What is the benefit of using Am-241 in smoke detectors even though it is a radioactive element? [closed]

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.