28. Dosimetry. Dose concepts.

Question 1

Define absorbed dose, explain how it is measured and give its formula

Answer 1

Absorbed dose (physical dose):

E of radiation that is absorbed by the unit mass of the absorber. It can be applied for any type of ionizing radiations and for any type of material of the absorber

→ measured indirectly by nuclear measurement techniques

Formula:** D = ∆E/∆m** in [J/kg] = [Gy] Gray, [Sv] Sievert

Nice to know: 6-8 Gy, Sv (in biological tissue) are lethal

Question 2

Define exposure, how it is measured and give its formula

Answer 2

Exposure (technical dose):

in e^- equilibrium (net charge exchange between air & material = 0) the exposure shows how much charge is produced in one unit mass of the air by X-ray or γ-radiation (< 3 MeV)

Formula: X = ∆Q/∆m in [C/kg]

→ measured by ionization chambers

Question 3

What is the difference between exposure and dose? Describe the stepwise conversion from exposure to absorbed dose in tissue.

Answer 3

Exposure characterizes the dose in air.

Conversion: exposure → dose in air

34/35 bV needed to produce 1 pair of ions in air

→ 34 Gy_air

_Conversion: dose in air → absorbed dose in tissue _

D ~ μ_m

→ D = μ_m,tissue/μ_m,air * f₀ * X

f₀ = 34 J/C

if E_photon < 0.6 MeV, for soft tissue: μ_m,tissue/μ_m,air = 1.1