It is, however, not the most useful parameter for measuring and calculating the penetrating ability of radiation. Therefore, the average distance (range) that photons penetrate a material is determined by the same factors that affect the rate of attenuation: photon energy, type of material (atomic number), and material density.Īverage photon range is a useful concept for visualizing the penetrating characteristics of radiation photons. Actually, the average photon range is equal to the reciprocal of the attenuation coefficient (µ):Īverage Range (cm) =1/Attenuation Coefficient (cm -1 ) Increasing the rate of attenuation by changing photon energy or the type of material decreases the average range of photons.
The average range of a group of photons is inversely related to the attenuation rate. Very few photons travel a distance exactly equal to the average range. The average range is the average distance traveled by the photons before they interact. In a given situation a group of photons have different individual ranges which, when considered together, produce an average range for the group. This means that the first layer encountered by the radiation beam attenuates many more photons than the succeeding layers. The nature of the exponential relationship is that each thickness of material attenuates the same fraction of photons entering it. The relationship between the number of photons reaching a specific point and the thickness of the material to that point is exponential. If we count the number of photons penetrating through each thickness of material, we begin to see a fundamental characteristic of photon penetration. Some of the photons travel a relatively short distance before interacting, whereas others pass through or penetrate the object. Let us consider a group of mono-energetic photons entering an object, as shownīelow. In fact, there is no way to predict the range of a specific photon. This distance can be considered the range of the individual photons.Ī characteristic of radiation is that all photons do not have the same range, even when they have the same energy. When photons enter an object, they travel some distance before interacting. It might be helpful in understanding the characteristics of radiation penetration to first consider the range, or distance, traveled by the individual photons before they are absorbed or scattered. Penetrating than low-energy photons, although there are limits and exceptions to this, which we discuss later. Increasing photon energy generallyĭecreases the probability of interactions (attenuation) and, The probability of photons interacting, especially with the photoelectric effect, is related to their energy.
The amount of penetration depends on the energy of the individual photons and the atomic number, density, and thickness of the object, as illustratedįactors That Affect the Penetration of Radiation through a Specific Object Penetration is the inverse of attenuation. The penetration can be expressed as the fraction of radiation passing through the object. When they are directed into an object, some of the photons are absorbed or scattered, whereas others completely penetrate the object. One of the characteristics of x- and gamma radiations that makes them useful for medical imaging is their penetrating ability.
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