Monitoring Flashcards
What are the two main types of personal monitoring detector types?
- Thermoluminescent dosimetry (TLD).
- Optically stimulated luminescence (OSL).
Explain thermoluminescent dosimetry (TLD). Provide some advantages and disadvantages.
- In these dosimeters, application of heat after irradiation (up to weeks or months) will stimulate the emission of photons when electrons are released from from meta-stable trapped states. These will give information on the original radiation dose.
- Typically formed from LiF doped with magnesium, copper and phosphorous.
- Readout follows heating across some sort of standard form to which response is understood. This is not a simple process.
- Response is uniform across different dosimeters.
- Response over a useful range of energies and radiation types (photons and betas).
- Up to ~ 500 ‘reads’ possible.
Explain optically stimulate luminescence (OSL) dosimetry. Provide some advantages and disadvantages.
- In these dosimeters, application of laser light after irradiation (up to weeks or months) will stimulate the emission of photons after electrons are released from meta-stable trapped states. These will give information on the original radiation dose.
- Typically formed from AlO3 doped with carbon.
- Response is uniform across different dosimeters.
- Response over a useful range of energies and radiation types (photons and betas).
- Lower limit of readout is less than that of TLDs.
What is direct ion storage (DIS)?
- These are not as common as TLDs or OSLs.
- They employ a solid state detector, much like a direct digital detector. They also incorporate a small ionisation chamber for improved dosimetry.
- Long-term recording is possible (months/years) due to non-destructive readout.
- Can be easily read out by the user rather than sending off for readout.
Other than TLD, OSL and DIS, what are some other examples of personal dosimetry?
- Film badge: Film stored within holder with some attenuation to determine exposure. These are now obsolete.
- Real time dosimetry: DIS technology is used to provide real time display of doses within a room for a specific interventional procedure, for example. These are expensive, however.
- Quartz fibre dosimeter: Small charged fibre moves over a scale depending on residual charge following ionisation. These can be read at any time and are resistant to harsh conditions.
- Electronic personal dosimeter (EPD): These employ GM technology. Gives real time indication of dose. Readout via PC is available. Limited by size/weight and battery life.
- Real time extremity dosimeter: Pin diode technology used to give a real time indication of finger dose. Has a restricted energy range.
- Blood tests: Biological tests following acute exposure. Tests for chromosome breaks and dicentrics caused by ionisation. Window of 24 hours to 6 weeks after exposure.
Why is personal monitoring undertaken?
- Personal monitoring is required for classified workers in IRR17.
- Personal monitoring for non-classified workers confirms that their status remains appropriate.
Aside from the values of dosimetry results, what else may need to be considered (consider nuclear medicine finger doses, for example)?
Where the dosimeters are worn and any corrections that may be required. In this example, ring dosimeters are worn at the base of the fingers. However, an increased dose will be apparent at the tips of the fingers. This has the potential to be the difference between non-classification and classification.
What is required of a monitoring service?
- Must be an approved dosimetry service (ADS).
- Must assess and store individual monitoring data where possible.
- Must retain records for 75 years (or 30 years after the last assessment).
- Must provide reports routinely (quarterly, annually), at termination of employment and on demand (from individual, from HSE).
What is required, as per IRR17, if an individual result is missing?
- For lost or damaged badges, the employer carries out an investigation and requests estimated or notional doses to be entered with a flag.
- Special entries are required when a badge result is thought to be wrong. A thorough employer/RPA investigation must be undertaken and the dose record approved by HSE. The individual must be informed.
What are some of the employer’s responsibilities regarding monitoring, as per IRR17?
- Takes advice from the RPA.
- Determines who should be monitored.
- Determines what monitoring is required.
- Determines monitoring frequency.
- Establishes an investigation level.
- Investigates high or suspect results.
- Ensures monitoring badges are worn correctly.
- Ensures monitoring badges are stored correctly when not worn.
- Estimates doses for dose records.
- Responds to information contained in dose reports.
- Liaises with ADS, other employers and regulators.
What are there requirements for where to wear personal dosimetry? Where are whole body, extremity and eye dosimeters typically worn?
- There are a range of locations in which personal dosimetry can be worn. This is determined with advice from the ADS and RPA. However, the wearing location of dosimeter should be consistent.
- Whole body dosimeters can be worn on the upper or lower torso, the collar and over or under lead aprons.
- Extremity and eye dosimeters will typically be worn close to the critical organ.
- Depending on the set location, the ADS will apply standard conversions to derive the appropriate dose.
What must be considered when calibrating personal dosimetry? How is calibration performed?
- Personal monitors are used to mimic the dose to different areas of an individual. They are no exposed ‘in-air’ during actual use. Rather, they are worn on the individual. As such, the area at which they are worn and the resultant scatter contribute significantly.
- To calibrate a personal monitor, they are attached to a standard phantom. The size and design of the phantom depends on the dosimeter type to be calibrated and the quantity to be determined (e.g. the whole body Hp(10) phantom is larger than the extremity Hp(0.07) or eye Hp(3) phantoms).
- The calibration factor could then be determined by comparing to primary data acquired through Monte Carlo simulation, via some direct air kerma measurement using an ion chamber or using a standard conversion.
What are some issues associated with whole body dosimetry results? What is a potential way around this?
- Shielded dosimeter (i.e. worn under leads) typically underestimates whole body dose by ~ 20%.
- Unshielded dosimeter typically overestimates dose by ~ x20.
- If more accurate dose readings are critical (e.g. for highly exposed staff doing some interventional work), two badges can be used. One is placed on the trunk under the lead apron, one on the collar over the apron. Various calculations can then be performed to provide a more accurate whole body dose value.
What are some issues associated with finger monitoring in practice?
It is difficult to directly monitor the fingertip (where the largest dose is received). Ring dosimeters are, instead, worn at the base of the finger where, for example, there is ~ 4x reduced dose for Tc-99m radio pharmacy staff. These means corrections have to be applied.
What does checking for surface contamination typically involve? What type of instrument is usually used? How are there results related to annual dose limits?
- Floors, surfaces, skin etc. must be checked for radioactive contamination, as per IRR17 and EPR16. This is typically performed routinely (e.g. daily in a nuclear medicine department).
- Contamination counting scintillation monitors are typically used. These provide an indication of counts per second or sometimes Bq/cm^2 if lookup tables are built in.
- To relate these to annual dose limits (e.g. skin dose limits), derived limits are used (e.g. Delacroix).
