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

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 [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.


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.

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

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.

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.

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.

Good article, (( I see Wiki needs bit of help )) But I think some people are wondering just how un / safe common radioactive elements like Americium 241are ? and with the average smoke detector containing 37 kBq, of Am241, is it a serious Gamma / Alpha emitting source, one should be afraid of ? Because what most people have been force fed by the media and at schools that Radioactivity is bad bad, run stay away (( just look at the new ionizing radiation warning signs with the skull and crossbones and the person running ))

I know that's what people are wondering. Because I work at a National Laboratory, I am necessarily hesitant to make apparently definitive statements which contradict current policy. What I can do is provide quantitative information, and pointers to reliable documentation, and expect my readers to exercise their own intelligence and use of calculators to come to conclusions.

...And I see that I haven't completely closed the loop on that. What I should add to my topic above is the conversion from Bq and Ci to Gy and Si or rem for some common isotopes (14C, 241Am, 60Co), and also give some guidance on how to compare those numbers to existing radiation we're all exposed to normally. Give me some time to get that done...

The best safety rule for any radioactive source, even the tiny amount (37 kBq is about 10 nCi) in a smoke detector, is to follow 1/r2: keep some distance away from it, and wear reasonable and practical protective equipment (latex gloves, dust mask) otherwise. If you touch it (get it on your skin) or ingest it (breathing dust, getting under a hangnail, whatever) then it will do more damage (1/0 = infinity :-).

I love the Banana reference, can a modern scantilator pick them out from other sources going across the north - south Korean boarder ? And I understand your cautious wording, there is so many variables that could make it safe to use and many more that would make it very dangerous, but with that said, would Am241 illuminate a older scantilator (( just wondering I have no plans on taking apart my smoke detectors, they are optical ones / rate of temperature rise anyhow, yep always hated how the ionization ones go off when you make toast ))

Well, scintillators are fine for detecting beta and gamma radiation (electromagnetic interactions), but their energy resolution tends to be kind of crappy. I'm not sure if the industrial models would be good enough to separate the 1.5 MeV40K -> 40Ar decay from other nearby lines.

A nuclear engineer would know this stuff much better than I do.

It depends on the scintillation crystal and the multiplier. We use a Sodium Iodide crystal and a cascading multiplier, which is OK, but there are better alternatives.

I have an ionization smoke detector, it once went off when I blew out a candle near it.

Something interesting about the ionization ones and the optical ones...

If you are in short supply of fuel and need to light something then a wick lighter will set them off if you fill it with normal petrol, spray tyre buffer (flammable and the old stuff could literally slough off skin, the new stuff is flammable and useless) and most other larger hydrocarbon based flammables, presumably incomplete combustion = more soot and smoke

Aftershave will set them off as well as deo in ionization versions but burning it won't unless there's powder in it.

An interesting one and a point to make, smoke coming directly from the end of a cigarette will set them off if it's held close enough for much smoke to get in, it tends to float up very fast and go around objects, however smoke being exhaled or even just puffed from the filter end but not having been through your lungs will not set it off, strangely cigar smoke seems to just not set them off, presumably the leaves burn cleaner but cigarettes have material added plus KNO3 which makes them burn faster...

One weird thing is that buildup has a habit of not setting the ionization ones off until it gets to extreme levels, however if you're dusting around them the dust can set them off, small insects and spiders can set either off if they get inside...

Most smoke detectors, as sold, should have it fairly well shielded, right? It is only if one tampers with the shielding that one can, um, get burnt.

Yes, indeed. But consider the community here. "Tampering" with closed-up consumer products is part of the culture. People will open things they probably shouldn't, so providing safety guidance isn't unreasonable.


Oh I understand that. And mistakes of my youth still hang around me also in the form of some scars that could have been worse then scars.

I was just adding my 2 cents so no one thought that having a smoke alarm would pose a danger. :-)

Thanks for clearing that up. (even though most of it was way over my head)

well on the Ionizing vs. non-ionizing radiation, units, and safety scale

you guys are up here
and I am down here

Did the introductory stuff help, or just make it more confusing for you? It's not easy to find the right balance between "details for the non-experts" and "talking down to people." I really do want this stuff to be useful and interesting.

I don't know about anyone else, but I really appreciate the level of detail you supply on this kind of thing. I found the introduction helpful. Sometimes a lot of it of it is over my head, but I like having all the information, because I can look up the things I don't understand, and I get frustrated with explanations that don't go as deep and leave a zillion questions unanswered.

I appreciate the thought but its not really a subject that I could get into immediately I will probably pick it up when I do something similar in school

...Just looked at your profile. Yes, I see your point. If you have a decent science teacher at your school (or even better, physics and chemistry teachers) I would encourage you to take this to them, and go over it with them. I know how intimidating teachers can be (it is a "power relationship," after all), but my universal experience is that the good ones really love to teach, and to be part of the process of discovery. Try seeing if they'd be available during lunch, or after classes. At the very least, get some age appropriate books at the library, so you can spread out information in front of you and compare things. Don't give up! The challenge of figuring something out is a great way to expand your mind.

"...Just looked at your profile. Yes, I see your point. " does my profile not please the great kelseymh. Must improve profile to please gods...

I'm sorry. That was not my point at all. In your profile, you listed your age as 14. I had (incorrectly) assumed that you, and others reading what I had written, were approximately college age. I made assumptions about what training you had which were incorrect. That is not in any way meant to be an insult or an offense. You had explained in your previous comment that there were things I wrote which you hadn't yet encountered in school. That's not a situation I had considered, but when I read your profile I realized my mistake.

Agreed. I'm going to go ask my old AP Bio teacher about the AM 241 (he also teaches Chem)

Thank you again for your comments! I've tried to add some better explanatory material, and have rearranged the text slightly. If there are specific things you don't understand, please consider yourself welcome to post your questions, ask for clarification or details, or (especially!) point out things you think are incorrect or inaccurate.

Thank you for the comment; I'll add some introductory information

It is also (as Adrian noted) useful to look up terms you don't understand. Wikipedia is an excellent starting point; just paste the word into their little search box.