Specifications

Radiaon Measurement Units

Several dierent units are used to measure radiaon, exposure and dosage.

Roentgen is the amount of X-radiaon or gamma radiaon that produces one electrostac unit of charge in

one cc of dry air at 0° C and 760 mm of mercury atmospheric pressure. One thousand milliroentgen (1,000

mR)= 1R. The Inspector EXP displays in milliroentgens per hour (mR/hr).

Rad is the unit of exposure to ionizing radiaon equal to an energy of 100 ergs per gram of irradiated material.

This is approximately equal to 1.07 roentgen.

Rem is the dosage received from exposure to a rad. It is the number of rads mulplied by the quality factor of

the parcular source of radiaon. The rem and millirem are the most commonly-used measurement units of

radiaon dose in the U.S. 1 rem= 1 rad.

Sievert is the standard internaonal measurement of dose. One sievert is equivalent to one hundred rems. A

microsievert (μSv) is one millionth of a sievert. A unit of dose equivalent. 1 Sv= 100 roentgens, 10 µSv/hr = 1

milliroentgen/hr.

Curie is the amount of radioacve material that decays at the rate of 37 billion disintegraons per second,

approximately the decay rate of one gram of radium. Microcuries (millionths of a curie) and picocuries

(trillionths of a curie) are also oen used as units of measurement.

Becquerel (Bq) is dened as the acvity of a quanty of radioacve material in which one nucleus decays per

second. 1 dps (one disintegraon per second).

Converng CPM to mR/hr

mR/hr =

cpm

sensivity

Sensivity is expressed in cpm per mR/hr (Counts Per Minute for every milliroentgen the GM tube can detect)

referenced to Cs-137. Mathemacally the cpm units cancel each other out leaving mR/hr, as shown below.

For example, if you have collected 200 CPM with the Radiaon Alert Inspector EXP, which has a typical gamma

sensivity of 3340 cpm per mR/hr, you would divide the 200 cpm by the 3340 cpm per mR/hr sensivity. The

cpm cancels out and you are le with 200/3340 mR/hr = 0.057 mR/hr

cpm

mR/hr

cpm

mR/hr

cpm

= mR/hr

200 cpm

cpm

mR/hr

= 0.057 mR/hr

3340