Distance travelled by Alpha Particles in air and other materials

The rule of thumb

A rule of thumb for alpha particles in real world radiation safety is that they travel 1-3 cm in air, and their penetration through other material is negligible. They are inherently more difficult to monitor for in the workplace when compared to beta, gamma and x-ray radiations.

An alpha particle would need an energy of at least 7.5 MeV in order to penetrate the protective (dead) layer of skin, which is taken to be 0.07 mm.

Some further thoughts on the alpha particle

In real world workplace situations, the rules of thumb given above are valid when you know the nature of the source. They are valid where you have a bonded / fixed source (one where the alpha material is not mobile). Where the source material is mobile (e.g. powdered Po-210) there is an entirely different and potentially significant internal radiation hazard. However, this is not for consideration in this section since the movement of radioactive material in the environment is very different from the ‘movement’ of the radiation particle being emitted.

Where radioactive materials emit more than just alpha particles (e.g. Am-241) they create additional external hazards from beta or gamma radiation. In the case of Am-241, except when using as an alpha monitor check source, alpha particles are irrelevant as the radioactive material will be fully encapsulated in stainless steel with a thin foil window. In this example it is the 60 keV gamma rays that are of use (e.g. for liquid fill level gauges in bottling plants).

Consider a 5 GBq encapsulated source of Am-241 (typical activity of a radiometric fill level gauge). If this source is not shielded then the photon (gamma and x-ray) dose rate would be about 20 micro Sv/h at 1 m. Assuming the source is in the gauge, and a radiation beam interacts with a test subject at 10 cm through a collimator, then the dose rate is likely to be up to 200 micro Sv/h (based on actual measurement and not the inverse square law). Now imagine your hand is 2 cm from the source where its unshielded, not collimated and not encapsulated, then the photon dose rate will be of the order of 5000 micro Sv/h. Your radiation exposure from alpha particles will be zero, even though some of them will be reaching your fingers.

What is the range of the alpha particle in air?

Am-241 has 25 possible alpha particle emissions, each with a different energy and probability of emission. The 5485 keV alpha (emission probability 85%) has a mean range in air (at standard temperature and pressure) of 4 cm (some will travel further but with a rapidly reducing intensity).

Po-210 has two possible alpha particle emissions, the most probable being 5304 keV (100%), and the mean distance travelled in air will be about 3.5 cm.

Some obscure or more exotic radioactive materials have much higher alpha particle emission energy. For example, Po-212 has an alpha particle emission of over 10 MeV with a mean range in air of 10.5 cm (half life of Po-212 is 45 seconds).

Practical issues monitoring for alpha emitting radioactive material

The distances travelled by alpha particles in air are based on measurements made in ideal conditions (i.e. experimentally). Workplace measurements (made by Ionactive and others) show that real world distances are much less, to a point where such direct monitoring is impracticable in some circumstances. Practical issues that will affect alpha particle monitoring include:

• The physical nature of the material emitting the alpha particles (dust, sludge, particulate)
• The type of surface the alpha emitting material is on (smooth, rough, pitted etc)
• Environmental factors (e.g. surface might be wet, or changing orientation)
• Type of monitoring probe (GM, scintillation)
• Probe window thickness, (protective) mesh spacing and shape, and overall detection area
• The skill of the monitoring surveyor (very important!)
• Other surface features (sharp edges, nails, screws) that could damage probe if too near the surface

Monitoring clothing for pure alpha emitters is particularly difficult due to the potential for folds and uneven surfaces (fabric dependent).