The underline on the dial (seen starting in 1963) was used to indicate still lower radiation levels, in line with <25 millicuries of tritium. The “exclamation point”, where a small dot is below the index marker at 6 o’clock (first seen in 1962), indicates that the piece is following Atomic Energy Commission regulations from 1960 and has lower radiation levels. Rolex (and Tudor) timepieces will have additional symbols to indicate different levels of radium. L Swiss L: optional marking indicating that the lume is photoluminescent (due to exciting the luminous radiation, as opposed to radioluminescent, which is due to the radioactivity of the material).Pm: contains promethium (may be designated by P, Pm, or Pm 0.5).Swiss TT Swiss T: Swiss made, contains tritium and emits less than 7.5 mCi.Swiss T: a watch that is Swiss made, and with less than 5.0 mCi (mCi is a millicurie, a unit of measuring radioactivity).R: at the same time “T” was added, dials “R” or “ Ra” were added, indicating radium.With the introduction of tritium came new markings on the dial, with “T” added to watch dials circa 1963. With promethium’s relatively short useful life span, it was replaced by tritium. Both tritium (H-3, half-life of 12.3 years) and promethium (Pm-147, 2.6 year half-life) are radionuclides, but emit much lower levels of radiation than radium (Ra-226). Tritium, introduced in the early 1960s, had replaced radium (Radium-226) in watches largely by the end of the 1960s, and although still radioactive and potentially hazardous, the beta particles are not able to escape through the watch glass or skin (but it is a health threat if ingested). T: indicates that tritium was used, as opposed to radium. After 1998, watches may have Swiss or Swiss Made on the dial, however by this time LumiNova was used instead of radium. If it has luminous markers, and made prior to the 1960s, then the watch most likely has radium. Outside of going to the trouble of purchasing a Geiger counter, text on your watch dial may provide some information on the level of radioactivity at time of production: Another route some collectors and watchmakers to save some dollars is to purchase old military surplus Geiger counters (just make sure you calibrate!). Others to check out include the Mazur Instruments PRM-9000 and SOEKS 01M. Reasonably accurate devices are not inexpensive, starting at the level of the RAXED 1053, for example. The easiest and most reliable way to know the level of radioactivity of your watch is to purchase a Geiger counter. How to know if your vintage watch has radioactive material By the late 1960s, radium was phased out and replaced with much safer alternatives. So don’t think just because your vintage watch doesn’t emit light, it no longer poses a hazard. The radium is still there (and will be for a long, long time… it has a 1,600 (!) year half-life), in amounts nearly as high as when the watch was made, though the compound no longer emits as much light. The paint (lume) glows due to mixing, though over time the glow of the paint will fade as the fluorescence of the mixture degrades due to the radium. When people refer to lume with radium, they are talking about radium salts mixed with a chemical phosphor, which results in a compound then developed into a luminescent paint. People accept different levels of risk related to radioactive material, so I can’t tell you whether it is “safe” to wear your watch, or if the radium in your watch can do harm (at the least, make sure the watch stays in one piece and remains closed…though even then, gamma particles can still make their way through crystal and metal). Those vintage hour numbers, indices, plots and hands that glow on their own (termed radioluminescent, as opposed to photoluminescent, which requires light to charge)? That might just be radium, and furthermore, it could be enough you should do a bit of research.
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