really normalized with respect to fabric density, and therefore practise non change with changes in density. Material density does accept a direct effect on linear attenuation coefficient values.

The full attenuation rate depends on the individual rates associated with photoelectric and Compton interactions. The respective attenuation coefficients are related as follows:

� _(full) = � _{(photoelectric)} + � _(Compton) .

Let us now consider the factors that affect attenuation rates and the contest between photoelectric and Compton interactions. Both types of interactions occur with electrons inside the fabric. The adventure that a photon will interact as it travels a i-unit distance depends on two factors.

1 factor is the concentration, or density, of electrons in the material. Increasing the concentration of electrons increases the chance of a photon coming close enough to an electron to interact. In a previous section (Characteristics and Structure of Matter) nosotros observed that electron concentration was determined past the physical density of the textile. Therefore, density affects the probability of both photoelectric and Compton interactions.

All electrons are not every bit attractive to a photon. What makes an electron more or less attractive is its binding free energy. The ii general rules are:

1. Photoelectric interactions occur most oftentimes when the electron binding energy is slightly less than the photon energy.

2. Compton interactions occur about oft with electrons with relatively low binding energies.

In the previous department referred to in a higher place we observed that the electrons with binding energies within the energy range of diagnostic x-ray photons were the Yard-trounce electrons of the intermediate- and loftier-atomic-number materials. Since an atom can have, at the most, two electrons in the Grand shell, the majority of the electrons are located in the other shells and have relatively low binding energies.