Ionising Radiation Flashcards
What does radiation carry enough energy to do?
To free electrons from atoms and make ions
How much energy is needed to free electrons?
Enough to overcome the electron binding energy
What is directly ionising?
Charged particles interacting directly via Coulomb’s law to free atomic electrons
e.g. alpha, electrons, protons
What is indirectly ionising?
Electrically neutral particles interacting to free electrons in secondary processes
e.g. neutrons, photons
What is important for radiotherapy?
Absorption : energy is delivered where the interaction occurs
Scatter is bad as it blurs out the volume receiving dose
Transmission delivers no dose
What is important for imaging?
Transmission and absorption are good for distinguishing different structures in the body
Too much absorption will give patient unnecessary dose
Scattering is bad as it blurs the image and means that shielding is required
How is the energy found of a photon?
E = hf = hv
How is the frequency found for a photon?
f = c / λ
What is the electron volt equation?
E = qv
What factors affect the interactions of photons with matter?
Photon energy
Target matter
Luck
Why is there a high chance that photons pass through atoms and won’t encounter electrons?
Due to empty space in atom
What makes it more likely for a photon to encounter an atomic electron?
The thicker the tissue
If there are more electrons per atoms
What is attenuation?
The reduction of intensity of a beam of photons as it passes through a thickness of tissue which is caused by absorption or scattering
What does the fractional reduction of intensity (attenuation) depend on?
thickness
d I ∝ - I dx
successive layers of material thickness will attenuate the same fraction each time
What is Beer’s law?
Intensity decreases with further distance into the tissue (as more atoms interact with photons)
I = I_0 e^ - μ x
What is the total linear attenuation coefficient μ?
A measure of likelihood of photon being attenuated within a given length (mm^-1)
(intensity before and after the material)
What does μ vary with?
Material type (density and atomic number Z)
Photon energy (higher energy = lower μ)
What is the mean free path?
Average distance travelled by a photon before interacting
1/μ
(generally less that original at depth)
What is the total attenuation coefficient made up of?
μ = τ + σ + κ + Φ
τ = Photoelectric effect
σ = Compton effect
κ = Pair production
Φ = Elastic scatter
What is the mass attenuation coefficient?
mass independent attenuation
linear attenuation coefficient / density of material
cm^2 g^ -1
What is the interaction cross section?
Effective area presented to an incoming photon beam (σ)
(larger areas = greater chance of interaction)
cm^2
What is the equation for attenuation and atomic cross section?
μ = N σ
N = Atomic density (cm^-3)
σ = Atomic cross section
What is the equation for atom density?
N = ρNa/ Mr
ρ = material density (g cm^-3)
Na = Avogadro’s number
Mr = relative atomic mass
What is the total linear attenuation coefficient for a compound or a mixture?
μ_total = N_1 σ_1 + N_2 σ_2 + …
What is the half-value layer (HVL)?
The thickness of any given material where 50% of the incident energy (intensity) has been attenuated
(number of photons in beam in reduced)
mm or cm
What does the HVL depend on?
Photon beam energy (not energy of photons)
(increase in energy = increase in material’s HVL)
How does the intensity change if the number of half value layers are 2, 3 or 4?
At 2 layers: I_0 / 4
At 3 layers: I_0 / 8
At 4 layers: I_0 / 16
What is the relationship between HVL and linear attenuation coefficient?
HVL is inversely proportional to the linear attenuation coefficient
HVL = ln2 / μ
How does HVL relate to density?
Higher atomic number leads to more interactions so higher HVL
What is fluence?
Number of photons incident on a particular area of tissue
m^-2
What is energy fluence?
The energy of photons passing through a particular area of tissue
Jm^-2
What is the intensity?
The energy fluence rate - the energy passing through a particular area of tissue in a particular time
Jm^-2 s^-1
Wm^-2
What is a narrow beam?
Thin beam which collimates like a pencil
What is a broad beam?
Beam is wide and often fan shape radiating outwards
