Sievert

The sievert is the ionizing radiation dose equivalence unit of the International System of Units (SI), equal to the joule per kilogram (symbol: Sv). It is a measure of the health effect of low levels of ionizing radiation on the human body. The sievert is of importance in dosimetry and radiation protection, and is named after Rolf Maximilian Sievert, a Swedish medical physicist renowned for his work in measuring radiation dose and investigating the biological effects of radiation.

One Sv is equivalent to one joule per kilogram (J/kg). This unit gives a numerical value with which the non-stochastic or deterministic effects of ionizing radiation can be quantified.

The sievert is used for radiation dose quantities such as equivalent dose and effective dose, which represent the risk of external radiation from sources outside the body, and committed dose, which represents the risk of internal irradiation due to inhaled radioactive substances or ingested. The sievert is intended to represent stochastic health risk, which for radiation dose assessment is defined as the probability of radiation-induced cancer and genetic damage. One sievert carries with it a 5.5% chance of developing cancer based on the linear model with no threshold.

To allow consideration of stochastic health risk, calculations are performed to convert the physical quantity absorbed dose into equivalent dose and effective dose, the details of which depend on the type of radiation and the biological context. For applications in radiation protection assessment and dosimetry, the International Commission on Radiological Protection (ICRP) and International Commission on Radiation Units and Measurements (ICRU) have published recommendations and data used to calculate them. These are subject to ongoing review, and changes to the "Reports" forms of those bodies.

Conventionally, the sievert is not used for high radiation dose rates that produce deterministic effects, which is the severity of acute tissue damage that is certain to occur, such as acute radiation syndrome; these effects are compared to the physical quantity absorbed dose measured by the unit gray (Gy).

Its difference with the gray (absorbed dose unit) is that the Sievert is corrected for the biological damage caused by radiation, while the gray measures the energy absorbed by a material.

Definition

CIPM sievert definition

The definition of SI given by the International Committee of Weights and Measures (CIPM) reads:

“The amount equivalent to the dose H is the product of the absorbed dose D ionizing radiation and non-dimensional factor Q (quality factor) defined according to the linear transfer of energy by the ICRU".

H = Q × D

The value of Q' is not defined by CIPM, but requires the use of relevant ICRU recommendations to provide this value.

The CIPM also says that:

"to avoid any risk of confusion between the absorbed dose D and the equivalent dose H, the special names of the respective units should be used, i.e. the gray name instead of joules per kilogram for the absorbed dose unit D and the name sievert instead of joules per kilogram for the equivalent dose unit H’’’”.

In summary:

The gray - quantity D

1 Gy = 1 July/kilogram - a physical amount. 1 Gy is the deposit of a July of radiation energy per kg of matter or tissue.

The sievert - amount H

1 Sv = 1 July/kilogram - a biological effect. Sievert represents the equivalent biological effect of the deposit of a July of radiation energy in a kilogram of human tissue. The equivalence with the absorbed dose is indicated by Q.

ICRP/CIPR sievert definition

The International Commission on Radiological Protection (ICRP) (also known as ICRP) definition of sievert is:

‘'The sievert is the special name for the SI unit of equivalent doses, effective doses and operating doses. The unit is July per kilogram."

The sievert is used for a series of dose quantities described in this article that are part of the international system of radiation protection devised and defined by the ICRP and ICRU.

External dose quantities

Numbers of external radiation doses used in radiological protection

The sievert is used to represent the stochastic effects of external ionizing radiation on human tissue. Received radiation doses are measured in practice with radiometric instruments and dosimeters and are called operational quantities. To relate these actual doses received to likely health effects, protective amounts have been developed to predict likely health effects using the results of large epidemiological studies. Consequently, this has required the creation of a number of different dose quantities within a coherent system developed by the ICRU in collaboration with the ICRP.

The amounts of external doses and their relationships are shown in the accompanying diagram. The ICRU has primary responsibility for operational dose quantities, based on the application of ionizing radiation metrology, and the ICRP has primary responsibility for protective quantities, based on modeling of dose absorption and sensitivity biology of the human body.

Naming Conventions

ICRU/ICRP dose amounts have specific purposes and meanings, but some use common words in a different order. There may be confusion between, for example, dose equivalent and dose equivalent’'.

Although the CIPM definition states that the ICRU linear energy transfer function (Q) is used to calculate the biological effect, the ICRP in 1990 developed the quantities of "protective" dose, "effective" dose, and "equivalent" doses that are calculated from more complex computer models and are distinguished by not having the phrase equivalent dose in their name. Only operational dose quantities that still use Q for calculation retain the phrase "dose equivalent". However, there are joint ICRU/ICRP proposals to simplify this system by changing the definitions of operational doses to harmonize them with those of protective quantities. These were outlined at the 3rd International Symposium on Radiation Protection in October 2015, and if implemented would make the names of the operating quantities more logical by introducing “eye lens dose” and “local skin dose” as equivalent doses.

In the United States there are dose quantities with different names that are not part of the ICRP nomenclature.

Physical quantities

These are directly measurable physical quantities in which biological effects have not been taken into account. Radiation fluence is the number of incident radiation particles per unit area per unit time, kerma is the ionizing effect on air of gamma rays and X-rays and is used for instrument calibration, and absorbed dose is the amount of radiation energy deposited per unit mass in the matter or tissue under consideration.

Operational quantities

Operational quantities are measured in practice, and are a means of directly measuring dose uptake due to exposure, or predicting dose uptake in a measured environment. Thus, they are used for practical dose control, providing an estimate or upper limit for the value of protective amounts associated with an exposure. They are also used in standard practice and guidelines.

Calibration of individual and area dosimeters in photon fields is performed by measuring the collision "free air to air kerma" under equilibrium conditions of secondary electrons. The proper operating quantity is then obtained by applying a conversion coefficient that relates the air kerma to the proper operating quantity. Conversion coefficients for photon radiation are published by the ICRU.

Use "phantoms" simple (non-anthropomorphic) to relate the operational quantities to the measured free air irradiance. The ICRU sphere phantom is based on the definition of a 4-element tissue equivalent ICRU material that does not actually exist and cannot be fabricated. The ICRU sphere is a "tissue equivalent" theoretical 30 cm diameter composed of a material with a density of 1 g cm⁻³ and a mass composition of 76.2% oxygen, 11.1% carbon, 10.1 % hydrogen and 2.6% nitrogen. This material is specified to closely approximate human tissue in its absorption properties. According to ICRP, the "ghost sphere" The ICRU in most cases adequately approximates the human body in terms of scattering and attenuation of the penetrating radiation fields under consideration. Therefore, radiation of a given energy fluence will have approximately the same energy deposition within the sphere as it would in the equivalent mass of human tissue.

To allow backscatter and absorption by the human body, the "ghost plate" used to represent the human torso for practical calibration of whole body dosimeters. The ghost slab is 300 mm × 300 mm × 150 mm deep to represent the human torso.

The joint ICRU/ICRP proposals outlined at the 3rd International Symposium on Radiation Protection in October 2015 to change the definition of operational quantities would not change the current use of calibration phantoms or reference radiation fields.

Amounts of protection

Protection quantities are calculated models and are used as "limit quantities" to specify exposure limits to ensure, in the words of the ICRP, "that the occurrence of stochastic health effects is kept below unacceptable levels and that tissue reactions are avoided". These quantities cannot be measured in practice, but their values are derived from models of external doses to internal organs of the human body, using anthropomorphic phantoms. These are 3D computer models of the body that take into account a number of complex effects, such as self-protection of the body and internal scattering of radiation. The calculation starts with the absorbed dose to the organ and then radiation and tissue weighting factors are applied.

Since protection quantities cannot be measured in practice, operational quantities must be used to relate them to the practical responses of radiation instruments and dosimeters.

Response to instrument and dosimetry

This is an actual reading obtained from a gamma environmental dose monitor, or from a personal dosimeter. These instruments are calibrated using radiation metrology techniques that relate them to a national radiation standard, and therefore to an operational quantity. Instrument and dosimeter readings are used to prevent uptake of excessive doses and to provide records of uptake doses to comply with radiation safety legislation, such as in the UK, the Ionising Radiations Regulations 1999.

Calculation of protective dose quantities

Chart showing the ratio of the amounts of “protection dose” in SI units

The sievert is used in external radiation protection for the equivalent dose (the external source, the effects of whole-body exposure, in a uniform field), and the effective dose (which depends on the parts of the body irradiated).

These dose quantities are weighted averages of absorbed doses designed to be representative of stochastic radiation health effects, and the use of the sievert implies that the appropriate weighting factors have been applied to the absorbed dose (expressed in grays).

The ICRP calculation provides two weighting factors to allow the calculation of protection quantities.

1. Radiation factor W_R, which is specific to the type of radiation R'- This is used to calculate the equivalent dose H_Twhich can be for the whole body or for individual organs.

2. The tissue weighting factor W_T, which is specific to the type of T tissue that radiates. This is used with W_R to calculate the doses of contributing organs to reach an effective dose E for uneven irradiation.

When the whole body is irradiated uniformly, only the radiation weighting factor W_R is used, and the effective dose is equal to the equivalent dose for the whole body. But if the irradiation of a body is partial or non-uniform, the tissue factor W_T is used to calculate the dose to each organ or tissue.. They are then added together to obtain the effective dose. In the case of uniform irradiation of the human body, these add up to 1, but in the case of partial or non-uniform irradiation, they add up to a lower value depending on the affected organs, reflecting the smaller overall effect. about health. The calculation process is shown in the attached diagram. This approach calculates the biohazard contribution to the whole body, taking into account total or partial irradiation, and the type or types of radiation. The values of these weighting factors are conservatively chosen to be greater than most of the experimental values observed for the most sensitive cell types, based on averages of those obtained for the human population.

Radiation type weighting factor WR

Since different types of radiation have different biological effects for the same deposited energy, a corrective radiation weighting factor W_R, which depends on the type of radiation and target tissue, is applied to convert the measured absorbed dose to the gray unit to determine the equivalent dose. The result is the sievert of the unit.

The equivalent dose is calculated by multiplying the absorbed energy, averaged by the mass over an organ or tissue of interest, by a radiation weighting factor appropriate to the type and energy of the radiation. To obtain the equivalent dose for a mixture of radiation types and energies, a sum is taken over all types of radiation energy doses.

HT=␡ ␡ RWR⋅ ⋅ DT,R{displaystyle H_{T}=sum _{R}W_{R}cdot D_{T,R} }

where

H_T is the equivalent dose absorbed by tissue T.

D_T,R is the dose absorbed in the tissue T by type of radiation R.

W_R is the radiation weighting factor defined by regulation

So, for example, an absorbed dose of 1 Gy from alpha particles will give rise to an equivalent dose of 20 Sv.

The radiation weighting factor of neutrons has been revised over time and remains controversial.

This may seem like a paradox. It implies that the energy of the incident radiation field in joules has increased by a factor of 20, thus violating the laws of conservation of energy. However, this is not the case. The sievert is used only to convey the fact that one gray of absorbed alpha particles would cause twenty times the biological effect of one gray of absorbed X-rays. It is this biological component that is expressed when the sieves are used instead of the actual energy supplied by the radiation absorbed by the incident.

WT fabric type weighting factor

The second weighting factor is the tissue factor W_T, but it is only used if there has been non-uniform irradiation of a body. If the body has been subjected to uniform irradiation, the effective dose is equal to the whole body equivalent dose, and only the radiation weighting factor W _{is used. R}'. But if there is partial or non-uniform body irradiation, the calculation must take into account the individual doses of each organ received, because the sensitivity of each organ to irradiation depends on its tissue type. This summed dose to only the affected organs gives the effective dose to the whole body. The tissue weighting factor is used to calculate individual organ dose contributions.

ICRP values for W_T are given in the table shown here.

The article on effective dose gives the calculation method. The absorbed dose is first corrected for the type of radiation to give the equivalent dose, and then corrected for the tissue receiving the radiation. Some tissues such as bone marrow are particularly sensitive to radiation, so they are given a weighting factor that is disproportionately large relative to the fraction of body mass they represent. Other tissues such as the hard bone surface are particularly insensitive to radiation and are given a disproportionately low weighting factor.

In summary, the sum of the tissue-weighted doses to each irradiated organ or tissue in the body adds up to the effective dose to the body. The use of the effective dose allows comparison of the total dose received regardless of the degree of body irradiation.

Operating quantities

Operating quantities are used in practical applications to monitor and investigate external exposure situations. They are defined for practical operational measurements and assessment of dose to the body. Three external operational dose quantities were designed to relate operational dosimeter and instrument measurements to calculated protection quantities. Two ghosts were also designed, the ICRU "slab" and "sphere", which relate these quantities to the quantities of incident radiation using the calculation Q(L).

Equivalent dose in the environment

Used for surface monitoring of penetrating radiation and is often expressed as the quantity H*(10). This means that the radiation is equivalent to that found 10 mm inside the ICRU sphere in the direction of origin of the field. An example of penetrating radiation is gamma rays.

Directional dose equivalent

Used to monitor low penetrating radiation and is often expressed as the quantity H'(0.07). This means that the radiation is equivalent to that found at a depth of 0.07 mm in the ICRU sphere. Examples of low penetrating radiation are alpha particles, beta particles, and low energy photons. This amount of dose is used for the determination of equivalent doses to, for example, the skin, the lens of the eye. In radiation protection practice, the omega value is not usually specified, since the dose is at most at the point of interest.

Dose Equivalent of Personal Dose

This is used for monitoring individual doses, such as with a personal dosimeter worn on the body. The recommended depth for evaluation is 10 mm, giving the quantity H_p(10).

Proposals to change the definition of protective dose quantities

In order to simplify the means of calculating operational quantities and to aid understanding of radiation protection quantities, ICRP Committee 2 and ICRU Report Committee 26 began in 2010 a review of the different means to achieve this through dose coefficients related to effective dose or absorbed dose.

Specifically;

1. To control the area of the effective dose of the whole body that would be:

H = Ω × conversion coefficient

The reason for this is that H^∗(10) is not a reasonable estimate of the effective dose due to high-energy photons, as a result of magnification of the particle types and energy ranges to be considered in ICRP report 116. This change would eliminate the need for the ICRU sphere and introduce a new quantity called E_max.

2. For individual monitoring, to measure the deterministic effects on crystalline and skin, it would be:

D = Ω × conversion coefficient for absorbed dose.

The driver for this is the need to measure the deterministic effect, which is suggested to be more appropriate than the stochastic effect. This would calculate the equivalent dose amounts H_lens and H_skin.

This would eliminate the need for the ICRU Sphere and the Q-L function. Any changes would supersede ICRU report 51 and part of report 57.

In July 2017, the ICRU/ICRP issued a draft final report for consultation.

Internal dose quantities

The sievert is used for human internal dose amounts in the calculation of the committed dose. This is a dose of radionuclides that have been ingested or inhaled into the human body, and therefore "compromised" to irradiate the body for a period of time. The concepts for calculating protection quantities described for external radiation are applicable, but since the radiation source is within the body tissue, the calculation of absorbed organ dose uses different irradiation coefficients and mechanisms.

The ICRP defines the committed effective dose, E(t) as the sum of the products of the equivalent organ or tissue committed doses and the appropriate tissue weighting factors W i>_T, where t is the integration time in the years following ingestion. The commitment period is considered to be 50 years for adults and 70 years for children.

The ICRP further states: "For internal exposures, committed effective doses are generally determined from an assessment of radionuclide intake from bioassay measurements or other quantities (for example, activity retained in the body or in the daily excreta). Radiation dose is determined from ingestion using recommended dose coefficients".

A committed dose from an internal source is intended to carry the same effective risk as the same amount of equivalent dose applied uniformly to the whole body from an external source, or the same amount of effective dose applied to a part of the body.

Immediate health symptoms

Symptoms in humans due to radiation accumulated during the same day (effects are reduced if the same number of Sieverts is accumulated over a longer period):

• 0 - 0.25 Sv: None
• 0,25 - 1 Sv: Some people feel nausea and loss of appetite, and may suffer damage to the bone marrow, lymph nodes or spleen.
• 1 - 3 Sv: mild and acute nausea, loss of appetite, infection, more severe bone marrow loss, as well as damage to lymph nodes, spleen, with recovery only likely.
• 3 - 6 Sv: severe nausea, loss of appetite, bleeding, infection, diarrhea, decamation, sterility, and death if not treated.
• 6 - 10 Sv: My symptoms, more deterioration of the central nervous system. Probably death.
• More than 10 Sv: paralysis and death.

Symptoms in humans due to cumulative radiation over a year, in millisieverts (1 Sv = 1000 mSv = 1000000 μSv):

• 2.5 mSv: Global average radiation.
• 5.5 - 10.2 mSv: Average natural values in Guarapari (Brazil) and in Ramsar (Iran). No harmful effects.
• 6.9 mSv: Scanner CT or CT.
• 50 - 250 mSv: Limit for prevention and emergency workers, respectively.

Maximum dose of radiation from astronauts

In space travel, and because there is radiation in space due to the solar wind and cosmic rays, NASA has a standard by which in 10 years of service, an astronaut should not receive more radiation than which would increase the probability of suffering a fatal cancer in the future by 3%.

Using this standard, NASA calculates the maximum amount of radiation an astronaut should receive in 10 years of service (based on rough estimates, with not much statistics available):

25-year-old men: 0.7 Sv; Women aged 25: 0.4 Sv
35-year-old men: 0.9 Sv; Women aged 35: 0.6 Sv
45-year-old men: 1.5 Sv; Women aged 45: 0.9 Sv
55-year-old men: 2.9 Sv; Women 55 years: 1.6 Sv

History

The sievert has its origin in the röntgen equivalent man (rem) which derives from the CGS units. The International Commission on Radiation Units and Measurements (ICRU) promoted a move to consistent SI units in the 1970s, announcing in 1976 that it planned to formulate a suitable unit for equivalent dose. The ICRP preempted the ICRU introducing the sievert in 1977.

The sievert was adopted by the International Committee for Weights and Measures (CIPM) in 1980, five years after adopting the gray. The CIPM then issued an explanation in 1984, recommending when sievert should be used instead of gray. This explanation was updated in 2002 to bring it closer to the ICRP definition of equivalent dose, which had changed in 1990. Specifically, the ICRP had introduced equivalent doses, renaming the quality factor (Q) as radiation weighting factor (W _R), and reduced another weighting factor 'N' in 1990. In 2002, the CIPM also removed the 'N' of his explanation, but kept other ancient terms and symbols. This explanation only appears in the SI brochure appendix and is not part of the sievert definition.

Health Effects

Ionizing radiation has deterministic and stochastic effects on human health. Deterministic events (acute tissue effect) are certain to occur, with resulting health conditions in every individual receiving the same high dose. Stochastics (cancer induction and genetic events) are inherently random, with the majority of individuals in a group not exhibiting any causal negative health effects after exposure, while an indeterminate random minority do, often with the subtle negative health effects that turn out to be observable only after large, detailed epidemiological studies.

The use of the sievert implies that only stochastic effects are considered, and to avoid confusion, deterministic effects are conventionally compared with absorbed dose values expressed by the SI unit gray (Gy).

Stochastic Effects

Stochastic effects are those that occur randomly, such as radiation-induced cancer. The consensus of nuclear regulators, governments and UNSCEAR is that the incidence of cancers due to ionizing radiation can be modeled linearly with effective dose at a rate of 5.5% per sievert. This is known as the sinless linear model. threshold (LNT model). Some commentators such as the French Academy of Sciences argue that this model of LNT is outdated and should be replaced by a threshold below which the body's natural cellular processes repair damage and/or replace damaged cells. There is general agreement that that the risk is much higher for infants and fetuses than for adults, higher for the middle-aged than for the elderly, and higher for women than for men, although there is no quantitative consensus on this.

Determined Effects

Chart that represents the effect of dose fractionation on the ability of gamma rays to cause cell death. The blue line is for the cells that were not given the opportunity to recover; the radiation was given in a session; the red line is for the cells that were allowed to stand for a while and recover. With the pause in childbirth conferring radirresistance.

Deterministic effects (acute tissue damage) that can lead to acute radiation syndrome occur only in the case of acute high doses (≳ 0.1 Gy) and high dose rates (≳ 0.1 Gy/h) and, conventionally, they are not measured using the unit sievert, but the unit gray (Gy) is used. A deterministic risk model would require different weighting factors (not yet established) than those used in the calculation of the equivalent and effective dose.

ICRP Dose Limits

The ICRP recommends a series of limits for dose assimilation in table 8 of report 103. These limits are "situational", for planned, emergency and existing situations. Within these situations, limits are established for the following groups;

• Planned exhibition – limits given for occupational, medical and public exposure.
• Emergency exposure – occupational and public exposure limits
• Existing Exhibition – All Exposed Persons

For occupational exposure, the limit is 50 mSv in a single year with a maximum of 100 mSv in a consecutive five-year period, and for the public at an average of 1 mSv (0.001 Sv) effective dose per year, not including medical and occupational exposures.

For comparison, natural radiation levels inside the United States Capitol are such that a human body would receive an additional 0.85 mSv/a dose rate, close to the regulatory limit, due to the uranium content of the granite structure. According to the ICRP's conservative model, someone who spent 20 years inside the Capitol building would have an extra chance of contracting cancer, above any other existing risk (calculated as: 20-20 y 0.85 mSv/a 0.001 Sv/mSv 5.5%/Sv ≈ 0.1%). However, that "existing risk" is much older; an average American would have a 10% chance of getting cancer during this same 20-year period, even without any exposure to artificial radiation (see natural cancer epidemiology and cancer rates). However, these estimates do not take into account the natural repair mechanisms of all living cells, evolved over a few billion years of exposure to environmental radiation and chemical threats that were greater in the past, and were exaggerated by the evolution of oxygen metabolism.

Examples of dosages

Table of doses of the United States Department of Energy 2010 in tamics for a variety of situations and applications (in English)

Several doses of sievert radiation, from trivial to lethal, expressed as comparative areas (in English)

Radiation dose comparison. It includes the amount detected on the Earth trip to Mars by the RAD in the MSL (2011-2013).

It is rare to find significant doses of radiation in daily life. The following examples may help to illustrate the relative magnitudes; these are just examples, not a complete list of possible radiation doses. An "acute dose" is one that occurs in a short and finite period of time, while a "chronic dose" it is a dose that continues over a long period of time, so it is best described by a dose rate.

Dosage Examples

Examples of Dose Rates

All conversions between hours and years have assumed continuous presence in a constant field, without accounting for known fluctuations, intermittent exposure, and radioactivity. Converted values are shown in parentheses.

Notes on the examples:

Common use of Sv

This unity of the International System is named in honor of Rolf Maximilian Sievert. In the units of the SI whose name comes from a person's own name, the first letter of the symbol is written with capital (Sv), while his name always starts with a tiny letter (sievert), except in case you start a sentence or a title.

Based on The International System of UnitsSection 5.2.

Frequently used SI prefixes such as millisievert (1 mSv = 0.001 Sv) and microsievert (1 μSv = 0.000001 Sv) and commonly used units for indications derived from time or "dose rate" on instruments and warnings for radiation protection are μSv/h and mSv/h. Regulatory limits and chronic doses are often given in units of mSv/a or Sv/a, where they are understood to represent an average over the entire year. In many occupational settings, the hourly dose rate can fluctuate at levels thousands of times higher over a short period of time, without violating annual limits. The conversion from hours to years varies due to leap years and exposure schedules, but approximate conversions do:

1 mSv/h = 8,766 Sv/a

114.1 μSv/h = 1 Sv/a

Conversion from hourly rates to annual rates is further complicated by seasonal fluctuations in natural radiation, decay from man-made sources, and intermittent proximity between humans and sources. Once the ICRP adopted the fixed conversion for occupational exposure, although these have not appeared in recent documents:

8 h = 1 day

40 h = 1 week

50 weeks = 1 year

Therefore, for occupancy exposures for that time period,

1 mSv/h = 2 Sv/a

500 μSv/h = 1 Sv/a

Conversion to other units

1 Sv = 100 rem

In applications that can be commonly found, their submultiples mSv and μSv are usually used. From 1 Sv the most important effects are deterministic, so the absorbed dose is used (hence the gray).

Amounts of ionizing radiation

Graphically show the relationships between radiation and detected ionizing radiation

The following table shows the amounts of radiation in SI and non-SI units:

Although the United States Nuclear Regulatory Commission allows the use of curie, rad, and rem units, along with SI units, the European Units of Measurement Directives of the European Union required that their use be for "reference purposes." public health..." will be phased out by December 31, 1985.

Rem equivalence

An older unit for equivalent dose, the rem, is still frequently used in the United States. One sievert is equal to 100 rem: