Attenuation of Gamma Radiation

When gamma-rays above the pair production threshold energy are considered, there may also be an annihilation gamma radiation contribution to the dose rate beyond the absorber. 

This phenomenon is referred to as huild-up and can be accounted for by a correction to Equation (2.16)  

The build-up factor, B, is the ratio of the total photons at a point to the number arriving there without being scattered. There are a number of empirical equations in use for estimation of the build-up factor, references to which are given later. 

---

V yugov, P., Goncharov, K., Dementii, V., and Mandrichenko, A. Attenuation of Gamma-Radiation by Concrete and Some Naturally Occurring Materials, J. Atomic Energy (Historical Archive) 10, no. 1 (1961) 78-80. 

Gamma-Ray Attenuation and Absorption. The attenuation of gamma radiation follows an approximately exponential law,... 

Table 2.1 Attenuation of gamma radiation by shielding materials...

Figure 7. The attenuation of dose rate of gamma-radiation (1) and neutrons (2) by the concrete shielding.

Electromagnetic radiation penetrates much deeper into matter than do charged particles. Attenuation of gamma rays in matter follows an exponential law, similar to the Beer-Lambert law for absorption of visible light. To decrease the intensity of a 1.0-MeV parallel beam of gamma rays to one-half its original value requires a... 

Reducing the sample mass and the amount of materials between the sample and the detector reduces radiation attenuation 1. This effort is most important for alpha particles and least important for energetic gamma rays, as discussed in Chapter 7. The fractional self-absorption within the sample can be estimated for beta particles and gamma rays by using Eq. (7.2). The fractional attenuation of gamma rays in... 

Gamma ray surveys are useful only for the detection and determination of radionuclides that are close to the surface due to attenuation of the radiation by soil, water, vegetation, and air. Ten meters of air or 10 cm of snow will reduce the radiation intensity by 1% (so from an altitude of 100 m the attenuation in air is 50%) while 2 cm of soil would attenuate 35% of the gamma rays and a 10% increase in soil moisture would increase the attenuation by about 10% (IAEA-TECDOC1363 2003). 

Attenuation of collimated beams of gamma radiation follows a simple exponential relationship involving the attenuation coefficient, p, ... 

Table 13.7 Attenuation factors for a beam of gamma radiation on lead...

Compton Effect—An attenuation process observed for x- or gamma radiation in which an incident photon interacts with an orbital electron of an atom to produce a recoil electron and a scattered photon whose energy is less than the incident photon. 

Density, level, and thickness measurements all depend upon the determination of the number of radiations per unit time penetrating the sample and producing a measurable signal in the radiation detector. When the amount of matter between the source and the detector increases, there usually is a decrease in the signal. The following relationship demonstrates the exponential natme of the attenuation of beta oi gamma radiation ... 

Microwave-based meters have also been used to monitor water content in emulsions (39). Microwave techniques can be used in two ways Either the attenuation of the microwave radiation due to absorption by the water phase is measured, or capacitance or resonance changes in a microwave cavity are noted. The capacitance-change method is much more sensitive, although both, like the gamma-ray absorption method, are limited in that solids content must be constant or zero in order to accurately interpret the information obtained. Both of these techniques are applicable to field situations and on-line monitoring. 

Gamma rays are electromagnetic radiation. The term photon is used when gamma rays are treated as particles with associated energies. The interaction of gamma rays with bed particles and a fluidization column can be treated separately as attenuation and scattering effects. 

Attenuation. Many interactions can occur between photons and matter. However, the photoelectric effect, the Compton effect, and the pair production effect are the three major interactions involved in the gamma radiation. The Compton effect is predominant when the gamma ray energy is in the range of 0.5-5 MeV, and the photoelectric and pair production effects are important only in lower and higher energy ranges, respectively. 

Gammas. A beam of gammas going through a material of thickness t is attenuated by a factor exp(-/x,r), where ju. is the total linear attenuation coefficient of the gamma in that medium. The higher the value of ft is, the smaller the thickness t that reduces the intensity of the beam by a desired factor. In theory, the beam cannot be attenuated to zero level. In practice, the attenuation is considered complete if the radiation level equals the background. 

This table gives mass attenuation coefficients for photons for all elements at energies between 1 keV (soft x-rays) and 1 GeV (hard gamma rays). The mass attenuation coefficient i describes the attenuation of radiation as it passes through matter by the relation... 

---