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Ionising Radiation > ... > Radioactive substances in watches
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Radioactive substances in watches
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Among others, luminous paints are used in watch dials to ensure that they can also be recognised in the dark. The luminous paints are stimulated to glow by a radioactive substance.
Until well in the 60ties, the luminous dials of wrist watches and alarm clocks were marked with radium(Ra-226)- or promethium (Pm-147)-containing luminous paints. Such watches are no longer produced today. This is not so much due to the radiation exposure to the persons wearing the watches but to the radiological risk for the persons manufacturing them.
Later on, until the middle of the 90ties, paints that were enriched with tritium (H-3), a radioactive isotope of hydrogen, were used for marking the luminous dials. Tritium is a beta-emitter with low energy of up to 19 keV and a half-life of 12.3 years. The paint was stimulated to glow by the tritium beta radiation. The radiation was nearly entirely absorbed in the luminous paint itself and in the watchcase or the watch glass. However, tritium as a volatile substance could diffuse through the underside of the watch casing, which was made from plastic in these watches, and could be incorporated through the skin by the person wearing the watch. Watches containing tritium-luminous paints show a tritium activity of about 0.2-0.3 GBq on average. The dose caused was frequently below 20 µSv per year. This corresponds to about 1/100 of the annual natural radiation exposure which is 2 mSv on average in Germany.
In watches manufactured today, tiny, narrow glass tubes filled with tritium gas (GTLS = gaseous tritium light sources) are used whose inner surface is coated with a special phosphorescent colour. The tritium beta-radiation stimulates this paint to produce continuous glowing. Additional to the glass tubes, the metallic watch case and the watch glass itself serve as further shielding. The wall material of the GTLS-glass tubes is much more impermeable to tritium than e. g. a plastic case. In the case of an assumed complete destruction of the watch, containing 1 GBq tritium (all tritium sources in the watch break), only a dose of 20 µSv would be incorporated through inhalation [3]. In general, during normal use such a damage scenario is unlikely, so the annual effective dose by everyday wearing of this type of wrist watch would be far below 0.1 µSv.
Some of the commercially available watches contain up to 15 GTLS and can have approximate activities between 1.1 to 1.9 GBq, exceeding the exemption limits of 1 GBq for H-3 by the German Radiation Protection Ordinance (StrlSchV). These watches could not legally be bought on the market.
Wrist watches are „consumer goods“. Consumer goods, to which radioactive substances have been added, can basically only be manufactured or marketed if radioactive substances are used whose activity does not exceed the exemption limit according to StrlSchV. In Germany, this requires a licence according to § 106 StrlSchV. According to § 108 StrlSchV, the same applies to the transboundary trafficking of such consumer goods for trading purposes or for commercial use.
Furthermore, according to § 4 StrlSchV, the principle of “justification” applies to the use of radioactive substances. This principle demands a consideration of the benefit of using radioactive substances compared with the possible health impairments. This consideration must especially take into account if – with justifiable effort - technical options are available for the same purposes that do not need to use radioactive substances.
Nowadays there are very good luminous, non-radioactive paints available for normal wrist watches manufactured on the basis of e.g. strontium aluminate (SrAl2O4).
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