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 Most Geiger Counters Are Calibrated To Cs137 
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However, unless Geiger counters are calibrated with the radiation source that they are intended to measure in the field, the mrem/hr readings will probably not be accurate. Most Geiger counters are calibrated with a radioactive source called Cesium-137 (Cs-137). This is because Cs-137 has a long radioactive half-life (about 30 years). Therefore, the calibration service companies do not have to purchase a new source (and dispose of the depleted old one) very frequently. Cs-137 emits gamma-rays when shielded for instrument calibration purposes. It is one of the sources that dirty bombs are thought to be composed of.

It is not possible to calibrate the Geiger counter for multiple types of radiation sources so that low and high energies are accounted for. It can be calibrated once and then ??correction factors?? can be supplied for other radiation energies (sources). The reading from the Geiger counter would be multiplied by these factors but, this is impractical for the first responder. Under most conditions, a responder would not know what radiation source or sources were involved in a particular incident making the correction factors useless and very difficult to apply in the chaos of a response.?? One must simply know the limitations of the Geiger counter while using it.

Other limitations of the Geiger counter include:

1. It cannot perform air-sampling measurements alone. A pump to draw contaminated air onto a filter is needed. The Geiger counter, under special conditions, can be used to measure the filter.

2. It cannot detect weak beta or gamma-emitting isotopes. Some isotopes such as tritium and iodine-125 emit weak radiation that the Geiger counter cannot.

3. It cannot be used to identify a radiation source i.e., it cannot tell you if the source is Cs-137 or Cobalt-60.

4. It may fail to respond in extremely high radiation fields unless it is equipped with special ??anti-saturation?? circuitry. Such circuitry can allow it to respond at radiation levels 100x its maximum reading.


http://www.fireworld.com/Archives/tabid/93/articleType/ArticleView/articleId/86863/Counting-on-the-Geiger-Counter.aspx

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MY OUTSIDE RADIATION MONITORING STATION:
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http://netc.com/chart/view.php?n=1%3AEB5A139C


Fri Dec 20, 2013 4:20 pm
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So looking above, it is easy to see that having a geiger counter calibrated to Cs137 will better detect that isotope than if it was not calibrated to that source. It is fine tuned to that isotope and therefore does not need to use the correction factors mentioned above.

Knowing this, it is worth noting that currently NETC EPA gamma stations are using channel 5 which is the energy range Cs137 is found in. These EPA stations can be seen as station ID 5: on NETC.

_________________
MY OUTSIDE RADIATION MONITORING STATION:
South Beloit, Illinois - GMC200 Outside on HEPA air purifier, ground level, facing West.
http://netc.com/chart/view.php?n=1%3AEB5A139C


Fri Dec 20, 2013 4:47 pm
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