Chapter 8: Unit Conversions - SE Radiation Alert Frisker Operation Manual

Chapter 8: Unit Conversions

Radiation Measurement Units
Several different units are used to measure radiation, exposure and dosage.
Roentgen is the amount of X-radiation or gamma radiation that produces one electrostatic 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 Radiation Alert® Frisker displays in milliroentgens per hour (mR/hr).
Rad is the unit of exposure to ionizing radiation 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 multiplied by the quality factor of
the particular source of radiation. The rem and millirem are the most commonly-used measurement units of
radiation dose in the U.S. 1 rem= 1 rad.
Sievert is the standard international 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 radioactive material that decays at the rate of 37 billion disintegrations per second,
approximately the decay rate of one gram of radium. Microcuries (millionths of a curie) and picocuries
(trillionths of a curie) are also often used as units of measurement.
Becquerel (Bq) is defined as the activity of a quantity of radioactive material in which one nucleus decays per
second. 1 dps (one disintegration per second).
Converting CPM to mR/hr
cpm
mR/hr =
sensitivity
Sensitivity is expressed in cpm per mR/hr (Counts Per Minute for every milliroentgen the GM tube can detect)
referenced to Cs-137. Mathematically the cpm units cancel each other out leaving mR/hr, as shown below.
cpm
cpm
=
cpm
mR/hr
For example, if you have collected 200 CPM with the Radiation Alert Radiation Alert® Frisker, which has a
typical gamma sensitivity of 3600 cpm per mR/hr, you would divide the 200 cpm by the 3600 cpm per mR/hr
sensitivity. The cpm cancels out and you are left with 200/3600 mR/hr = 0.056 mR/hr
200 cpm
= 0.056 mR/hr
cpm
3600
mR/hr
12
mR/hr
X = mR/hr
1
cpm