Measurement of Radiation
There are a few scales that one can use to measure radiation. Depending upon
your application, one scale may be better than the others.
Roentgen: Is the measurement of energy produced by Gamma or X-Ray radiation in a
cubic centimeter of air. It is abbreviated with the capital "R". One
milliroentgen, abbreviated "mR" is one-thousandth of a roentgen. One
microroentgen, abbreviated uR is one-millionth of a roentgen.
RAD: Radiation Absorbed Dose. Original measuring unit for expressing the
absorption of all types of ionizing radiation (alpha, beta, gamma, neutrons,
etc) into any medium. One rad is equivalent to the absorption of 100 ergs of
energy per gram of absorbing tissue.
REM: Roentgen Equivalent Man is a measurement that correlates the dose of any
radiation to the biological effect of that radiation. Since not all radiation
has the same biological effect, the dosage is multiplied by a "quality factor"
(Q). For example, a person receiving a dosage of gamma radiation will suffer
much less damage than a person receiving the same dosage from alpha particles,
by a factor of three. So alpha particles will cause three times more damage than
gamma rays. Therefore, alpha radiation has a quality factor of three. Following
is the Q factor for a few radiation types
|Radiation:||Quality Factor (Q)|
|Beta, Gamma and X-rays||1|
|Fast n, a, and protons||10|
|Heavy and recoil nuclei||20|
The difference between the rad and rem is that the rad is a measurement of the
radiation absorbed by the material or tissue. The rem is a measurement of the
biological effect of that absorbed radiation.
For general purposes most physicists agree that the Roentgen, Rad and Rem
may be considered equivalent.
System International (SI) of Units
The System International of unit for radiation measurements is now the official
system of measurements. This system uses the gray (Gy) and sivert (Sv) for
absorbed dose and equivalent dose respectively.
The conversion from one system to another is simple:
|1 Sv = 100 rem||1 rem = .01 Sv|
|1 mSv = 100 mR (mrem)||1 mR = .01 mSv|
|1 Gy = 100 rad||1 rad = .01 Gy|
|1mGy = 100 mrad||1 mrad = .01 mGy|
How Much Radiation is Safe?
In the United States the U.S. Nuclear Regulatory Commission (NRC) determines
what radiation exposure level is considered safe. Occupational exposure for
worker is limited to 5000 mrem per year. For the general population, the
exposure is 500 mrem above background radiation in any one year. However for
long term, multi-year exposure, 100 mrem above background radiation is the limit
set per year.
Lets extrapolate the 100 mrem number to an hourly radiation exposure rate.
There are 365 days/yr x 24 hr/day equals 8760 hours. Divide 100 mrem by 8760
hours equals .0114 mrem/hr or 11.4/hr microrem. This is an extremely low
radiation level. The background radiation in my lab hovers around 32 uR/hr. Am I
in trouble? No. Typically background radiation in the United States averages 300
mrem/yr, or 34 microrem/hr. The NRC specifications is for radiation above this
34 urem/hr background radiation.
Notice that my lab readings are in microrad (uR/hr) and the exposure limit is
given in microrem (urem/hr). I do not know what type of radiation (a , b or y)
the geiger counter is reading in my lab at any particular instant, so I do not
know the Q factor of the radiation and therefore can not calculate the mrem.
However for general purposes I consider them the one and the same. Remember,
the digital geiger counters are calibrated using a Cs-137 radioactive source.
Therefore the highest accuracy in reading radiation levels will be from Cs-137
Common Radiation Exposure (General Population)
|Exposure Source||Dose(conventional)||Dose (SI)|
|Flight from LA to NY||1.5 mrem||.015 mSv|
|Dental X-ray||9 mrem||.09 mSv|
|Chest X-ray||10 mrem||0.1 mSv|
|Mammogram||70 mrem||0.7 mSv|
|Background Radiation||620 mrem/year||6.2 mSv/year|
Background radiation consists of three sources; Cosmic radiation from the sun
and stars. Terrestrial radiation from low levels of uranium, thorium, and their
decay products in the soil, air and water. Internal radiation from radioactive
potassium-40, carbon-14, lead-210, and other isotopes found inside our bodies.
Testing Radiation Levels for Safety
Our line of Digital Geiger Counters are extremely sensitive and will detect
and measure background radiation in addition to detecting and measuring
radioactivity above background radiation. To test for small increases in
radioactivity that may be present in food and other materials to cause
a increase in the background radiation one must first establish the
background radiation level.
To establish background radioactivity level, set the geiger counter to
1 minute mode. Then record the next 5 (one minute) readings. Average the
reading to determine a approximate background radiation level for your
area. Also record the (Counts Per Minute) CPM at the end of each minute
run. Record the CPM number at the last second, before the screen updates
for the next minute reading. The equivalent radiation level will stay on
the second display line of the LCD making that easier to record, until the
line is updated at the end of the next run.
The more readings you take, say 25, the more accurate will be your average
background radiation level. Take the average of your CPM readings. The highest
and lowest CPM count will establish your minimum and maximum CPM. These
numbers will establish a baseline so that you will be able to determine
if the background radiation has changed, or to detect trace amounts of
To run a test position the probe (or geiger counter) very close to the top surface
of the material you are testing, and run the counter in its 1 minute or 5 minute
mode to check for radiation above your establish background radiation.
Fukushima Radioactive Fallout Article
U.S. Nuclear Regulatory Commission
CDC - Center for Disease Control maintains a radiation emergency web site:
Health Physics Society
U.S. Environmental Protection Agency
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