I. Dose and dose rate
From Intamap
I. Dose and dose rate
The measure of the energy deposited in a medium by radioactivity is called absorbed dose, which is equivalent to the energy deposited per unit mass of medium. The absorbed dose is expressed in the unit J/kg, which is called gray (Gy).
The equivalent dose is a measure of the radiation dose to tissue takeing into accout different relative biological effects of the different types of ionizing radiation. Equivalent dose is calculated by multiplying the absorbed dose with the quality factor, which is 1 for gamma-rays and beta-particles.
The equivalent dose describes the health hazard caused by radiation and is expressed in units of sieverts (Sv). The dose is often given in thousandths of sieverts, i.e. millisieverts (mSv) or in millionths, i.e. microsieverts (µSv).
| Dose | What the dose causes | ||
| 6000 mSv | The dose which may lead to death when received all at once | ||
| 1000 mSv | The dose which may cause symptoms of a radiation sickness (e.g. tiredness and nausea) if received within 24 hours | ||
| 100 mSv | The highest permitted dose for a radiation worker over a period of five years | ||
| 4 mSv | The average annual radiation dose for Finns caused by indoor radon, X-ray examinations, etc | ||
| 2 mSv | The annual dose of cosmic radiation received by a person working in an aeroplane | ||
| 0,1 mSv | The radiation dose received by a patient having his/her lungs X-rayed | ||
| 0,01 mSv | The radiation dose received by a patient having his/her teeth X-rayed |
Table 1 Examples of radiation doses (http://www.stuk.fi)
Dose rate is the dose absorbed in unit time and indicates the amount of radioactive dose received by a person within a certain period of time. For the measurement of radioactivity the most frequently used detector typs are proportional counters or Geiger-Müller tubes. Depending on the calibration procedures used by the manufacturer, the detectors provide information about the strength of a radioactive source. Using the calibration relation the count rate in counts per second (cps) can be converted into the absorbed dose rate (Gy/h) or the dose equivalent rate in units of sieverts per hour (Sv/h). The dose rate is often given in thousandths of sieverts per hour, i.e. millisieverts per hour (mSv/h) or in millionths, i.e. microsieverts per hour (µSv/h).
Ambient dose equivalent rate dH*(10)/dt
The ambient dose equivalent H*(10) is an operational quantity for area monitoring. It is defined as the dose equivalent that would be produced by a given the radiation field in the ICRU sphere at a depth of 10 mm. This sphere adequately approximates the human body. The ambient dose gives an conservative estimation of the effective dose a person would receive when staying at the point of the monitoring instrument.
The corresponding ambient dose rate dH*(10)/dt describes the ambient dose equivalent per unit time see also: http://w3.tue.nl/fileadmin/sbd/Documenten/IRPA_refresher_courses/External_Dosimetry__Operational_Quantities_and_their_Measurement.pdf
| Dose rate | Example | ||
| 100 µSv/h | It is necessary to take protective measures (e.g. to shelter indoors) | ||
| 30 µSv/h | The dose rate measured at a distance of one metre of a patient that has undergone isotope treatment. When the dose rate is less than 30 µSv/h, the patient can be discharged. | ||
| 5 µSv/h | The highest dose rate measured in Finland during the Chernobyl accident | ||
| 5 µSv/h | The dose rate in an aeroplane flying at an altitude of 12 kilometres | ||
| 0,3 µSv/h | If this dose rate threshold is exceeded, a dose rate measurement station of the german network triggers an alarm. | ||
| 0,04-0,24 µSv/h | Natural background radiation in Germany |
Table 2 Examples of external dose rates
As has been described in the AIRDOS project, different types of detectors are used in european countries, those which are calibrated with respect to ambient dose rate and others with respect to ambient dose equivalent rate. Also in the German dose rate network both types of detectors are used since the equipment has to be changed from measuring data in ambient dose rate to ambient dose equivalent rate, which due to the EU legislation has to be used definetly in all european states in 2011. It is intended to have a homogenous set of detectors within the next years.
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