Ionising Radiation

Question 1

What does radiation carry enough energy to do?

Answer 1

To free electrons from atoms and make ions

Question 2

How much energy is needed to free electrons?

Answer 2

Enough to overcome the electron binding energy

Question 3

What is directly ionising?

Answer 3

Charged particles interacting directly via Coulomb’s law to free atomic electrons

e.g. alpha, electrons, protons

Question 4

What is indirectly ionising?

Answer 4

Electrically neutral particles interacting to free electrons in secondary processes

e.g. neutrons, photons

Question 5

What is important for radiotherapy?

Answer 5

Absorption : energy is delivered where the interaction occurs

Scatter is bad as it blurs out the volume receiving dose

Transmission delivers no dose

Question 6

What is important for imaging?

Answer 6

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

Question 7

How is the energy found of a photon?

Answer 7

E = hf = hv

Question 8

How is the frequency found for a photon?

Answer 8

f = c / λ

Question 9

What is the electron volt equation?

Answer 9

E = qv

Question 10

What factors affect the interactions of photons with matter?

Answer 10

Photon energy

Target matter

Luck

Question 11

Why is there a high chance that photons pass through atoms and won’t encounter electrons?

Answer 11

Due to empty space in atom

Question 12

What makes it more likely for a photon to encounter an atomic electron?

Answer 12

The thicker the tissue

If there are more electrons per atoms

Question 13

What is attenuation?

Answer 13

The reduction of intensity of a beam of photons as it passes through a thickness of tissue which is caused by absorption or scattering

Question 14

What does the fractional reduction of intensity (attenuation) depend on?

Answer 14

thickness

d I ∝ - I dx

successive layers of material thickness will attenuate the same fraction each time

Question 15

What is Beer’s law?

Answer 15

Intensity decreases with further distance into the tissue (as more atoms interact with photons)

I = I_0 e^ - μ x

Question 16

What is the total linear attenuation coefficient μ?

Answer 16

A measure of likelihood of photon being attenuated within a given length (mm^-1)

(intensity before and after the material)

Question 17

What does μ vary with?

Answer 17

Material type (density and atomic number Z)

Photon energy (higher energy = lower μ)

Question 18

What is the mean free path?

Answer 18

Average distance travelled by a photon before interacting

1/μ

(generally less that original at depth)

Question 19

What is the total attenuation coefficient made up of?

Answer 19

μ = τ + σ + κ + Φ

τ = Photoelectric effect

σ = Compton effect

κ = Pair production

Φ = Elastic scatter

Question 20

What is the mass attenuation coefficient?

Answer 20

mass independent attenuation

linear attenuation coefficient / density of material

cm^2 g^ -1

Question 21

What is the interaction cross section?

Answer 21

Effective area presented to an incoming photon beam (σ)

(larger areas = greater chance of interaction)

cm^2

Question 22

What is the equation for attenuation and atomic cross section?

Answer 22

μ = N σ

N = Atomic density (cm^-3)

σ = Atomic cross section

Question 23

What is the equation for atom density?

Answer 23

N = ρNa/ Mr

ρ = material density (g cm^-3)

Na = Avogadro’s number

Mr = relative atomic mass

Question 24

What is the total linear attenuation coefficient for a compound or a mixture?

Answer 24

μ_total = N_1 σ_1 + N_2 σ_2 + …

Question 25

What is the half-value layer (HVL)?

Answer 25

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

Question 26

What does the HVL depend on?

Answer 26

Photon beam energy (not energy of photons)

(increase in energy = increase in material’s HVL)

Question 27

How does the intensity change if the number of half value layers are 2, 3 or 4?

Answer 27

At 2 layers: I_0 / 4

At 3 layers: I_0 / 8

At 4 layers: I_0 / 16

Question 28

What is the relationship between HVL and linear attenuation coefficient?

Answer 28

HVL is inversely proportional to the linear attenuation coefficient

HVL = ln2 / μ

Question 29

How does HVL relate to density?

Answer 29

Higher atomic number leads to more interactions so higher HVL

Question 30

What is fluence?

Answer 30

Number of photons incident on a particular area of tissue

m^-2

Question 31

What is energy fluence?

Answer 31

The energy of photons passing through a particular area of tissue

Jm^-2

Question 32

What is the intensity?

Answer 32

The energy fluence rate - the energy passing through a particular area of tissue in a particular time

Jm^-2 s^-1

Wm^-2

Question 33

What is a narrow beam?

Answer 33

Thin beam which collimates like a pencil

Question 34

What is a broad beam?

Answer 34

Beam is wide and often fan shape radiating outwards