Question

What is the danger of alpha, beta, and gammas in terms of their relative internal (biological) damage?

Answer 1

For internal exposure, ingestion or inhalation, Alpha and Beta are the larger dangers.Each alpha, beta and gamma particle carries enough energy to knock electrons loose from atoms and molecules. This is what causes what we call of as radiation poisoning.

Alpha particles, helium nuclei, are not very penetrating because of their high mass and charge. The rule of thumb is that alpha particles can be stopped by a sheet of paper.
But If you have an alpha emitter inside your body, it will decay in place and send alpha particles through whatever's nearby, which will be cells of your body. Because alpha particles are brought to a stop very quickly, they deposit all their energy in a very short track and do lots of damage over the course of their travel.

Gamma rays are photons. They penetrate everything to some extent, and deposit relatively little energy per unit distance traveled. As a result, most of the trouble is caused by external emitters. An internal source, such as the potassium-40 in your bones, will contribute some dose, but a lot of the gamma rays will actually escape your body before depositing any energy.

Beta particles are electrons. They are intermediate in range and their penetrating power. Beta particles from an external source can penetrate the skin and affect the body, if the energy is high enough. If you ingest a beta radiation source, most of the energy will be deposited inside your body, and to a good first approximation, all the radiation dose will affect you.

The danger caused by the three particles can be summarized as follows:

Type.......................Internal......................External
Alpha........................High...........................Low
Beta...................Intermediate...............Intermediate
Gamma....................Low............................High