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The existence of radioactive natural stones has been widely known. Our laboratory occasionally accepts inquiries about the influence of such radiation on the human body or about the radiation dose that is subject to regulation. We introduce here the basic knowledge of radiation and examples of actual measurement of radiation amount of some gem mineral. |
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It has been commonly known that some gemstones are radioactive and several stones were introduced in the GEMMOLOGY December 2002 issue. The radiation may influence the human body and radioactive gemstones have been treated with special care, but people are actually not familiar with the influence of radiation on our body or with the amount of radiation that should be regulated. We go through a basic comprehension of radiation and then introduce actual examples of radiation measurement on radioactive gemstones. To start with, consider the radiation measurement. Two types of unit below are used to measure radiation of radioactive substances. Bq; a value to show how much a sample contains radioactive element(s), Sv; a value that reflects the biological effects of radiation when the human body is exposed to it. Radioactivity is the ability to emit radiation hence Bq value means the measurement of radioactivity. According to the Nuclear Reactor Regulation Law a substance that has radiation concentration 370 Bq or over per gram and weighs 900g or over is subject to regulation and it should be reported. It is unlikely to deal with such large amount of radioactive substances and we will need to pay more attention on radiation amount in Sv unit that shows the biological effect of radiation. There are common examples to understand the SV scale. We are exposed to 0.6 mSv at stomach X-ray mass examination. Or, we receive radiation from cosmic ray, daily foods or radioactive substances that are contained in soil or air in slight amount (= natural radiation). Such natural radiation amount varies in places and in Japan we receive about 1.5 mSv per year in average. The radiation dose we receive in various situation are listed in the Table 1. Now, where does the safety limit of radiation dose to our body lies? According to reports of various researches to date, acute exposure to radiation of 200 mSv or under in total showed no influence to a human body. Based on these results, the International Commission on Radiological Protection (ICRP) regulates in its recommendation issued in 1990 (ICRP Publication 60) that the annual dose limit for members of the general public is 1 mSv (mSv/y) (except natural radiation and medical exposure). Generally radiological measuring unit measures the radiation amount per hour so that the regulation value above can be converted to the hourly value for the sake of comparison by dividing it by [1 day (24 hours) ×365 days = 8760 hours], and the resulting value is about 0.114 μSv/h (μSv= a thousandth of mSv). This is significantly larger than the radiation amount measured in a state of nature (About 0.030 μSv/h in Tokyo) and this becomes a safety criterion for radioactivity of a substance.
(by a brochure on nuclear and radiation safety ensuring issued by Nuclear Safety Technology Center) |
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| Some of naturally occurred minerals are radioactive because of radioactive elements such as U (Uranium) or Th (thorium), contained in them. Certain gemstones such as zircon emit only a trace amount or radiation because they contain radioactive elements as trace elements, whereas some others such as ekanite or thorianite contain those elements as major components and they are rather strongly radioactive. Our laboratory recently had an opportunity to measure radiation amount of a mineral sample of autunite (Figure 1). This mineral is known to contain U as a major component and show strong fluorescence under UV light (Figure 2). In the measurement, the strongest radiation amount on some surface area was 3.2 μSv/h (=27.6 mSv/y) and the average value of about 2.5 μSv/h (=21.9 mSv/y) was detected. At the point 10 cm away from the sample, radiation amount was 0.12~0.13 μSv/h (=1.05~1.14 mSv/y). In this case, the radiation amount does not reach to the protection limit as far as you keep 10 cm away from the sample and proper storage will pose no problem on keeping it as a collector’s stone or selling it as a sample stone. However, when it is worn in close contact with the skin the radiation dose may largely exceed the protection limit. The dose is not so high as to cause effect immediately on the human body, but it is wise to refrain from wearing it constantly or handling it directly for a long period. Many radioactive natural stones such as autunite are brittle (hardness of autunite, for example, is 2~21/2) and care should be taken to prevent fragments attaching to the skin or being inhaled when handling them. Some gemstones that are constantly worn as jewellery also need attention. For example, we occasionally encounter slightly radioactive deep blue apatite among those set in a necklace. Such apatite pieces contain a trace of Th and the radiation of each piece is not hazardous but when many pieces are set in a necklace the total radiation dose of the jewellery may slightly exceed the ICRP protection limit. The limit value is a total dose during continuous contact for a full year and the actual radiation amount received from such jewellery will be less than this, but its use with direct skin contact requires caution. Not all blue apatite is such radioactive, but its risk should be borne in mind.
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| At the GAAJ-ZENHOKYO Laboratory, all items we receive from our customers are measured their residual radioactivity using a radiological measuring unit (Aloka Scintillation survey meter PDR-101), which can detect radiation as weak as ICRP protection limit level, to confirm their safety before issuing their gem reports. For further information, please contact: GAAJ-ZENHOKYO Laboratory E-mail: laboratory@gaaj-zenhokyo.co.jp |
