No, Geiger counters do not incorporate any type of circuit to discriminate among different kinds of radiations. Some counters have an open window over the Geiger-Mueller tube: this allows for energy discrimination between gamma and X-rays (intense enough to pass through the housing) and beta and/or alpha radiation, which can be read through the open window. However, when discrimination is required, it is better to use proportional counters or ionization chambers (gas-filled or solid-state).

As for the measurement unit, Geiger counters simply measure the counts per minute (CPM). Counters are usually calibrated with Cs137: in the case of GM-10 counter, for example, 120 CPM  corresponds to about 1 microSievert per hour.

Sievert is a unit of risk, not dose. A Sievert estimates risk of cancer as effective dose. Instruments are calibrated to Sieverts based on one of several definitions. These calibrations have little resemblance to actual exposure situations. Nevertheless, rules and practices are such that the measurements with an appropriate instrument will meet regulatory guidelines. Regulatory guidelines are set sufficiently low that there should be no concern about the actual risk, including instrument flaws. The suggested reference by Pedro Almendral more or less embraces this condition, but does not answer the question in the reference.

Compensated GM detectors do a better job than uncompensated GM detectors. Most detectors, in particular GM detectors, measure correctly only for the calibration conditions. Detectors can be constructed to and with effort used to measure actual dose conditions. In general the effort is huge and not worth the effort.

An uncompensated GM is one of the worst choices for knowing the approximate dose rate for a location. Some scintillation detectors are worse. It all depends on the energy mix and the direction of radiation.

In https://www.researchgate.net/publication/260188262_Measurement_of_Effective_Dose

I show the definitions of dose and the response of detectors to calibration conditions and simple environmental exposure.

Chapter Measurement of Effective Dose

A Geiger counter can be used only for a single component radiation field such as gamma rays and must be calibrated for such a field. In a mixed field a number of instruments are required and/or more sophisticated equipment such as Tissue Equivalent Proportional Counters,

It has to be calibrated to a known dose rate for a particular spectrum of gamma/ or beta's. geiger tubes are overly sensitive at low energy. Dose rate measurement is best with ionization survey meter. Hope this helps.

As pointed out by my colleagues G-M cannot measure dose equivalent( Sv or Rem).

They can give an indication of the presence of radiation, specially for contamination measurements in a radioactive laboratory.They can give a precise exposure reading for high energy gamma radiation ( Co-60 or Cs-137). G-M are not suitable for pulsed radiation and for low energy radiation.

Usually, if you want to use a GM to measure H*(10) you will need to use an energy compensated GM. All major producers offer now "radiameters" with compensated GM and usually for the energy range between 40 keV and 1.5 MeV the measurement is accurate enough for radiological protection purposes, i.e. it should be in the range of +/- 15% of the conventionaly true value. Many people use this type of compensated GMs for radiation protection survey because they have a very fast response and they will help you identify the hot spots or any places where the shielding is not good enough. Once you found the weak areas, you can measure the dose rate with a higher accuracy using an instrument with an ion chamber or with a proportional counter.

As I pointed out before, I would not use the G_M for pulsed radiation ( X-rays or linacs), even for external shielding surveys.I prefer to use the ATOMTEX 1121 tissue equivalent scintillation detector, that has afast response and gives the readings in Sv or mSv or nSv..

@ Sergio: the Radiagem 2000 also gives the answer in Sv, and it uses a compensated GM. It can be used (and it is used, in many instances) also shielding surveys at Linacs or other radiotherapy installations because at the repetition rate the Linacs have the detector will see the field as continuous. Furthermore, if you use the special TTC counting technique, than you extend very much the dose rate range. I agree with you that a scintillation detector is even more sensitive, but the energy dependence problem still remains and you have to compensate it somehow (even for a issue equivalent detector) and, added to that, you have a problem of saturation at higher dose rates - but of course you should not get there, at 10 mSv/h because it would mean the shielding is really bad.

Bottom line, both the GM and the scintillation detectors should be used only for fast survey and then the real dose rates should be recorded with an instrument with an ion chamber.

Dear Radu,

you are right but you urgently need a calibration of you GM counter in equivalent dose rate for your radiation quality. I would always prefer calibrated scintillation detectors.

As a matter of fact a radiameter like the Radiagem comes with a factory calibration giving the calibration factor for Cs-137 and the energy dependence factors for Co-60 and Am-241 The correction factors are within +/- 15% of the calibration for Cs-137 confirming that it has a good comensation filter, so it can be used with enough confidence for dosimetry surveys - in fact it is meant just for that.

It is good practice to use a fast response detector for an initial survey and a more accurate instrument for actual dose rate. Nevertheless, no instrument will measure close to the actual dose rate, as there is no exposure situation that equals

• The quantity Hp(d) in soft tissue, at a point in a radiation field is the dose equivalent that would be produced by the corresponding expanded field in the ICRU slab at a depth, d, below a specified point on the body. The unit of personal dose equivalent is joule per kilogram (J kg-1) and its special name is sievert (Sv).
• Furthermore, Hp(d) is a calculated quantity and cannot be measured in practice.

Instrument calibration in Sv is a convenience that allows measurement in the same unit for all types of detectors. It is equivalent to the practice of calibrating all types of instruments in counts per minute. This was a common and often prescribed practice. 

What every measurement you make in Sv will be wrong. It is your job to know what it really means.

This is correct, the only quantity that you can really measure is the absorbed dose. Survey instruments, however, are routinely calibrated in H*(10) ("ambient dose equivalent") because this is the operational quantity established by ICRP as relevant for such surveys.

The ambient dose equivalent H*(10) at the point of interest in the actual radiation field is the dose equivalent which would be generated in the associated oriented and expanded radiation field at a depth of 10 mm on the radius of the ICRU sphere which is oriented opposite to the direction of incident radiation. 

This means that the calibration will be done in a broad paralel beam and the reference detector should be placed in an appropriate phantom and thus transfer the calibration factor to the survey instrument. If the calibration is correct, and the compensation filters are built in a proper manner, than the survey meter will give a correct enough answer in terms of ambient dose. In most cases the result of the measurement will only be used for comparison with the limits imposed by the norms and for the decision whether the shielding is good enough or not.

If one wishes to go further and calculate risks, than it is better to go back to measuring air kerma, than go through all the steps necessary to calculate the effective dose (not quite straight forward) and then calculate risks by taking a risk model into account - yet another source of errors.

But I am affraid we are straying from the original question, for which the answer is pretty simple. The GM has to have a good energy compensation filter and then to be properly calibrated in terms of ambiental dose (it cannot be calibrated in effective dose).

Radu

I agree there was straying in a commercial direction. I stand by both my answers, the first to the original question, the second to your comment concerning Hp(10).

Nobody, including standards, can measure kerma as defined, so let us not go there.

Well, in the end it all depends on how you define a measurement. Strictly speaking, what you really measure with any detection system is a current or a voltage. How you relate that current or voltage to the quantity you really want to measure, that's another story.

But my comment was not about Hp(10), it was about H*(10). Hp(10) is no longer measured with GM dosemeters, as far as I know. All active personal dosemeters are now using silicone detectors nowadays.

Radu

My error on H*(10). The comment is the same. H*(10) cannot be physically measured so it is calculated. It can not be physically measured because the definition cannot be constructed.

GM dosemeters vs silicon dosemeters? Leading us astray?

I believe it would be worth to have a discusssion on what can and cannot be measured. Theoretically you are right, the expanded and aligned fields do pose practical problems but in practice one uses models that can be close enough to the definition. This is why virtualy all survey meters in Europe are calibrated in H*(10).

Radu

As you suggested, I have proposed a question

https://www.researchgate.net/post/How_practical_are_the_operational_quantities_for_Sievert

I would like to add the following elucidation to the answers given by others by commencing from fundamentals to fully answer Malihe Rostampour’s question.

The detection of x and gamma rays by GM tubes is by the production of secondary electrons by the radiation. These electrons mainly originate in the metal wall of the counter. The intrinsic efficiency of a GM tube for gamma counting (𝟄) may be defined as the number of secondary electrons entering the sensitive volume of the GM tube per photon striking the wall. This efficiency is a linear function of energy in the range 200keV – 2MeV for cathode materials such as aluminum, copper or brass. So we can write:

Count rate (C) is proportional to: N𝟄 = Nk E𝞬 (N is the photon flux on the counter) -------(1)

where E𝞬  is the energy of the gamma photon and k is the proportionality constant

Now the gamma exposure rate (D, mR/h) at the counter is proportional to:  NE𝞬 ( µen/ρ) -----(2)

where E𝞬   is the (mean) gamma ray energy of the beam (MeV) and

µen/ρ is the mass energy absorption coefficient of air for E𝞬 , which is nearly constant in the above energy range.

So we get from (1) and (2) above:

C/D =k/ (µen/ρ) (ρ is the density of air) --------------(3) which is also a constant in view of the reasoning’s above.

Hence the count rate can be calibrated in terms of exposure rate in mR/h

Now for body tissue, Roentgen(R), Rad and Rem are nearly same for x and gamma radiation and 1Rem =   10-2  Sv(Actually 1R corresponds to 0.89 rads in air and 0.97 rads in tissue, and this can be taken into account in the proportionality constant above).

NOW IT IS OBVIOUS HOW THE DOSE EQUIVALENT RATE CAN BE CALIBRATED IN TERMS OF COUNT RATE MEASURED BY A GM COUNTER. HENCE THE METER DISPLAYS DIRECTLY DOSE EQUIVALENT RATE (mSv or µSv/h).

Now perforated filters of various types (thickness and number of holes) are employed surrounding the metal wall of the GM counter (materials mentioned above), to extend the energy range (to make equation 3 above constant) from 60 keV to 2 MeV with an accuracy of about ±15%. This is done experimentally. Our group in BARC (India) had done extensive work in this field.

It may be added here that GM counters used for radiation protection monitoring are generally of halogen type of small size with a very small resolution time (< 10 µsec) so that the proportionality of count rate and dose rate are preserved over a wide range (e.g. 1mR/h to 1000R/h) over the energy range above.

Please note that C/D above is proportional to and not exactly equal to the term on the right hand side. Also on the RHS, the concept of energy flux per roentgen also can be used. Also, most of the present day monitors give the x and gamma radiation levels in terms of mR/h, mRad/h(air Kerman rate), mrem/h or uSv/h, since the quality factor is 1. What I wanted to show was, any of these quantities can be proportionately related to count rate and displayed on the survey monitor. I also wish to point out that air/tissue equivalent scintillators coupled to photodiodes/miniature PM tubes operated in the current mode(not pulse mode)can also be successfully used to display the above quantities over a very wide range and such instruments are commercially available.

Kindly note I have given two answers here on different dates. They should be read together chronologically for consistency. Thank you all.