Dosimetry Equipment

Question 1

Explain how an ion chamber measures dose.

Answer 1

• X-Rays excite air molecules inside chamber forming ions and free electrons.
• Electric field gradient attracts ions.
• Charge collected and measured (some ions recombine prior to collection)
  (Efficiency depends on E-field strength and rate of irradiation)
  (Sensitivity depends on chamber size and air pressure)
• Mass of air in chamber calculated using STP correction.
• Energy required to create ion pair known (33.7eV)
• Dose can be calculated (D=E/m)

Question 2

What are the advantages of an ion chamber?

Answer 2

• Measuring Air KERMA, so give linear response across a large dynamic range.
• Changes with X-Ray energy slowly, predictably, and by no more than a few %.
• Chamber size can vary: large - increases sensitivity, small - increases spat. res.

Question 3

What are the disadvantages of an ion chamber?

Answer 3

• Needs electrometer for readout - modern electrometer can only read down to 1pC (limited by leakage current). This is approximately 10^6 ion pairs.

Question 4

Explain how a GM tube measures ionisation events.

Answer 4

• Ion pair in tube accelerated by large voltage (800V).
• Increased energy from ion pair creates further ion pairs (2-3 each) causing a chain reaction until there is saturation, and a pulse is detected by the electronics.
• After 10ms quenching gas absorbs ion pairs, resetting device.

Question 5

What are the advantages of a GM tube?

Answer 5

• Can count individual ionisation events.
• Increased sensitivity over ion chamber (x1000)
• Best for low-level measurements.
• Cheap!

Question 6

What are the disadvantages of a GM tube?

Answer 6

• Reduced dynamic range due to dead time.

- Poor energy response.

Question 7

Explain how a scintillation device measures ionisation events.

Answer 7

• Due to arrangement of electrons in material (NaI, plastic scintillators), light is emitted when irradiated.
• Light is amplified by a PMT, and measured.

Question 8

What are the advantages of a Scintillation device?

Answer 8

• Can measure individual events.
• Similar sensitivity to GM tube (x1000 ion chamber)
• Size of pulse can determine energy of radiation.
• Good for low-level measurement

Question 9

What are the disadvantages of a Scintillation Device?

Answer 9

• Reduced dynamic range.

- Poor energy response.

Question 10

Explain how a solid state device measures dose?

Answer 10

• Charge carriers in P-N diode increase upon irradiation.
• Allows for current to be measured using suitable electronics.
• Mass of diode known.
• Energy required to create ion pair known.
• Dose can be calualted (D=E/m)

Question 11

What are the advantages of a Solid State Device?

Answer 11

• Increased sensitivity compared to ion chamber (10x)
• Robust design
• Small size - good spat. res.
• No need to temperature/pressure correction.
• Energy dependence can be corrected out.

Question 12

What are the disadvantages of a Solid State Device?

Answer 12

• Not a counting device - so sensitivity limited by detector size and electrometer.
• Significant energy dependence

Question 13

Explain how dose can be measured from a Thermo-Luminescent Dosimeter (TLD).

Answer 13

• Irradiation of TLD (LiF in Teflon matrix) causes electrons to be stored in meta-stable states within the crystal.
• Once heated the electrons are released and measured as scintillation events measured using PMTs

Question 14

What are the advantages of TLDs?

Answer 14

• Small and light (used for personal monitoring)

Question 15

What are the disadvantages of TLDs?

Answer 15

• Sensitivity limited by efficiency of phosphor, lowest radiation levels not recorded.