Radiation sources & measurement Flashcards
What percentage of death is radiation responsible for?
1%
What percentage of cancer deaths is due to radiation?
4%
How many people are killed by radiation in the UK per year?
6588
What percentage NUMBER of medical radiographic examination are dental?
30%
What of radiation is environmental or artificial?
17% - environmental
83% - artificial
What percentage of medical exposures are due to dental?
3-9%
What percentage of medical and dental doses are avoidable?
45-50%
What affect does additional doses have on the human population?
Additional doses increase the number of cases seen.
Not likely to result in new disease and malformations as we have been exposed to radiation since the beginning.
Why is it important to measure radiation dose?
Hazards result from radiation doses, thus need to assess risk planned doses against benefit from the dose.
What properties of x-rays allow use to measure dose?
- Excitation
- Ionisation
Name the dose meters that measure via the amount of ionisation?
- Free ionisation chamber
- Thimble ionisation chamber
- Chemical conversion (Frickes’ Dose Meter)
- Photographic ionisation
How does a Thimble ionisation chamber work?
Measures the amount of discharge due to ionisation.
How does a chemical conversion (Frickes Dose Meter) work?
Measures chemical changes due to ionisation.
What does a photographic density dose meter work?
Assesses dose through level of black caused by ionisation of Silver Bromide.
How doses a calorimetry dose meter work?
Measures heating due to excitation
Name the dose meters that measure exitation?
- Calorimetry
- Thermoluminescent dose meter
In a sentence how does a free air ionisation chamber measure dose?
Collects electrons and measures charge.
In a sentence how does a thermoluminescent dose meter measure dose?
Assesses dose by measuring released stored energy, modified type on excitation.
How does a freer air ionisation chamber work?
-Arrangements of the plates producesa measurable volume between plates
-Free electrons are liberated from air by ionising radiation
- Liberated electrons are allows to pass through the plate to a meter
-Measuring how many electrons shows how many ionisation events have occurred
Thus able to calculate the total energy transferred to the sample
Why is the free air ionisation chamber not used to measure dose in practice?
Not practical as the machine is too large to be used everyday
It is a primary standard laboratory method.
What can used to record personal dose?
Thermoluminescent dose meter (TLD)
How does a thermoluminescent dose meter work?
-Work on modified type of excitation
-Electrons do not return directly to their original positions but are suspended in a state of increased energy
-Exposure of radiation raises the energy levels of electrons (excitation) from valency band
-These electrons are trapped by energy anomalies
-Electrons remain here until released by heating they then fall giving off light that can be measured.
Intensity of light is proportional to the dose received.
What are the advantages of TLDs?
- Tissue equivalent so dose reading is direct
- Can be reused
- Measures HP7 and HP10 (surface, deep)
- Relatively inexpensive
- Widely dynamic range
- Reading can be recorded as effective dose
- Can be produced in a wide variety of sizes
What are the disadvantages of TLDs?
- No permanent dose record
- Sensitive to extreme environments
What is radiation exposure?
Number of ionisation events (C/kG) - off little use
What is absorbed dose?
Total energy absorbed (J/kg- Grays Gy) - most common but not very informative.
What is equivalent dose?
Weighted by radiation hazard but adds little in terms of x-ray assessment. (Sieverts -Sv)
What is effective dose
Absorbed (Gy) x Radiation weighting factor (WR) x Tissue weighting factor (Sum of WT)
Most important but has little use in practice.
Measure in Sieverts (Sv)
Why is effective dose an important measurement?
It is the only measurement that gives a realistic assessment of potential harm.
What are the 15 tissue weighting factors?
- Bone marrow
- Colon
- Lung
- Stomach
- Breast
- Gonads
- Bladder
- Oesophagus
- Liver
- Thyroid
- Brain
- Salivary glands
- Skin
