ASNT

thickness of a specific material that reduces the intensity of radiation from the source to one-half the original intensity. The equation for intensity becomes:

electrons

0

10

20

⎛ ⎝ ⎜

⎞ ⎠ ⎟

ρ

I 0 2

−

I =

= I 0 e

×ρ× HVL

(Eq. 5)

µm

30

40

⎞ ⎠ ⎟

⎛ ⎝ ⎜

ρ

HVL = In 2 ( ) /

×ρ

(a)

(Eq. 6)

or

1

0.639 × ⎞ ⎠ ⎟ The density ρ is material dependent, and the mass attenuation (µ/ ρ ) is both energy and material dependent. Applications in radiation shielding also employ HVL. Tables are maintained for use in designing apparatus and configuration of shielding walls to keep down unwanted scatter into detectors and to protect operators and the public from dosages. A radiation field from a source is known. Applying the inverse square law, half-value layers for materials such as concrete, lead, and steel are added to the design of the imaging system’s room or vault. The resulting dose or f lux/area can be calculated for the far side of the shielding using the linear mass ρ ×ρ ⎛ ⎝ ⎜

θ 2

θ 1

2

L 1

L 2

electrons

(b)

ATTENUATION IN RADIOGRAPHIC INSPECTION The attenuation law for x-rays is often expressed by the half-value layer or tenth-value layer. X-ray intensity is also reduced by the inverse square law. Half-Value Layer Uniform sheets of material are sometimes used to harden the spectrum of the source or remove low-energy scatter from entering detectors. The lower-energy X-rays (or "softer" X-rays) are absorbed by the material and the resulting spectrum is "harder." The filtered spectrum contains a larger fraction of more energetic X-rays compared to the original spectrum. One way of specifying the effect of the filter is to state the half-value layer (HVL). HVL corresponds to the Figure 6 Angular distribution of X-rays emerging from solid target: (a) 250 keV electron trajectories in tungsten. As electrons lose energy and interact with target nuclei, X-rays are generated over a range of depths; (b) path through the target material L 1 for radiation emitted at angle θ 1 is shorter than for θ 2 . The absorbed radiation will be greater for θ 2 than for θ 1 .

attenuation equations. Tenth-Value Layer

Much like the half-value layer, the tenth-value layer (TVL) corresponds to the thickness of a specific material that reduces the intensity of radiation from the source to one-tenth of the original intensity. The equation for intensity becomes:

⎛ ⎝ ⎜

⎞ ⎠ ⎟×ρ× TVL

ρ

I 0 10

−

I =

= I 0 e

(Eq. 7)

⎞ ⎠ ⎟

⎛ ⎝ ⎜

ρ

TVL = In 10 ( ) /

×ρ

(Eq. 8)

or 2.30 / ρ ⎛ ⎝ ⎜

⎞ ⎠ ⎟

×ρ

CHAPTER 2

46

Part 2