Geiger counter calibration question

bitrex

I'm wondering if anyone could point me to a reference that explains how commercial digital geiger counters convert from the natural counts/sec unit to units of mR or uR/hr?

Astronuc

One would need to convert counts/s or per min or hr, to R/hr, by knowing the energy per count. That requires calibration based on the strength of the pulse generated by the path of the ionizing radiation.

From - http://www.geigercounters.com/Definitions.htm

"CPM - Counts Per Minute, i.e. radiation counts per minute, a unit of measurement for a Geiger counter, on many models corresponding directly to the audible beeps or clicks per minute. CPM is the standard unit of measurement for alpha and beta radiation, and is also commonly used to express background radiation in numerical terms.

mR/hr - milli-Roentgens per hour, or 1/1000 of a Roentgen per hour, a standard unit of measurement for radioactivity, popular in the United States and Israel.

µSv/hr - micro-Sieverts per hour, a standard unit of measurement for radioactivity, popular in Canada and overseas. If you want a Geiger counter that reads out in µSv/hr, look for this feature, standard on some models, and optional on others."

http://www.geigercounters.com/AboutGgr.htm
How does one determine the type of radioactivity?

• Position the Geiger counter near the suspected source of radioactivity, without aiming the open window at the source. If you are getting a reading, it is most likely Gamma and/or X-radiation or high energy Beta, and can be read in terms of mR/hr or µSv/hr.
• Next, place an 1/8" thick piece of aluminum between the instrument and the source. If the radioactive indication stops or decreases, it is most likely Beta radiation, and can be read in terms of CPM. Most common isotopes emit both Gamma and Beta radiation.
• Now aim the open window of the Geiger counter at and immediately next to the suspected source of radioactivity. If this gives you a reading versus no detection through the housing itself, then the radioactivity is from Alpha, Beta, or low energy Gamma. Next, place a piece of aluminum foil between the open window and the radioactive source. If the radioactive indication stops, it is likely Beta radiation, and should be read in terms of CPM
• Now do the same test with a sheet of paper between the open window and the radioactive source. If the radioactive indication stops, it is most likely Alpha, and should be read in terms of CPM.

"In the course of your readings, be careful not to contaminate the detector with radioactivity by physically touching the radioactive source or by holding the source above the open window of the Geiger counter.

Unless otherwise mentioned, all of our Geiger counters come pre-calibrated from the factory. You should re-calibrate your Geiger counter as often as your regulations require, or in any case, at least once a year. Follow the manufacturer's instructions in the operating manual that comes with your unit for guidance on re-calibration."

More information

http://hps.org/publicinformation/ate/q7972.html
http://www.hps.org/publicinformation/ate/q3497.html
http://hps.org/publicinformation/ate/cat1.html#80
http://hps.org/publicinformation/ate/q8495.html

JJThompson

This site that you referenced says that different instruments will get different mR/hr readings from the same source. Is that right?

Rajini

Hi..GM counter gives counts per sec.
Actually to convert counts per sec to R/hr or any dose equivalent are mainly based on which part of your body gets exposed..it is different for tissue from stomach, hand, etc. Because not all part of your body will absorb counts per sec. as same...it is different for different part of body..
So counts per sec. is universal value (All gm counters should give the same value) but different R/hr ..

I think the manufacturers of gm should have their own standereds..

Bob S

Hi-
GM tubes give readings in counts per second, which is not linear in R per sec, rads per sec, or REM per sec. In general, the counting rate and sensitivity (gain) of a GM tube increases as the voltage is increased, and eventually it will start counting noise and saturate (which paralyzes the tube because each time it counts, the voltage on the center wire is reduced by several hundred volts). The best thing to do is use a calibrated radioactive source, and set the GM counter reading or the counting rate to the mfgr's recommended sertting.

If you have a clean calibration gamma like cesium-137 (661 KeV), you can sometimes plateau the voltage by carefully increasing the tube voltage and plotting the counting rate. You will need to minimize low-energy backscatter gammas from nearby shielding. This procedure does not ensure that the setting will be the mfgr's recommended setting, however.
Bob S

[added] Use this site to get the x-ray attenuation coefficients for your GM tube and calculate approximate counting rate.
http://physics.nist.gov/PhysRefData/...oef/cover.html