One sievert refers to the dose when one Joule of energy is absorbed per one kilogram of a body of a human (or a mammal animal which is supposed to have similar absorption rate) In this sense ,
\(\rm 1 Sv = 1 J / kg\)
Usually it is assumed that the ionizing particles arise at the spontaneous decay of unstable isotopes produced at the nuclear accidents. However, the same quantity and the same unit may refer to the exposure to radiation of the personnel of the nuclear plants, radio-chemical laboratory of workers of the particle accelerator facilities.
The same unit may be called also Grey. The gray is used with quantities of absorbed dose in any material, while the sievert is used with equivalent, effective, and committed dose in biological tissue. Qualitatively, Grey can be considered as equivalent of sievert.
The exposure of one sievert is believed to cause a significant probability of cancer; 100 sievert are considered sufficient dose to cause the fast death of a human exposed; however, this sequences depend also on the time of the exposition. The same dose received in a short time is assumed to be more dangerous. After the Fukushima disaster (since March 2011), many people worry about radiation; so, the estimates of the orders of magnitude of the related quantities are important.
In 2013, the maximal appropriate dose of 500 microSv/hour is suggested as criterion for the evacuation or staying indoors for the residents within 5 km from the nuclear plant, and 20 microsieverts per hour for those who live farther .
The background radiation is estimated to deliver of order of 4 milisievert/year. An average Gregorian year is 365.2425 days = 52.1775 weeks = 8765.82 hours = 525949.2 minutes = 31556952 seconds. So, roughly, the background radiation provides of order of 7 nanosievert/minute or order of 0.5 microsievert per hour.
In such a way, the official regulation consider as dangerous the raise of the level of radiation for two or three orders of magnitude. Roughly, the level above corresponds to the 10 clicks/minute of an ordinary Geiger counter. However, this rate depends on the type of the device, kind and size of the sensitive element.
The historical experience indicates, that in the case of nuclear accidents, the maps of the radioactive contamination prepared by the professionals late for many years (case of the Chernobyl disaster, 1986) or at least several weeks (case of the Fukushima disaster, 2011), and are already outdated when they become available. Such a delay can be attributed to the sabotage of the dosimetric staff and their supervisors, who usually hide the valuable information from the people in order to get benefits of the exclusive personal use. Usually, the official use the standard pretext "to avoid panic" justifying the sabotage .
The residents usually have no possibility to measure quantitatively the radiation dose, nor the radioactivity of the products they consume, by themselves. In the best case, a family is supposed to have a personal Geiger counter or some equivalent. Such a device does not measure the energy spectrum of the ionizing particles, nor even distinguish beta–particles from gamma–particles, and has poor sensitivity to the alfa–particles of low energy. The estimate of the absorbed radiation dose, measured on sieverts, becomes difficult, if at all. Instead, the average counting rate should be used.
The significant raise of the counting rate say, 100 clicks/minute instead of normal 10 clicks/minute should be considered as serious warning. Such a criterion should be used instead of more accurate criterion based on the laboratory measurements of the absorbed radiation dose expressed in sieverts. Due to the poor sensitivity of the ordinary Geiger counters, the increase of the radiation level for an order of magnitude may be qualified as reason for the evacuation.
In such a way, the raise of the counting rate of a Geiger counter, measured, for example, in clicks/minute, should be used instead of the more precise criterion of absorbed radiation dose, measured in sievert.
The unit is named after Rolf Maximilian Sievert (6 May 1896 – 3 October 1966).
- http://ajw.asahi.com/article/0311disaster/fukushima/AJ201301220068 JIN NISHIKAWA. Watchdog to set strict evacuation rules for nuclear accidents. January 22, 2013. Residents would be told not to drink water or milk if they contain 300 becquerels of radioactive iodine per kilogram or more, or 200 becquerels of radioactive cesium or more.
- http://www.japantimes.co.jp/opinion/2013/01/26/commentary/read-all-about-it-how-kan-do-attitude-averted-the-meltdown-of-japan/ BY ROGER PULVERS. How Kan-do attitude averted the meltdown of Japan. 2013.01.26. .. Kan takes up the narrative: “It was at 3 a.m. when Minister of Economy, Trade and Industry Banri Kaeda came to the Residence with the news that Masataka Shimizu, president of Tepco, had put in a request to withdraw from the nuclear plant site.// “If I (had let this happen), 50 million people would have to be evacuated within a few weeks. … The very announcement to evacuate would result in mass panic.” ..