3. Attenuation

Attenuation is the reduction in the intensity of gamma ray or X-ray beam, as it traverses matter either by the absorption of photons or by deflection (scattering) of photons from the beam.

In diagnostic energy range, m decreases with increasing energy except at

• The number of photons removed from the beam traversing a very small

coefficient (μ), and it is typically expressed in cm�<sup>1</sup>

on the material; X = the thickness of material.

• The density (μ) of material affects this number.

• The process of fraction of photons removed from a monoenergetic beam of Xray or gamma ray per unit thickness of material is called linear attenuation

where n = the number removed from beam; N = the number of photons incident

• For a given thickness of material, the probability of interaction depends on the number of atoms which the X-rays or gamma rays encounter per unit

• -Linear attenuation coefficient is proportional to the density of the material:

• For a given thickness, the probability of interaction relies on the number of

• Dependency can be overcome by normalizing linear attenuation coefficient for

μwater=ρwater ¼ μice=ρice ¼ μwater vapour=ρwater vapour (13)

4. In radiology, we usually differentiate between regions of an image that

N ¼ Noe

5. In density, the mass contained within a given volume plays an important role.

� <sup>μ</sup>

.

n ¼ μNΔx (10)

μwater>μice>μwater vapour (11)

linear attenuation coefficient ð Þ μ density of material ð Þρ

/g units.

ð Þ<sup>ρ</sup> <sup>ρ</sup><sup>x</sup> (14)

(12)

absorption edges (e.g., K-edge) [9].

DOI: http://dx.doi.org/10.5772/intechopen.85502

3.3.1 Linear attenuation coefficient

Basic Modes of Radioactive Decay

thickness μx:

distance.

3.3.2 Mass attenuation coefficient

atoms per volume.

thickness of material:

3.For a given photon energy:

33

mass attenuation coefficient ð Þ¼ μ=ρ

1.Mass attenuation coefficient ordinarily can be seen in cm<sup>2</sup>

2.Mass attenuation coefficient is autonomous of density.

correspond to irradiation of adjacent volumes of tissue.

Attenuation results from the interaction between penetrating radiation and matter, as it is not a simple process. These interactions include the photoelectric effect, scattering, and pair production [8].

#### 3.1 HVL

Half-value layer (HVL): It is defined as the thickness of material required to reduce intensity of gamma ray or X-ray beam to one-half of its initial value (as shown in Figure 11).

#### 3.2 Mean free path

The range of a single photon in matter that cannot be predicted. The distance traveled some time recently interaction can be calculated from direct attenuation coefficient or the HVL of the beam.

Mean free path (MFP) of photon beam is:

$$\text{MFP} = \frac{1}{\mu} = \frac{1}{0.693/\text{HVL}} = 1.444 \text{HVL} \tag{8}$$

#### 3.3 Linear attenuation coefficient

The linear attenuation coefficient (μ) can be characterized as the division of a beam of X-rays or gamma rays that's retained or scattered per unit thickness of the absorber.

This esteem accounts for the volume of number of atoms in a cubic cm of material and the probability of a photon of being scattered or absorbed from the nucleus or an electron of one of these atoms.

Linear attenuation coefficient is the sum of individual linear attenuation coefficients for each type of interaction:

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$$
\mu = \mu\_{\text{coherent}} + \mu\_{\text{photo}} + \mu\_{\text{Compton}} + \mu\_{\text{pair}} \tag{9}
$$

Figure 11.

Monoenergetic photons under narrow-beam geometry conditions. The probability of attenuation remains the same for each additional HVL thickness placed in the beam.

In diagnostic energy range, m decreases with increasing energy except at absorption edges (e.g., K-edge) [9].

3.3.1 Linear attenuation coefficient

3. Attenuation

3.1 HVL

absorber.

Figure 11.

32

photons from the beam.

shown in Figure 11).

3.2 Mean free path

coefficient or the HVL of the beam.

3.3 Linear attenuation coefficient

cients for each type of interaction:

nucleus or an electron of one of these atoms.

same for each additional HVL thickness placed in the beam.

Mean free path (MFP) of photon beam is:

MFP <sup>¼</sup> <sup>1</sup>

effect, scattering, and pair production [8].

Use of Gamma Radiation Techniques in Peaceful Applications

Attenuation is the reduction in the intensity of gamma ray or X-ray beam, as it traverses matter either by the absorption of photons or by deflection (scattering) of

Attenuation results from the interaction between penetrating radiation and matter, as it is not a simple process. These interactions include the photoelectric

Half-value layer (HVL): It is defined as the thickness of material required to reduce intensity of gamma ray or X-ray beam to one-half of its initial value (as

The range of a single photon in matter that cannot be predicted. The distance traveled some time recently interaction can be calculated from direct attenuation

The linear attenuation coefficient (μ) can be characterized as the division of a beam of X-rays or gamma rays that's retained or scattered per unit thickness of the

Linear attenuation coefficient is the sum of individual linear attenuation coeffi-

μ ¼ μcoherent þ μphoto þ μCompton þ μpair (9)

This esteem accounts for the volume of number of atoms in a cubic cm of material and the probability of a photon of being scattered or absorbed from the

Monoenergetic photons under narrow-beam geometry conditions. The probability of attenuation remains the

<sup>0</sup>:693=HVL <sup>¼</sup> <sup>1</sup>:44HVL (8)

<sup>μ</sup> <sup>¼</sup> <sup>1</sup>


$$m = \mu \text{N} \Delta x \tag{10}$$

where n = the number removed from beam; N = the number of photons incident on the material; X = the thickness of material.


$$
\mu\_{\text{water}} > \mu\_{\text{ice}} > \mu\_{\text{water vapour}} \tag{11}
$$
