An expert explains Geiger-Müller counters
(email 19.3.2011, from a Western nuclear-medical expert known to Piotr Bein)
Alpha radiation attenuates in air within a couple of inches.
The Electromagnetic Energy of alpha decay can be detected if a detector has the right proximity to the radioactive source material. This is true for beta, gamma and neutron radiation. They have to reach the detector. In air, some EM radiation travels inches, some travels feet and some travels miles (alpha, beta, gamma, neutron respectively)
These small all purpose GM counters you are asking about are used to measure Becquerels and use a formula to calculate Greys and Sieverts.
Any detected alpha radiation in an air born sweep or a swipe of a human body is the result of coming within a couple of centimetres of the source material. In the case of alpha EM radiation, the the radioactive source material will be floating or passing, stuck to clothing, skin or hair in the form of particulate. Uranium and plutonium particulate has an affinity for clothing due to filter effects of the surface material, static charging and humidity of the body.
If the air-born particulate or fall out is an alpha emitter it will be detected by the hand held detector if its close enough (an inch or so) and the rate of decay is frequent enough to emit an alpha decay within the immediate area of the detectors reading face/the detector pauses long enough at that point if the source is stationary.
No GM counter can identify the type of radiation it is detecting. An alpha probe which ignores all other types of radiation is needed. That is what I used in Iraq and Afghanistan. Also, no GM counter can identify the nuclide source. A nuclide identifier is required (about $15,000 for a basic field portable identifier). I used an Exploranium in Iraq.
The GM counters like you are asking about can only read EM pulses, not the type of radiation (i.e. it cannot specify that it is alpha). They are set to give an accumulated reading of decays and or translate (unreliably) into biological effectiveness.