Why beta particles are potentially harmful to skin than alpha particles?

Answer 1

Alpha particles have high mass and energy and therefore momentum compared to beta particles. An alpha is a helium nucleus (2+) charge, beta is an electron or positron (1- or 1+ charge). The relative high momentum of the alpha particle means there is potential for more ionisation events compared to beta. The 2+ charge increases the probability of each event.

Alphas dump their energy in a shorter distance than beta particles. The property that describes this is called stopping power, S, which is both particle (ie alpha/beta) and stopping material (ie skin or plastic, etc) dependant. Stopping power is the change in energy with respect to the penetration depth in the material.

S = dE/dx It's usually given in Mev/cm or eV/cm (as opposed to the J/m SI unit). There are tables of this available on the internet for various particle and material combinations.

This has potentially useful and harmful implications. Usefully, a known alpha source can be stopped readily to avoid harmful effects - by using a shielding material. For alpha this could be a think layer of plastic or foil (or even paper). Alphas stop so readily they are significantly impeded by smoke... and hence the smoke detector still found in many houses. Beta takes a bit more material to stop - 1 cm of acrylic plastic is usually sufficient.

Now to the question proper - why beta could damage skin more than alpha. Alpha stops in a really short distance. The top portion of one's "skin" is actually dead cells. The incident alphas are likely to be stopped by the dead cells on the top layer of skin, rather than depositing any significant energy in live tissue. Betas on the other hand will travel a few millimetres or a centimetre into human tissue, so a significant dose would be received by the skin from an external beta source. Of course a high energy alpha may actually have enough energy to penetrate the dead layer in the first place, or you may have just ex-foliated or you may have "thin" skin, so don't try this at home.

Is the moral of the story than betas are more harmful? NO. Alphas are pretty safe in a box, but don't swallow them! Alphas may have a reputation of not delivering a large skin dose, but if they get to a place like the bloodstream or the stomach, there's no "dead skin shielding" to stop them so you could be in for some radiation poisoning.

Don't eat a smoke detector... the batteries make you really ill.