# Are Geiger counters isotope-specific?

I was talking with an employee at a company that does I-131 therapy for hyperthyroidism and they said that the Geiger counters they use are "tuned" for I-131, implying that regular Geiger counters are less effective in some way than these special "tuned" Geiger counters.

Is this true? Can a radiation detector be "tuned" in a useful way for specific isotopes, or is this just a marketing ploy used by manufacturers to sell into the medical community?

• I suppose that one could 'tune' the sweet spot by tube geometry, pressure, voltage, and signal processing (e.g. rejecting low pulses) but you really can't make them sensitive to only a single isotope. – Jon Custer Jan 15 '16 at 14:46
• I'm surprised that one would "tune" a GM tube for anything. A GM tube is a cheap but crude tool for detecting ionizing radiaion. They're usually found in hand-held survey meters, and in radiation alarms. For precise dose(rate) measurements, you should be using ion chambers, diodes, or TLD. – Solomon Slow Jan 15 '16 at 15:34
• james is right, and so is everybody else. One would not use a Geiger counter for anything that requires any level or precision. They are, at best, "hardy" (?) survey instruments and threshold detectors for situations in which the nature of the radiation is well understood. I have never used one for anything other than "fun with radiation". – CuriousOne Jan 15 '16 at 18:25

The gas inside the geiger tube will have a different response according to what kind of radiation is entering and what energy it has. For example, here's Helium's absorption for electrons , and here's its X-ray absorption . The tube only gives you a particle count, but if you have a fixed distribution of incoming particles then you can calculate/calibrate a direct relation between count and dose.

If you are measuring varying energies and particles, you can try to selectively attenuate certain energies in order to get a flat response so that you get a count that is proportional to dose.

• I see, sort of. So, using I-131 as an example, it emits both beta and gamma radiation with particular energy profiles. So if you use a detector that looks for specific energy levels for each of those two types of radiation, you can essentially measure only the radiation coming from I-131 and filter out any background radiation? – Ambrose Swasey Jan 15 '16 at 18:52
• @TylerDurden The way you take advantage of this difference is by not using a Geiger counter (because they are intentionally run in a mode without energy discrimination), but by using a track resolving detector of some kind and following the track for long enough to figure it's energy loss profile. – dmckee Jan 16 '16 at 0:38

True, Geiger counters can be tuned to be more sensitive for certain types of decay and for the amount of radiation emitted.

They can detect ionizing radiation in the form of alpha particles, beta particles and gamma rays. Given the different nature and energy of these types of radiations (alphas are Helium nuclei, betas are electrons or positrons, gammas are photons) a Geiger counter could be tuned to detect with more precision a certain type of radiation than another.

In addition they can be tuned to be more sensitive to measure a low amount of radiation or to measure huge amount of radiation.

In the case of Iodium-131 the decay is $\beta^-$, so the Geiger should be tuned to detect electrons.

• This is mostly wrong. Geiger counter run in saturation and are pretty close to digital device (either respond or don't respond). Gas ionization counter running in the linear range give you energy sensitivity but you don't call one a "Geiger counter". Nor do these device distinguish between the source of the ionization. – dmckee Jan 15 '16 at 18:32

Short answer: turned ionization detectors are easy enough, but they are not "Geiger counters".

The core of a Geiger counter is a gas ionization detector that runs in a saturated cascade mode. They respond to ionization in the gas and are very nearly digital in nature.

Notice that Geiger counters are often rigged to emit an audible click when it respond, and there is no difference between the clicks. That's because the saturated pulse generated by the cascade is the same regardless of the source of amount of radiation.

This energy-insensitive mode of operation has one really big advantage: it's doesn't require careful calibration, which makes is very suited for a basic hand-help survey instrument but completely useless for a particle ID.

You can run the core detector in a linear gain mode, but then it isn't a "Geiger counter", and you need rather more sophisticated electronics backing it. In fact, this is the preferred way to build a sophisticated detector, but it requires careful calibration an the requisite electrons used to be pretty bulky and power-hungry.

There is one thing you can do for resolving different classes of radiation, and that is take advantage of the different penetration depths. At nuclear decay energies (a few 10 of keV up to a few MeV), gammas penetrate more deeply than betas which penetrate more deeply than alphas. By engineering the thickness of the walls you can exclude the alphas or both the alphas and the betas.

However, that is not accurate enough to let you sort middle energy gammas from higher energy gammas (that is to pick out individual lines) because the penetration of any particle ray is subject to considerable random variation.

Most off the shelf Geiger counters have a "thin" window designed to allow betas in. Designing a Geiger style counter to accept alphas is delicate and of limited use because most decay alphas range out completely in a few mm of air.