Dose and Dose Rate

There is a lot of confusion going on these days about Dose and Dose Rate.  Japan data are generally reported in mSv/hr (milliSieverts per hour) or just plain mSv.  When data is properly communicated mSv/hr indicates rate.  It’s like driving a car at 50 kilometers per hour.  In an hour you will have driven 50 kilometers.  If you are exposed to 1 mSv (one milliSievert) per hour for one hour, you will get a dose of 1 mSv.

The amount of radioactivity in a given volume of air, water or tissue is generally expressed in becquerels (Bq).  One becquerel means one disintegration (radiation event) takes place every second.  So when you hear a sample of water contains 10,000 becquerel of a certain isotope per cubic centimeter it means every second 10,000 radioactive events are happening in that sample.

When translating from Bq to dose or dose rate, it is important to understand the capabilities of the detector and the characteristics of the radiation.  Alpha radiation is considered to be 20 times more harmful than beta, gamma, x radiation.  It is also not detected by many instruments.  The same holds true for beta and neutron radiation. So dose and dose rate can be under-reported if the capabilities of the instrument are not understood.  In an emergency situation, confusion and danger can often lead to misinterpretation, or improper reporting of data.  It is important to know how to ask the right questions about the data.

Here’s a quick guide to understanding the terminology:

Activity (quantity) is measured in Becquerel (Curies in the U.S.)

Absorbed Dose is measured in gray (Gy) (Rads in the U.S.)

Dose Equivalent is measured in sievert (Sv) (Rem in the U.S.)

Converting between Conventional Units (used in U.S.) and SI Units (used in Japan and Europe)

To convert millirems  (mrem) to microsieverts (uSv)  multiply by 10.

To convert microsieverts to millirem multiply by 0.1

Caution: Geiger Counters that are energy compensated do not detect alpha and beta radiation, so any dose rate or dose information will not include alpha or beta dose (only energetic gamma).

Geiger Counters with mica windows will detect alpha and beta, but calibration accuracy in mR/hr or uSv/hr will be affected by the mix of radioactive materials present.