Units for Radiation Protection

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Notes from Riaz’s lecture: Units for radiation protection, 8th August 2008

Exposure
Ionisation of air by xrays and gamma rays
Unit is relaed to amount of + or – charge liberated:
Exposure = charge liberated / mass of air = ΔQ / Δm
Unit = C/kg or R (Roentgen)
1 R = 2.58 x 10-4 C/kg
Exposure rate: mR/h, mR/h
(This definition is no longer used in radiation protection. “Exposure” is now used in the standard sense.)

Absorbed dose
Energy deposited by ionising radiation passing through a material medium
Absorbed dose = energy deposited / mass of the body = ΔK / Δm
Unit = J/kg or Gy (Gray)
1 Gy = 1 J/kg
1 Gy = 100 rad
Absorbed dose rate: mGy/h, mGy/h

Radiation weighting factor
Different types of radiation have different capabilities of causing biological damage.
The radiological hazard may be different (for the same amount of absorbed dose) depending on the type of radiation involved.
WR = radiation weighting factor
(Used to be called quality factor.)

Equivalent dose
Equivalent dose = (radiation weighting factor) x (absorbed dose)
HT = WR x DT,R
or
HT = SR (WR x DT,R), for different radiations R
Unit = Sv (Sievert) or rem (rad equivalent of man)
1 Sv = 100 rem (may vary due to changes in weighting factors)

Tissue (organ) weighting factor
Some parts of the human body are more sensitive to radiation than others.
The same radiation causes different effects in different tissues.
WT = tissue (organ) weighting factor.
Whole body WT = 1.0
Specific organs WT = 0.2, 0.12, 0.05, …
(Listed in ICRP publication 60, 1991)

Effective dose
Effective dose = (tissue weighting factor) x (absorbed dose)
ET = WT x HT = WT x WR x DT,R
or
ET = ST (WT x HT), for different tissues T
or
ET = ST (WT x (SR (WR x DT,R)), for different tissues and different radiations
Unit = Sv (Sievert)
Effective dose rate: mSv/h, mSv/h
(The same unit is used for equivalent and effective dose.)

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