[Laser] Semi OT

[Glenn Thomas]

Kurt -

What Kerry says is correct but incomplete. Most of the smoke 
detectors I know of use some isotope on Americium. Unfortunately, I 
don't know which isotope. My very old edition (1968) of the Chemical 
Rubber Handbook lists a total of nine isotopes - Am237 through AM246. 
Most of them have half lifes ranging from seconds to hours, so they 
would be inappropriate for smoke detectors. Of the three longer lived 
isotopes, Am241 (458 years), Am242 (152 years) and Am243 (7950 
years), some are alpha emitters as Kerry points out (241, 243) and 
some are beta emitters (242). However, ALL are gamma emitters, with 
gamma ray energies ranging from 0.03 to .4 Mev.

For comparison, the gammas from Radium226 (one of the things U238 
becomes as it decays to lead) are between .2 and .6 Mev.

Kerry points out that it doesn't take much to stop an alpha particle, 
and the plastic case on the smoke detector should be more than 
adequate for this. The plastic case seem unlikely to stop the beta 
particles, but the metal case that the source is usually contained in 
(at least in all the smoke detectors I've seen) will likely to do the job.

However, neither will do much to a gamma. One approach is to find a 
detector that is easily upset by ionizing radiation (like a gamma). 
Also note that as the Americium decays, it's decay products will also 
produce alphas, betas and gammas of their own. If these emissions are 
more energetic and the half lifes are brief, they may be easier to 
detect than the initial Am decay event.

As for a detector, the diode junction might be your friend. Rather 
than reverse biasing it close to breakdown (like a zener diode), I 
might try to put just enough forward bias on it so that no current is 
actually flowing and wait for an ionizing decay product to bridge the 
now reduced diode gap. Also, consider that even though the metal and 
plastic case will not stop the gamma rays, they are still subject to 
inverse square. Put your detector as close to the pellet as you can.

Of course, at this point, this thread has gone a bit off topic for 
this list. No one here (I hope!) is trying to do communications with 
modulated gamma rays...

73 de Glenn wb6w


At 04:20 PM 8/22/2006, you wrote:

>Kurt - I believe the smoke detectors emit Alpha particles that are 
>stopped by most any solid material ( even a sheet of paper) so the 
>particle may not make it much  past the outside surface of your 
>detector.  I have been purchasing some of the cheap detector units 
>available from Russia  (off Ebay)  for less than $20 including 
>shipping. These are the entire Geiger tube plus electronics. These 
>will detect the smoke detector source up close as well as the older 
>lantern mantles that are radio active.
>  - Kerry N6IZW -
>
>At 04:07 PM 8/22/2006, you wrote:
>>Seems to me this should be possible and I have heard it may be...
>>
>>How would I detect radiation from a source such as the pelet in a smoke
>>detector using a piece of semiconductive material?  I've tried putting a
>>solar cell on an amplifier, but don't hear any ticks as the particle passes
>>through.  I've tried LEDs, capacitors, anything I can think of than may
>>produce a slight amount of energy.  What about a large transistor?  A CDS
>>cell?  One idea I have, that I have yet to try is to run voltage through a
>>diode in reverse, just below the breakdown point and see if an energy
>>particle is enough to push it over the edge for a second.  I may be wrong
>>entirely, but there must be something along these lines that may work.
>>
>>Any ideas?
>>
>>Kurt
>
>
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