Chapter 10: Unit Conversions; Radiation Measurement Units; Converting Cps To Mr/Hr - Radiation Alert AM-7149 Operation Manual

Chapter 10: 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 Area Monitor can also displays in microroentgens per hour (millionths--µR/h), milliroentgens per
hour (throusandths--mR/hr) and kiloroentgens per hour (thousands—kR/h).
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 CPS to mR/hr

cps
mR/hr =
sensitivity
Sensitivity, or Calibration Constant, is expressed in cps per mR/hr (Counts Per Second for every milliroentgen
the detector can detect) referenced to Cs-137. Mathematically the cps units cancel each other out leaving mR/
hr, as shown below.
cps
cps
=
cps
mR/hr
For example, if you have collected 200 CPS with a GM tube that has a typical gamma sensitivity of 60 cpm per
mR/hr, you would divide the 200 cps by the 60 cpm per mR/hr sensitivity. The cps cancels out and you are left
with 200/60 mR/hr = 3.33 mR/hr (Note, however, that the Area Monitor performs this conversion internally.)
200 cps
= 3.33 mR/hr
cps
60
mR/hr
X mR/hr
= mR/hr
1
cps
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