UNITS OF RADIATION

Radioactivity is measured in Becquerel (Bq) per second. 1 Bq means one disintegration per second. It is also measured in Curie (Ci), named for Madam Curie, who shared Nobel Prize with her husband. 1 Curie = 3.7 x 10¹⁰ Bq or disintegrations per second.  The radiation absorbed dose is measured in Gray, rad, rem and Sievert (Sv).

In the United States, absorbed dose is commonly given in rad or Gray and other protection quantities, such as equivalent dose and effective dose, are given in rem. The following table is provided to help avoid confusion among persons not familiar with these quantities. The use of the newer system of units would be particularly useful during radiological incidents involving international responders.

The REM weights internal organs more heavily than the skin, this is from standards being derived from high dose atomic bomb testing.  In the case medical x-rays softer x-rays are filtered out and damage from the soft x-rays are avoided.  Airport backscatter use predominantly soft x-rays that are very destructive to skin, breast and testicles.   The rem measurement offered by IAEA TSA is a measure of internal organ risk, as such greatly underestimates the damage to the skin by perhaps 10-20x.  Soft x-rays used by the scanners are typicly filtered out for medical imaging to keep images sharp.  Soft x-rays used by airport backscatter are particularly damaging to DNA due to photo electrons generated by the x-ray interacting with phosphorus atoms making up the DNA back bone.   

Conversions for Effective Dose, Equivalent Dose, Dose Equivalent, and ambient dose equivalent
0.001 rem = 1 mrem = 0.01 mSv
0.01 rem = 10 mrem = 0.1 mSv
0.1 rem = 100 mrem = 1 mSv = 0.001 Sv
1 rem = 1000 mrem = 10 mSv = 0.01 Sv
10 rem = 100 mSv  = 0.1 Sv
100 rem = 1000 mSv = 1 Sv (Sievert)
1000 rem = 10 Sv

Conversions for Absorbed Dose
0.001 rad = 1 mrad = 0.01 mGy
0.01 rad = 10 mrad = 0.1 mGy
0.1 rad = 100 mrad  = 1 mGy = 0.001 Gy
1 rad = 1000 mrad = 10 mGy = 0.01 Gy
10 rad = 100 mGy = 0.1 Gy
100 rad = 1000 mGy = 1 Gy (Gray)
 1000 rad = 10 Gy

• TEDE - Total Effective Dose Equivalent (TEDE)- means the sum of the deep-dose equivalent (for external exposures) and the committed effective dose equivalent (for internal exposures).

• DDE - Deep-dose equivalent (Hd)- which applies to external whole-body exposure, is the dose equivalent at a tissue depth of 1 cm (1000 mg/cm2).

• EDE - Effective dose equivalent (HE)- is the sum of the products of the dose equivalent to the organ or tissue (HT) and the weighting factors (WT) applicable to each of the body organs or tissues that are irradiated (HE = ΣWTHT).

• CDE - Committed dose equivalent (HT,50)- The dose equivalent to organs or tissues of reference (T) that will be received from an intake of radioactive material by an individual during the 50-year period following the intake.

• CEDE - Committed Effective Dose Equivalent - The sum of the products of the weighting factors applicable to each of the body organs or tissues that are irradiated and the committed dose equivalent to these organs or tissues (HE,50 = ΣWTHT.50).

• Rem - the special unit of any of the quantities expressed as dose equivalent. The dose equivalent in rems is equal to the absorbed dose in rads multiplied by the quality factor (1 rem=0.01 sievert).

• Sievert - the SI unit of any of the quantities expressed as dose equivalent. The dose equivalent in sieverts is equal to the absorbed dose in grays multiplied by the quality factor (1 Sv=100 rems).

• Gray (Gy) - the SI unit of absorbed dose. One gray is equal to an absorbed dose of 1 Joule/kilogram (100 rads).

• RAD - the special unit of absorbed dose. One rad is equal to an absorbed dose of 100 ergs/gram or 0.01 joule/kilogram (0.01 gray).

Definitions from 10 CFR 20

Measured Dose (Temporary Measurements) – gamma radiation or X-rays
1 R (roentgen) = 0.01 Gy = 0.01 Sv

References:

NCRP Report No. 138, Oct. 24, 2001, “Management of Terrorist Events Involving Radioactive Material,” Recommendations of the National Council on Radiation Protection and Measurements, Bethesda, MD

“An Evaluation of Radiation Exposure Guidance for Military Operations, Interim Report,” Committee on Battlefield Radiation Exposure Criteria, Fred A. Mettler, Jr, Chairman, Medical Follow-up Agency, Institute of Medicine of the National Academy of Sciences, National Academy Press, Wash., DC, 1997.