(picture: “Geiger Tube” by “dh” on Flickr, showing Russian SBM-20 hard beta and gamma GM tube)
On the internet there lots of DIY people showing off their homebuilt Geiger counter circuits, complete with microprocessors and LCD’s calculating and displaying count rates and often the dose rate in microsieverts/roentgen per hour.
The last thing surely makes a project look more impressive with the floating point number with a weird unit that most people are unfamiliar with. It’s not a big deal with DIY setups but there are lots of mentions of sending projects to Japan to help people over there “measure” contamination.
I would like to explain something I think is overlooked when implementing these projects:
You cannot convert counts per minute accurately to sieverts or roentgen per hour
If you look up information of a tube it will often specify how sensitive it is to gamma radiation of a certain isotope. For example, the specification of the widely used LND712 end window tube reads:
Gamma sensitivity: 18 counts per second = 1 mR/hr (Co60)
The tube is calibrated against a known gamma source, and the counts per minute to R/hr is only valid for gamma radiation emitted by Co-60.
In real life, all kinds of radiation of different isotopes cause GM tube discharges. A GM tube does not give information about the particle/ray energy, it only indicates that something was able to ionize the gas in the tube. It doesn´t matter whether the tube detected a high energy secondary cosmic ray, or some weak beta particle just being able to penetrate the tube, it will give the same loud click.
However, this does not mean GM tubes are useless. They will detect most beta and gamma radiation and alphas (pancake, end window tubes) so most of the time they will give you information about whether something is radioactive or not, and how radioactive, relative to the background. For example. I was able to measure that a porcelain mug I have is very slightly above background.
There are also other purposes for geiger counters in experiments. If you make an array of tubes and connect them to a microcontroller you can actually count particles that come from a certain direction, from space for example. It should also be possible to detect radon daughters decaying by pushing air through a filter and measure the radioactivity of the filter afterwards.
So, even with a `simple` geiger muller tube you can perform interesting experiments.
Disclaimer: I am not a scientist. When you want to accurately measure radiation for some official purpose, consult a nuclear physicist.
I completely agree with you.
But some electronic dosimeter manufacturing companies are using GM tube as detector to measure dose. Can you guide me how they are converting pulses to dose?
And they give a wide range of energy where their product can be used safely.
Zeeshan, it is possible to improve the linearity of the GM tube so that it responds more equal to radiation of varying energy. This is called energy compensation: https://en.wikipedia.org/wiki/Geiger%E2%80%93M%C3%BCller_tube#Energy_compensation.
Also, when a dosimeter is used in a specific situation it can be quite accurate. For example, a dosimeter calibrated to Cs-137 will be accurate when measuring the gamma dose in Chernobyl or the contaminated areas around Fukushima, as Cs-137 is the main contaminant.
The point I was trying to make is that dose rates of the background radiation are not really useful on most geiger counters, but often included and interpreted by users as being accurate.