Dosimetry Equipment Flashcards

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1
Q

Explain how an ion chamber measures dose.

A
  • 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)
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2
Q

What are the advantages of an ion chamber?

A
  • 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.
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3
Q

What are the disadvantages of an ion chamber?

A
  • Needs electrometer for readout - modern electrometer can only read down to 1pC (limited by leakage current). This is approximately 10^6 ion pairs.
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4
Q

Explain how a GM tube measures ionisation events.

A
  • 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.
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5
Q

What are the advantages of a GM tube?

A
  • Can count individual ionisation events.
  • Increased sensitivity over ion chamber (x1000)
  • Best for low-level measurements.
  • Cheap!
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6
Q

What are the disadvantages of a GM tube?

A
  • Reduced dynamic range due to dead time.

- Poor energy response.

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7
Q

Explain how a scintillation device measures ionisation events.

A
  • Due to arrangement of electrons in material (NaI, plastic scintillators), light is emitted when irradiated.
  • Light is amplified by a PMT, and measured.
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8
Q

What are the advantages of a Scintillation device?

A
  • 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.
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9
Q

What are the disadvantages of a Scintillation Device?

A
  • Reduced dynamic range.

- Poor energy response.

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10
Q

Explain how a solid state device measures dose?

A
  • 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)
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11
Q

What are the advantages of a Solid State Device?

A
  • 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.
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12
Q

What are the disadvantages of a Solid State Device?

A
  • Not a counting device - so sensitivity limited by detector size and electrometer.
  • Significant energy dependence.
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13
Q

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

A
  • 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.
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14
Q

What are the advatages of TLDs?

A
  • Small and light (used for personal monitoring)
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15
Q

What are the disadvantages of TLDs?

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