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Flashcards in Dosimetry Equipment Deck (15)
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Explain how an ion chamber measures dose.

- 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 calualted (D=E/m)


What are the advantages of an ion chamber?

- 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.


What are the disadvantages of an ion chamber?

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


Explain how a GM tube measures ionisation events.

- 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.


What are the advantages of a GM tube?

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


What are the disadvantages of a GM tube?

- Reduced dynamic range due to dead time.
- Poor energy response.


Explain how a scintillation device measures ionisation events.

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


What are the advantages of a Scintillation device?

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


What are the disadvantages of a Scintillation Device?

- Reduced dynamic range.
- Poor energy response.


Explain how a solid state device measures dose?

- 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)


What are the advantages of a Solid State Device?

- 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.


What are the disadvantages of a Solid State Device?

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


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

- 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.


What are the advatages of TLDs?

- Small and light (used for personal monitoring)


What are the disadvantages of TLDs?

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