Gamma-rays attenuation coefficients

The third correction factor, which is the ratio of the adsorbed dose buildup factors in the sample and the dosimeter, is usually ignored, but is shown in this paper to be very important. The absorbed dose buildup factor is defined in this paper analogous to the dose buildup factor, a notation used when the unit roentgen was still the unit of radiation dose. This paper shows the magnitude of this third correction factor, which is caused by differences in gamma-ray attenuation coefficients and softening of the gamma-ray spectrum. As an illustrative example, the dose in different dosimeters is calculated as a function of the distance from a point isotropic cobalt-60 source in water. 

Berger MJ, Hubbell JH (1987) NIST X-ray and gamma-ray attenuation coefficients and cross sections database, NIST standard reference database 8. National Institute of Standards and Technology,... 

Fig. 2.5 Influence of an iterative mass attenuation coefficient determination on the precision of wet bulk densities. The gamma ray attenuation log of gravity core PS 1725-2 was used as test data set. (a) Wet bulk densities calculated with a constant mass attenuation coefficient ( processing porosity =50%) are displayed versus the data resulting from die iteration. A pore fluid density of 1.024 g cm and a constant grain density of 2.7 g cm were used, and the iteration was stopped if densities of two successive steps differed by less than 0.1%o (b) Cross plot of wet bulk densities measured on discrete samples versus wet bulk densities calculated from gamma ray attenuation with a constant mass attenuation coefficient (O) and with the iterative scheme (+). (c) Influence of grain density on iteration. Three grain densities of 2.65, 2.75 and 2.1 g cm were used to calculate wet bulk densities. Modified after Gerland (1993).

Beta radiation has a finite range, whereas in theory x- and gamma rays are exponentially attenuated. It should be noted that x-radiation of each energy exhibits sharp changes in absorption coefficients for certain absorbers. 

The attenuation coefficient for a beam of gamma rays is related to the number of gamma rays removed from the beam, either by absorption or scattering. For the Compton effect, the absorption cross section is determined by the energy absorbed by the electron, which is the total collision energy minus the average scattered photon... 

Overton TR, Snyder RE, Hangartner TN, et al. 1992. Changes in the linear attenuation coefficient of canine appendicular bone following intravenous infusion of strontium lactate, measured using gamma-ray computed tomography. Calcif Tissue Int 50 350-356. 

The total probability of an interaction can be represented by the total linear attenuation coefficient m, which is a function of the gamma ray energy. 

The specific Compton mass attenuation coefficient (p) is a material constant. It depends on the energy of the gamma rays and on the ratio (Z/A) of the number of electrons (Z) to the atomic mass (A) of the material (Ellis 1987). For most sediment and rock forming minerals this ratio is about 0.5, and for a Cs source the corresponding mass attenuation coefficient (p ) for sediment grains is 0.0774 cm g. However, for the hydro-... 

Thus, the mean free path is simply the inverse of the total linear attenuation coefficient. If = 10 m for a certain y-ray traveling in a certain medium, then the distance between two successive interactions of this gamma in that medium is A = 1/p = 1/10 m = 0.10 m. 

Table D1 Total Mass Attenuation Coefficients in cm / for Gamma Rays ...

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... 

One effective medium in common use for gamma-ray measurements is thallium-activated sodium iodide [Nal(Tl)]. The relatively high-Z iodine atom provides a high attenuation coefficient for interacting with energetic gamma radiation. It... 

Although gamma rays are much less subject to attenuation than alpha and beta particles, a density correction is needed if the density of the sample deviates significantly from the density of the calibration standards. The effect of density on self-absorption for both the standard and the sample is estimated by Eq. (7.2) [x for this purpose is the photon attenuation coefficient in cm /g and x is the sample area density in g/cm. Values for ix in some common materials are listed in Table 2.2 and in its cited reference. If a large set of samples with consistent density is analyzed, it may be possible to prepare radioactivity standards at the same density to avoid the need for correction. Interpolating efficiency values as a function of density is feasible at energies above 0.1 MeV because the effect of minor density difference on counting efficiency is small. 

The intrinsic efficiency for detecting gamma rays in the Ge detector, estimated in terms of the attenuation coefficient, usually is less than 100% except for low energies. Table 8.2 shows the percent efficiency in a typical 6-cm-thick detector as a function of gamma-ray energy. The figures are based on Eq. (2.12). 

Figure 2.1 Attenuation coefficient of materials as a function of gamma-ray energy...

This is the parameter plotted in Figure 2.2, comparing attenuation and absorption. The attenuation coefficient only expresses the probability that a gamma-ray of a particular energy will interact with the material in question. It takes no account of the fact that as a result of the interaction a photon at a different energy may emerge as a consequence of that interaction. The total absorption coefficient, niust, of course, take into account those incomplete interactions ... 

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