Gamma absorption

Question 1

What is γ-radiation?

Formula for E of a γ-photon.

Its intensity depends on … ?

Answer 1

EM radiation of high photon E (E_γ > 100 keV)

E_γ = h*f

• h = Planck’s constant
• f = frequency

intensity J is proportional to the no. of photons in case of monochromatic radiations (= photons of equal energy)

Question 2

What is the photoeffect/photoelectric effect?

Formula.

Answer 2

an incident γ-photon removes a photoelectron from the bound e^- of an atom, while being absorbed

E_γ = A + E_e

• E_γ = energy of incident γ-photon
• A = work needed to remove e^-
• E_e = energy of removed photoelectron

Question 3

What is Compton scattering?

Formula.

Answer 3

incident γ-photon removes a Compton e^- from the outer shell e^- of the atom, while a Compton photon of lower E (longer λ) is emitted
⇒ E is split btw Compton e^- and Compton photon

E_γ = A + E_e + E_γ‘

• E_γ = energy of incident γ-photon
• A = work needed to remove e^-
• E_e = energy of Compton e^-
• E_γ’ = energy of Compton photon

Question 4

What is pair production?

Formula.

Answer 4

incident γ-photon of sufficiently high E (h*f > 1022 keV) is absorbed near the nucleus and and electron-positron pair is created

E_γ = 2 m_ec² + E_e + E_p

• E_γ = energy of incident γ-photon
• 2 m_ec² = combined rest mass of electron-positron
• E_e = energy of e^-
• E_p = energy of e⁺

Question 5

What is annihilation?

Answer 5

process when a subatomic particle collides with its antiparticle

ex: positron + electron → 2 γ-photons of 511 keV

Question 6

What is elastic scattering?

Answer 6

γ-photon changes only its direction while its E remains the same

Question 7

What is the formula of the general attenuation law?

Answer 7

J = J₀ * e^-μ*x

• J = attenuated intensity
• J₀ = incident intensity
• μ = attenuation coefficient
• x = layer thickness

⇒ attenuation is dependent on layer thickness

Question 8

The (linear) attenuation coefficient depends on.. ?

Unit.

Answer 8

• type + density of the material
• radiation energy

in [1/cm]

Question 9

What is half/tenth value thickness resp.?

Formulas.

Answer 9

distance at which the incident intensity J₀ decreases

• by half → D = ln 2/μ
• by a factor of 10 → = 3.33 D

Question 10

The mass attenuation coefficient depends on.. ?

Formula + unit.

Answer 10

depends on:

• type of the material
• radiation energy

⇒ NOT: on density of the material

μ_m = μ/⍴
in [cm²/g]

• μ = linear attenuation coefficient
• ⍴ = density

Question 11

What is surface density?

Formula + unit.

Answer 11

mass that is behind the 1 cm² surface area of the absorbent

x_m = x * ⍴
in [g/cm²]

• x = layer thickness
• ⍴ = density

_{(NOTICE: unit is reciprocal of mass attenuation coefficient)}

Question 12

What is another formula for the attenuation law?

Answer 12

J = J₀ * e^-μm*xm

• J = attenuated intensity
• J₀ = incident intensity
• μ_m = mass attenuation coefficient
• x_m = surface density

⇒ attenuation is dependent on surface density

Question 13

What is half value mass?

Formula + unit.

Answer 13

mass of the material behind 1 cm² surfaces that decreases the incident intensity by half

D_m = D * ρ
in [g/cm²]

• D = half value thickness
• ρ = density

Question 14

Since the mass attenuation coefficient μ_m is basically a compound coefficient it is the sum of… ?

Why?

Answer 14

μ_m = τ_m + σ_m + κ_m

• τ_m = mass attenutation coeff. of photoeffect
• σ_m = mass attenuation coeff. of Compton scattering
• κ_m = mass attenuation coeff. of pair production

each material absorbs different types of radiation with a different amount, hence absorption diagrams can be constructed

Question 15

Which material is usually used to shield from gamma radiation?

Why?

Answer 15

lead

→ element with highest atomic number Z that is not radioactive