Hi Bob, hi all! RS> Hi All -- RS> I spent several years fooling around in the medical X-ray field back in the RS> '80s. Most detectors did, indeed, involve scintillation screens and visible RS> light receptors, but I did run into one real oddball direct detector. It RS> was a "line scan" type device whose mission was to form a verification image RS> of the placement of lead blocks used to shield healthy tissue adjacent to RS> tumor masses being treated with high energy radiation beams. If I recall RS> the source was a linear accelerator and the output was high energy electrons RS> in the Mev range, not photons (X-rays). I may be wrong here, the electrons RS> may have been slammed into a target which emitted high energy photons.
RS> Uwe, help! I'm in over my head here! RS> The detector was nothing but an array of 512 very ordinary silicon diodes RS> (the 1N914 variety) placed side by side and wired, one-by-one, to charge RS> detectors. The array, which formed a line detector, was scanned across the RS> field of interest and sampled every few mm of travel to build up, RS> pixel-by-pixel, a scanned 2-D image. You description makes sense! First, yes, you can create X-rays by slamming high-energy electrons into a solid target - that's the principle of every X-ray source in the world, however, they differ in the type and energy of acceleration. (Synchrotron radiation is caused by electrons being accelerated in vacuum rather than stopped in a solid, but that again is the same principle...) It is also possible that the electrons (i.e. beta rays) were used directly for the treatment, as their absorption in body tissue is much higher and the beam is more easily focused. If you slam MeV electrons into a solid target you will probably end up with a broad cascade of secondary radiation, mostly high-energy X-rays. And you'll need to have a certain energy to make it through the glass package of the 1N914, but you will certainly get a signal out of it - given the devices are shielded from light and kept at a moderate, constant temperature. In fact you can use a 1N914 (or 1N4148) as a poor photodiode! The package is transparent and the edge of the silicon crystal inside is exposed to the ambient light. I have no idea about the actual junction geometry in these devices, but the leakage current is heavily influenced by room light levels (and temperature of course). Uwe. -- Author: Uwe Zimmermann INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
