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The following archive was created by hippie-mail 7.98617-22 on Fri Aug 21 1998 - 17:20:08 ADT
Mike Rosing (eresrch@msn.fullfeed.com)
Mon, 6 Jul 1998 21:37:19 -0500 (CDT)
On Tue, 7 Jul 1998, Antonomasia wrote:
> Timing gets messed up by the counter dead time. This is the interval
> following a detection during which no further detections can be made.
> (It arises from the fact that an ionisation is amplified to detectable size,
> swamping the tube momentarily. Ordinary counts correct for this easily, but
> it will interfere with stats by imposing a rough minimum interval.)
So, instead of using a Geiger counter, what if we use an ionization chamber
and watch the current fluctuate. For example, I have a 1 microcurie source
of Americium connected to a series of op amps. It comes from a Radio Shack
smoke detector. The ionizaton current gets amplified by several thousand
times so I get a voltage fluctuation of about 2.5 volts p-p. I heavily
filter the thing so only frequencies in the range of 200 to 10000 HZ pass
thru.
When I look at the signal on the scope, it is obviously random noise. I
would like some good ideas on how to convert that signal to a random bit
stream. There are interesting aspects to this, because I see the
ionization current created by the alpha decays there is no dead time in
the detector, but the signal is "periodic" because the probability of
decay is uniform.
> I think I remember the standard deviation of the decay rate being
> the square root of the decay rate. All this derived from single
> atoms having the same probability of decay in a given time.
> If you used the count rate to feed your program and it had say
> 100 counts per second (sd=10 c/s) then you'd be using 100 counts
> to get several bits [maths omitted], but I think the bits per count
> would be lower than the method you are already considering.
The decay rate of the Americium is on the order of 5k/sec. What makes
this particular detection method interesting is that decays which are
close in time create a higher voltage signal (more current getting to the
anode) and decays which are far apart have a much lower signal. This
gives lots of peak fluctuations as well as cross over fluctuations.
The next step I have to do is to digitize the signal and analyze it. I'm
building a toy to do that. If anyone is interested in staring at
ionization chamber decay current I'd appreciate some ideas on how to get
random bits out of it. Will be a while tho, this is all a "just for fun"
hobby gig.
Patience, persistence, truth,
Dr. mike
The following archive was created by hippie-mail 7.98617-22 on Fri Aug 21 1998 - 17:20:08 ADT