From: utzoo!decvax!cca!ima!ism780!mark Newsgroups: net.physics Title: Re: (Usenet) two questions on REMs/RADs/RBEs and xrays and crts Article-I.D.: ism780.5 Posted: Fri Feb 4 09:16:00 1983 Received: Sun Feb 6 01:04:51 1983 It has been a long time since I hung around a nuke lab, but I once had to know what all of those were. This is all from memory (I don't have my health-physics text anymore). Measures such as Curies and Roentgens are not directly relevent to radiation dosage - since they merely count ionization events without considering the damaging potential of those events. RAD - Measure of Ionizing energy absorbed This is the basic measure of radiation absorbtion. One RAD corresponds to the absorbtion 100 ergs of ionizing enerby per gram of tissue. This measure is misleading however ... RBE - Relative Biological Effect (occasionally R B Potency) Different forms of radiation have a differing ability to harm animals. Alpha radiation is EXTREMELY damaging to the extent that it is absorbed. Fortunately a few feet of air or good piece of paper will considerably attenuate it. Beta radiation will penetrate into skin, doing a little damage - but is stopped by a piece of metal foil. Gamma radiation (photons of nuclear origin) is highly energetic and will penetrate much protective clothing - but will not do as much damage per absorbed erg as would alpha radiation. In order to compensate for the different degrees of havoc wrought by different types of absorbed radiation, the RBE is used as an adjustment which attempts to map received energy into a more-or-less linear measure of absorbed damage. For a simple source of radiation, RBE is a number between zero and one. For real sources, it is computed as the weighted average of the RBE's of the individual radiation components. REM - Roentgen Equivalent Man This is an attempt to measure the actual degree of damage to which a subject has been exposed. It is the dosage multiplied by the RBE. This is the more meaningful number when considering the potential harm that a radiation field might do. A Geiger counter measures ionization incidents, and thus gives a measure of the ionizing particle flux. This is useful in making simple determinations such as this area is hot and this area is clean. For measuring actual radiation hazard, there are more complex devices (such as Nemo spheres) which attempt to simulate the absorbtion of particles in the body. As to your real question, a little more data is required to ascertain the likely threat to terminal users. (1) You estimate the energy of the particles hitting the screen, but not their flux. I understand that you needed to know the energy per particle to figure out the wavelength of the emitted radiation, but the actual hazard comes from the total energy absorbed, not the energy per particle. (2) The absorbing, scattering and reradiation properties of the CRT are crucial to the question. The user is not struck by the beam from the CRT, but rather by particles which pass through the thick screen and photons which are emitted from the collision. This computation could be non-trivial: There is probably a 15-20 degree variation in incidence angle between the electron stream and the screen. The incomming stream of particles does not have a velocity, but rather a distribution. The likelihood of capture is probably highly dependent on the velocity of the incidental particle. A computation based on mean velocity may yield a good approximation of the energy absorbed by the screen - but a meaningless statement about the spectrum of re-radiated energy. The screen is not perfectly uniform, geometrically or compos- itionally. The phosphor is probably a combination of more and less exotic materials, each of which has different absorbtion cross-section and re-radiation properties. I never got beyond basic cross-section/absorbtion computations and always ignored second-order emissions. I suspect that the whole of your problem deals with second-order emissions. I'm not even competent to guess how important the above considerations are. I would suggest that unless you are looking for a thesis topic, that you would do better to: a) Contact OSHA. They have been preparing standards for radiation emission of EDP equipment for the last few years. They can probably send you reams of repors and the relevent standards to boot. b) If you have an "in" to someone who works in highly-hush-hush military work or (dare I say it) "No Such Agency", you might try to find out if the "Tempest" requirements for terminals mention radiation passing through the screen of a CRT. They weren't really interested in biological hazzards, but you can bet that they know everything about ALL forms of radiation which might be emitted from a terminal. c) Don't bother trying to measure the radiation with equipment in your physics department. I am fairly certain that the basic radiation safety equipment won't tell you much at all. The really sensitive equipment (like gamma spectrometers) won't work unless you can put the CRT into the bottom of a small lead cylinder. d) If you want to try a simple "Mr. Wizzard" experiment get some fast b&w film (ASA 600-1000) and a few beta-gamma badges. Make about a dozen test packets with each and set them up in contact with the screen, atop and behind the CRT, below and around the HV transformer and suspended 12-24 inches away from the terminal. Then put another few under lead bricks and a few more on top of a desk, five feet from the terminal (with lead foil between them and the terminal). Expose them all for a month, have the badges processed and develope the film in boiling DEKTOL. Compare them and see if you mesured anything significant. ---mark--- --------