The curie is a non-SI unit of radioactivity defined in 1910. According to a notice in Nature at the time, it was named in honour of Pierre Curie, but was considered at least by some to be in honour of Marie Curie as well, it was defined as "the quantity or mass of radium emanation in equilibrium with one gram of radium" but is defined as: 1 Ci = 3.7×1010 decays per second after more accurate measurements of the activity of 226Ra In 1975 the General Conference on Weights and Measures gave the becquerel, defined as one nuclear decay per second, official status as the SI unit of activity. Therefore: 1 Ci = 3.7×1010 Bq = 37 GBqand 1 Bq ≅ 2.703×10−11 Ci ≅ 27 pCiWhile its continued use is discouraged by National Institute of Standards and Technology and other bodies, the curie is still used throughout the government and medicine in the United States and in other countries. At the 1910 meeting which defined the curie, it was proposed to make it equivalent to 10 nanograms of radium, but Marie Curie, after accepting this, changed her mind and insisted on one gram of radium.
According to Bertram Boltwood, Marie Curie thought that'the use of the name "curie" for so infinitesimally small quantity of anything was altogether inappropriate.'The power in milliwatts emitted by one curie of radiation can be calculated by taking the number of MeV for the radiation times 5.93. A radiotherapy machine may have 1000 Ci of a radioisotope such as caesium-137 or cobalt-60; this quantity of radioactivity can produce serious health effects with only a few minutes of close-range, unshielded exposure. Ingesting a millicurie is fatal. For example, the LD-50 for ingested polonium-210 is about 53.5 nanograms. The typical human body contains 0.1 μCi of occurring potassium-40. A human body containing 16 kg of carbon would have about 24 nanograms or 0.1 μCi of carbon-14. Together, these would result in a total of 0.2 μCi or 7400 decays per second inside the person's body. Units of activity refer to a quantity of radioactive atoms; because the probability of decay is a fixed physical quantity, for a known number of atoms of a particular radionuclide, a predictable number will decay in a given time.
The number of decays that will occur in one second in one gram of atoms of a particular radionuclide is known as the specific activity of that radionuclide. The activity of a sample decreases with time because of decay; the rules of radioactive decay may be used to convert activity to an actual number of atoms. They state that 1 Ci of radioactive atoms would follow the expression: N × λ = 1 Ci = 3.7 × 1010 Bqand so, N = 3.7 × 1010 Bq / λ,where λ is the decay constant in s−1. We can express activity in moles: 1 Ci = 3.7 × 10 10 N A moles × t 1 / 2 in seconds ≈ 8.8639 × 10 − 14 moles × t 1 / 2 in seconds ≈ 5.3183 × 10 − 12 moles × t 1 / 2 in minutes ≈ 3.1910 × 10 − 10 moles × t 1 / 2 in hours ≈ 7.6584 × 10 − 9 moles × t 1 / 2 in days ≈ 2.7972 × 10 − 6 moles × t 1 / 2 in years where NA is Avogadro's number and t1/2 is the half life. The number of moles may be converted to grams by multiplying by the atomic mass. Here are some examples, ordered by half-life: The following table shows radiation quantities in SI and non-SI units: Geiger counter Ionizing radiat
Absorbed dose is a measure of the energy deposited in an irradiated medium by ionizing radiation per unit mass. Absorbed dose is used in the calculation of dose uptake in living tissue in both radiation protection, radiology, it is used to directly compare the effect of radiation on inanimate matter. The SI unit of measure is the gray, defined as one Joule of energy absorbed per kilogram of matter; the older, non-SI CGS unit rad, is sometimes used, predominantly in the USA. The quantity absorbed. However, absorbed dose is a physical quantity and used unmodified is not an adequate indicator of the health effects in humans. For stochastic radiation risk, defined as the probability of cancer induction and genetic effects occurring over a long time scale, consideration must be given to the type of radiation and the sensitivity of the irradiated tissues, which requires the use of modifying factors. To represent stochastic risk the quantities equivalent dose H T and effective dose E are used, appropriate dose factors and coefficients are used to calculate these from the absorbed dose.
Equivalent and effective dose quantities are expressed in units of the sievert or rem which implies that biological effects have been taken into account. The derivation of stochastic risk is in accordance with the recommendations of the International Committee on Radiation Protection and International Commission on Radiation Units and Measurements; the coherent system of radiological protection quantities developed by them is shown in the accompanying diagram. Conventionally, in radiation protection, unmodified absorbed dose is only used for indicating the immediate health effects due to high levels of acute dose; these are tissue effects, such as in acute radiation syndrome, which are known as deterministic effects. These are effects; the measurement of absorbed dose in tissue is of fundamental importance in radiobiology as it is the measure of the amount of energy the incident radiation is imparting to the target tissue. The absorbed dose is equal to the radiation exposure of the radiation beam multiplied by the ionization energy of the medium to be ionized.
For example, the ionization energy of dry air at 20 °C and 101.325 kPa of pressure is 33.97±0.06 J/C. Therefore, an exposure of 2.58×10−4 C/kg would deposit an absorbed dose of 8.76×10−3 J/kg in dry air at those conditions. When the absorbed dose is not uniform, or when it is only applied to a portion of a body or object, an absorbed dose representative of the entire item can be calculated by taking a mass-weighted average of the absorbed doses at each point. More D T ¯ = ∫ T D ρ d V ∫ T ρ d V Where D T ¯ is the mass-averaged absorbed dose of the entire item T T is the item of interest D is the absorbed dose as a function of location ρ is the density as a function of location V is volume Non-uniform absorbed dose is common for soft radiations such as low energy x-rays or beta radiation. Self-shielding means that the absorbed dose will be higher in the tissues facing the source than deeper in the body; the mass average can be important in evaluating the risks of radiotherapy treatments, since they are designed to target specific volumes in the body a tumour.
For example, if 10% of a patient's bone marrow mass is irradiated with 10 Gy of radiation locally the absorbed dose in bone marrow overall would be 1 Gy. Bone marrow makes up 4% of the body mass, so the whole-body absorbed dose would be 0.04 Gy. The first figure is indicative of the local effects on the tumour, while the second and third figure are better indicators of the overall health effects on the whole organism. Additional dosimetry calculations would have to be performed on these figures to arrive at a meaningful effective dose, needed to estimate the risk of cancer or other stochastic effects; when ionizing radiation is used to treat cancer, the doctor will prescribe the radiotherapy treatment in units of gray. Medical imaging doses may be described in units of coulomb per kilogram, but when radiopharmaceuticals are used, they will be administered in units of becquerel. Wilhelm Röntgen first discovered X-rays on November 8, 1895, their use spread quickly for medical diagnostics broken bones and embedded foreign objects where they were a revolutionary improvement over previous techniques.
Due to the wide use of X-rays and the growing realisation of the dangers of ionizing radiation, measurement standards became necessary fo
Relative biological effectiveness
In radiobiology, the relative biological effectiveness is the ratio of biological effectiveness of one type of ionizing radiation relative to another, given the same amount of absorbed energy. The RBE is an empirical value that varies depending on the particles, energies involved, which biological effects are deemed relevant. In ionizing radiation dosimetry RBE is now represented in the recommendations of the International Commission on Radiological Protection by the radiation weighting factor, for each type of radiation; these weighting factors convert absorbed dose into formal biological equivalent dose for radiation exposure. The higher the RBE or weighting factor for a type of radiation, the more damaging it is, this is incorporated into the calculation to convert from gray to sievert units; the relative biological effectiveness for radiation of type R on a tissue of type X is defined as the ratio R B E = D X D R where DX is a reference absorbed dose of radiation of a standard type X, DR is the absorbed dose of radiation of type R that causes the same amount of biological damage.
Both doses are quantified by the amount of energy absorbed in the cells. Different types of radiation have different biological effectiveness because they transfer their energy to the tissue in different ways. Photons and beta particles have a low linear energy transfer coefficient, meaning that they ionize atoms in the tissue that are spaced by several hundred nanometers apart, along their path. In contrast, the much more massive alpha particles and neutrons leave a denser trail of ionized atoms in their wake, spaced about one tenth of a nanometer apart; the concept of RBE is relevant in medicine, such as in radiology and radiotherapy, to the evaluation of risks and consequences of radioactive contamination in various contexts, such as nuclear power plant operation, nuclear fuel disposal and reprocessing, nuclear weapons, uranium mining, ionizing radiation safety. Radiation weighting factors that go from physical energy to biological effect must not be confused with tissue weighting factors.
The tissue weighting factors are used to convert an equivalent dose to a given tissue in the body, to an effective radiation dose, a number that provides an estimation of total danger to the whole organism, as a result of the radiation dose to part of the body. To bypass the complexity of tissue dependence, the ICRP defined standard radiation weighting factors, independently of tissue type, to be used for risk and exposure assessment in radiology and the nuclear industry; these values are conservatively chosen to be greater than the bulk of experimental values observed for the most sensitive cell types, with respect to external sources. Values for internal sources for heavy ions, such as a recoil nucleus, have not been developed; the ICRP 2007 standard values for relative effectiveness are given below. Radiation weighting factors WR used to represent relative biological effectiveness according to ICRP report 103 Thus, for example, a given amount of energy absorbed in the form of 15 keV neutrons should be assumed to produce 10 times the damage caused by an equal amount of energy absorbed as X-rays or gamma rays.
The evaluation of relative biological effectiveness is done on various types of living cells grown in culture medium, including prokaryotic cells such as bacteria, simple eukaryotic cells such as single celled plants, advanced eukaryotic cells derived from organisms such as rats. The doses are adjusted to the LD-50 point; the types R of ionizing radiation most considered in RBE evaluation are X-rays and gamma radiation, alpha radiations, beta radiation, neutron radiation, heavy nuclei, including the fragments of nuclear fission. For some kinds of radiation, the RBE is dependent on the energy of the individual particles. Early on it was found that X-rays, gamma radiation, beta radiation were equivalent for all cell types. Therefore, the standard radiation type X is an X-ray beam with 250 keV photons; as a result, the relative biological effectiveness of beta and photon radiation is 1. For other radiation types, the RBE is not a well-defined physical quantity, since it varies somewhat with the type of tissue and with the precise place of absorption within the cell.
Thus, for example, the RBE for alpha radiation is 2–3 when measured on bacteria, 4–6 for simple eukaryotic cells, 6–8 for higher eukaryotic cells. According to one source it may be much higher on ovocytes; the RBE of neutrons is 4–6 for bacteria, 8–12 for simple eukaryotic cells, 12–16 for higher eukaryotic cells. In the early experiments, the sources of radiation were all external to the cells that were irradiated. However, since alpha particles cannot traverse the outermost dead layer of human skin, they can do significant damage only if they come from the decay of atoms inside the body. Since the range of an alpha particle is about the diameter of a single eukaryotic cell, the precise lo
International System of Units
The International System of Units is the modern form of the metric system, is the most used system of measurement. It comprises a coherent system of units of measurement built on seven base units, which are the ampere, second, kilogram, mole, a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units; the system specifies names for 22 derived units, such as lumen and watt, for other common physical quantities. The base units are derived from invariant constants of nature, such as the speed of light in vacuum and the triple point of water, which can be observed and measured with great accuracy, one physical artefact; the artefact is the international prototype kilogram, certified in 1889, consisting of a cylinder of platinum-iridium, which nominally has the same mass as one litre of water at the freezing point. Its stability has been a matter of significant concern, culminating in a revision of the definition of the base units in terms of constants of nature, scheduled to be put into effect on 20 May 2019.
Derived units may be defined in terms of other derived units. They are adopted to facilitate measurement of diverse quantities; the SI is intended to be an evolving system. The most recent derived unit, the katal, was defined in 1999; the reliability of the SI depends not only on the precise measurement of standards for the base units in terms of various physical constants of nature, but on precise definition of those constants. The set of underlying constants is modified as more stable constants are found, or may be more measured. For example, in 1983 the metre was redefined as the distance that light propagates in vacuum in a given fraction of a second, thus making the value of the speed of light in terms of the defined units exact; the motivation for the development of the SI was the diversity of units that had sprung up within the centimetre–gram–second systems and the lack of coordination between the various disciplines that used them. The General Conference on Weights and Measures, established by the Metre Convention of 1875, brought together many international organisations to establish the definitions and standards of a new system and standardise the rules for writing and presenting measurements.
The system was published in 1960 as a result of an initiative that began in 1948. It is based on the metre–kilogram–second system of units rather than any variant of the CGS. Since the SI has been adopted by all countries except the United States and Myanmar; the International System of Units consists of a set of base units, derived units, a set of decimal-based multipliers that are used as prefixes. The units, excluding prefixed units, form a coherent system of units, based on a system of quantities in such a way that the equations between the numerical values expressed in coherent units have the same form, including numerical factors, as the corresponding equations between the quantities. For example, 1 N = 1 kg × 1 m/s2 says that one newton is the force required to accelerate a mass of one kilogram at one metre per second squared, as related through the principle of coherence to the equation relating the corresponding quantities: F = m × a. Derived units apply to derived quantities, which may by definition be expressed in terms of base quantities, thus are not independent.
Other useful derived quantities can be specified in terms of the SI base and derived units that have no named units in the SI system, such as acceleration, defined in SI units as m/s2. The SI base units are the building blocks of the system and all the other units are derived from them; when Maxwell first introduced the concept of a coherent system, he identified three quantities that could be used as base units: mass and time. Giorgi identified the need for an electrical base unit, for which the unit of electric current was chosen for SI. Another three base units were added later; the early metric systems defined a unit of weight as a base unit, while the SI defines an analogous unit of mass. In everyday use, these are interchangeable, but in scientific contexts the difference matters. Mass the inertial mass, represents a quantity of matter, it relates the acceleration of a body to the applied force via Newton's law, F = m × a: force equals mass times acceleration. A force of 1 N applied to a mass of 1 kg will accelerate it at 1 m/s2.
This is true whether the object is floating in space or in a gravity field e.g. at the Earth's surface. Weight is the force exerted on a body by a gravitational field, hence its weight depends on the strength of the gravitational field. Weight of a 1 kg mass at the Earth's surface is m × g. Since the acceleration due to gravity is local and varies by location and altitude on the Earth, weight is unsuitable for precision
The roentgen or röntgen 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, was an replicated method of measuring air ionization directly by using an ion chamber, it is named after the German physicist Wilhelm Röntgen. Although easy to measure, the roentgen 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, hence tissue damage, was linked to energy absorbed, not just radiation exposure. New radiometric units for radiation protection were defined which took this into account. In 1953 the International Commission on Radiation Units and Measurements recommended the rad, equal to 100 erg/g, as the unit of measure of the new radiation quantity absorbed dose.
The rad was expressed in coherent cgs units. In 1975 the unit gray was named as the SI unit of absorbed dose; the gray was equal to the cgs unit. Additonally, a new quantity Kerma was defined for air ionisation, is the modern metrological, but not radiation protection, successor to the roengten, 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, an indication of acute tissue effects occurring at high dose rates, from low levels of absorbed dose the equivalent dose, representing the stochastic health risk, can be calculated; the roengten has been redefined over the years. It was last defined by the US National Institute of Standards and Technology 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 of absorbed 0.0096 Gy in soft tissue. One roentgen of X-rays may deposit anywhere from 0.01 to 0.04 Gy 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, ambiguous in the latest NIST definition; the roentgen has its roots in the Villard unit defined in 1908 by the American Roentgen Ray Society as "the quantity of radiation which liberates by ionisation one esu of electricity per cm3 of air under normal conditions of temperature and pressure." Using 1 esu ≈ 3.33564×10−10 C and the air density of ~1.293 kg/m³ at 0 °C and 101 kPa, this converts to 2.58 × 10−4 C/kg, the modern value given by NIST. 1 esu/cm3 × 3.33564 × 10−10 C/esu × 1,000,000 cm3/m3 ÷ 1.293 kg/m3 = 2.58 × 10−4 C/kg This definition was used under different names for the next 20 years. In the meantime, the French Roentgen was given a different definition which amounted to 0.444 German R. In 1928, the International Congress of Radiology defined the roentgen as "the quantity of X-radiation which, when the secondary electrons are utilised and the wall effect of the chamber is avoided, produce in 1 cc of atmospheric air at 0 °C and 76 cm of mercury pressure such a degree of conductivity that 1 esu of charge is measured at saturation current."
The stated 1 cc of air would have a mass of 1.293 mg at the conditions given, so in 1937 the ICR rewrote this definition in terms of this mass of air instead of volume and pressure. The 1937 definition was extended to gamma rays, but capped at 3 MeV in 1950; the USSR all-union committee of standards had meanwhile adopted a different definition of the roentgen in 1934. GOST standard 7623 defined it as "the physical dose of X-rays which produces charges each of one electrostatic unit in magnitude per cm3 of irradiated volume in air at 0 °C and normal atmospheric pressure when ionization is complete." The distinction of physical dose from dose caused confusion, some of which may have led Cantrill and Parker report that the roentgen had become shorthand for 83 ergs per gram of tissue. They named this derivative quantity the roentgen equivalent physical to distinguish it from the ICR roentgen; the introduction of the roentgen measurement unit, which relied upon measuring the ionisation of air, replaced earlier less accurate practices that relied on timed exposure, film exposure, or fluorescence.
This led the way to setting exposure limits,and the National Council on Radiation Protection and Measurements of the United States established the first formal dose limit in 1931 as 0.1 roentgen per day. The International X-ray and Radium Protection Committee, now known as the International Commission on Radiological Protection soon followed with a limit of 0.2 roentgen per day in 1934. In 1950, the ICRP reduced their recommended limit to 0.3 roentgen per week for whole-body exposure. The International Commission on Radiation Units and Measurements took over the definition of the roentgen in 1950, defining it as "the quantity of X or γ-radiation such that the associated corpuscular emission per 0.001293 gram of air produces, in air, ions carrying 1 electrostatic unit of quantity of electricity of either sign." The 3 MeV cap was no longer part of the definition, but the degraded usefulness of this unit at high beam energies was mentioned in the accompanying text. In the meantime, the new concept of roentgen equivalent man (rem
The kilogram or kilogramme is the base unit of mass in the International System of Units. Until 20 May 2019, it remains defined by a platinum alloy cylinder, the International Prototype Kilogram, manufactured in 1889, stored in Saint-Cloud, a suburb of Paris. After 20 May, it will be defined in terms of fundamental physical constants; the kilogram was defined as the mass of a litre of water. That was an inconvenient quantity to replicate, so in 1799 a platinum artefact was fashioned to define the kilogram; that artefact, the IPK, have been the standard of the unit of mass for the metric system since. In spite of best efforts to maintain it, the IPK has diverged from its replicas by 50 micrograms since their manufacture late in the 19th century; this led to efforts to develop measurement technology precise enough to allow replacing the kilogram artifact with a definition based directly on physical phenomena, now scheduled to take place in 2019. The new definition is based on invariant constants of nature, in particular the Planck constant, which will change to being defined rather than measured, thereby fixing the value of the kilogram in terms of the second and the metre, eliminating the need for the IPK.
The new definition was approved by the General Conference on Weights and Measures on 16 November 2018. The Planck constant relates a light particle's energy, hence mass, to its frequency; the new definition only became possible when instruments were devised to measure the Planck constant with sufficient accuracy based on the IPK definition of the kilogram. The gram, 1/1000 of a kilogram, was provisionally defined in 1795 as the mass of one cubic centimetre of water at the melting point of ice; the final kilogram, manufactured as a prototype in 1799 and from which the International Prototype Kilogram was derived in 1875, had a mass equal to the mass of 1 dm3 of water under atmospheric pressure and at the temperature of its maximum density, 4 °C. The kilogram is the only named SI unit with an SI prefix as part of its name; until the 2019 redefinition of SI base units, it was the last SI unit, still directly defined by an artefact rather than a fundamental physical property that could be independently reproduced in different laboratories.
Three other base units and 17 derived units in the SI system are defined in relation to the kilogram, thus its stability is important. The definitions of only eight other named SI units do not depend on the kilogram: those of temperature and frequency, angle; the IPK is used or handled. Copies of the IPK kept by national metrology laboratories around the world were compared with the IPK in 1889, 1948, 1989 to provide traceability of measurements of mass anywhere in the world back to the IPK; the International Prototype Kilogram was commissioned by the General Conference on Weights and Measures under the authority of the Metre Convention, in the custody of the International Bureau of Weights and Measures who hold it on behalf of the CGPM. After the International Prototype Kilogram had been found to vary in mass over time relative to its reproductions, the International Committee for Weights and Measures recommended in 2005 that the kilogram be redefined in terms of a fundamental constant of nature.
At its 2011 meeting, the CGPM agreed in principle that the kilogram should be redefined in terms of the Planck constant, h. The decision was deferred until 2014. CIPM has proposed revised definitions of the SI base units, for consideration at the 26th CGPM; the formal vote, which took place on 16 November 2018, approved the change, with the new definitions coming into force on 20 May 2019. The accepted redefinition defines the Planck constant as 6.62607015×10−34 kg⋅m2⋅s−1, thereby defining the kilogram in terms of the second and the metre. Since the second and metre are defined in terms of physical constants, the kilogram is defined in terms of physical constants only; the avoirdupois pound, used in both the imperial and US customary systems, is now defined in terms of the kilogram. Other traditional units of weight and mass around the world are now defined in terms of the kilogram, making the kilogram the primary standard for all units of mass on Earth; the word kilogramme or kilogram is derived from the French kilogramme, which itself was a learned coinage, prefixing the Greek stem of χίλιοι khilioi "a thousand" to gramma, a Late Latin term for "a small weight", itself from Greek γράμμα.
The word kilogramme was written into French law in 1795, in the Decree of 18 Germinal, which revised the older system of units introduced by the French National Convention in 1793, where the gravet had been defined as weight of a cubic centimetre of water, equal to 1/1000 of a grave. In the decree of 1795, the term gramme thus replaced gravet, kilogramme replaced grave; the French spelling was adopted in Great Britain when the word was used for the first time in English in 1795, with the spelling kilogram being adopted in the United States. In the United Kingdom both spellings are used, with "kilogram" having become by far the more common. UK law regulating the units to be used when trading by weight or measure does not prevent the use of either spelling. In the 19th century the French word kilo, a shortening of kilogramme, was imported into the English language where it has been used to mean both kilogram and kilometre. While kilo is acceptable in many generalist texts
The becquerel is the SI derived unit of radioactivity. One becquerel is defined as the activity of a quantity of radioactive material in which one nucleus decays per second; the becquerel is therefore equivalent to an inverse second, s−1. The becquerel is named after Henri Becquerel, who shared a Nobel Prize in Physics with Pierre and Marie Curie in 1903 for their work in discovering radioactivity; as with every International System of Units unit named for a person, the first letter of its symbol is uppercase. However, when an SI unit is spelled out in English, it should always begin with a lowercase letter —except in a situation where any word in that position would be capitalized, such as at the beginning of a sentence or in material using title case. 1 Bq = 1 s−1A special name was introduced for the reciprocal second to represent radioactivity to avoid dangerous mistakes with prefixes. For example, 1 µs−1 could be taken to mean 106 disintegrations per second: 1·−1 = 106 s−1. Other names considered were hertz, a special name in use for the reciprocal second, fourier.
The hertz is now only used for periodic phenomena. Whereas 1 Hz is 1 cycle per second, 1 Bq is 1 aperiodic radioactivity event per second; the gray and the becquerel were introduced in 1975. Between 1953 and 1975, absorbed dose was measured in rads. Decay activity was measured in curies before 1946 and in rutherfords between 1946 and 1975. Like any SI unit, Bq can be prefixed. For practical applications, 1 Bq is a small unit. For example, the 0.0169 g of potassium-40 present in a typical human body produces 4,400 disintegrations per second or 4.4 kBq of activity. The global inventory of carbon-14 is estimated to be 8.5×1018 Bq. The nuclear explosion in Hiroshima is estimated to have produced 8×1024 Bq; the becquerel succeeded the curie, an older, non-SI unit of radioactivity based on the activity of 1 gram of radium-226. The curie is defined as 3.7 · 1010 s 37 GBq. Conversion factors: 1 Ci = 3.7×1010 Bq = 37 GBq 1 μCi = 37,000 Bq = 37 kBq 1 Bq = 2.7×10−11 Ci = 2.7×10−5 μCi 1 MBq = 0.027 mCi For a given mass m of an isotope with atomic mass m a and a half-life of t 1 / 2, the radioactivity can be calculated using: A B q = m m a N A ln t 1 / 2 With N A = 6.02214179×1023 mol−1, the Avogadro constant.
Since m / m a is the number of moles, the amount of radioactivity A can be calculated by: A B q = n N A ln t 1 / 2 For instance, on average each gram of potassium contains 0.000117 gram of 40K that has a t 1 / 2 of 1.277×109 years = 4.030×1016 s, has an atomic mass of 39.964 g/mol, so the amount of radioactivity associated with a gram of potassium is 30 Bq. The following table shows radiation quantities in non-SI units. Background radiation Banana equivalent dose Counts per minute Ionizing radiation Orders of magnitude Radiation poisoning Relative Biological Effectiveness Rem Rutherford Sievert Derived units on the International Bureau of Weights and Measures web site