17. Law of radiation attenuation.

Question 1

Define attenuation

Answer 1

Attenuation is the loss of intensity as radiation propagates through a medium. It is caused by absorption and scattering.
Scattering is negligible in dilute solutions.

→ E “lost” is absorbed

→ remaining E is transmitted

Question 2

Define absorbance, give 2 different terms, and its formula

Answer 2

Absorbance is the logarithmic ratio of the amount of radiation falling upon a material to the amount of radiation transmitted through the material.

On the atomic scale this means that E_photon is used to excite an outer shell e^- of the absorbing material.

Absorption is wavelength dependent.

Also referred to as:

1. optical density (OD)
2. extinction (E)

Formula: A = lg(J₀/J)

BUT: the amount of absorbance can be calculated from the general radiation attenuation law

Question 3

Define transmittance and give its formula

Answer 3

Transmittance is the fraction of incident light (EM radiation) at a specified wavelength that passes through a sample.

Formula: T = J/J₀ * 100%

Question 4

Explain how the differential form of the law of radiation attenuation is obtained, its validity, and give its formula. Refer to the diagram below.

Answer 4

Differential form of the law of radiation attenuation:

the decrease of intensity is proportional to the thickness of absorber Δx and J what is the atten

BUT:

• only parallel beams perpendicular to the surface are considered (simplification)
• only applicable for very thin layers

Formula: ΔJ = J₀ − J = −μ ⋅ Δx ⋅ J

• μ = linear attenuation coefficient
• J₀ = intial intensity of radiation

Question 5

Explain how the integral form of the law of radiation attenuation is obtained, its validity, and give its formula. Refer to the diagram below.

Answer 5

Integral form of law of radiation attenuation:

in fact intensity decreases exponentially with the increase of thickness, only in case of very thin linearly propotional

Formula: J = J₀ ⋅ e^−μx

• μ = linear attenuation coefficient
• J₀ = intial intensity of radiation

Question 6

The linear attenuation coefficient μ depends on.. ? What is its unit?

Answer 6

• quality of the material
• wavelength

Unit: 1/m

Question 7

Define half-value thickness and give another term

Answer 7

Half-value thickness is the thickness of the material at which the intensity of radiation entering it is reduced by one half

• also related to as half-value layer HVL

Question 8

On which 3 qualities does radiation attenuation depend on?

Answer 8

Dependence of radiation attenuation:

• photon E of radiation
• material of absorber (→ μ)
• density / concentration of absorber