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PHYS853: Radiation Protection in Medicine > Measurement in Radiation Protection > Flashcards

Measurement in Radiation Protection Flashcards

(67 cards)

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1
Q

Legislation exists defining permitted dose limits to _________ and _____.

A

Individuals
Areas

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2
Q

How can radiation dose to areas and people be monitored?

A

Using radiation monitoring instruments, including area survey meters and personal dosimeters.

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3
Q

Why do dose levels need to be measured?

A

To control access to areas, demonstrate compliance with legislation, and keep people safe.

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4
Q

Define absorbed dose

A

The basic physical dosimetry quantity that represents the specific energy (energy per unit mass) deposited by ionizing radiation in living matter. Absorbed dose is measured in J/kg or Gy.

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5
Q

Why isn’t absorbed dose satisfactory for radiation protection purposes?

A

It doesn’t consider the effects of different types of ionising radiation or biological effects to different tissues.

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6
Q

Define equivalent dose

A

The basic quantity in radiation protection that considers the absorbed dose, weighted for the biological effectiveness of the radiation type used (using a radiation-weighting factor). Equivalent dose is measured in J/kg or Sv.

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7
Q

Define effective dose

A

A dosimetric quantity which can be related to risk and is calculated as the weighted sum of mean equivalent doses to organs, using organ weighting factors to account for organ radiosensitivity. Effective dose is measured in J/kg or Sv.

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8
Q

Define absorbed organ dose

A

The mean absorbed dose (physical dose) in a specified tissue or organ of the human body.

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9
Q

Give the equation for absorbed organ dose

A

D_T = absorbed organ dose
ε_T = total energy imparted by radiation into that tissue/organ
m_T = mass of the tissue/organ

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10
Q

Give the equation for equivalent dose

A
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11
Q

What is the radiation-weighting factor for X-rays?

A

1

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12
Q

What is the radiation-weighting factor for gamma rays?

A

1

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13
Q

What is the radiation-weighting factor for electrons?

A

1

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14
Q

What is the radiation-weighting factor for protons?

A

2 (ICRP 103, 2007)

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15
Q

What is the radiation-weighting factor for alpha particles?

A

20

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16
Q

What is the radiation-weighting factor for neutrons?

A

It varies depending on energy

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17
Q

Give the equation for effective dose

A
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18
Q

Is effective dose directly measureable?

A

No

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19
Q

What is an operational quantity?

A

Practical measurements that can be made as a reasonable estimate of effective dose when monitoring and investigating situations involving external exposure.

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20
Q

What are operational quantities used for?

A

Calculating the dose equivalent at a point in a human body/phantom based on the type and energy of the radiation existing at that point. These calculations are on the basis of the energy fluence at that point.

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21
Q

What are the 3 types of operational dose quantity?

A
  • Ambient dose equivalent
  • Directional dose equivalent
  • Personal dose equivalent
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22
Q

Define ambient dose equivalent, H*(d)

A

The dose that would be measured at depth, d, in a sphere which has a unidirectional field incident upon it.

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23
Q

Define directional dose equivalent, H’(d,α)

A

The dose that would be measured at depth, d, in a sphere which has a unidirectional field incident upon it at angle, α.

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24
Q

Define personal dose equivalent, Hₚ(d)

A

The dose equivalent in soft tissue at an appropriate depth, d, below a specified point on the human body. It is normally used to demonstrate compliance with dose limits

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25
What distances are chosen when defining operational personal dose quantities to the skin, eyes, and when using strongly penetrating radiation?
Skin = 0.07mm Eyes = 3mm Strongly penetrating = 10mm
26
State 3 uses of dose measurements
1) Completing radiation surveys 2) Assessing risks and writing risks assessments 3) Writing local rules
27
What is a radiation survey?
Taking a series of measurements to verify the calculations, structures, and safety features of a radiation source.
28
What is a critical exam?
A mandatory review of newly installed or modified radiation-emitting equipment to ensure it operates safely and provides adequate protection from radiation exposure.
29
State 5 typical measurements made in a radiation survey
1) Energy 2) Dose rate 3) Field-size and direction 4) Scatter 5) Positions of measurements
30
What needs to be considered when choosing the appropriate type of equipment to measure radiation?
- Radiation type - Energy - Dose rate - Duration of exposure - Geometrical precision
31
What are the common features of radiation monitors that make them more useable?
- Preliminary function checks - Scale zeroing - Audible alarm - Portability
32
To ensure that radiation monitoring instruments are accurate, they must be _______ and traceable to National Standards. This must be regularly _______.
Calibrated Verified
33
What are the 4 main categories of radiation detectors?
- Film: passive chemical film - Gas: gas filled detectors such as ionisation chambers - Scientillation detectors: utilise either a liquid or solid state scintillator as a detection medium - Semiconductor detectors: an elemental or compound semiconductor crystal is used as the detection medium
34
How does traditional film work?
It contains silver bromide crystals suspended in cellulose. Ionising radiation releases free silver to form a latent image that is then developed, providing blackening on the film.
35
Traditional film is most commonly used for ______ dose measurements, but can be calibrated by exposure to ____ doses for _______ dose measurements.
Relative Known Absolute
36
What are the advantages of using traditional film?
+ Very high spatial resolution + Provides a 2D dose map + Produces a permanent record of dose + No electrical connections required
37
What are the disadvantages of using traditional film?
- It requires processing - There is a finite latitude so can only assess a range of doses - It has a high atomic number relative to tissue so the response doesn't necessarily correspond to patient dose - It requires processing with specialised equipment which can result in output variation between batches
38
How do gas detectors work?
An inert gas is sealed in a chamber with an electrode running through the middle and another connected to the wall. These generate an electric field in the chamber. When ionising radiation interacts with the gas, it ionises them and charged particles are collected. This produces a detectable charge which we can convert into a dose measurement.
39
How do secondary ionisations occur in gas detectors?
When initial ionised particles pass by other inert gas atoms, they can ionise that gas, creating more ionised particle and leading to an amplification of the initial ionization effect.
40
What are the 3 types of gas detectors?
1) Ionisation chamber 2) Proportional chamber 3) Saturation detector (Geiger-Muller tubes)
41
Describe the graph of increase in number of ion pairs with increase in applied voltage
Ionisation chamber = region B Proportional chamber = region C Saturation detector = region E
42
What are ionisation chambers?
Gas detectors that can be used to measure dose by applying a relatively low voltage. The voltage is chosen as it is in a relatively stable region where charge is proportional do dose. Ionisation chambers come in many sizes based on their purpose.
43
What voltage is used in ionisation chambers?
< 400 V
44
What are the advantages and disadvantages of ionisation chambers?
+ High accuracy + Well studies and understood + Low dose-rate dependence + Linear response + Stable +No secondary ionisation - Sensitivity: small chambers have poor sensitivity so require high doses and large chamber have good sensitivity but poor spatial resolution
45
What are proportional counters?
Gas detectors that can be used to measure dose with a greater charge multiplication than ionisation chambers. This means that they have good sensitivity so are suitable for low intensities and neutrons. However, some secondary ionisations are produced which can impact the response time, making them less suitable for high dose rates.
46
What is a Geiger-Müller counter?
A gas detector that counts events, with each event completely ionising the gas. The potential is so strong here that when one ionisation event happens, the charged particle created ionises the gas nearby which in turn ionises more gas. This means that it is very sensitive but not suitable for high dose rates or pulsed beams due to the high recovery times. The signal for GM tubes is independent of the initiating energy.
47
What are gas detectors best used for?
- Some radiation surveys to measure scatter - Leakage measurements - Quality assurance - Contamination monitoring (routine proactive and reactive monitoring, and emergency scenarios)
48
What are the 3 types of solid state detectors?
1) Semiconductors 2) Scintillators 3) Thermo-luminescent detectors (TLDs)
49
What are semiconductor detectors?
Solid state detectors that act in a similar way to ionisation chambers, with incident radiation interacting and producing charge which is collected and read out. They are typically much more sensitive than ion chambers as they require less energy to make ion pairs and have a much higher density. This means that they have a good spatial resolution as they can be made much smaller.
50
What are the advantages and disadvantages of semiconductor detectors (diodes)?
+ Smalland sensitive volume + High gain (low threshold energy for ionisation) + Instant read-out - Temperature dependent - Subject to radiation damage so require regular calibration - High atomic number relative to water
51
What are semiconductor detectros best used for?
In radiotherapy: - In-vivo point-dose measurements - Arrays for quality assurance measurements In diagnostics: - In arrays for imaging - In dose measuring devices
52
What are scintillators?
Detectors based on scintillation (light emission). They contain certain organic and inorganic crystals containing activator atoms and emit scintillations upon absorption of radiation.
53
How do scintillation probes work?
A scintillator converts energy lost by ionising radiation into pulses of light. These are then converted into photoelectrons and accelerated down a photomultiplier tube to an anode where the charge is measured and analysed. The energy of the pulse emitted is proportional to the energy absorbed.
54
How can the signal of a scintillaton detector be improved?
By doping the scintillator (as it gives the excited electrons more potential energy states, releasing more photons)
55
What are the properties of an ideal scintillator?
- High efficiency - Linear (light yield is proportional to the energy) - Good light collection (the crystal should be transparent to its own light) - Short decay time (rate of electrons decaying back to a stable state limits system sensitivity) - Size - Index of refraction (as close of possible to glass to permit efficient coupling to the PMT) - Low cost
56
What are the 4 types of scintillator?
- Inorganic - Organic - Pure - Activated
57
What are the pros and cons of inorganic scintillators?
+ Good light output + Good linearity - Relatively slow decay time
58
What type of radiation are inorganic scintillators good for?
Gamma rays
59
What are the pros and cons of organic scintillators?
+ Usually much faster than inorganic scintillators - Lower light yield than inorganic scintillators
60
What type of radiation are organic scintillators good for?
Beta Fast neutrons
61
What are thermoluminescent dosimeters?
Small crystals that have electrons in the valence band that, when irradiated, move up to the condution band then get trapped. By heating up these crystals, the electron is released and light is emitted. This light is proportional to the dose delivered.
62
What are the advantages and disadvantages of TLDs?
+ Small size and variety of forms + No electronic connections required + Approximately tissue-equivalent - Time involved in preparation and reading (processing time) - No permanent record - Potential light output fades with time - Low accuracy
63
What are the typical uses of TLDs?
- Point dose measurements in/on phantoms - Personal dosimetry - Dose-profile measurements - Environmental monitoring
64
What are the two types of personal dosimetry?
- Long term 'film badges' - Real time electronic badges
65
When are long term 'film badges' used?
For all radiation workers to provide an integrated dose over a long period. They contain filters to differentiate the different energies/types of radiation that the individuals were exposed to.
66
When are real time electronic badges used?
In high dose rate situations.
67
What is environmental monitoring?
A measurement of the dose delivered to a specific, fixed location over an extended period. It is useful for demonstrating compliance with legislation and demonstrating teh accuracy of risk assessments.

PHYS853: Radiation Protection in Medicine (19 decks)

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