Note: This was originally intended as a reply in the Americium-241 Science forum topicWhen people talk about "radiation," they are referring to many different things, and are probably thinking of some things that don't even apply. "Radiation," the invisible energy given off by radioactive materials, can be either "subatomic" particles or electromagnetic. The most common particles emitted are "beta rays," high-energy electrons, and "alpha particles," helium nuclei. Some sources can emit neutrons, protons, or "positive beta rays" (anti-electrons, or positrons), but those are much less common.The units we use to measure radioactivity are becquerels (Bq, decays per second) or curies (Ci, 3.7 x 1010 decays per second). Since the effects of radiation depend on their energy, another unit of interest is absorbed dose, the energy deposited per unit mass of target, measured in grays (Gy).Safety experts classify radiation into "ionizing," meaning there is enough energy to knock electrons out of atoms or molecules, and "non-ionizing." Infrared and ultraviolet light are non-ionizing, as are neutrons. Alpha particles (helium nuclei), beta particles (electrons) and gammas (as well as lower energy X-rays) are all ionizing radiation. The three have substantially different effects on biological systems, even at the same absorbed dose. Consequently, for radiation safety purposes, scaling factors are applied to produce numerical measures (sieverts, Sv) of "effective" or "equivalent" dose, that can be compared across different kinds of sources.Here's a small table with information for some commonly encountered sources. Isotope Source Activity Dose rate Am-241 smoke detector 35 kBq (1 ÂµCi) 11 ÂµSv/yr @ 1m Te-99m MRI contrast 740 MBq (20 mCi) 1.6 Sv/hr @ 1cm C-14 atmosphere, body 0.23 Bq 10 ÂµSv/yr K-40 bananas, body 4.4 kBq 200 ÂµSv/yrWhat you should see clearly from this is that the natural radioactivity in your body is comparable or larger than that in a common smoke detector. At SLAC, the limit for exposure to sources at the lab by most staff (including me) is 20 ÂµSv/yr (5 mrem).As I noted above, neutrons are sometimes lumped in with ionizing radiation in non-technical "radiation safety" classes (we call them "the photon is your friend" training :-). That is not really accurate -- neutrons don't interact with electrons(*), and so cannot ionize directly. They can interact with hydrogen nuclei (protons), knocking them out of complex organic molecules, and leave behind ionized fragments and free radicals. The can also be absorbed by otherwise stable nuclei, making them radioactive; those new nuclei may in turn give off ionizing radiation.Neutrons lose energy much more slowly in passing through material, and so can penetrate much farther than ionizing particles or gammas. The nuclear interaction [http://en.wikipedia.org/wiki/Cross_section_(physics) cross-section] is much more important here than dE/dx (ionization) energy loss. Materials rich in carbon and hydrogen (for example, paraffin) are far more effective at neutron shielding than dense metals like lead.(* for the expert readers) Yes, there is n-e scattering through W and Z exchange, but the cross-section and energy scales are completely irrelevant to this discussion.