Personal Dosimetry Flashcards
What is Absorbed dose?
The energy absorbed by a medium per unit mass (measured in J/Kg).
What unit is Absorbed dose measured in?
GRAY
What is Equivalent Dose?
The dose obtained by multiplying the absorbed dose (Gy) by a radiation weighting factor (WR)
What unit is Equivalent Dose measured in?
Sievert (Sv)
What unit is Effective Dose measured in?
Sievert (Sv)
What is Tissue weighting factor (WT)?
A relative measure of the risk of stochastic effects that might result from irradiation of that specific tissue
Which organs/tissues has a Tissue Weighting Factor (WT) of 0.20?
Gonad
Which organs/tissues has a Tissue Weighting Factor (WT) of 0.12?
Red Bone Marrow
Colon
Lung
Stomach
Which organs/tissues has a Tissue Weighting Factor (WT) of 0.05?
Bladder Breast Liver Oesophagus Thyroid
Which organs/tissues has a Tissue Weighting Factor (WT) of 0.01?
Skin
What is the Linear No-Threshold (LNT) Dose-effect Model
If all radiation dose is minimised then individuals are unlikely to be exposed to any unnecessary risk
What’s the drawbacks of the LNT Model
It does not consider human defense mechanisms
What is a classified person?
Employees who are likely to receive:
An effective dose greater than 6 mSv per year or
An equivalent dose greater than 15mSv per year for the lens of the eye or
A dose greater than 150mSv per year for the skin or the extremities (hands, forearms, feet or ankles
What is Effective Dose?
The dose given to the whole body that would produce the same biological effect as the equivalent dose(s) received by each organ or tissue
Women who are pregnant or breastfeeding should not recieve an equivalent dose to the fetus at above what dose?
1 mSv
Name the 5 Characteristics of Dosimeters
Able to measure doses over the whole range of radiation types & energies encountered by the wearer
Show an appropriate response over that range
Provide sufficient information to enable the radiation type & energy to be identified and suitable correction factors applied
Capable of measuring doses in the range of fractions of milli Sieverts to tens of Sieverts.
Be issued for a specific period
What does TLD stand for?
Thermoluminescent Dosimeter Badges
How does a Thermoluminescent Dosimeter Badge (TLD) work?
When the radiation falls on TLD, the electrons are excited and store energy.
The TLD reader consists of a heater. On getting heated, the excited electrons again come back to the ground state and emit light which is then read by a photomultiplier.
The light output is proportional to the radiation exposure.
What dose range can TLDs measure in Sieverts and Gray?
- 01 mGy to 10 Gy^3
0. 05 mSv to 10 Sv
Where are Extremity TLD Dosimeters worn?
Worn on fingers or wrists
What dose range can Extremity TLD measure in Sieverts?
0.2 mSv to 10 Sv
What radiation type does TLD’s not read?
Alpha radiation
What dose range can Photographic Film Dosimeters measure in Sieverts for beta and gamma compared to x-rays?
- 1 mSv to 10 Sv for beta & gamma radiation
0. 1 mSv to 400 mSv X-ray radiation
How does a Photographic Film Dosimeter work?
The badge consists of two parts: Photographic film and the film holder.
The silver film emulsion is sensitive to radiation and forms the latent image in the film.
Exposed areas increase in optical density (film blackening) in response to incident radiation.
The photo densitometry analysis is used to compare the worker’s exposed film with the standard films exposed to calibrated radiation doses
How does an Optically Stimulated Luminescence (OSL) Badge Dosimeter work?
Uses a lase to stimulate an aluminumoxide material.
A tiny crystal traps and stores energy from exposure to
ionizing radiation
Amount of exposure can be determined by shining a green light on the crystal and measuring the intensity of the blue light emitted
What dosimeter badges can be re-used and which cannot be?
Thermoluminescent Dosimeter (TLD) badge - Cannot be re-used
Optically Stimulated Luminescence (OSL) badge dosimeters - Can be re-used
What dose range can Direct Ion Storage (DIS) Dosimeter measure in Sieverts?
0.01 mSv to 5 Sv
Give 8 characteristics of A Geiger counter (Geiger-Muller tube)
- GM inexpensive, since amplification in detector means simple amplification post detector
- GM tubes are more robust than ionisation chambers
- GM tubes have reasonable response time
- End mica window is fragile
- Relationship between counts per second and dose rate is energy dependent
- Cannot differentiate different radiations being detected
- Cannot be used to determine exact energy of the detected radiation
- Have a smaller size and very low efficiency.
Describe how a Geiger-Muller Tube (Counter) works
Ionising radiation particles enter the tube and ionise some of the gas molecules in the tube.
The tube consists of a pair of electrodes surrounded by a gas, with a high voltage across the electrodes.
Filled with Helium or Argon
An electron is knocked out of the ionised atom, and the remaining atom is positively charged.
The high voltage in the tube produces an electric field inside the tube.
The electrons that were knocked out of the atom are attracted to the positive electrode.
Positively charged ions are attracted to the negative electrode.
This produces a pulse of current in the wires connecting the electrodes, and this
pulse is counted
What 3 advantages does an Ionisation Chamber have over a GM tube?
Have linear response
Have a dose rate scale
Usually more robust than end-window GM tubes
List 4 characteristics of Ionisation Chambers
- Sensitive to humidity, temperature & shock
- Expensive
- Response time slow
- For given detector size, less sensitive than other types
Describe how a Ionisation Chamber works
Ionising radiation passing through the gas produces free electrons and positive ions.
These are attracted towards electrodes and generate current and measures the direct dose rate
Describe how a Scintillation Detector works
When an ionising particle passes into the scintillator material, atoms are ionised along a track
The photon from the scintillation strikes a photocathode and emits an electron which accelerated by a pulse and produces a voltage across the external resistance
This voltage is then amplified and recorded by the photomultiplier tube (Electronic counter).
Give 3 advantages of Scintillation Detectors
- Very sensitive for given detector volume
- Alpha and Beta particles can be detected
- Fast response time
Give 3 disadvantages of Scintillation Detectors
- No dose scale because signal varies with radiation energy
- Window vulnerable to damage
- Expensive
