Geiger Counter Measurement: Sieverts/Hour?

[kiskrof]

I have been looking for an answer about what a Geiger counter actually measures. I am puzzled by the claim that it can actually give measures in sieverts/hour, i.e, measure the dose equivalent received by the body. On Wikipedia (Geiger counter article), I read: "a G-M tube can detect the presence of radiation, but not its energy which also influences the radiation's ionising effect. Consequently, dose rate measurement requires the use of an energy compensated G-M tube, so that the dose displayed relates to the counts detected.[2] The electronics will apply known factors to make this conversion, which is specific to each instrument and is determined by design and calibration."
Obviously, the device cannot know that I am in a cellar full a Radon inhaling alpha-emitters, which is obviously much worse than walking on a field of beta-emitters.
Furthermore, if "a G-M tube can detect the presence of radiation, but not its energy", it should mean that the result should be in becquerels (number of desintegration per second) and not grays (J/kg).
Everytime I read that some people live in a place with, let's say, 1,000,000 Bq/m², I wonder how many sieverts it might be yearly for the inhabitants and I always think it is impossible to calculate that, because it depends on the quantity of dust inhaled, the water they drink, the food they eat, the kind of houses they live in.
Now super little Geigers seem to be able to provide this tricky conversion automatically! Waoo!

 

[Vanadium 50]

They are calibrated for betas and gammas. You're right that they wouldn't have the right correction factors for alphas. They are also unlikely to detect alphas at all, since they have to penetrate the tube walls.

 

[montoyas7940]

A G-M with a thin mica window is used to detect alphas.

Edit: Additionally, since alpha, beta and gamma have different penetrating ability, different materials can be used to shade the detector allowing the instrument operator to discover how many counts are due to each.

 

[kiskrof]

thank you but I still have a question. If Wikipedia is right in saying "a G-M tube can detect the presence of radiation, but not its energy", then it means that the Geiger counter can measure Becquerels. Since some betas are more energetic than others, how can we turn the Becquerels into Grays or Sieverts? I think it is only possible to calibrate the Geiger counter if you know where you are. For example, if you are 10km from Fukushima Daiichi, you know most of the radioactivity is Cs137 (512 keV for each Bq). If you are in your cellar, you know you are looking for Radon (5.48MeV for each Bq).
Furthermore, the Geiger counter will tell me about the dose I receive from the background radiation, and not about the possible later radiation from dust I have inhaled or dust sticking to my clothes. So if you decide to resettle a place, you must know that you'll get more Sieverts than what your Geiger tells you. I think it is an important thing to know.
I am not a specialist at all. So please correct me if I talk nonsense.

 

[kiskrof]

One more question. In articles, they often mention the number of Bq/m². Is there a equation to turn that into Sieverts?

 

[montoyas7940]

You are correct. If you are contaminated externally or internally you will receive dose not measured in background surveys. I would assume if an organized resettlement of a contaminated region took place, whole body counts and personal contamination monitoring would be part of the program.

Bq is a measurement of the number of decay events per second and won't tell you about the energy of each event. So as far I know there is no direct conversion from Bq to sieverts. But like you mentioned previously knowing the radiation source and counts per second would allow you to calculate dose.

 

[Morbius]

thank you but I still have a question. If Wikipedia is right in saying "a G-M tube can detect the presence of radiation, but not its energy", then it means that the Geiger counter can measure Becquerels. Since some betas are more energetic than others, how can we turn the Becquerels into Grays or Sieverts? I think it is only possible to calibrate the Geiger counter if you know where you are.

kiskrof,

You are correct! Since a G-M tube goes into saturation by design, it's not energy sensitive. It really only measures Becquerels.

However, for an assumed source and assumed deposition medium; it can be "calibrated" to give dose. That is they make an assumption about the energy distribution, and they also make the assumption that you want dose in biological material.

If you are interested in dose in material other than living tissue, or the source doesn't have the assumed energy distribution; then the "calibrated" dose isn't meaningful under those conditions.

Greg

 

[kiskrof]

Let there be a point A situated at height h above the ground. How much radiation does it get from the ground, if every point on the ground emits a certain quantity of power per surface P (J/s/m²)?

Let's define a circle of radius r, whose center is right under A. If we can calculate the circle, we'll integrate from r=0 to r=infinity and we'll have our answer.
The radiations decay exponentially as they go through air, according to a decay constant L. I think the contribution from each circle is 2pi*r*exp(-L*sqrt(r²*h²)). If I integrate that with a machine, I get a final result of P*(L*h+1)*exp(-L*h)/L². But I am not sure at all it is correct.
Of course, my result is in J/s/m². So if you want a power as a result, you must further integrate a certain quantity of point a define a horizontal surface(just multiply by the surface, it is a trivial intergral). Then you can integrate a pile of surfaces and you'll get volume (this time it is a difficult integral because the different surfaces get different amount of radiation). However, the result would be wrong if the volume is made of water (simplified model of a person), because the radiations decay faster in water than in air.

 

[kiskrof]

Actually I have just realized that the result will depend on the decay constant inside the body of the person too. If you are walking on a gamma emmiter, than a lot a the radiation will go through. The radiation intake will depend more or less linearly on the volume of the person. If we are walking on beta, then then radiation intake will depend on the surface exposed to the radiations, since most beta is absorbed in the first few centimeters.