Dosimetry

Radiation dosimetry in the fields of health physics and radiation protection is the measurement, calculation and assessment of the ionizing radiation dose absorbed by an object, usually the human body. This applies both internally, due to ingested or inhaled radioactive substances, or externally due to irradiation by sources of radiation.

Internal dosimetry assessment relies on a variety of monitoring, bio-assay or radiation imaging techniques, whilst external dosimetry is based on measurements with a dosimeter, or inferred from measurements made by other radiological protection instruments.

Dosimetry is used extensively for radiation protection and is routinely applied to monitor occupational radiation workers, where irradiation is expected, or where radiation is unexpected, such as in the aftermath of the Three Mile Island, Chernobyl or Fukushima radiological release incidents. The public dose take-up is measured and calculated from a variety of indicators such as ambient measurements of gamma radiation, radioactive particulate monitoring, and the measurement of levels of radioactive contamination.

Other significant areas are medical dosimetry, where the required treatment absorbed dose and any collateral absorbed dose is monitored, and in environmental dosimetry, such as radon monitoring in buildings.

Measuring radiation dose

External dose

There are several ways of measuring absorbed doses from ionizing radiation. People in occupational contact with radioactive substances, or who may be exposed to radiation, routinely carry personal dosimeters. These are specifically designed to record and indicate the dose received. Traditionally, these were lockets fastened to the external clothing of the monitored person, which contained photographic film known as film badge dosimeters. These have been largely replaced with other devices such as the TLD badge which uses Thermoluminescent dosimetry or optically stimulated luminescence (OSL) badges.

A number of electronic devices known as Electronic Personal Dosimeters (EPDs) have come into general use using semiconductor detection and programmable processor technology. These are worn as badges but can give an indication of instantaneous dose rate and an audible and visual alarm if a dose rate or a total integrated dose is exceeded. A good deal of information can be made immediately available to the wearer of the recorded dose and current dose rate via a local display. They can be used as the main stand-alone dosimeter, or as a supplement to such as a TLD badge. These devices are particularly useful for real-time monitoring of dose where a high dose rate is expected which will time-limit the wearer's exposure.

The International Committee on Radiation Protection (ICRP) guidance states that if a personal dosimeter is worn on a position on the body representative of its exposure, assuming whole-body exposure, the value of personal dose equivalent Hp(10) is sufficient to estimate an effective dose value suitable for radiological protection.[1] Such devices are known as "legal dosimeters" if they have been approved for use in recording personnel dose for regulatory purposes. In cases of non-uniform irradiation such personal dosimeters may not be representative of certain specific areas of the body, where additional dosimeters are used in the area of concern.

In certain circumstances, a dose can be inferred from readings taken by fixed instrumentation in an area in which the person concerned has been working. This would generally only be used if personal dosimetry had not been issued, or a personal dosimeter has been damaged or lost. Such calculations would take a pessimistic view of the likely received dose.

Internal dose

Internal dosimetry is used to evaluate the committed dose due to the intake of radionuclides into the human body.

Medical dosimetry

Medical dosimetry is the calculation of absorbed dose and optimization of dose delivery in radiation therapy. It is often performed by a professional health physicist with specialized training in that field. In order to plan the delivery of radiation therapy, the radiation produced by the sources is usually characterized with percentage depth dose curves and dose profiles measured by a medical physicist.

In radiation therapy, three-dimensional dose distributions are often evaluated using a technique known as gel dosimetry.[2]

Environmental dosimetry

Environmental Dosimetry is used where it is likely that the environment will generate a significant radiation dose. An example of this is radon monitoring. Radon is a radioactive gas generated by the decay of uranium, which is present in varying amounts in the earth's crust. Certain geographic areas, due to the underlying geology, continually generate radon which permeates its way to the earth's surface. In some cases the dose can be significant in buildings where the gas can accumulate. A number of specialised dosimetry techniques are used to evaluate the dose that a building's occupants may receive.

Measures of dose

Dose quantities and units
External radiation protection dose quantities in SI units
SI Radiation dose units
Graphic showing relationship of SI radiation dose units

To enable consideration of stochastic health risk, calculations are performed to convert the physical quantity absorbed dose into equivalent and effective doses, the details of which depend on the radiation type and biological context. For applications in radiation protection and dosimetry assessment the (ICRP) and the International Commission on Radiation Units and Measurements (ICRU) have published recommendations and data which are used to calculate these.

Units of measure

There are a number of different measures of radiation dose, including absorbed dose (D) measured in:

  • grays (Gy) energy absorbed per unit of mass (J·kg−1)
  • Equivalent dose (H) measured in sieverts (Sv)
  • Effective dose (E) measured in sieverts
  • Kerma (K) measured in grays
  • dose area product (DAP) measured in gray centimeters2
  • dose length product (DLP) measured in gray centimeters
  • rads a deprecated unit of absorbed radiation dose, defined as 1 rad = 0.01 Gy = 0.01 J/kg
  • Roentgen a legacy unit of measurement for the exposure of X-rays

Each measure is often simply described as ‘dose’, which can lead to confusion. Non-SI units are still used, particularly in the USA, where dose is often reported in rads and dose equivalent in rems. By definition, 1 Gy = 100 rad and 1 Sv = 100 rem.

The fundamental quantity is the absorbed dose (D), which is defined as the mean energy imparted [by ionising radiation] (dE) per unit mass (dm) of material (D = dE/dm)[3] The SI unit of absorbed dose is the gray (Gy) defined as one joule per kilogram. Absorbed dose, as a point measurement, is suitable for describing localised (i.e. partial organ) exposures such as tumour dose in radiotherapy. It may be used to estimate stochastic risk provided the amount and type of tissue involved is stated. Localised diagnostic dose levels are typically in the 0-50 mGy range. At a dose of 1 milligray (mGy) of photon radiation, each cell nucleus is crossed by an average of 1 liberated electron track.[4]

Equivalent dose

The absorbed dose required to produce a certain biological effect varies between different types of radiation, such as photons, neutrons or alpha particles. This is taken into account by the equivalent dose (H), which is defined as the mean dose to organ T by radiation type R (DT,R), multiplied by a weighting factor WR . This designed to take into account the biological effectiveness (RBE) of the radiation type,[3] For instance, for the same absorbed dose in Gy, alpha particles are 20 times as biologically potent as X or gamma rays. The measure of ‘dose equivalent’ is not organ averaged and now only used for "operational quantities". Equivalent dose is designed for estimation of stochastic risks from radiation exposures. Stochastic effect is defined for radiation dose assessment as the probability of cancer induction and genetic damage.[5]

As dose is averaged over the whole organ; equivalent dose is rarely suitable for evaluation of acute radiation effects or tumour dose in radiotherapy. In the case of estimation of stochastic effects, assuming a linear dose response, this averaging out should make no difference as the total energy imparted remains the same.

Radiation weighting factors WR (formerly termed Q factor)
used to represent relative biological effectiveness
according to ICRP report 103[6]
Radiation Energy WR (formerly Q)
x-rays, gamma rays,
beta rays, muons
  1
neutrons < 1 MeV 2.5 + 18.2·e−[ln(E)]²/6
1 MeV - 50 MeV 5.0 + 17.0·e−[ln(2·E)]²/6
> 50 MeV 2.5 + 3.25·e−[ln(0.04·E)]²/6
protons, charged pions   2
alpha rays,
Nuclear fission products,
heavy nuclei
  20

Effective dose

Effective dose is the central dose quantity for radiological protection used to specify exposure limits to ensure that the occurrence of stochastic health effects is kept below unacceptable levels and that tissue reactions are avoided.[7]

It is difficult to compare the stochastic risk from localised exposures of different parts of the body (e.g. a chest x-ray compared to a CT scan of the head), or to compare exposures of the same body part but with different exposure patterns (e.g. a cardiac CT scan with a cardiac nuclear medicine scan). One way to avoid this problem is to simply average out a localised dose over the whole body. The problem of this approach is that the stochastic risk of cancer induction varies from one tissue to another.

The effective dose E is designed to account for this variation by the application of specific weighting factors for each tissue (WT). Effective dose provides the equivalent whole body dose that gives the same risk as the localised exposure. It is defined as the sum of equivalent doses to each organ (HT), each multiplied by its respective tissue weighting factor (WT).

Weighting factors are calculated by the International Commission for Radiological Protection (ICRP), based on the risk of cancer induction for each organ and adjusted for associated lethality, quality of life and years of life lost. Organs that are remote from the site of irradiation will only receive a small equivalent dose (mainly due to scattering) and therefore contribute little to the effective dose, even if the weighting factor for that organ is high.

Effective dose is used to estimate stochastic risks for a ‘reference’ person, which is an average of the population. It is not suitable for estimating stochastic risk for individual medical exposures, and is not used to assess acute radiation effects.

Weighting factors for different organs[8]
Organs Tissue weighting factors
ICRP30(I36)
1979
ICRP60(I3)
1991
ICRP103(I6)
2008
Gonads 0.25 0.20 0.08
Red Bone Marrow 0.12 0.12 0.12
Colon - 0.12 0.12
Lung 0.12 0.12 0.12
Stomach - 0.12 0.12
Breasts 0.15 0.05 0.12
Bladder - 0.05 0.04
Liver - 0.05 0.04
Oesophagus - 0.05 0.04
Thyroid 0.03 0.05 0.04
Skin - 0.01 0.01
Bone surface 0.03 0.01 0.01
Salivary glands - - 0.01
Brain - - 0.01
Remainder of body 0.30 0.05 0.12

Dose versus source or field strength

Radiation dose refers to the amount of energy deposited in matter and/or biological effects of radiation, and should not be confused with the unit of radioactive activity (becquerel, Bq) of the source of radiation, or the strength of the radiation field (fluence). The article on the sievert gives an overview of dose types and how they are calculated. Exposure to a source of radiation will give a dose which is dependent on many factors, such as the activity, duration of exposure, energy of the radiation emitted, distance from the source and amount of shielding.

Background radiation

The worldwide average background dose for a human being is about 3.5 mSv per year [1], mostly from cosmic radiation and natural isotopes in the earth. The largest single source of radiation exposure to the general public is naturally occurring radon gas, which comprises approximately 55% of the annual background dose. It is estimated that radon is responsible for 10% of lung cancers in the United States.

Calibration standards for measuring instruments

Because the human body is approximately 70% water and has an overall density close to 1 g/cm3, dose measurement is usually calculated and calibrated as dose to water.

National standards laboratories such as the National Physical Laboratory, UK (NPL) provide calibration factors for ionization chambers and other measurement devices to convert from the instrument's readout to absorbed dose. The standards laboratories operates as a primary standard, which is normally calibrated by absolute calorimetry (the warming of substances when they absorb energy). A user sends their secondary standard to the laboratory, where it is exposed to a known amount of radiation (derived from the primary standard) and a factor is issued to convert the instrument's reading to that dose. The user may then use their secondary standard to derive calibration factors for other instruments they use, which then become tertiary standards, or field instruments.

The NPL operates a graphite-calorimeter for absolute photon dosimetry. Graphite is used instead of water as its specific heat capacity is one-sixth that of water and therefore the temperature increase in graphite is 6 times higher than the equivalent in water and measurements are more accurate. Significant problems exist in insulating the graphite from the surrounding environment in order to measure the tiny temperature changes. A lethal dose of radiation to a human is approximately 10–20 Gy. This is 10-20 joules per kilogram. A 1 cm3 piece of graphite weighing 2 grams would therefore absorb around 20–40 mJ. With a specific heat capacity of around 700 J·kg−1·K−1, this equates to a temperature rise of just 20 mK.

Dosimeters in radiotherapy (linear particle accelerator in external beam therapy) are routinely calibrated using ionization chambers[9] or diode technology or gel dosimeters.[10]

Radiation-related quantities

The following table shows radiation quantities in SI and non-SI units.

Radiation related quantities
Quantity Unit Symbol Derivation Year SI equivalence
Activity (A) curie Ci 3.7 × 1010 s−1 1953 3.7×1010 Bq
becquerel Bq s−1 1974 s−1
rutherford Rd 106 s−1 1946 1,000,000 Bq
Exposure (X) röntgen R esu / 0.001293 g of air 1928 2.58 × 10−4 C/kg
Fluence (Φ) (reciprocal area) m−2 1962 m−2
Absorbed dose (D) erg erg⋅g−1 1950 1.0 × 10−4 Gy
rad rad 100 erg⋅g−1 1953 0.010 Gy
gray Gy J⋅kg−1 1974 J⋅kg−1
Dose equivalent (H) röntgen equivalent man rem 100 erg⋅g−1 1971 0.010 Sv
sievert Sv J⋅kg−1 × WR 1977 SI

Although the United States Nuclear Regulatory Commission permits the use of the units curie, rad, and rem alongside SI units,[11] the European Union European units of measurement directives required that their use for "public health ... purposes" be phased out by 31 December 1985.[12]

Radiation exposure monitoring

Records of legal dosimetry results are usually kept for a set period of time, depending upon the legal requirements of the nation in which they are used.

Medical radiation exposure monitoring is the practice of collecting dose information from radiology equipment and using the data to help identify opportunities to reduce unnecessary dose in medical situations.

See also

References

  1. ^ ICRP pub 103 para 138
  2. ^ C Baldock, Y De Deene, S Doran, G Ibbott, A Jirasek, M Lepage, KB McAuley, M Oldham, LJ Schreiner 2010. Polymer gel dosimetry. Physics in Medicine and Biology 55 (5) R1
  3. ^ a b International Commission on Radiation Units and Measurements (ICRU).Options for Characterizing Energy Deposition. Journal of the ICRU Vol 11 No 2 (2011) Report 86
  4. ^ Feinendegen LE. The cell dose concept; potential application in radiation protection. 1990 Phys. Med. Biol. 35 597
  5. ^ The ICRP says "In the low dose range, below about 100 mSv, it is scientifically plausible to assume that the incidence of cancer or heritable effects will rise in direct proportion to an increase in the equivalent dose in the relevant organs and tissues" ICRP publication 103 paragraph 64
  6. ^ "The 2007 Recommendations of the International Commission on Radiological Protection". Annals of the ICRP. ICRP publication 103. 37 (2–4). 2007. ISBN 978-0-7020-3048-2. Archived from the original on 16 November 2012. Retrieved 17 May 2012.
  7. ^ ICRP publication 103, paragraph 112
  8. ^ UNSCEAR-2008 Annex A page 40, table A1, retrieved 2011-7-20
  9. ^ Hill R, Mo Z, Haque M, Baldock C, 2009. An evaluation of ionization chambers for the relative dosimetry of kilovoltage x-ray beams. Medical Physics. 36 3971-3981.
  10. ^ Baldock C, De Deene Y, Doran S, Ibbott G, Jirasek A, Lepage M, McAuley KB, Oldham M, Schreiner LJ, 2010. Polymer gel dosimetry. Phys. Med. Biol. 55 R1–R63.
  11. ^ 10 CFR 20.1004. US Nuclear Regulatory Commission. 2009.
  12. ^ The Council of the European Communities (1979-12-21). "Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC". Retrieved 19 May 2012.

External links

  • Ionization chamber
  • [2] - "The confusing world of radiation dosimetry" - M.A. Boyd, U.S. Environmental Protection Agency. An account of chronological differences between USA and ICRP dosimetry systems.
  • Tim Stephens and Keith Pantridge, 'Dosimetry, Personal Monitoring Film' (a short article on Dosimetry from the point of view of its relation to photography, in Philosophy of Photography, volume 2, number 2, 2011, pp. 153–158.)
Absorbed dose

Absorbed dose is a measure of the energy deposited in a medium by ionizing radiation. The unit of measure derived from the SI system is the gray (Gy), which is defined as one Joule of energy absorbed per kilogram of matter. Absorbed dose is used in the calculation of dose uptake in living tissue in both radiation protection (reduction of harmful effects), and radiology (potential beneficial effects for example in cancer treatment). It is also used to directly compare the effect of radiation on inanimate matter.

The non-SI CGS unit rad is sometimes also used, predominantly in the USA.

CR-39

CR-39, or allyl diglycol carbonate (ADC), is a plastic polymer commonly used in the manufacture of eyeglass lenses. The abbreviation stands for "Columbia Resin #39", which was the 39th formula of a thermosetting plastic developed by the Columbia Resins project in 1940.The first commercial use of CR-39 monomer was to help create glass-reinforced plastic fuel tanks for the B-17 bomber aircraft in World War II, reducing weight and increasing range of the bomber. After the War, the Armorlite Lens Company in California is credited with manufacturing the first CR-39 eyeglass lenses in 1947. CR-39 plastic has an index of refraction of 1.498 and an Abbe number of 58. CR-39 is now a trade-marked product of PPG Industries.An alternative use includes a purified version that is used to measure neutron radiation, a type of ionizing radiation, in neutron dosimetry.

Although CR-39 is a type of polycarbonate, it should not be confused with the general term polycarbonate, a tough homopolymer usually made from bisphenol A.

Computational human phantom

Computational human phantoms are models of the human body used in computerized analysis. Since the 1960s, the radiological science community has developed and applied these models for ionizing radiation dosimetry studies. These models have become increasingly accurate with respect to the internal structure of the human body.

As computing evolved, so did the phantoms. Graduating from phantoms based on simple quadratic equations to voxelized phantoms, which were based on actual medical images of the human body, was a major step. The newest models are based on more advanced mathematics, such as Non-uniform rational B-spline (NURBS) and polygon meshes, which allow for 4-D phantoms where simulations can take place not only 3-dimensional space but in time as well.

Phantoms have been developed for a wide variety of humans, from children to adolescents to adults, male and female, as well as pregnant women. With such a variety of phantoms, many kinds of simulations can be run, from dose received from medical imaging procedures to nuclear medicine. Over the years, the results of these simulations have created an assortment of standards that have been adopted in the International Commission on Radiological Protection (ICRP) recommendations.

Dosimeter

A radiation dosimeter is a device that measures exposure to ionizing radiation. As a personal dosimeter it is normally worn by the person being monitored, and is a record of the radiation dose received. Older dosimeters, such as a film badge, require processing after use to reveal the cumulative dose received. Modern electronic personal dosimeters can give a continuous readout of cumulative dose and current dose rate, and can warn the person wearing it when a specified dose rate or a cumulative dose is exceeded.

E. Gail de Planque

Eileen Gail de Planque (also Eileen Gail de Planque Burke, best known as E. Gail de Planque; 1944 – September 8, 2010) was an American nuclear physicist. An expert on environmental radiation measurements, she was the first woman and first health physicist to become a Commissioner at the US government's Nuclear Regulatory Commission (NRC). Her technical areas of expertise included environmental radiation, nuclear facilities monitoring, personnel dosimetry, radiation shielding, radiation transport, and solid state dosimetry.

Equivalent dose

Equivalent dose is a dose quantity H representing the stochastic health effects of low levels of ionizing radiation on the human body. It is derived from the physical quantity absorbed dose, but also takes into account the biological effectiveness of the radiation, which is dependent on the radiation type and energy. In the SI system of units, the unit of measure is the sievert (Sv).

Foton-M No.2

Foton-M No.2 was an unmanned Foton-M spacecraft which carried a European payload for the European Space Agency (ESA). It was placed into orbit by a Russian Soyuz-U rocket launched at 12:00 UTC on 20 June 2005 from the Baikonur Cosmodrome in Kazakhstan by the Russian Space Agency (RKA). The Foton-M No.2 mission was a replacement for the failed Foton-M No.1 mission, which was lost in a launch failure on 15 October 2002.

The spacecraft carried a 600-kilogram (1,300 lb) payload, including 385 kilograms (849 lb) of experiments; consisting of 39 experiments in fluid physics, biology, material science, meteoritics, radiation dosimetry and exobiology (BIOPAN-5). Some of the experiments were designed by the ESA's student programme.

One notable experiment tested the ability of lichen to survive in space. It was successful, as the lichen survived over 14 days of exposure to space.

Gray (unit)

The gray (symbol: Gy) is a derived unit of ionizing radiation dose in the International System of Units (SI). It is defined as the absorption of one joule of radiation energy per kilogram of matter.It is used as a unit of the radiation quantity absorbed dose which measures the energy deposited in a unit mass at a certain position, and of the radiation quantity kerma, which is the amount of energy that is transferred from photons to electrons per unit mass at a certain position.

The corresponding cgs unit to the gray is the rad (equivalent to 0.01 Gy), which remains common largely in the United States, though "strongly discouraged" in the style guide for U.S. National Institute of Standards and Technology authors.The gray was named after British physicist Louis Harold Gray, a pioneer in the measurement of X-ray and radium radiation and their effects on living tissue. It was adopted as part of the International System of Units in 1975.

Health physics

Health physics is the applied physics of radiation protection for health and health care purposes. It is the science concerned with the recognition, evaluation, and control of health hazards to permit the safe use and application of ionizing radiation. Health physics professionals promote excellence in the science and practice of radiation protection and safety. Health physicists principally work at facilities where radionuclides or other sources of ionizing radiation (such as X-ray generators) are used or produced; these include hospitals, government laboratories, academic and research institutions, nuclear power plants, regulatory agencies, and manufacturing plants.

Internal dosimetry

Internal dosimetry is the science and art of internal ionising radiation dose assessment due to radionuclides incorporated inside the human body.Radionuclides deposited within a body will irradiate tissues and organs and give rise to committed dose until they are excreted from the body or the radionuclide is completely decayed.

The internal doses for workers or members of the public exposed to the intake of radioactive particulates can be estimated using bioassay data such as lung and body counter measurements, urine or faecal radioisotope concentration, etc. The International Commission on Radiological Protection (ICRP) biokinetic models are applied to establish a relationship between the individual intake and the bioassay measurements, and then to infer the internal dose.

Ionization chamber

The ionization chamber is the simplest of all gas-filled radiation detectors, and is widely used for the detection and measurement of certain types of ionizing radiation; X-rays, gamma rays, and beta particles. Conventionally, the term "ionization chamber" is used exclusively to describe those detectors which collect all the charges created by direct ionization within the gas through the application of an electric field. It only uses the discrete charges created by each interaction between the incident radiation and the gas, and does not involve the gas multiplication mechanisms used by other radiation instruments, such as the Geiger counter or the proportional counter.

Ion chambers have a good uniform response to radiation over a wide range of energies and are the preferred means of measuring high levels of gamma radiation. They are widely used in the nuclear power industry, research labs, radiography, radiobiology, and environmental monitoring.

List of International Organization for Standardization standards

This is a list of published International Organization for Standardization (ISO) standards and other deliverables. For a complete and up-to-date list of all the ISO standards, see the ISO catalogue.The standards are protected by copyright and most of them must be purchased. However, about 300 of the standards produced by ISO and IEC's Joint Technical Committee 1 (JTC1) have been made freely and publicly available.

National Nuclear Energy Commission

The National Nuclear Energy Commission (Portuguese: Comissão Nacional de Energia Nuclear; CNEN) is the Brazilian government agency responsible for the orientation, planning, supervision, and control of Brazil's nuclear program. The agency was created on 10 October 1956. The CNEN is under the direct control of the Ministry of Science and Technology.

National Voluntary Laboratory Accreditation Program

National Voluntary Laboratory Accreditation Program (NVLAP) is a National Institute of Standards and Technology (NIST) program in the USA which provides an unbiased third-party test and evaluation program to accredit laboratories in their respective fields to the ISO 17025 standard. NVLAP is in compliance with ISO 17011.

Radiation Protection Dosimetry

Radiation Protection Dosimetry is a monthly peer-reviewed scientific journal covering radiobiology, especially dosimetry and radiation monitoring for both ionizing and non-ionizing radiation. The editor-in-chief is J. Hans Zoetelief (Delft University of Technology).

According to the Journal Citation Reports, the journal had a 2013 impact factor of 0.861.

Roentgen (unit)

The roentgen or röntgen () (symbol R) is a legacy unit of measurement for the exposure of X-rays and gamma rays. It is defined as the electric charge freed by such radiation in a specified volume of air divided by the mass of that air.

In 1928 it was the first international measurement quantity for ionising radiation to be defined for radiation protection, and was an easily replicated method of measuring air ionization directly by using an ion chamber. It is named after the German physicist Wilhelm Röntgen, who discovered X-rays.

Although relatively easy to measure, it had the disadvantage that it was only a measure of air ionisation and not a direct measure of radiation absorption in other materials. As the science of radiation dosimetry developed, it was realised that the ionising effect, and hence damage, was linked to energy absorbed by irradiated materials, and new radiometric units for radiation protection were defined from 1953 onwards which took this into account. A new quantity Kerma was defined which can measure air ionisation, and is the modern metrological successor to the roengten, and from this the absorbed dose can be calculated using known coefficients for specific target materials. In radiation protection the absorbed dose is the energy absorption which is an indication of likely acute tissue effects occurring at high dose rates, and from low levels of absorbed dose the equivalent dose, representing the stochastic health risk, can be calculated; for which the current SI units used are the gray (Gy) and the sievert (Sv) respectively.

The roengten has been redefined over the years. It was last defined by the US National Institute of Standards and Technology (NIST) in 1998 as 2.58×10−4 C/kg, with a recommendation that the definition be given in every document where the roentgen is used. One roentgen deposits 0.00877 grays (0.877 rads) of absorbed dose in dry air, or 0.0096 Gy (0.96 rad) in soft tissue. One roentgen of X-rays may deposit anywhere from 0.01 to 0.04 Gy (1.0 to 4.0 rad) in bone depending on the beam energy. This tissue-dependent conversion from kerma to absorbed dose is called the F-factor in radiotherapy contexts. The conversion depends on the ionizing energy of a reference medium, which is ambiguous in the latest NIST definition.

Sievert

The sievert (symbol: Sv) is a derived unit of ionizing radiation dose in the International System of Units (SI) and 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 work on radiation dose measurement and research into the biological effects of radiation.

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 or ingested radioactive substances. The sievert is intended to represent the 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 eventually developing cancer based on the linear no-threshold model.To enable 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 radiation type and biological context. For applications in radiation protection and dosimetry assessment the International Commission on Radiological Protection (ICRP) and International Commission on Radiation Units and Measurements (ICRU) have published recommendations and data which are used to calculate these. These are under continual review, and changes are advised in the formal "Reports" of those bodies.

Conventionally, the sievert is not used for high dose rates of radiation that produce deterministic effects, which is the severity of acute tissue damage that is certain to happen, such as acute radiation syndrome; these effects are compared to the physical quantity absorbed dose measured by the unit gray (Gy).One sievert equals 100 rem. The rem is an older, non-SI unit of measurement.

Thermoluminescent dosimeter

A thermoluminescent dosimeter, or TLD, is a type of radiation dosimeter. A TLD measures ionizing radiation exposure by measuring the intensity of visible light emitted by a crystal inside the detector when the crystal is heated. The intensity of light emitted is dependent upon the radiation exposure. Materials exhibiting thermoluminescence in response to ionizing radiation include calcium fluoride, lithium fluoride, calcium sulfate, lithium borate, calcium borate, potassium bromide, and feldspar. It was invented in 1954 by Professor Farrington Daniels of the University of Wisconsin-Madison.

X. George Xu

Xie George Xu is currently the Edward E. Hood Chair Professor of Engineering at Rensselaer Polytechnic Institute (RPI), Troy, New York.

George Xu received a B.S. in Physics from Xidian University in Xi'an, China in 1983. After working several years, he came to study in the United States where he received a Ph.D.in Nuclear Engineering from Texas A&M University in 1994. He then joined RPI as Assistant Professor (with a joint appointment as the Director of the Office Radiation and Nuclear Safety) and was promoted to Associate Professor with tenure in 2001 and then Professor in 2006. From 2011 to 2013, he served as the Head of Nuclear Engineering Program at RPI. Xu is a Fellow of the American Nuclear Society, Health Physics Society, and American Association of Medical Physicists.

Xu leads the Rensselaer Radiation Measurement and Dosimetry Group (RRDMG). He and his colleagues are interested in novel computational and experimental methods that have important and diverse applications in radiation protection, radiation measurement, shielding design]], reactor modeling, medical imaging, and radiotherapy. In particular, he uses Monte Carlo simulations as a research tool and has nearly 30 years of experience in various production Monte Carlo codes. His recent research projects have included such diverse topics as parallel Monte Carlo computing using GPU/CUDA, nanomaterials-based x-ray sources, X-ray computed tomography (CT) and proton radiotherapy, compressive sensing. Xu has directed numerous projects, with a total of about $20 million in grant funding from agencies such as the National Science Foundation, Department of Energy, National Institutes of Health, National Institute of Standards and Technology, and Electric Power Research Institute, as well as private nuclear power industry. Xu has graduated 21 Ph.D. and 12 M.S. students at RPI. He has authored or co-authored 170 peer-reviewed journal papers and books chapters, 370 conference abstracts, 120 invited seminars and presentations, 5 patents/disclosures and 5 software packages. An internationally recognized leading expert in Monte Carlo computation and radiation protection dosimetry, Xu is a co-founder of the International Consortium of Computational Human Phantoms and co-edited Handbook of Anatomical Models for Radiation Dosimetry.

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