Hi All --
I spent several years fooling around in the medical X-ray field back in the
'80s. Most detectors did, indeed, involve scintillation screens and visible
light receptors, but I did run into one real oddball direct detector. It
was a "line scan" type device whose mission was to form a verification image
of the placement of lead blocks used to shield healthy tissue adjacent to
tumor masses being treated with high energy radiation beams. If I recall
the source was a linear accelerator and the output was high energy electrons
in the Mev range, not photons (X-rays). I may be wrong here, the electrons
may have been slammed into a target which emitted high energy photons.
Uwe, help! I'm in over my head here!
The detector was nothing but an array of 512 very ordinary silicon diodes
(the 1N914 variety) placed side by side and wired, one-by-one, to charge
detectors. The array, which formed a line detector, was scanned across the
field of interest and sampled every few mm of travel to build up,
pixel-by-pixel, a scanned 2-D image.
The image quality was pretty rotten - certainly not useful for medical
diagnostics - but it did build up a pretty good picture of the lead blocker
masks for verification of alignment.
As I recall the output of the diode junctions were connected to small
capacitors and clamping FETs. The detectors were simple charge sensitive
op-amps. At the end of each sample, the clamp FETs were fired to discharge
the array and inititate charge accumulation for the next sample.
While the device worked well, within the limits of its resolution, there
were major practical problems involving keeping stray, mostly secondary,
radiation out of the electronics which caused an effect similar to "fogging"
of an image.
Bob Smith
--- Avoid computer viruses, Practice safe hex ---
-- Specializing in small, cost effective
embedded control systems --
Robert L. (Bob) Smith
Smith Machine Works, Inc.
9900 Lumlay Road
Richmond, VA 23236 804/745-1065
[EMAIL PROTECTED]
----- Original Message -----
To: "Multiple recipients of list CHIPDIR-L" <[EMAIL PROTECTED]>
Sent: Monday, January 20, 2003 3:54 PM
>
> >
> > Hello Markus,
> >
> > sounds veeeeery strange to me.
> >
>
> Hello Jan and all the others,
>
> yes, this article is quite extraordinary, but PIN-Diodes ARE INDEED used
> to detect radiation. I don't know whether they are directly coated by a
> szintillation substance. The author replaced the glass window with a 10�
> Al-coated foil to keep the light outside (but I don't know wheter this
> foil also acts as a szintillatior). I would really like to give this
> circuit a try before looking for such a special detection diode.
>
> Further researches about the author braught up the following results:
> The article was published in "Electronics Designer's Casebook No.3 for
> the period Feb 16, 1978 to Jan 4, 1979". The author doesn't work for the
> indicated institute in poland any more, not quite a surprise. So I can't
> contact him directly.
>
> But this tells us that the diodes are at least 25 years old and probably
> of eastern europe type (former CCCP I think, but not sure). So if anyone
> of the list members has an old data book on the shelf, please take a
> look ...
>
> Today's photodiodes should have a higher performance/sensitivity, so
> would a general purpose type (which one?) be ok for a simple test of the
> circuit?
>
> Best Regards
> Markus
>
> --
> Author: Markus Rohe
> INET: [EMAIL PROTECTED]
>
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--
Author: Robert Smith
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