Dose Units

The SI unit for activity is the Becquerel

1 Bq = 1 Disintegration per second

An older unit that is still in use is the Curie

One Curie (Ci) is equal to the activity of one gram of radium-226

1 Bq = 2.7 x 10-11 Ci
1 Ci = 3.7 x 1010 Bq
10 µCi = 370 kBq

The Absorbed dose rate is measured in grays (Gy), 1 Gy = Energy deposition of 1 Jkg-1

As neutrons and alpha particles have mass they cause more ionising damage per gray than gamma or beta radiation.

Therefore another unit is used to set radiological protection standards - the sievert. The sievert measures the biological effect of the absorbed dose in tissue taking into account the type and energy of the incident radiation. A weighting factor WR, the radiation weighting factor, is introduced. The equivalent dose HT is therefore

HT = Σ WRDT,R

DT,R = absorbed dose averaged over the tissue organ due to radiation R.

Type of Radiation WR
Photons 1
Electrons and Muons, all energies 1
Neutrons < 10 keV 5
Neutrons 10 - 100 keV 10
Neutrons 100 keV - 2 MeV 20
Neutrons 2 - 20 MeV 10
Neutrons > 20 MeV 5
Protons other than recoil protons, energy > 2 MeV 5
Alpha Particles, fission fragments, heavy nuclei  20


Another factor that has to be taken into account is that different types of body tissue have different sensitivities to radiation. This is accounted for by the tissue weighting factor W
T. The effective dose is therefore

E = Σ WTWRDT,R

Tissue or Organ WT
Gonads 0.20
Bone Marrow 0.12
Colon 0.12
Lung 0.12
Stomach 0.12
Bladder 0.05
Breast 0.05
Liver 0.05
Oesophagus 0.05
Thyroid 0.05
Skin 0.01
Bone Surface 0.01
Remainder 0.05


The unit of equivalent and effective dose is the sievert.

This means that the radiological dose can be compared for different activities

Activity Dose (mSv)
Living in Cornwall 7.8
From a brain scan 5
Average UK dose 2.6
Average additional dose for airline crews 2
Average annual dose received by a coal miner 1.2
Dose from return flight from London to Los Angeles 0.14
Dose from return flight from London to Spain 0.02
Dose from 1 week holiday in Cornwall 0.1
Dose from drinking a glass of mineral water every day for 1 year 0.065
Dose from a single X-ray 0.02
Dose limit for registered radiation workers 20
Average dose for a Sellafield worker from occupational exposure per year 1.3 


For γ radiation

• A short term dose of 1 Sv causes temporary radiation sickness
• 5 Sv would kill about half the people receiving it within a month
• 10 Sv would be fatal to all exposed within days