Dosimetry equipment

Question 1

When do we measure radiation in healthcare?

Answer 1

• Patient dose (risk): effective and organ dose.
• Occupational dose: ALARP.
• Area monitoring: room shielding, “other persons” dose.
• Calibration in NM: how much isotope?
• Equipment performance: QA/calibration.

Question 2

What are the physical dose quantities and how can we link them to protection dose quantities?

Answer 2

• Exposure [dQ/dm].
• Air KERMA.
• Absorbed Dose [dE/dm].
• calculation and phantoms links to protection quantities.

Question 3

What are the operational dose quantities and how can we link them to protection dose quantities?

Answer 3

• Ambient dose equivalent H*(d)
• Personal Dose equivalent Hp(d)
• Measurable approximation to link to protection quantities.

Question 4

What are the protection dose quantities?

Answer 4

• Equivalent organ dose (HT)

- Effective dose (E)

Question 5

What is the ICRU sphere?

Answer 5

• Theoretical sphere, 0.3m diameter with density of 1000kg/m^3 and a mass composition equivalent to tissue.
• Radiation field expanded and aligned so that it encompasses the sphere and the quantity is independent of the angular distribution of the radiation field.

Question 6

What is the ambient dose equivalent and how is it obtained?

Answer 6

• Operational quantity for area monitoring (strongly penetrating radiation only).
• H*(10) in a radiation field is the dose equivalent that would be produced by the corresponding ‘expanded and aligned field’ in the ICRU sphere at a depth of 10mm, on the radius opposing the direction of the aligned field.
• Good estimate for effective dose E [Sv].
• Obtained through calibration chain and published conversion factors (convert photon fluence, air KERMA or exposure to H(10) or H(0.07).

Question 7

What is the personal dose equivalent and which depths should be used for effective dose, skin dose and eye dose?

Answer 7

• Operational quantity for personal dose.
• The equivalent dose that would be generated in a depth ‘d’ from the surface of a person.
• Hp(10): good estimate of effective dose to a person.
• Hp(0.07): for skin.
• Hp(3): for lens of eye.
• Measured with a dosimeter that is worn at the surface of the body and covered with tissue equivalent material.

Question 8

Describe how an ion chamber works.

Answer 8

• Contains air at normal atmospheric pressure.
• 2 electrodes with p.d of ~ few hundred volts.
• Radiation causes ionization of air - creates charge pairs.
• e- and ions accelerated towards electrodes.
• Charge collected and measured.
• Current is proportional to dose.
• Ion recombination can occur.
• Sensitivity is proportional to the mass of air.

Question 9

How can we relate the exposure measured with an ion chamber to air KERMA?

Answer 9

• 33.7eV to make an ion pair.

- KERMA = 33.7(Q/em) eVKg^-1 (convert to mGy)

Question 10

State the positives and negatives of using an ion chamber.

Answer 10

Good:
-Measures air KERMA
-Gives linear response across large dynamic range.
-Variety of chamber sizes: large increases sensitivity, small increases spatial resolution.
-Excellent for standards: cross calibration.
-Can measure dose rate or dose accumulate.
Bad:
-Electrometer readout: limited by leakage current.
-Temperature and pressure corrections required.

Question 11

Describe how a GM tube works.

Answer 11

• Sealed chamber contains very low pressure gas.
• 2 electrodes with very high p.d. between them ~400-800V.
• Radiation causes creation of ion pairs.
• e- accelerated by high voltages and causes further ionization.
• Chain reaction until saturation and a pulse is detected by the electronics.
• Measures in CPS but can be calibrated to measure μSv/hr within specific E range.

Question 12

State the positives and negatives of using a GM tube.

Answer 12

Good:
-~1000 x increased sensitivity c.f. ion chamber.
-Can detect all radiation types.
-Low dose measurements - contamination monitoring.
-Cheap.
Bad:
-Poor at high doserates: dead time, 10^4-10^5 CPS limit.
-Poor energy response output (pulse always the same regardless of energy of radiation).

Question 13

Describe how a scintillation detector works.

Answer 13

• Phosphor as detection medium e.g. NaI crystal.
• Radiation excites e-s, releases flash of light.
• light hits photo cathode: e-s released.
• PMT: e-s accelerated by dynodes, amplifying signal.

Question 14

State the positives and negatives of using a scintillation detector.

Answer 14

Good:
-High sensitivity, ~1000 x ion chamber.
-Size of pulses can be related to energy of radiation.
Bad:
-Low intrinsic efficiency of detection for high energy gamma rays.
-Reduced resolution in large crystals since some of the light photons will not reach the detector.
-Crystals sensitive to temp. changes, may crack.

Question 15

Describe how a solid state detector works.

Answer 15

• Radiation creates electron-hole pairs in semiconductor.
• no of electron-hole pairs proportional to energy of rad.
• e-s and holes move to electrodes: measurable voltage.

Question 16

Describe how a TLD badge works.

Answer 16

• e.g. LiF
• When irradiated, e-s stored in meta-stable states within crystal structure.
• Heat badge to ~few hundred degrees to release e-s from traps.
• e-s fall back to ground state releasing visible, UV and IR light which is measured using PMT.
• Intensity of light released is related to dose.

Question 17

State the positives and negatives of using a TLD badge.

Answer 17

Good:
-Linear response to dose.
-High sensitivity to dose.
-Response is energy independent.
-Reusable after readout.
Bad:
-Needs to be sent off to ADS, no immediate readout.
-Doses cannot be re-read.

Question 18

Describe how Gafchromic film works.

Answer 18

• Contains dye that changes colour upon exposure.
• Dose obtained via a lookup table or calibration curve.
• Results can be obtained almost instantly.
• Very poor response <1cGy - good for RT and primary beam.