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I don't understand why Am-241 is used in smoke detectors even though it is a radioactive element.

The only benefit I can see is that it has a long half life.

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closed as off-topic by airhuff, Jon Custer, aventurin, Tyberius, Mithoron Oct 22 '18 at 17:37

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    $\begingroup$ You would have to explain how to make a good smoke detector without a radioactive element. $\endgroup$ – Martin Argerami Oct 22 '18 at 0:18
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    $\begingroup$ "even though it is a radioactive element" - can you elaborate on why you think this is a negative? By going through your exact concerns you may be able to answer your own question. $\endgroup$ – thosphor Oct 22 '18 at 10:20
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    $\begingroup$ Is this not off-topic? Am241 is an artificial element made in fission reactors and cannot be synthesized by chemical reactions. It's use in smoke-detectors does not depend upon chemistry either. Americium, and this use for it, is nuclear physics all the way down... $\endgroup$ – Martin James Oct 22 '18 at 11:29
  • $\begingroup$ My body has carbon in it, some of which is naturally C-14, a radioactive isotope of carbon. Still, it'd probably be best for me to hang onto that carbon. :) Point is, don't get too concerned just because something is radioactive. The amount might be very limited, or in the case of a detector, they manage it properly to limit risk. A question like this can be one piece of figuring out when to worry about it, and when not to. It's about pros and cons, too, how many people (if any) are harmed by a bit of radioactive material in a detector vs. how many are harmed when there is no smoke detector? $\endgroup$ – Don Branson Oct 22 '18 at 16:54
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The usage in ionizing smoke detectors requires a radioactive isotope to work. In addition to a sufficient half-life to make a smoke detector with a suitable service life, Am-241 emits mostly alpha radiation (which is used for the ionizing mechanism) with relatively little of the more hazardous and useless gamma radiation. See the Applications section in https://en.m.wikipedia.org/wiki/Americium.

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    $\begingroup$ It is possible to ionize air directly using an electrical potential difference. But this requires high voltages and wouldn't be supported by a battery. $\endgroup$ – Nayuki Oct 22 '18 at 4:31
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The radioactive substance is required for the smoke detector to work

Americium 241 based smoke detectors work because of the radioactivity of the 241Am isotope. The basic mechanism inside the detector relies on measuring the amount of ionisation in two compartments in the detector. One compartment is a closed sample of air; the other is open to household air. Detectors in both compartments measure the number of ions created by the alpha radiation from the 241Am by measuring the current between two electrodes. If the household air contains smoke particles, the current is altered in the open compartment because the smoke particles (rather than oxygen or nitrogen molecules in air) tend to pick up the ionisation and this significantly alters the current. This makes a very sensitive detector for smoke particles.

So the whole point of the 241Am is to provide a source of the ionising radiation. And this isn't a problem as you don't need much of the isotope to do the job and it is easy to manufacture in a safe form (were the 241Am is safely contained in a gold foil and can't escape). And the half life is longer than the lifetime of the detector so isn't really a constraint.

There are other types of smoke detector. Some use scattering of infra-red light by particles in air. But they are more sensitive to things like water vapour or mist which give more false alarms.

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When we talk of radioactive substances, the main three types of radiation are alpha radiation, beta radiation and gamma radiation.

Alpha radiation is the radiation of a particle which can only travel a very short distance, except in a vacuum. But if an alpha emitting substance is inside your body it will do a lot of damage.

Beta radiation can travel further in air. It is a much smaller particle, and does less damage inside you than alpha, but more than gamma.

Gamma radiation can travel through almost anything (except thick lead), but the particles are very small. We are all being hit by some gamma radiation all the time. In high doses it is deadly. When your body is hit by gamma, a small proportion of the gamma is stopped: it is that portion that is stopped that causes the damage, the majority that passes straight through does no harm. And our bodies can cope with/repair small damage.

Diagnostic medicines are given containing gamma emitting substances for the purposes of performing scans. The type of substance given would be one with a short half-life... long enough to have the scan. Because it is a fairly small amount of gamma radiation the benefit to the patient outweighs the risk of damage.

You could have a ton of alpha emitting substance sitting in your living room and it probably would do you no harm until you started to crush it up and inject it inside you. But if it was a high-dose gamma emitting substance you wouldn't be reading this.

Smoke detectors use alpha radiation. When buying a smoke detector for installation, remember that as long as you don't try to crush up the smoke detector and try to eat it or inject the tiny amount of emitting substance, a life insurance policy probably isn't necessary. Finally, it isn't a good idea to break the detector, crush up the emitting chemical, get it somehow into a liquid, and inject into the wife (or husband), because it would be tricky to explain it away as an accident, either to her or, in the preferred scenario, to the legal authorities.

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Photons are blocked by water vapour, but the alpha particle radiation from A-241 continues through only stopped by smoke.

It's therefore much more sensitive to a single particle of smoke. Water vapour on the other hand, is only able to stop the photons of a photo-electric smoke detector, which uses no Americium-241 at all.

Americium is also likely the most toxic substances on the planet around with a teaspoon able to kill hundreds of thousands if taken intravenous. It's mixed with Gold and beaten into an aluminium container. You could go around burnt buildings to collect them perhaps, with a radiation detector.

The batteries last longer.

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    $\begingroup$ This doesn't answer the question. $\endgroup$ – matt_black Oct 22 '18 at 8:33
  • $\begingroup$ @matt_black It's actually the only answer which attempts to answer the (implicit) question: What's the unique property of Americium that we need it in smoke detectors (even though it's radioactive)? If I understand correctly, the alpha particles aren't stopped by water vapor which would (wrongly, I assume) trigger a light based detector. Admittedly the answer could be better worded. $\endgroup$ – Peter A. Schneider Oct 22 '18 at 12:41
  • $\begingroup$ Yes, but the answer doesn't explain why or how smoke detectors work. eg "It's extremely sensitive..." What is extremely sensitive? Plus other random but irrelevant facts that don't point usefully to an answer. $\endgroup$ – matt_black Oct 22 '18 at 15:43

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