Ionizing vs. non-ionizing radiation, units, and safety (updated)

Note: This was originally intended as a reply in the Americium-241 Science forum topic

When 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 µCi)    11 µSv/yr @ 1m     Te-99m   MRI contrast       740 MBq (20 mCi)  1.6 Sv/hr @ 1cm     C-14     atmosphere, body   0.23 Bq           10 µSv/yr     K-40     bananas, body      4.4 kBq           200 µSv/yr

What 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 µSv/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 [ 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.

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Pazzerz8 years ago
Quite frankly the Americium scare is just that: a scare. The low dose gamma is insignificant and the alpha particles??? Well, if they truly ARE high enough energy to cause harm, then why do they use Americium in nuclear medicine to show pictures of the blood system? This is direct injection. The precautions that are handed out these days seem to be taken without question. I guess it is assumed that the general public is too stupid to use common sense (and in many, many cases, yes!), so the word out is to lean toward the super safe side, as in, extreme.
kelseymh (author)  Pazzerz8 years ago
Not too stupid, but too uneducated. Where in high school do you get any of the background necessary to understand radiation issues, or especially the different biological effects of alphas vs. betas, etc.?
Where in high school do you get any of the background necessary to understand radiation issues

one of my elective classes in the 9th grade was Nuclear Energy (although most of the class was made up of seniors).

I don't know if they still carry that class or not.
I remember that kind of class in my 6th grade, enough to know about the kinds of radiation and their types of effects.
The class I attended would have been a bit much for the 6th grade (except for the greatly advanced) as they used 2nd year college texts for the class. We did learn an awful lot about what is harmful and when, concerning radioactive isotopes and the reason Alpha, beta, and gamma radiation pose different threats when either inside or outside the body. I loved that class .... why I didn't pursue that line of study, I'll never know...
Well, you should be around the folks I have to deal with.... =P
Pazzerz8 years ago
Actually the carbon is insignificant in shielding. Its the hydrogen that does the work for you. Think about it. They're the same size as a neutron. They bounce back. Water is excellent neutron shielding as are some plastics. If this weren't so, our nuclear reactors couldn't achieve a steady state so easily.
kelseymh (author)  Pazzerz8 years ago
The hydrogen doesn't shield, it moderates. As you say, the neutrons "bounce off", and thereby slow down. The carbon has a high absorption cross-section 12C(n,13C) (if I got the notation right).
In the reactor, this bouncing is what causes a higher reactivity at the outer part of the core. In fact, the water jacket around surrounding the reactor room at prototype sites IS the shield for the surrounding area.
kelseymh (author)  Pazzerz8 years ago
Ah, good point. Same as with the borated polyethylene we used for a while to "shield" our PMTs. It's not an absorber-type shield, but rather a thick scattering.
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