Interaction with matter

Question 1

What are the types of interactions

Answer 1

Absorption
Scatter
Half value thickness (HVT) and attenuation coefficients

Question 2

What is attenuation?

Answer 2

A reduction in the intensity (energy) of the beam

Question 3

What is absorption?

Answer 3

the transfer of energy from the radiation beam to the irradiated material (e.g. The patient)

Question 4

What is scatter?

Answer 4

radiation in a direction other than the primary beam, with or without a loss of energy

Question 5

The amount of attenuation depends upon what?

Answer 5

The thickness of the material
The density of the material
The energy (wavelength) of the primary beam

Question 6

Differential attenuation

Answer 6

The various structures of the body attenuate by differing amounts

Across the radiation field, the beam leaving the body has an intensity profile according to the amount of attenuation

Question 7

What is HVT and what does it mean?

Answer 7

HVT IS A MEASURE OF THE PENETRATING POWER OF THE BEAM OF RADIATION (beam quality)

Definition:
The thickness of attenuator which, when inserted into the path of a homogeneous beam, will reduce its intensity by 50%

Question 8

What is HVT used for?

Answer 8

Used to work out how much radiation protection we need depending on the intensity of the beam.

HVT differs for different materials and energy of the beam.

You can use it to plot a graph, which has an exponential curve (never reaches zero)

Question 9

What are the types of attenuation coefficients?

Answer 9

Linear attenuation coefficient

Mass attenuation coefficient

Question 10

What does attenuation coefficients mean?

Answer 10

Attenuation coefficients indicate the effectiveness of materials to attenuate radiation

This is useful in radiation protection, clinical buildings design etc.

(Basically attenuation unit)

Question 11

What does Linear Attenuation Coefficient (L.A.C) mean

Answer 11

The fractional reduction in intensity of a narrow beam of radiation per unit thickness of the material

i.e. the amount per centimetre

Question 12

What is the disadvantage of L.A.C

Answer 12

The major disadvantage of the linear measurement is that it relies on the number of atoms in the path of the beam

It does not take into account the density of the material

Question 13

LAC formula

Answer 13

μ=0.693/HVT

Question 14

What does Mass Attenuation Coefficient (M.A.C) mean?

Answer 14

The fractional reduction in intensity of a narrow beam of radiation per unit cross section per unit mass of the material
i.e. The amount of reduction per gram of material

Question 15

MAC formula

Answer 15

MAC=LAC/Density

Units=m²/kg or cm²/kg