Dosimetry

Question 1

What is ionizing radiation?

Answer 1

radiation that has enough E to produce directly or indirectly ion pairs while absorbed in the matter

Question 2

Differentiate btw direct and indirect effect

Answer 2

• direct effect: ionizing radiation causes directly damage in biologically important macromolecules
• indirect effect: ionizing radiation absorbed in H₂0 and causes indirectly damage in biologically important macromolecules via free radicals

Question 3

Differentiate btw stochastic and deterministic effect.

Answer 3

• stochastic effect: probability of damage is proportional to dose (no threshold!)
  → e.g. cell mutations
• deterministic effect: probability of damage increases abruptly above threshold dose, above this level severity proportional to dose
  → e.g. erythema

Question 4

Give the formula for absorbed dose (D) and its unit.

It is valid for… ?

Answer 4

energy absorbed by mass Δm

D = ΔE/Δm

in Gray = [Gy]

• valid for any ionizing radiation

Question 5

Give the formula for exposure (X) and its unit.

What is the conversion factor to absorbed dose?

It is valid for… ?

Answer 5

positive charge produced in Δm of air by ionization (at e^- equilibrium)

X = Δq/Δm

in [C/kg]

• conversion factor = 34
• valid for X-rays and γ-rays smaller than 3 MeV (α- and β- doesn’t propagate long distances in air)

Question 6

Give the formula for dose equivalent (H_T), its unit and explain what it is.

It is valid for… ?

What happens in case of more than 1 type of radiation?

Answer 6

absorbed dose D_{T, R} * radiation weighting factor (w_R), ⇒ characteristic for the type and energy of the radiation

H_T = D_{T, R} * w_R

in Sievert = [Sv]

• w_R = 1 refers to X-rays, γ-rays
• H_T must be added in case of more than 1 type of radiation

Question 7

Give the formula for effective dose equivalent (E), its unit and explain what it is.

It is valid for… ?

What happens in case of affection of more than 1 organ?

Answer 7

equivalent dose * tissue weighting factor (w_T)
⇒ characteristic for the sensitivity of the organ for the radiation damage

E = H_T * w_T

in Sievert = [Sv]

• E must be added in case of affection of more than 1 organ

Question 8

Give the formula for the dose rate and some examples for possible units

Answer 8

Strength of the ionizing radiations

dose rate = D/t = dose per unit time

in [Gy/h], [mGy/h], [Sv/h], etc.

Question 9

How can the dose in case of γ-radiation be calculated?

Give the formula.

What about other types of radiation?

Answer 9

D_air = K_γ * At/r²

• K_γ = characteristic for the type of isotope [(µGy*m₂)/(h+GBq)]
• other types of radiation not considered, because they cannot rly penetrate deep enough into the body

Question 10

Explain the ALARA-principle w/r/t workplace security and medical applications.

Give some interesting numbers.

Answer 10

​As low as reasonably achievable = ALARA

• received dose must not exceed effective dose limit
• limit for whole body irradiation:
  
  • 100 mSv/5 years
  • 50 mSv/year

⇒ expected benefit of diagnostic/therapeutic procedure should outweigh estimated risk of radiation damage

Question 11

How does a film dosimeter work?

Are energy and type of radiation measurable?

Answer 11

measures darkening of the photographic film that was exposed to ionizing radiation

→ filters used to determine type and energy of the ionizing radiation

Question 12

Explain how an ionization chamber works.

What can be measured? Give formuals.

Name some types of ionization chambers.

Answer 12

1. ionizing radiation produces charges (ion pairs) in air btw the charged plates of a capacitor
2. charges Q move towards the electrodes, and produce an electric current, which is proportional to the dose rate
3. → change of the charge of the capacitor = dose
   → change of charge of the resistor = dose rate

⇒ according to Ohm’s law:

• dose measurement: U = Q/C ~ X
• dose rate measurement: U = I * R = Q/t * R ~ X/t

Types of ionization chamber:

• thimble ionization chamber
• pocket chamber dosimeter
• GM-tube

Question 13

What is the difference btw a “normal” ionization chamber and a GM-tube.

It is sensitive to what kind of radiation?

Answer 13

GM tube:

• filled with noble gas instead of air
• high voltage accelerates e- → avalanche of ionization → current pulse without amplifier
• value shown proportional to no. of ionizing particles (dose rate!)
• extremely sensitive to α-, β-radiation, but no information about type/energy of radiation

Question 14

How does a thermoluminescent dosimeter (TLD) work?

It is sensitive to what kind of radiation?

Answer 14

1. e^- produced by the ionizing radiation are trapped on the impurity E levels of the insulator crystal
2. Evaluation = heating the crystal, to allow the e^- to get back to the conduction band
3. e^- back down to the valence band → fluorescence

• sensitive to every kind of radiation

⇒ no. of emitted photons is proportional to the absorbed dose

Question 15

How does a semiconductor diode dosimeter work?

What is their advantage?

Answer 15

In the impurity semiconductor diode ionizing radiation generates e^--hole pairs which produce a current pulse → proportional to the dose rate

advantage:
because of larger density sensitivity is much larger

Question 16

How does a scintillation dosimeter work?

Answer 16

e^- are excited by ionizing radiation into the conduction band, and they decay through the impurity levels to the valence band, while emitting light.

no. of emitted photons per unit time is proportional to the dose rate