Geiger counters are able to measure that a certain level of radiating energy has been detected within the Geiger tube. They cannot detect the absolute level of the energy or whether the event was alpha, beta, or gamma. By analogy, they can determine that a car has passed, but it cannot say what type of car ir was or how fast it was going.
Geiger counters can give results in counts per second, CPS, or counts per minute, CPM.
Geiger counters do report in rems (radiologic equivalent in man) which reflect the impact on biological tissues or in Sieverts. One Sievert equals 100 rem.
By convention, 1cpm may equivalent to .01 microSieverts per hour, but there is no accurate way of doing this, as they are separate issues, such as rocks and apples.
If one monitors a Geiger counter, one sees that there is significant variability from moment to moment, but that over 5 to 10 minutes of sampling, the average becomes quite stable. Also, as samples get large, say 1000, the average becomes stable. Other instruments such as dosimeters give information over longer periods of time, and can reflect the total dose. Since the effects of radiation are cumulative over time, one would like to measure the total amount of exposure. By analogy with a car, average rate times time equals distance traveled. Thus, an ideal instrument would have both the capacity to measure rate, in CPM, but also the ability to measure total number of events that occurred.
For my first Geiger counter I selected a Mazur PRM 8000 Geiger. To use a MacIntosh I used Parrallels to get a Windows system, and then I downloaded a driver. The driver can be found at http://www.ftdichip.com/Drivers/VCP.htm. Information could be collected by the Geiger Counter, It can report maximum, minimum, average, duration, total count, time, duration, and time of maximun count. It can download to a computer, and upload to GeigerGraph, where results can be posted on a national data base in real time at RadiationNetWork. On my Geiger counter, I can switch units, from counts per minute, CPM, to microRems per hour to find out equivalence and determined that 14 CPM equals 14 microrem per hour. Hence, 1 CPM equals 1 microrem per hour. Or 60 counts per hour equals 1 microrem per hour. Since there are 24 hours a day, and 365 days per year, then there are 24X365=8,760 minutes per year, then there are 60X8,760=525,000 or 5.25 times 10 to the 5th power counts per year is equivalent to 8760 microrems or 8.76 mrem per year. Rounding off, 600,000 C are equivalent to 10 mrem is roughly what would accumulate over one year at the rate of 1 count per minute. Since my Geiger counter mostly reads 13 CPM, then this should convert to 130 mrem per year, in the same order of magnitude of the exposure of 300 mrem per year predicted by the EPA calculation. I have recently learned from Vince Mazur, maker of the Geiger counter I use, that the pancake model is 3.5 times more sensitive and is thus useful for low level detection and for use with food.
For the Mazur 9000 the average count is about 31 cpm, while at the same time the Mazur 8000 is 13 cpm.
I found information about the Inspector radiation alert:
This shows that there is more sensitivity of the window style Geiger Counter at low levels compared to the end style, but that these differences disappear at higher counts. There is also variable efficiency for the different isotopes. Only 33% of radioactive phosphorous is identified. This would explain the difference between counts per minute, and discharges per second.
For more information about how to set up a citizens’ monitoring of radiation, particularly around a nuclear reactor, see this post.