L 5: Measurement of ionizing radiation

Question 1

Exposure definition

Answer 1

• Measurement of ionization in air produced by photons.
• Value of total charge of ions of one sign (+/-) produced in air when all electrons liberated are completed stopped in air.
• Cannot be used for photons above 3MeV energy

Question 2

Unit of measurement of ionizing radiation exposure

Answer 2

Roentgen

Question 3

1R=

Board Question

Answer 3

2.58 x 10 to the power -4c/Kg in air

Question 4

Exposure formula

Answer 4

X=dQ/dm

dQ= charge
dm= mass

Question 5

Exposure units per formula

Answer 5

C/Kg

Question 6

Ionization can be measured only in

Answer 6

Air and only for photons

Question 7

Ionization cannot be measured in

Answer 7

Electrons or protons

Question 8

Free air ionization chamber

Answer 8

• Primary Std chamber for calibration of secondary chambers.
• A condition of electron equilibrium.
• ionization loss = ionization gain.

Question 9

Accurate measurements require corrections of

Answer 9

1. Air attenuation
2. Recombination on ions
3. Temp
4. Pressure
5. Humidity
6. Ionization produced by scattered photons
7. Non-uniform electric field

Question 10

Upper limit of photon energy to measure Roentgen

Answer 10

3 MeV

Question 11

Thimble Chamber

Answer 11

They are field instruments used in RT
They are still sensitive to air pressure, tempt and humidity

Question 12

Stem Effect

Answer 12

In Thimble chamber
Effect of ionization in the body of the stem
Between the end of the chamber and the metal cap

Question 13

Farmer Chamber

Answer 13

They are used to calibrate low energy beam (<2MeV) used in RT

Question 14

Electrometer

Answer 14

It is a charge measuring device
Eg: Negative feedback operational amplifier

Question 15

Secondary chambers

Answer 15

These chambers require calibration from std chambers
eg: Farmers Chamber. Farmer-type chamber

Question 16

Parallel plate chamber
Extrapolation chamber

Answer 16

Used to measure surface dose or dose in the build up region where dose gradients are high

Question 17

Polarity effect

Answer 17

Chamber readings changing with chane in polarity
ideally should be <0.5% for a well designed chamber

Question 18

Biological T 1/2 life

Answer 18

t 1/2 and effective t1/2 will be given

Question 19

Pneumbra

Answer 19

Increases as SDD decreases

Question 20

Best way to measure pacemaker dose

Answer 20

• OSLD
  Optically stimulated luminescent dosimeter
• TLD
• =Thermoluminescent dosimeters

Question 21

TG-51 Ion chamber reading

Answer 21

• Mraw = raw chamber reading
• Pion = incomplete collection efficeincy

Question 22

SIlicon Diodes

Answer 22

Small enough to measure small fields

Question 23

Well chamber

Answer 23

Used for radioactive sources callibration

Question 24

Portal Dosimetry

Answer 24

• MV image detector in linac
• Used for image verification for IMRT treatment.
  *

Question 25

Ionization chamber

Answer 25

* directly proportional to pressure * Inversly proportional to temp

Question 26

AS the energy increase the width of the pneumbra

Answer 26

Increases due to secondary electrons

Question 27

CTDI

Answer 27