| MadSci Network: Physics |
Heather, This was the hardest question to answer that I have received as a mad scientist. Not that I didn't have a good answer from my years as a nuclear scientist and consultant, but finding published data was extremely difficult. Over 1,000 search results (examples: "x-rays + fluorescent lights", "health physics + fluorescent lights", "fluorescent light + dosimetry + x-rays", and "x-ray exposure + fluorescent light") were evaluated. Apparently, no one has published the results of placing an appropriate radiation detector next to comercially available fluorescent lights. Most of the dosimetry was concerned with the UV (ultraviolet) emission from fluorescent lights. The one piece of data that indicates the exposure from "strong" x-rays-- those being able to pass through the glass--is not of concern is from Laboratory for Calibration of Radiation Protection Instruments Individual and environmental Dosimetry Laboratory http://www.ifj.edu.pl/old- html/htdocs/Introductions/NPP/tld%20research.htm They manufacture ultra-high sensitive thermoluminescent dosimeters, and also supply the material for other investigators to use. The data sheet for the Ultra-High Sensitivity material gives: ULTRA-HIGH SENSITIVITY LiF:Mg,Cu,P TL PHOSPHOR & PELLETS Fluorescent Light effect on fading and zero reading: Negligible at standard laboratory light intensity Standard laboratory fluorescent lights are usually 2 to 4 in a bank about 5 feet above bench height. This material is sensitive to environmental radiation, so I conclude that there is no difference between environmental (background) radiation with the lights on or off. Personally, I have used calibrated microR-meters and found no difference in background radiation from fluorescent lights. Now, let's fill in the rest of your question. Gas discharge lamps are under pressure while fluorescent lights operate at about 0.3% of atmospheric pressure, and mercury vapor is present at about 1 part per thousand of that. The main gases in fluorescent lights are argon and, occasionally, neon or krypton. Because the electrons are accelerated to a low energy, the x-rays from these gases are very soft, and are stopped by the glass of the tube. There is a high potential created to start the tube, but once the plasma (current conducted by ionization of a gas) is established, the voltage is reduced, and the current limited to prevent overheating (the job of the ballast for fluorescent lights). Gas discharge tubes are quartz so that the ultraviolet light as well as the visible will be emitted, and are usually at or above atmospheric pressure. Neon and argon x-rays will be absorbed by the quartz. X-rays are produced when an inner electron of an atom is removed by a collision with an energetic electron. One source of x-rays is the electrode that the electrons hit--it is usually metallic. The other source of x-rays is heavy gases where removing an electron takes considerable energy--100,000 electron volts or more (100 keV) in the inelastic collision. When this inner shell vacancy is created in an element such as mercury, an electron from a higher shell "falls" into the potential hole and in so doing gives off an x-ray. For mercury, this x- ray is on the order of 80 kEv, and is a moderately strong x-ray that will go through the glass of a fluorescent light. But most commercial lighting systems do not work at 100 keV or 100,000 volts potential. They operate at 10kV starting potential, and lower for operation. Now to the question about the Van der Graaf (VdG) generator, when a high potential electric field is generated (as high as 500 keV or even greater), electrons can be accelerated. If the field is strong enough, it will ionize the air, and this can be seen as a corona discharge from the VdG. In a fluorescent light, this ionization produces UV and some x-ray emission. The UV emission causes the tube to fluoresce. The high potential may remove some inner electrons from mercury atoms causing emission of mercury x-rays. However, this ionization is on an individual atom basis; there is no plasma generated carrying a current of high- velocity electrons because there is minimal potential difference between the electrodes in the fluorescent tube, and nothing to carry off the charge buildup on an electrode (you don't see sparks flying out of the end of the tube). The principles of Health Physics to reduce exposure are time, distance, and shielding. In the case of the fluorescent light: the exposure is for a very short time, and the exposure rate is very low; the hand holding the tube is only over a short section of the tube, so any exposure is mainly to a short part of the tube; and the glass envelope provides shielding for almost all of the soft x-rays, and there is not a significant dose from the mercury x-rays in this experiment. One very interesting example of this phenomenon, with pictures, underneath 400 kV electric power transmission lines is http://www.zen32868.zen.c o.uk/r/press.htm A field of 1301 fluorescent lights was set up as a performance art piece by Richard Box, Artist in Residence, Physics Department, Bristol University, UK. The description of how a fluorescent light works is not totally correct, but that is journalism. The Guardian G2 26.02.04 How does this field of lights work? Ian Sample The 1301 fluorescent tubes are powered only by the electric fields generated by overhead powerlines. Richard Box, artist-in-residence at Bristol University’s physics department, got the idea for the installation after a chance conversation with a friend. ‘He was telling me he used to play with a fluorescent tube under the pylons by his house,’ says Box. ‘He said it lit up like a light sabre.’ Box decided to see if he could fill a field with tubes lit by powerlines. After a few weeks hunting for a site, he found a field, slipped the local farmer £200 and planted 3,600 square metres with tubes collected from hospitals. A fluorescent tube glows when an electrical voltage is set up across it. The electric field set up inside the tube excites atoms of mercury gas, making them emit ultraviolet light. This invisible light strikes the phosphor coating on the glass tube, making it glow. Because powerlines are typically 400,000 volts, and Earth is at an electrical potential voltage of zero volts, pylons create electric fields between the cables they carry and the ground. Box denies that he aimed to draw attention to the potential dangers of powerlines, ‘For me, it was just the amazement of taking something that’s invisible and making it visible,’ he says. ‘When it worked, I thought: ‘This is amazing.’’ General references for how fluorescent lights work: howthingswork.virginia.edu/fluorescent_lamps.html en.wikipedia.org/wiki/Fluorescent_light
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