[Laser] Semi OT

[wa4qal at ix.netcom.com]

> Date: Tue, 22 Aug 2006 17:44:54 -0700
> From: Glenn Thomas <glennt at charter.net>
> Subject: Re: [Laser] Semi OT
> To: Free Space LASER Communications <laser at mailman.qth.net>
> Message-ID: <7.0.1.0.2.20060822164825.0349ac78 at charter.net>
> Content-Type: text/plain; charset="us-ascii"; format=flowed
> 
> 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. 

Americium-241 is the favoured isotope:

http://en.wikipedia.org/wiki/Smoke_detector

> My very old edition (1968) of the Chemical Rubber Handbook lists 
> a total of nine isotopes - Am237 through AM246. 

There are a bunch more that have been synthesized, all the way from
231 to 249:

http://www.nndc.bnl.gov/chart/reCenter.jsp?z=91&n=142

> 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). 

241 is the main one of interest, and it's an alpha emitter, with a peak
alpha particle energy of 5.485 MeV (which is a pretty hot alpha particle).
However, alpha particles are very easily stopped.

> However, ALL are gamma emitters, with gamma ray energies ranging 
> from 0.03 to .4 Mev.

And, Am-241 spews out a whole bunch of gamma rays, although the
peak energies are 13.9 KeV (which is pretty mild, since most CRTs 
spit out more energetic X-rays), and 59.5 KeV, which is moderately
warm (but still less warm than most dental X-rays).

http://www.nndc.bnl.gov/chart/decaysearchdirect.jsp?nuc=241AM

> 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. 

Most detectors have the Am-241 source in a metal chamber, which
will surely stop the alphas.  Under no circumstances should you even
think about opening that chamber!

> 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.

Since Am-241 is primarily an alpha emitter, there shouldn't be too 
many beta particles flying around, so that's likely a moot point.  There
may be some secondaries caused by those hot alpha particles, but
they would be relatively low energy and almost certainly filtered out
by any enclosure.  It would seem that the best approach would be to
try to detect the gammas, since those are the only ones which should
make it out of the metal chamber.

> 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). 

The traditional detector for gammas is a Geiger-Muller tube, in which
a single gamma ionizing a gas atom causes a chain reaction avalanche 
and subsequent amplification.  It's hard, awfully hard, to detect a single
electron produced by a gamma encountering an atom.  

Another possibility is a scintillation counter with a photomultiplier tube.
The idea behind these are that the ionizing radation causes a phosphorescent
screen to emit a photon of light, which is then detected and amplified
by the photomultiplier tube.

> Also note that as the Americium decays, it's decay products will also 
> produce alphas, betas and gammas of their own. 

While this is true, unfortunately, Am-241 decays (via the alpha particle)
to Np-237, and this isotope of Neptunium has a half-life of over 2 million
years, so there won't be much radiation produced from this (and, it's
an alpha emitter, too):

http://www.nndc.bnl.gov/chart/decaysearchdirect.jsp?nuc=237NP

> If these emissions are more energetic and the half lifes are brief, 
> they may be easier to detect than the initial Am decay event.

The alphas are slightly weaker from Np-237, and most of the gammas
are weaker, too, so there won't be much chance of detecting it if 
the Am-241 is too weak to detect.

> As for a detector, the diode junction might be your friend. Rather 
> than reverse biasing it close to breakdown (like a zener diode), 

If you try to use a diode junction, you'll need to be especially careful
about dark current, since you're going to be trying to detect either
single electrons or small groups of electrons.  That will almost certainly
require a cooled junction, careful shielding, and ultimate purity of
the junction itself.  It might be possible, but it won't be easy.  That's
why the typical device is a GM tube, which has the ability to amplify
the singly produced electrons via the avalanche effect in the gas.

> 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. 

I really doubt if this would work.  Remember that diode current is an
exponential function involving temperature, and since we're trying
to count individual electrons, any dark current at all will almost certainly
swamp the desired current.  It might work if you could reduce the 
exponential function by chilling the junction down to near zero degrees K, 
but not many of us have vats of liquid Helium to use for such purposes.

> 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.

Excellent point.  

I don't remember exactly how much Am-241 is used in a typical smoke
detector, but there's not very much of it.  So, even though the half-life
is reasonably short, there still isn't going to be all that many gammas
to count.  Thus, it's important to collect as many as you can, which 
means minimizing the distance from the source to the detector.

> Of course, at this point, this thread has gone a bit off topic for 
> this list. 

Probably, although some of the detection theories are still the same.

> No one here (I hope!) is trying to do communications with modulated 
> gamma rays...

And why not?  ;-)  All we really need is a gamma-laser (a gaser?).

> 73 de Glenn wb6w

Dave
WA4QAL

> 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
>>
>>
>>WAR IS PEACE!
>>FREEDOM IS SLAVERY!
>>IGNORANCE IS STRENGTH!
>>(be seeing you!)
> 
>