28 | PRM-7000 / 8000 / 9000 Users Guide
clides from these incidents have been deposited in a non-uniformly around the world.
Isolated pockets with elevated concentrations of man-made radionuclides can still be
found. For example, ventilation systems that were installed prior to 1963 collected fallout
radionuclides. If these systems are still in use and the ducts have not been thoroughly
cleaned, there is a potential for elevated background radiation. Another potential source
of elevated background radiation from man-made radionuclides is wood ash. Trees fi lter
and store some airborne pollutants, including 137Cs from fallout. When the wood is
burned the 137Cs is concentrated in the wood ash. Materials or equipment associated
with the ash could have elevated levels of background radiation.
Cosmic Radiation
Cosmic radiation consists of highly energetic particles that are believed to originate from
phenomena such as solar fl ares and supernova explosions. The Earth’s atmosphere
serves as a shield for these particles, although on rare occasions a solar fl are is strong
enough to produce a signifi cant radiation dose in the lower reaches of the atmosphere.
Cosmic radiation is also responsible for the production of radioactive elements called
cosmogenic radionuclides. These radionuclides are produced from collisions between
the highly energetic cosmic radiation with stable elements in the atmosphere. Cosmo-
genic radionuclides include 3H, 7Be, 14C, and 22Na.
14.1.2 Measuring Background Radiation
Once the PRM Instrument is moved to a new area, a new background measurement
should be established as follows:
1. Place the PRM Instrument away from items that could be radioactive,
such as metals, pottery, granite counter tops, etc. Ideally, locate the
instrument in the center of the area that you want to monitor and locate
it table height above the fl oor.
2. Reset the Average Counter Values - see “9.1 Resetting Counter Values
(Reset Counters)” - or power-cycle the unit off and on to reset the
counters.
3. Simply allow the unit to run overnight and the next morning, look at the
Average measurement and note the reading. This will be the normal
background count.
4. While measuring, feel free to look at the Average value to see the
value it is moving towards, but do not move the unit from the current
location until the background measurement is complete.
5. Alternately, a Timed Measurement can be used. See “6. Performing a
Timed Measurement (Timed Measure)”. A 10-minute timed measure-
ment will be reasonably accurate, but an hour will be more accurate.
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