Smoke Detector disassembly

[myID]

Hi-

Finally I got hold of an old DICON Micro 300 smoke detector. Manual says: 0,9 myC Am 241- sealed source. It also says: ANY STRAY PARTICLES WOULD BE UNABLE TO PENETRATE TROUGH THE DEAD LAYER OF SKIN- SO NO NO HAZARD... sorry- but what about my lungs...
I am wondering how the Am is bonded inside it. Any chance that debris falls of when you just transport/ shake it? And when I dissasemble it? I do not need lung cancer but want a (safe) Am source for several experiments....

Thanks
Roman

[Carl Willis]

Hi Roman,

First the usual disclaimer: disassembly of smoke detectors for the purpose of harvesting the sources is unlawful in many parts of the world. This might be why you don't find explicit discussion too often, despite the common hobby exploitation of detector sources.

As I understand, most of these sources use small spherules of ceramic containing AmO2 bonded together by gold or another noble metal. This is then sandwiched by pure metal films. Separating significant numbers of the spherules from the substrate would require aqua regia or similar chemical techniques to dissolve the sandwich and the substrate. Separating Am from the ceramic would likewise be difficult, essentially impossible to do by accident and almost impossible to do effectively even on purpose. I suspect a method involving concentrated hydrofluoric and sulfuric acids might do it. But rest assured, just shaking or disassembly (or burning, or crushing, etc.) is far from sufficient to dislodge significant Am from the source. They're designed to be rugged and extremely safe.

-Carl

[Chris Trent]

Well, It's illegal to tamper with smoke detectors so you're totally on your own with that one.

From what I've found on the internet, The americium is bonded to a metal foil, then is usually pressed into a aluminum mounting disk. It's apparently quite safe and stable, just don't eat it or burn it.

A nice example: http://www.theodoregray.com/periodictab ... index.html


If you're worried about the potential hazards of the inner workings of your new smoke alarm:

We have the alarmist site: http://www.iicph.org/docs/radioactive_f ... ectors.htm
The government site: http://www.epa.gov/radiation/radionucli ... icium.html

[myID]

Sorry-

the idea was obviously not to disassemble mine- just was a question of general interest- and a safety concern.... And as I sayed: I want a safe source- which means- "leave it like it is" and hope the small dose on the outside is enough for experiments with my MCA.

Thanks for the answers anyways!

[enochthered]

It's OK to remove the sealed source from the machine - Illegal, in some localities, so obey your local laws - but it won't kill you.

Now, you've got that little sealed source capsule. Don't try and open it, or dissolve it, or remove the foil containing the source material, just use the sealed source capsule as a check source, or whatever you want it for, as is.

Now, I hate to sound like Helen Caldicott, but when they say "any stray particles would be unable to penetrate through the dead layer of skin - so no hazard" that's a little bit mendacious.

Whilst Am-241 is primarily an alpha emitter, it's also emitting some photons, primarily a gamma ray at 59 keV. If you have a reasonably sensitive instrument capable of detecting gammas, such as a NaI scintillator maybe, you'll definitely detect gamma radiation from the smoke detector, even without the source removed, even from the outside of the smoke detector casing. A piece of paper, even a thin piece of metal, won't stop those gammas.

Now, don't be alarmed - it's not dangerous. You can look up the gamma ray dose constant for Am-241, and work out how many nanosieverts per hour of dose you're getting from such a source, at a distance of one metre or whatever distance. It's very little dose, and it won't hurt you.

There's no radioactive debris that falls off the source or anything like that, it's a well-made sealed source.

[David D Speck MD]

Just to confirm for my own uncertainty -- if the Am-241 emits a 59 KeV gamma photon, is that in any way different from the photon produced by an X-ray tube driven at a potential of 59 kV?

Thanks,
Dave

[Carl Willis]

Hi Dave,

The 59 keV Am-241 gamma ray is more energetic than the mean energy of diagnostic x-ray beams, which is typically about 1/3 of the applied voltage. If you applied 59 kV to an x-ray tube, you'd expect a flux-weighted mean energy of ~20 keV in the output radiation. The other big difference is that x-ray tubes produce broad spectra, and the gamma emission from Am is a line emission.

Attached is a radiograph showing what a CDV-700 Geiger counter tube looks like to Am-241 gamma rays. The graininess results from the low gamma flux and short exposure. Some features of the tube can be made out, but you also get a sense that the contrast is poor--a result of the Am gamma's energy being well over the "k-edge" of the common light elements.


-Carl

[David D Speck MD]

Neat radiograph, Carl! Thanks for posting it.
Can you give a simple explanation of why X-ray energies are only 1/3 of the voltage applied to the tube, and why there is a broad range of resulting output energies?

Thanks,
Dave

[Carl Willis]

Hi David,

X-rays from a tube are caused by beam electrons being rapidly decelerated by the fields from electrons and nuclei in the target (this statement is overly simplistic in some ways). Beam electrons come under varying degrees of influence from target charges depending on closeness of approach, and hence varying degrees of deceleration. This produces broadband "bremsstrahlung." The peak energy of this radiation is constrained by conservation of energy to be below the incoming particle energy. Much lower energies (x-rays of a few keV, visible light, radio waves) are in theory produced as well, but the target is opaque to these and they are produced in low yield.

X-ray tubes also produce some line radiation by a different mechanism--the excitation of the electrons bound in target atoms. Since the electronic energy levels of these atoms are quantized, de-excitation results in some "characteristic" radiation peaks that may or may not be prominent in the spectrum, depending on how the tube is built and what beam energy is in use.

Hope this answer does some justice to your question.

-Carl

[Richard Hull]

It always amazed me that the nuclear beta's are all spread spectrum, splattered over a vast range of energies, while the gammas are all precise line spectra. (That old photonic quantization slips in again). The helium atoms (alpha's) also blow out as if quantized, again, hinting at a shell model of the nucleus based on helium nuclei.

Richard Hull

[myID]

Ok-

this is the spectrum so far the am source gives (600 sec run time):
only thing it shows: programming and building a cable MCA - PC made sense- now I can export data from my MCA to EXCEL
but: still no clue why spectrum looks like it does...
whatever- background had no "peaks" and was subtracted for this plot..
I think energy resolution (if one can call it such) is crap (resolution at this low energy can be expected to be not too got with NaI??)...
Peaks (best case) could be expected at 26,35 keV and 59,54 keV (which goes not too bad with maximum at ch 184 / 409 I guess)
Will work on it when I have more time... (I read a lot about the theory- but still this is what it looks like...)

Greets- and any tipps for better resolution are welcome!!!!
Roman

[Richard Hull]

Roman, that is a pretty expanded scale and looks just fine to me. Try dialing the gain back to where channel #59 is the Am peak. The resolution is limited by the crystal and PMT combo. What you have is what you have to live with for the most part.

Stuff in the 0kv to almost 40kev is best ignored on a NaI:tl xtal as this is a region where all manner of photons based on X-ray fluorescense (K and L shell stuff) and lower energy nuclear electrons create all manner of x-ray peaks.

Richard Hull