ALERT: More from an expert on radioactivity from Japan
from Tedd Weyman, Uranium Medical Research Center, Toronto, Canada, small corrections and additions to previous email 19.3.2011 to Piotr Bein
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). Some will pass through concrete and lead; some cannot pass through skin.
Alpha radiation attenuates in air within a couple of inches. 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 radioactive source material will be floating or passing, stuck to clothing, skin or hair in the form of particulate (sub-micron sized radioactive dust particles). Uranium and plutonium particulate has an affinity for clothing due to filter effects of the surface material, static charging and humidity of the body. The most radioactive secondary sources in Iraq were the clothing of troops in the battle field.
If the air-born particulate or fall-out is an alpha emitter it will be detected by the hand held detector provided it is 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 detector’s reading face (window) or the detector pauses long enough at that point if the source is stationary. Frequently the operator will scan too fast to catch the decays when the frequency is low or the distance is greater than a centimetre. Proximity/distance, frequency and density make the hand held detection a 3 dimensional task. The type of detector and the type of emission must be compatible as well.
No GM counter can identify the type of radiation it is detecting. An alpha probe designed to ignore all other types of radiation is needed for alpha-only detection. That is what I used in Iraq and Afghanistan, where alpha decay is accompanied by gamma emissions.
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. We used gamma and beta detectors to find the hot spots and original sources and then narrowed in with alpha probes.
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.
The emissions of radioactivity from the Japanese supercritical reactor cores and the spent fuel cooling ponds that are causing a threat to the liquidators on site are two kinds: gamma and neutron radiation. Both are extremely “penetrating” and extremely damaging as they pass through the human body like a microscopic bullet, slicing and cutting cellular material, and super-heating sub-cellular molecular targets (just as if they are a miniature grenade exploding inside a cell).
The second type of radiation is the emissions from any air-born particles carried out the plant, lofting into the atmosphere and travelling across Japan — these have now reached Canada and the US. These particles are entrained by the mechanical disturbances of the fire-fighters’ water being poured on the reactors, the hydrogen explosions, etc. These air-born reactor fission products (man made products inside the nuclear reactor) such as radioactive Iodine and Cesium are mostly beta radiation emitters. They do not exist in nature and they are accompanied by a host of alpha radiation emitters from the original fuel: Depleted Uranium and Enriched Uranium and artificial nuclide, Plutonium. Beta, gamma and alpha radiation will be emitted by the contents of these air-born materials spread in the vapors, steams and smoke plumes leaving the plants and travelling around the globe.
A single particle of these air-born radiation emitters entering the lungs can enter the blood stream, incorporate in human tissue and start a biological chain reaction that can cause cancer and other systemic illnesses. To claim they are not a threat to human health is a fallacious statement. It is statement of statistics regarding the likelihood of exposure and consequent illness, and nothing to do with the biological effects of contamination. These radiations emitting materials (dusts and particles) are extremely dangerous when internalized. They should not be breathed into the body, ingested in food or water or allowed to be absorbed through the skin.