Dosimetry

Dosimetry

Overview
Radiation dosimetry is the measurement and calculation of the absorbed dose
Absorbed dose
Absorbed dose is a measure of the energy deposited in a medium by ionizing radiation per unit mass...

 in matter and tissue resulting from the exposure to indirect and direct ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

. It is a scientific subspecialty in the fields of health physics
Health physics
Health physics is a field of science concerned with radiation physics and radiation biology with the goal of providing technical information and proper techniques regarding the safe use of ionizing radiation...

 and medical physics
Medical physics
Medical physics is the application of physics to medicine. It generally concerns physics as applied to medical imaging and radiotherapy, although a medical physicist may also work in many other areas of healthcare...

 that is focused on the calculation of internal and external doses from ionizing radiation.

Dose is reported in gray
Gray (unit)
The gray is the SI unit of absorbed radiation dose of ionizing radiation , and is defined as the absorption of one joule of ionizing radiation by one kilogram of matter ....

 (Gy) for matter or sievert
Sievert
The sievert is the International System of Units SI derived unit of dose equivalent radiation. It attempts to quantitatively evaluate the biological effects of ionizing radiation as opposed to just the absorbed dose of radiation energy, which is measured in gray...

s (Sv) for biological tissue, where 1 Gy or 1 Sv is equal to 1 joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

 per kilogram.
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Encyclopedia
Radiation dosimetry is the measurement and calculation of the absorbed dose
Absorbed dose
Absorbed dose is a measure of the energy deposited in a medium by ionizing radiation per unit mass...

 in matter and tissue resulting from the exposure to indirect and direct ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

. It is a scientific subspecialty in the fields of health physics
Health physics
Health physics is a field of science concerned with radiation physics and radiation biology with the goal of providing technical information and proper techniques regarding the safe use of ionizing radiation...

 and medical physics
Medical physics
Medical physics is the application of physics to medicine. It generally concerns physics as applied to medical imaging and radiotherapy, although a medical physicist may also work in many other areas of healthcare...

 that is focused on the calculation of internal and external doses from ionizing radiation.

Dose is reported in gray
Gray (unit)
The gray is the SI unit of absorbed radiation dose of ionizing radiation , and is defined as the absorption of one joule of ionizing radiation by one kilogram of matter ....

 (Gy) for matter or sievert
Sievert
The sievert is the International System of Units SI derived unit of dose equivalent radiation. It attempts to quantitatively evaluate the biological effects of ionizing radiation as opposed to just the absorbed dose of radiation energy, which is measured in gray...

s (Sv) for biological tissue, where 1 Gy or 1 Sv is equal to 1 joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

 per kilogram. Non-SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

 units are still prevalent as well, where dose is often reported in rads
Rad (unit)
The rad is a unit of absorbed radiation dose. The rad was first proposed in 1918 as "that quantity of X rays which when absorbed will cause the destruction of the malignant mammalian cells in question..." It was defined in CGS units in 1953 as the dose causing 100 ergs of energy to be absorbed by...

 and dose equivalent in rems
Röntgen equivalent man
Named after Wilhelm Röntgen , the roentgen equivalent in man or rem is a unit of radiation dose equivalent...

. By definition, 1 Gy = 100 rad and 1 Sv = 100 rem.

In the United States, radiation dosimeters measure and display dose in rems rather than sieverts. One rem = 1 centisievert = 1/100 sievert. Popular emergency-level personal radiation dosimeters are DOSE-GARD and EPD N2, made by General Atomics Inc. and Thermoscientific, respectively.

Radiation effects on living tissue


The distinction between absorbed dose (Gy) and dose equivalent (Sv) is based upon the biological effects of the weighting factor (denoted wr) and tissue/organ weighting factor (WT) have been established, which compare the relative biological effects of various types of radiation and the susceptibility of different organs.

Organ dose weighting factors


By definition, the weighting factor for the whole body is 1, such that 1 Gy of radiation delivered to the whole body (i.e. an evenly distributed 1 joule of energy deposited per kilogram of body) is equal to one sievert (for photons with a radiation weighting factor of 1, see below). Therefore, the weighting factors for each organ must sum to 1 as the unit gray is defined per kilogram and is therefore a local effect. As the table below shows, 1 gray (photons) delivered to the gonads is equivalent to 0.08 Sv to the whole body—in this case, the actual energy deposited to the gonads, being small, would also be small.
Organ or tissue WT
Gonads 0.25
Breasts 0.15
Red Bone Marrow 0.12
Lung 0.12
Thyroid 0.03
Bone surfaces 0.03
Remainder 0.30
Whole body 1.0

Radiation weighting factors


By definition, x-rays and gamma rays have a weighting factor of unity, such that 1 Gy = 1 Sv (for whole-body irradiation). Values of wr are as high as 20 for alpha particles and neutrons, i.e. for the same absorbed dose in Gy, alpha particles are 20 times as biologically potent as X or gamma rays.

Dose versus activity


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
Becquerel
The becquerel is the SI-derived unit of radioactivity. One Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. The Bq unit is therefore equivalent to an inverse second, s−1...

, Bq). Exposure to a radioactive source will give a dose which is dependent on the activity, time of exposure, energy of the radiation emitted, distance from the source and shielding. The equivalent dose is then dependent upon the weighting factors above. Dose is a measure of deposited dose, and therefore can never go down—removal of a radioactive source can only reduce the rate of increase of absorbed dose, never the total absorbed dose.

The worldwide average background dose for a human being is about 3.5 mSv per year http://www.arpansa.gov.au/pubs/baseline/bg_rad.pdf, mostly from cosmic radiation and natural isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

s in the earth. The largest single source of radiation exposure to the general public is naturally-occurring radon
Radon
Radon is a chemical element with symbol Rn and atomic number 86. It is a radioactive, colorless, odorless, tasteless noble gas, occurring naturally as the decay product of uranium or thorium. Its most stable isotope, 222Rn, has a half-life of 3.8 days...

 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.

Measuring dose


There are several ways of measuring doses from ionizing radiation. Workers who come in contact with radioactive substances or may be exposed to radiation routinely carry personal dosimeters. In the United States, these dosimeters usually contain materials that can be used in thermoluminescent dosimetry (TLD) or optically stimulated luminescence
Optically stimulated luminescence
In physics, optically stimulated luminescence is a method for measuring doses from ionizing radiation.The method makes use of electrons trapped between the valence and conduction bands in the crystalline structure of certain types of matter . The trapping sites are imperfections of the lattice -...

 (OSL). Outside the United States, the most widely-used type of personal dosimeter is the film badge dosimeter
Film badge dosimeter
The film badge dosimeter, or film badge, is a dosimeter used for monitoring cumulative exposure to ionizing radiation. The badge consists of two parts: photographic film, and a holder. The film is removed and developed to measure exposure....

, which uses photographic emulsion
Emulsion
An emulsion is a mixture of two or more liquids that are normally immiscible . Emulsions are part of a more general class of two-phase systems of matter called colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion is used when both the dispersed and the...

s that are sensitive to ionizing radiation. The equipment used in radiotherapy (linear particle accelerator
Linear particle accelerator
A linear particle accelerator is a type of particle accelerator that greatly increases the velocity of charged subatomic particles or ions by subjecting the charged particles to a series of oscillating electric potentials along a linear beamline; this method of particle acceleration was invented...

 in external beam therapy) is routinely calibrated using ionization chamber
Ionization chamber
The ionization chamber is the simplest of all gas-filled radiation detectors, and is used for the detection or measurement of ionizing radiation...

s or the new and more accurate diode technology.

Dose standards


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 NPL
National Physical Laboratory, UK
The National Physical Laboratory is the national measurement standards laboratory for the United Kingdom, based at Bushy Park in Teddington, London, England. It is the largest applied physics organisation in the UK.-Description:...

 provide calibration factors for ionization chambers and other measurement devices to convert from the instrument's readout to absorbed dose. The standards laboratories operate a Primary Standard, which is normally calibrated by absolute calorimetry
Calorimetry
Calorimetry is the science of measuring the heat of chemical reactions or physical changes. Calorimetry is performed with a calorimeter. The word calorimetry is derived from the Latin word calor, meaning heat...

, 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
National Physical Laboratory, UK
The National Physical Laboratory is the national measurement standards laboratory for the United Kingdom, based at Bushy Park in Teddington, London, England. It is the largest applied physics organisation in the UK.-Description:...

 in the UK 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 rises in graphite are 6 times more than the equivalent in water and measurements are more accurate. Significant problems exist in insulating the graphite from the laboratory 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.

Medical dosimetry


Medical dosimetry is the calculation of absorbed dose and optimization of dose delivery in radiation therapy
Radiation therapy
Radiation therapy , radiation oncology, or radiotherapy , sometimes abbreviated to XRT or DXT, is the medical use of ionizing radiation, generally as part of cancer treatment to control malignant cells.Radiation therapy is commonly applied to the cancerous tumor because of its ability to control...

. It is often performed by a professional medical dosimetrist with specialized training in the field. In order to plan the delivery of radiation therapy, the radiation produced by the sources is usually characterized with percentage depth dose curve
Percentage depth dose curve
In Radiotherapy, a percentage depth dose curve relates the absorbed dose deposited by a radiation beam into a medium as it varies with depth along the axis of the beam. The dose values are divided by the maximum dose, referred to as dmax, yielding a plot in terms of percentage of the maximum dose...

s and dose profile
Dose Profile
In External Beam Radiotherapy, transverse and longitudinal dose measurements are taken by a radiation detector in order to characterise the radiation beams from medical linear accelerators. Typically, an ionisation chamber and water phantom are used to create these radiation dose profiles...

s measured by medical physicists
Medical physics
Medical physics is the application of physics to medicine. It generally concerns physics as applied to medical imaging and radiotherapy, although a medical physicist may also work in many other areas of healthcare...

.

Dosimetry in NMR


Improperly said NMR dosimeter, the device shown in picture is a sampler for monitoring the static magnetic field in Nuclear Magnetic Resonance. This device is an electrical SMD device that store data for post processing analysis and evaluation of risk exposed operators.

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