sorrell@triton.unm.edu (05/02/91)
In a recent thread about x-ray detection via ion chambers, film, scintillators and photo-multipliers some misconceptions have appeared. X-rays can be detected directly by discharge tubes, i.e. geiger- mueller or proportional counters (in addition to ion chambers), and by carrier production in semiconductors, both doped and intrinsic. Ads for companies making and selling such can be found in the annual equipment issues of various journals, e.g. AAAS, Physics Today, etc. In addition, it is possible to fabricate many other kinds of x-ray detectors. See, for example, Review of Scientific Instruments, Physical Abstracts, etc. One of the most simple x-ray detectors to build, is the scintillation detector. It can be as simple as a chunk of lead glass, optically coupled to a photo detector, even a cadmium selenide photo conductive type, the whole covered by a light tight enclosure. As was pointed out in the thread, calibration is the really difficult part. In fact, for the amateur it would be very near impossible. Relative measurements could be made, however. As for the concern over the 5 kv vacuum tube generating x-rays, it does, of course. But they have a maximum energy of 5 kv. That means they could penetrate a few cm in air, a mm or so in tissue, and almost certainly could not get through the glass envelope of the vacuum tube. If a dosimeter placed next to the 5 kv tube indicated any exposure, one should suspect the environment, and perhaps conduct an experiment with the 5 kv turned off. An occasional film badge (or TLD) failing to indicate a known exposure could be expected from the fact that film was being used. Film is generally cheaper than TLDs, and the precision is considered to be lower. While reading a TLD is far from a trivial task, because of its complexity, it is generally done by a completely automated process. Film often involves some human implementation, even if only checking and maintaining the developer solution strength. Humans are after all human. OBTW, the VDT you are probably reading this on employs an electron beam at 15 to 30 kv, and definitely produces x-rays at that voltage, right there on the screen. Fortunately, the glass is fairly thick, and the current is only a few micro amperes, and I'm not planning on any more children anyway. SAO I disclaim everything.