6.1.7Radiation
Equivalent Dose
5 min read 5 parts Lesson 7 of 14
Section 1 of 5Why Equivalent Dose is Needed
Equivalent dose combines the energy absorbed with how harmful each type of radiation is to tissue.
Different radiations
- The same absorbed dose of alpha, beta or gamma radiation does not do the same amount of damage to living tissue.
Weighting factor
- Each type of radiation is given a radiation weighting factor wR that shows how damaging it is compared with gamma.
Values
- Gamma and beta have wR = 1, while alpha has wR = 20 because its dense ionisation causes concentrated damage.
Equation
- H = D × wR, where H is equivalent dose in sieverts (Sv), D is absorbed dose in grays and wR has no unit.
The sievert
- The sievert measures the biological effect of radiation, so doses of different radiations can be compared fairly.
Section 2 of 5Dose Rate and Total Dose
Dose Rate and Total Dose
Equivalent dose rate
- Dose rate = H ÷ t, the equivalent dose received per unit time, usually quoted in μSv per hour or mSv per year.
Total dose
- Equivalent dose from several sources or radiation types is found by calculating each one and then adding the results.
Exposure time
- For a constant dose rate, the total equivalent dose is dose rate multiplied by time in the field.
Units
- Doses in mSv or μSv must be converted into sieverts if the equation requires sieverts.
Comparison
- Equivalent dose is compared with the annual limits to decide whether a working practice is safe.
| Radiation | Weighting factor wR |
|---|---|
| Gamma | 1 |
| Beta | 1 |
| Alpha | 20 |
Section 3 of 5Calculating Equivalent Dose
1
Identify the type of radiation and look up its radiation weighting factor from the data sheet.
2
Calculate or read the absorbed dose in grays, converting from mGy or μGy if necessary.
3
Multiply the absorbed dose by the weighting factor to find the equivalent dose in sieverts.
4
For several radiations, find each equivalent dose separately and add them for a total.
5
Divide by the time if a dose rate is required, and state the unit, for example mSv per year.
Section 4 of 5Worked Example
Question
- A worker's lung tissue absorbs a dose of 0.20 mGy from alpha radiation.
- The weighting factor for alpha is 20.
- Calculate the equivalent dose, and say how it compares with the same absorbed dose of gamma radiation.
- Convert0.20 mGy = 2.0 × 10⁻⁴ Gy.
- H = D × wR = 2.0 × 10⁻⁴ × 20.
- H = 4.0 × 10⁻³ Sv = 4.0 mSv.
- For gamma (wR = 1) the same absorbed dose gives H = 0.20 mSv, twenty times less.
Answer H = 4.0 mSv for alpha, twenty times larger than the equivalent dose from the same gamma absorbed dose.
Section 5 of 5Exam Checklist & Tips
6.1.7 – Key Points for the Exam
- Equivalent dose The absorbed dose multiplied by the radiation weighting factor, a measure of biological harm.
- Sievert (Sv) The unit of equivalent dose.
- Weighting factor A number that scales the absorbed dose according to the type of radiation.
- Dose rate The equivalent dose received per unit of time.
- Biological effect The damage done to living tissue by radiation.
Common Mistakes
- Using grays when sieverts are required, or the other way round.
- Forgetting that alpha radiation has a weighting factor of 20.
- Adding equivalent doses without multiplying by the weighting factors first.
- Mixing mSv and Sv in the same calculation.