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Show lengths and return times so short ( 10- 9 to 10- 10 second) that they probably lie beyond the scope of present radar technology. 2.6.8 Dielectric Effect The dielectric constant of water ( the human body- approximately) and that of ice are nearly the same for the very high and very low ends of the electromagnetic spectrum. They differ widely over a middle range of the spectrum in the neighborhood of 10 mHz ( short- wave radio frequencies). C. Jaccard of the Swiss Avalanche Institute has pointed out some possible detection schemes which take advantage of this difference, but at the present they have not yet been brought to practical trial. Category B includes some promising technical means of locating avalanche victims, but all of these encounter a basic practical difficulty in that they require the potential avalanche victim to equip himself in advance with some special device. Technological and cost problems raised by this requirement may be solvable, but the psychological problem is formidable. Just those persons most in need of avalanche accident protect ion- the untrained, the unaware or the over- confident- are the ones least likely to seek special protective devices. 2.6.9 Magnetic Detectors If the avalanche victim carries a strong permanent magnet he can readily be located at burial depths up to 8 feet or more by a portable magnetometer. The feasibility of this system has been recognized for a number of years. Practical search systems are currently offered by Varian in the United States and Foerster in Germany. The magnets are inexpensive and require no maintenance. Rather careful scanning of the avalanche debris is required, for the radius of the detectable magnetic signal is comparatively small. Some training and practice on the part of the magnetometer operator facilitates rapid search, but the instrument is very simple to operate. The principal disadvantage of the magnetic search system is the very high cost of the magnetometer, from $ 5,000 to $ 8,000 at current United States prices. This severely limits the number of search instruments which can be stocked in avalanche caches. In many instances the cost rules out use of the magnetometer. 2.6.10 Radioactive Tracers The idea of using Geiger- Mueller counters or scintillometers to search for a radioactive source carried by an avalanche victim has been repeatedly advanced. In fact, while theoretically feasible, this method is practically one of the least attractive of the various search schemes. Water, including water in the form of snow particles, is an efficient absorber of gamma radiation. ( Alpha and beta rays for practical purposes do not penetrate water at all.) Any gamma ray source strong enough to penetrate several feet of avalanche snow and be detectable at the surface would have to be so strong that it would pose a serious radiation hazard not only to the bearer but to anyone else in his vicinity. This obstacle cannot be overcome by 51 |