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The gray and the sievert

Radiation has to be absorbed to count towards your dose

If a gamma photon passes straight through you then you don’t absorb any of its energy and your dose from it is zero.  Radiation dose is only concerned with the radiation that is actually absorbed by your body.

Absorbed dose, measured in grays

The ‘absorbed dose’ is the energy in joules absorbed by each kilogram of your body.  Say someone absorbs 2 joules of energy by exposure to nuclear radiation.  This is the amount transferred by someone gently passing a football.

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If a person has a mass of 50 kilograms then their absorbed dose is 2 joules divided by 50 kilograms, which is 0.04 joules per kilogram.  The unit for absorbed dose is the gray, named after the 20th century British physicist Hal Gray.  1 gray is one joule absorbed per kilogram.  So here your absorbed dose is 0.04 grays.

This doesn’t depend on the type of radiation.  Alpha, beta or gamma, you’ll still absorb 0.04 grays.  However 0.04 grays of alpha will do you more harm than 0.04 grays of gamma.

Committees of scientists have decided that alpha should be thought of as being 20 times more harmful than beta or gamma.

Biological equivalent dose, measured in sieverts

So the biological equivalent dose if your 0.04 gray were from alpha radiation would be 0.04 times 20, which is 0.8.  The unit of equivalent dose is the ‘sievert’.

The equivalent dose in sieverts is the absorbed dose in grays multiplied by a number that is bigger the more damaging the radiation is.  This number is normally given the symbol ‘Q’ for ‘quality factor’.  Different types of radiation have different values of Q.

If you ask what dose of radiation someone received then the answer will normally be given in sieverts.  This means you don’t need to worry about what kind of radiation caused this dose because the Q factor has taken this into account.

back to Lesson 5: How Radiation Harms