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"Becquerel" and "sievert" are the most common units of radiation. Becquerel is a unit of radioactivity and focuses on where radiation comes from. It is used to express the amount of radioactive materials contained in soil, foods, tap water, etc. The higher the value expressed in becquerels, the larger the radiation being emitted. Sievert is a unit of radiation exposure dose that a person receives and is used with regard to what is exposed to radiation, i.e. the human body. The larger the value expressed in sieverts, the larger the effects of radiation to which the human body is exposed (p.40 of Vol. 1, "Concepts of Doses: Physical Quantities, Protection Quantities and Operational Quantities"). The extent of radiation effects on the human body varies according to the types of exposure, i.e., internal or external exposure, or whole-body or local exposure (for details, refer to Vol. 1, "2.1 Exposure Routes"), and according to the types of radiation (for details, refer to Vol. 1, "1.3 Radiation"). By using sieverts to express all types of exposure, it is possible to compare their effects on human body. External exposure of 1 mSv and internal exposure of 1 mSv are deemed to have equal effects on the human body. Exposure to 1 mSv of radiation from outside the body and exposure to 1 mSv of radiation from within the body mean exposure to a total of 2 mSv of radiation.
Included in this reference material on March 31, 2013 Updated on March 31, 2019
The unit "sievert" is named after Rolf Sievert, a Swedish researcher on radiological protection. He served as the chairman of the International X-ray and Radium Protection Committee (IXRPC), the predecessor of the International Commission on Radiological Protection (ICRP), and participated in founding the ICRP*^1. Millisieverts (one millisievert = a thousandth of sievert) and microsieverts (one microsievert = a millionth of sievert) are mostly used to express radiation doses that people receive in their daily lives. Becquerel (unit of radioactivity), curie (former unit of radioactivity) and gray (unit of absorbed dose) are all named after researchers who made significant contributions to the study of radiation.
*1: It is said that George Kaye at the National Physical Laboratory played a central role in founding the ICRP. (Source: ICRP Publication 109, The History of ICRP and the Evolution of its Policies, ICRP, 2009)
Included in this reference material on March 31, 2013 Updated on March 31, 2015
To calculate the effective dose that expresses the effects of radiation exposure on the whole body, it is necessary to first determine the absorbed doses of individual tissues and organs exposed. The equivalent dose (expressed in sieverts) is obtained by multiplying the absorbed doses of individual tissues and organs by their respective radiation weighting factors (W (^) R ) for taking into account the types of radiation. The value of the radiation weighting factor is larger for the types of radiation having larger effects on the human body (ʸ-particles: 20; ʹ-particles and ʺ-rays: 1). Once the equivalent doses for individual tissues and organs exposed to radiation are determined, they are then multiplied by the respective tissue weighting factors (W (^) T ) for taking into account differences in sensitivity among organs, and the products are summed. The tissue weighting factors are for weighting the radiation sensitivity of individual tissues and organs. Any organ or tissue where radiation is likely to induce fatal cancer is given a higher factor. The tissue weighting factors summate to 1. Thus, the effective dose can be considered as the weighted average of the equivalent doses of all organs and tissues. Effective doses can be calculated similarly for both internal and external exposures. (Related to p.38 of Vol. 1, "Various Factors")
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Recommendations issued by the International Commission on Radiological Protection (ICRP) in 2007 presented new radiation weighting factors and tissue weighting factors. It is stated that ʸ-particles have 20 times larger effects on the human body than ʺ-rays and ʹ-particles with the same absorbed doses. Neutron beams are also given high radiation weighting factors and are expected to have 2.5 to 21 times larger effects on the human body than ʺ-rays and ʹ-particles depending on the energy quantities (p.37 of Vol. 1, "Conversion from Gray to Sievert"). A survey on the health effects of radiation on atomic bomb survivors revealed which organs and tissues are more prone to the cancer-causing effects of radiation (p.114 of Vol. 1, "Tissues and Organs Highly Sensitive to Radiation"). These tissues are assigned high tissue weighting factors. Surveys on the health effects of radiation were also conducted on the children and grandchildren of atomic bomb survivors but no heritable effects of radiation were observed (p.109 of Vol. 1, "Chromosomal Aberrations among Children of Atomic Bomb Survivors"). Therefore, the ICRP lowered the tissue weighting factor for the gonads from 0.2 in the 1990 Recommendations to 0.08 in the 2007 Recommendations. In this way, the factors used in the calculation of effective doses are updated to accommodate new findings.
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To control radiation effects on the human body, it is necessary to take into account the effects of exposure on multiple parts of the body and the effects of previous exposures. The equivalent dose and the effective dose were invented for that purpose. The equivalent dose is obtained by weighting effects on individual organs and tissues according to the types of radiation. The effective dose is obtained by converting the effects on individual tissues to a value for the whole body. It is not the simple average of equivalent doses for individual organs but the result of weighting according to differences in sensitivity to radiation among organs. A factor for weighting radiation effects on individual organs is called the tissue weighting factor. Thus, protection quantities are calculated based on doses for organs and tissues in the human body. They are therefore different from physical quantities such as the radiation intensity (unit: becquerel) and absorbed dose (unit: gray) and cannot be measured directly with instruments. To indicate effects on the human body, operational quantities are defined. Some survey meters use sieverts in their readings. They do not directly measure a protection quantity but show approximate values defined based on measured physical quantities, i.e., operational quantities. Operational quantities include the ambient dose equivalent used in environment monitoring and the personal dose equivalent used in personal monitoring (p.41 of Vol. 1, "Dose Equivalents: Measurable Operational Quantities for Deriving Effective Doses"). To provide conservative (on the safe side) estimates of protection quantities, operational quantities are defined to assume slightly larger numerical values than the values of protection quantities in most cases.
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Operational quantities for approximating effective doses that cannot be actually measured (p.40 of Vol. 1, "Concepts of Doses: Physical Quantities, Protection Quantities and Operational Quantities") are defined, such as the ambient dose equivalent H*(d) (d is depth) for evaluating ambient doses in a work environment, etc., the personal dose equivalent H (^) p (d) for evaluating personal exposure, and the directional dose equivalent H' (d,ʸ) (ʸ is the angle of incidence) as a quantity for use when there is a need to evaluate the depth and directions of incidence as well, as in the case of exposure of the lens of the eye to ʹ-particles or soft X-rays. Generally, both the ambient dose equivalent and the personal dose equivalent are also called 1 cm dose equivalents because a depth of 1 cm is used in the case of exposure to ʺ-rays. However, while the ambient dose equivalent is measured using measuring instruments that are less affected by directivity, such as a stationary ionization chamber and a survey meter, the personal dose equivalent is measured using a small personal dosimeter worn on the trunk of the body, so incidence from the back is evaluated while a self-shielding effect is always at work. Therefore, in the case of exposures only from the front direction, such as exposures in laboratories, the ambient dose equivalent and the personal dose equivalent are equal, but in the case of exposures from all directions, personal dose equivalents are always smaller than the values measured with a survey meter, etc. Calculation of an effective dose for incidence from all directions is made under the condition of "rotational irradiation" in which the human body is rotated, and the calculated value will be exactly the same as the personal dose equivalent. In other words, the calculated value will generally be larger than the effective dose.
Included in this reference material on March 31, 2017 Updated on March 31, 2021
Sievert is used as the unit for (i) radiation dose to the whole body (effective dose) (p.42 of Vol. 1, "Difference between Values of Effective Dose and Dose Equivalent"), (ii) radiation dose due to internal exposure (committed effective dose) (p.56 of Vol. 1, "Committed Effective Doses"), and (iii) dose from local exposure, in which exposure to radiation is limited to a certain location (equivalent dose). They are common in that they all take into account the risks of cancer and heritable effects on individuals or tissues exposed. Sievert may also be used for (iv) the readings of survey meters. The relevant value shows a value converted to an ambient dose equivalent (p.44 of Vol. 1, "Various Measuring Instruments").
Included in this reference material on March 31, 2013 Updated on March 31, 2019
