----- Original Message -----
From: "Alice Campbell" <electrifry@ALTAVISTA.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Friday, December 21, 2001 1:40 PM
Subject: [EE]: Cheerful holiday radiation detector project


> Oops, this time with topic tag :o
>
>
>              For my holidays I thought I'd build a nice PIC broadband
radiation detector.
>               I've researched the web and asked around, and have found two
>              non-Geiger circuits that look promising.  Both involve a
diode
>              of some sort for a detector, followed by a FET-style opamp,
>              followed by an audio or comparator stage for final output
processing.
>
>
>              My question involves the diode detector:  One seems to call
>              for a 'large-area photodiode', something that I don't offhand
>              have in the junkpile, and the other for what looks like a
high-voltage
>              automotive diode 1N3191.  Can anyone tell me why an
automotive
>              diode would work for this, OR whether something else fairly
common
>              might work?  I have a no-brand small-area photodiode with a

>            Here is the link to one of the circuits:
>
>              http://members.tripod.com/xexorz/schematics/dosage.html

I don't know how you plan to test it but 100 RADs/minute is a lethal level
of radiation. Any reverse-biased diode works as a radiation detector. High
energy particles passing through the pn junction trigger avalanche
breakdown, so it would appear that the diode needs to be at a voltage close
to its reverse breakdown potential. The area covered by the pn junction in
the diode determines the efficiency of the detector. The glass/plastic
surrounding the diode will attenuate alphas and betas much more than gamma:
filing off the outer covering may improve matters. Background radiation is
going to give you almost no signal, though. That's why Geiger counters are
used for low level detection. They saturate at high levels and so diodes are
used in that regime. I have tried using ordinary diodes for low level
detection but it seems that the thermal noise swamps any signal from
radiation.
/\/\/\/*=Martin

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