Radiation Protection Measurements

Question 1

Why is radiation measurement important?

Answer 1

Radiation is not apparent to our senses and can be harmful so it needs to be controlled and measured with technology

Question 2

What are the two classes of radiation monitoring equipment?

Answer 2

1. Area survey meters
2. Personal dosimeters

Question 3

What is absorbed dose?

Answer 3

Absorbed dose (D) is the energy absorbed per unit mass

D = E/m (J/kg = Gy)

Question 4

Why is absorbed dose not useful for radiation protection?

Answer 4

It isn’t related to risk because it doesn’t account for different types of radiation or biological effects

Question 5

What is equivalent dose?

Answer 5

Equivalent dose (H) accounts for the different effects of different radiation

H = D * Wr (J/kg = Sv)

Question 6

What is the radiation weighting factor?

Answer 6

Scales the absorbed dose by how ionising the radiation type is compared to photons

Question 7

What is the effective dose?

Answer 7

Effective dose (E) is a dosimetric quantity which can be related to risk

E = H * Wt (J/kg = Sv)

Calculated as a weighted sum of mean equivalent doses

Question 8

What are operational quantities?

Answer 8

They are based on dose equivalent to that at a point in the human body/phantom which relate to the type and energy of the radiation at that point. They are calculated on the basis of the energy fluence at that point

Question 9

Where are operational quantities defined?

Answer 9

ICRU

Question 10

What dose quantity is the skin dose limit of 500 mSv?

Answer 10

Equivalent dose

It is a skin dose limit so it is to a specific organ, therefore it cannot be effective dose. Effective dose is the weighted sum of all organs in the whole body (i.e. cannot have an effective dose limit for one organ)

Question 11

What is a radiation survey?

Answer 11

Performed as the first step after installation which verifies the construction and calculations as part of the prior risk assessment. This is in addition to the critical examination

Question 12

What should be considered when choosing an instrument for radiation protection measurements?

Answer 12

• Radiation type
• Energy
• Dose rate
• Duration
• Geometrical precision

Question 13

How are instruments calibrated?

Answer 13

Field instruments (tertiary standards) are calibrated to secondary standards, which are calibrated to a national primary standard (e.g. at NPL). National primary standards are calibrated against global standards

Question 14

Why are instruments calibrated?

Answer 14

So that we know 1 Sv in one centre is the same as 1 Sv in another, as well as 1 Sv in one country is the same as 1 Sv in another

Question 15

What are 4 detector technologies?

Answer 15

1. Film
2. Gas
3. Scintillation detectors
4. Semiconductor detectors

Question 16

How does film work?

Answer 16

1. AgBr crystals on a cellulose base
2. Radiation releases free Ag to form a latent image
3. The latent image is developed which blackens the film

Question 17

When is film used to measure dose?

Answer 17

For relative dose measurements, but can be calibrated for absolute measurements

Question 18

What are the advantages of film?

Answer 18

1. High spatial resolution
2. 2D dose map
3. Permanent record
4. No electronic connections

Question 19

What are the disadvantages of film?

Answer 19

1. Requires processing
2. Finite latitude (range of dose)
3. High atomic number relative to tissue (different energy response)
4. Optical density dependent of processing chemistry - variable between batches

Question 20

How do gas detectors work?

Answer 20

An inert gas is sealed in a chamber with an electrode running through the middle- and another connected to the wall which generates an electric field in the chamber.
When an incident photon interacts with the gas, it ionises the gas producing an electron/positive ion pair which are accelerated by the field and collected by the electrodes producing a detectable charge which can be converted to a dose measurement

Question 21

What are some examples of gas detectors?

Answer 21

• Ionisation chambers
• Geiger-Muller tubes/Saturation detector
• Proportional counters

Question 22

How can the sensitivity be improved of a gas detector for high or low energy photons?

Answer 22

Add a build-up for high energies
Use a thin window for low energies

Question 23

What are the 6 regions of the gas chamber response graph?

Answer 23

1. Recombination region
2. Ionisation chamber region
3. Proportional region
4. Region of limited proportionality
5. GM counter region
6. Region of continuous discharge

Question 24

What is the relationship between charge and dose in an ionisation chamber?

Answer 24

Proportional

Question 25

When is an ionisation chamber used

Answer 25

For lower dose rates

Question 26

What are the disadvantages of ionisation chambers?

Answer 26

Gain is dependent on mass or volume of sensitive region - need to balance spatial resolution with sensitivity (i.e. small chambers have good resolution but poor sensitivity and vice versa)

Question 27

What are the advantages of ionisation chambers?

Answer 27

1. High accuracy 2. Well studied and understood 3. Low-dose rate dependent (recombination) 4. Linear response 5. Stable

Question 28

What is a proportional counter and when is it used?

Answer 28

Secondary ionisation is produced meaning there is a charge multiplication (~10^3-4) which gives good sensitivity but may need recovery time between events making it unsuitable for high dose rates

Question 29

When are gas detectors used?

Answer 29

1. Radiation surveys (scatter) 2. Leakage measurements 3. QA 4. Contamination monitoring

Question 30

What is a Geiger-Muller counter?

Answer 30

Each event completely ionises the gas meaning recovery time is needed and the signal is independent of initial energy. They have a high sensitivity but not suitable for high-dose rates

Question 31

Name 3 types of solid state detectors

Answer 31

1. Semiconductors 2. Scintillators 3. Thermo-luminescent detectors

Question 32

What are the advantages of solid state detectors?

Answer 32

1. High sensitivity (~10^4 times higher than ionisation chambers of the same volume) 2. Good spatial resolution 3. Miniaturisation

Question 33

What are the advantages of diodes?

Answer 33

1. Small sensitive volume (good for high dose gradients) 2. High gain (good for low dose rates) 3. Instant read-out (no processing)

Question 34

What are the disadvantages of diodes?

Answer 34

1. Temperature dependent (energy gap decrease with increasing temperature - bad for in-vivo) 2. Subject to radiation damage (regular calibration) 3. High atomic number relative to water (higher response to low energy scattered photons)

Question 35

What are scintillator detectors?

Answer 35

Detectors based on scintillation (light emission) using organic or inorganic atoms Incident radiation creates light emission

Question 36

How do scintillator detectors work?

Answer 36

Incident photon interacts with the scintillator which converts photon energy into light Light passes through the optical coupling producing electrons which are amplified by the dynodes (electron multiplication) in the PMT and the signal is detected Energy of pulse is proportional to the energy absorbed

Question 37

How does doping improve signal in a scintillator?

Answer 37

Undoped system - incident radiation excited an electron to a higher state, then drops down releasing one photon Doped system - additional states are introduced for the electron to occupy, releasing multiple photons as the electron drops to each state producing more signal, with the overall energy being the same as the undoped system

Question 38

What are the feature of an ideal scintillator?

Answer 38

1. High efficiency (to convert photon to light - high density and atomic number) 2. Linear (light proportional to energy deposited) 3. Good light collection 4. Scintillation material transparent to its own light 5. Short decay time (fast pulses can be generated) 6. Size 7. Index of refraction as close to glass (efficient coupling to PMT) 8. Low cost

Question 39

What are TLDs?

Answer 39

Thermoluminescent dosimeters

Question 40

How do TLDs work?

Answer 40

Valence band electrons raised to the conduction band when irradiated Electrons drop to an electron trap in the crystal lattice Upon heating, crystal gives the trapped electron enough energy to escape and fall back to the valence band, emitting light Amount of light produced is proportional to the dose delivered

Question 41

How do scintillators differ from TLDs?

Answer 41

In scintillators, the electrons fall rapidly back to the valence band without heating

Question 42

What are the advantages of TLDs?

Answer 42

1. Small 2. Variety of forms (chip, rod, powder) 3. No electrical connections 4. Approximately tissue equivalent

Question 43

What are the disadvantages of TLDs?

Answer 43

1. Time involved in preparation and reading 2. No permanent record 3. Potential light output fades with time 4. Low accuracy (~5%)

Question 44

What are TLDs typically made off?

Answer 44

LiF

Question 45

A member of staff has suspected radioactive contamination on their person (99mTc). For each of the following radiation detection media (radiographic film, Scintillation crystal contamination monitor and 1800cc ionisation chamber dose-rate meter), list 3 features of their use relevant to localising the contamination (defining each as either an advantage or disadvantage). Also clearly state the preferred choice of detection medium for this task. i) Radiographic film ii) Scintillation crystal contamination monitor iii) 1800cc ionisation chamber dose-rate meter

Answer 45

i) Radiographic Film Passive Detection – Disadvantage Radiographic film does not give real-time feedback, so contamination cannot be quickly localised or confirmed. Spatial Resolution – Advantage Film can provide a permanent image with good spatial resolution, showing the general area of exposure after development. Sensitivity to Gamma Radiation – Disadvantage Not very sensitive to low-energy gamma emitters like ⁹⁹ᵐTc, requiring long exposure times and high activity. ii) Scintillation Crystal Contamination Monitor (e.g., NaI(Tl) Probe) High Sensitivity to Gamma Radiation – Advantage Very effective for detecting low-energy gamma emissions from ⁹⁹ᵐTc, allowing for rapid and sensitive measurements. Real-Time Feedback – Advantage Provides immediate audio/visual feedback, useful for quickly scanning and pinpointing contamination. Directional Sensitivity – Advantage Can be maneuvered close to the body to narrow down the contamination site with high spatial precision. iii) 1800cc Ionisation Chamber Dose-Rate Meter Dose Rate Measurement Rather Than Localisation – Disadvantage Designed for measuring ambient dose rate, not precise source localisation. Slow Response Time – Disadvantage Typically has a slower response compared to scintillation detectors, making real-time scanning less effective. Large Volume Detector – Disadvantage The large chamber averages out readings over a wide area, reducing spatial resolution.