The radioactivity , term invented towards 1898 by Marie Curie, is a natural physical phenomenon during which atomic nuclei unstable disintegrate while releasing from energy in the form of various radiations, to transmute into more stable atomic nuclei. The radiations thus emitted are called, according to the case, of the '' rays '' α, of the '' rays '' β or of the '' rays '' γ.

The most frequent radioelements in the terrestrial rocks are the Isotope 238 of the Uranium (238U), isotope 232 of the Thorium (232Th), and especially isotope 40 of the Potassium (40K). In addition to these natural radioactive isotopes still relatively abundant, there exists in the nature of the radioactive isotopes in abundances much lower. They are in particular the unstable elements produced at the time of the continuation of disintegrations of the isotopes mentioned, for example of various isotopes of the Radium and the Radon.

One of the natural radioelements most used by the man is isotope 235 of the uranium (235U) which is in nature in weak concentration (<1%) associated with the isotope 238U, but which one modifies the concentration by techniques of Uranium enrichment so that it can be used for the production of nuclear energy civil and military.

Another natural radioisotope is the radiocarbon, i.e. the isotope 14 of carbon (14C). This last is constantly produced in the upper atmosphere by cosmic rays interacting with nitrogen, and is destroyed by radioactive decays about at the same rate that it is produced, so that it occurs a dynamic balance which makes that the concentration of the 14C remains more or less constant during time in the air and the living organisms which breathe this air. Once a died organization, the concentration in 14C decreases in its fabrics, and makes it possible to date the moment from death. This dating with radiocarbon is research tools very snuffed in Archéologie and makes it possible to date with a good precision from the organic objects whose age does not exceed fifty to a hundred and thousand years.

One speaks about radioactivity alpha (α) to indicate the emission of a helium core or helium nucleus :

$\left\{\right\} ^ \left\{has\right\} _ \left\{Z\right\} \ hbox \left\{X\right\} \; \ to \; ^ \left\{A-4\right\} _ \left\{Z-2\right\} \ hbox \left\{Y\right\} \; + \left\{\right\} ^ \left\{4\right\} _ \left\{2\right\} \ hbox \left\{He\right\}$

These helium nuclei, still called particles alpha , have a load 2 E, and a Masse of 4,001 505 8 units of Atomic mass.

The radioactivity beta less -) affects nuclides X presenting an excess of neutrons. It appears by the transformation in the core of a Neutron into Proton, the phenomenon being accompanied by the emission of a electron (or particle beta less ) and of an electronic antineutrino ν:

$\left\{\right\} ^ \left\{has\right\} _ \left\{Z\right\} \ hbox \left\{X\right\} \; \ to \; ^ \left\{has\right\} _ \left\{Z+1\right\} \ hbox \left\{Y\right\} + e^- + \ naked bar \left\{\\right\} _e$

The radioactivity beta more +) relates to only nuclides which present an excess of protons. It appears by the transformation in the core of a proton into neutron, the phenomenon being accompanied by the emission of a Positon (or positron , or particle beta more ) and of a electronic Neutrino ν:

$\left\{\right\} ^ \left\{has\right\} _ \left\{Z\right\} \ hbox \left\{X\right\} \; \ to \; ^ \left\{has\right\} _ \left\{Z-1\right\} \ hbox \left\{Y\right\} + e^+ + \left\{\ naked\right\} _e$

Disintegrations α, β- and β+ are often accompanied by the emission of photons by high energy or Gamma rays , of which the wavelengths are generally even shorter than those of the x-rays, being about 10-9 m or lower. This emission gamma (γ) results from the emission of photons at the time of transitions nuclear: rearrangement of the internal loads of the core lately formed, or of the deep layer of the disturbed electronic procession, starting from excited energy levels with concerned energies about the MeV.

The α radiations, β and γ produced by the radioactivity are Ionizing rays which interact with the matter by causing a Ionization. The Irradiation of an organization involves effects which can be more or less harmful for health, according to the amounts of radiation received and the type of Rayonnement concerned.

## History

While making pass the rays α in a tube to discharge and by studying the spectral lines thus produced, one could conclude that the α radiation consists of helium nuclei, in other words from helium cores (4He). Other experiments made it possible to establish that the rays β are composed of electrons like the particles in a Cathode tube, and that the γ are, just like x-rays, of the very energy photons. Thereafter, it was discovered that many other chemical elements have radioactive isotopes. Thus, by treating tons of Pitchblende, a uranium-bearing rock, Marie Curie succeeds in isolating a few milligrams from radium whose chemical properties are completely similar to those of the Baryum (both are alkaline-earth metals), but that one manages to distinguish because of the radioactivity of radium.

An unspecified radionuclide has as many chances to disintegrate at a given time than another radionuclide of the same species, and disintegration does not depend on the physicochemical conditions under which the nuclide is. In other words, the law of radioactive decay is a statistical law . That is to say NR (T) the number of radionuclides of a species given present in a sample at one moment T unspecified. As the probability of disintegration of any of these radionuclides does not depend on the presence on the other radionuclides nor on the surrounding medium, the full number of disintegrations DNN during an amount of time dt at the moment T is proportional to the number of of the same radionuclides species NR present and to the dt duration of this interval: it is a law of exponential Decay. One has indeed:

$d NR = - \ lambda NR D t$

where the constant of proportionality λ, called constant radioactive of the radionuclide considered, has the dimension of the reverse of a time. The minus sign is put (-) because NR decreases during time, so that the constant λ is positive.

By integrating the preceding differential equation, one finds the number NR (T) of radionuclides present in the body at one moment T unspecified, knowing that at a moment given T = 0 there were N0 of it:

$NR \left(T\right) = N_0 e^ \left\{- \ lambda T\right\}$

One calls “radioactive half-life” (or Demi-vie ) $T_ \left\{\ frac \left\{1\right\} \left\{2\right\}\right\}$ the duration at the end of which the number of radionuclides present in the sample is tiny room of half. One thus has:

$\ frac \left\{N_0\right\} \left\{2\right\} = N_0 e^ \left\{- \ lambda T_ \left\{\ frac \left\{1\right\} \left\{2\right\}\right\}\right\} \ Rightarrow e^ \left\{\ lambda T_ \left\{\ frac \left\{1\right\} \left\{2\right\}\right\}\right\} = 2 \ Rightarrow T_ \left\{\ frac \left\{1\right\} \left\{2\right\}\right\} = \left(\ ln \left\{2\right\}\right) \ lambda^ \left\{- 1\right\} \ Rightarrow T_ \left\{\ frac \left\{1\right\} \left\{2\right\}\right\} = 0.693 \ lambda^ \left\{- 1\right\}$

## Pénétratives properties of the α radiations, β and γ

It was seen above that the radiations produced by radioactive substances can take three different forms:

• Radiation α: an unstable atomic nucleus emits a heavy particle positively charged (a core of Hélium -4) that a sheet of Papier can stop;

• Radiation β: an unstable atomic nucleus emits a light particle (a electron or a Positon) that a sheet of Aluminum can stop;
• Radiation γ: an atomic nucleus which does not suffer from a baryon imbalance , but which is in an unstable state of energy, emits a Photon very energy, therefore very penetrating, to reach a stable state of energy; one needs several centimetres of Plomb to stop it. There is hardly difference between hard x-rays and the γ radiation - only their source differentiates them. In general, the emission of rays γ follows a disintegration α or β, because it corresponds to a rearrangement of the Nucléon S, and in particular to a reorganization of the electric charge inside the new core. One thus meets frequently a radioactive core emitting several types of radiation simultaneously: for example, isotope 239 of the Plutonium (239Pu) is a transmitter α-γ, isotope 59 of the Fer (59Fe) is a transmitter β-γ.

## Interaction of the radiations with the matter

The ionizing rays cause all within the matter of ionizations and on the excitation S. the way in which these ionizations occur depends on the type of radiation considered.

The Gamma ray is a beam of Photon S without load nor Masse. While crossing the matter it causes three type of interactions: the photoelectric Effect, the Creation of pairs and the Effect Compton. These mechanisms will produce, in fine , of the excitations and ionizations in crossed material. The gamma ray has a strong capacity of penetration in the matter (several tens of meters of Béton).

The Rayonnement alpha is a beam of particles heavy and charged, generally of high energy. While crossing the matter, this beam of particles strikes the electrons of the periphery of the atoms of crossed material what excites them or ionizes them. This mechanism occurs on a very short distance: the capacity of penetration of the alpha radiations is weak (a simple sheet of Papier stops them completely) and consequently the deposit of energy per crossed unit of length will be high. This energy dissipated in the crossed matter will result in excitations and ionizations.

The Radiation beta, made up of electron S or positron S is a beam of particles light and charged. It interacts with the matter while causing, him also, of the excitation S and of the Ionization S. the Parcours of the electrons and the positrons in the matter is more important than that of the particles alpha (about a few meters maximum in the air). The loss of energy of the beta radiation per crossed unit of length will be, anything else being equal, less than that of the alpha radiation. It will be thus the same number of excitation and ionization produced per unit of length.

The nature of the physical laws making it possible to calculate the courses or the attenuation of the radiations in the matter differ according to the radiations considered. The gamma rays never are completely stopped by the matter. This is why the emerging flow of photons of a screen will be weak, even quasi undetectable, but never no one. The physical laws which translate the Parcours radiations alpha and beta show that beyond certain distance, it is impossible that particles can be found. The incidental radiation can thus be completely blocked by a material which plays the part of screen.

### Objective sizes

The radioactive half-life, for a radioactive isotope, is the duration (generally expressed in years) during which its radioactive activity decrease of half for a mode of disintegration given. It is more often called Demi-vie. The Activité of a radioactive body at a given moment is the number of disintegrations a second at this moment, in other words the intensity of its radioactivity. The activity of a given number of atoms of a radioactive isotope is proportional to this number and inversely proportional to the radioactive half-life of the isotope.

The Activité of a radioactive sample (radioactive source) is measured in Becquerel S (Bq), unit which corresponds to the number of disintegrations into 1 second, in homage to Henri Becquerel. Sometimes one uses (in biology for example) the number of disintegrations per minute. The curie (Ci) was formerly: it is defined as the activity of one gram radium, either 37 × 109 disintegrations a second, or 37 Bq = 1 N Ci.

One can also use the Coulomb per kilogram (C/kg) which measures the exposure to the radiations X and gamma (the load of Ion S released in the mass of air).
L' old unit equivalent was the Roentgen which corresponds to the number of ionizations per kilogram of air.

For the ionizing rays, the amount absorptive by the target is defined like the energy received per unit of mass, in Joule S per kilogram, i.e. in Gray S (Gy) in the system IF. The old unit was the rad. 1 Gy = 100 rad.

During a durable exposure, one defines the dose rate, i.e. the energy absorptive per kilogram and unit of time. Indeed relatively slow biological phenomena can be disturbed by the radiations, thus the original cells of the system Hématopoïétique in court of divisions are destroyed so irradiated, but are not affected when quiescent. The unit of the dose rate is Gray a second (Gy/s).

These sizes, activity, amount and rate dose are measurable sizes, which can be measured using apparatuses of physics (meters, calorimeters, clock).

Conversion of the various objective units:

1 Ci = 3,7 1010Bq
1 Bq = 0,027 nCi
1 rad = 0,01 Gy = 10 mGy

### Subjective sizes

The first subjective size is the absorptive amount by the organization. The unit of amount of radiation absorptive of the international System (SI) is the Gray, which replaces the rad.

Among the subjective sizes of the radioactivity, some evaluate the health risk. All the radiations not having same harmfulness, one defines a equivalent amount in which each radiation must be balanced to take account of the differences.

When the rad was used as unit of absorptive amount, the unit of equivalent amount was the Rem, acronym of “rad are equivalent man”. Currently, the rem is replaced by the Sievert (Sv), which is “equivalent Gray Homme” and is a unit of the system IF.

The equivalent amount is not measurable, but it is evaluated according to the received amount, of the sensitivity of irradiated fabric and the nature of the radiation.

$E = D \ times S \ times Q$
• E is the equivalent amount,
• D is the absorptive physical amount,
• S depends on the sensitivity of fabric, weak for the muscles or the skin, but important for the gonades, the nervous system, the cells of osseous marrow or the intestine,
• Q is a parameter which depends on the nature of the radiation. It is equal to 1 for the gamma rays and beta, to 5 for the rays alpha, and to 20 during an irradiation by the neutrons.

The equivalent dose rate is expressed in sieverts a second. This size is relevant to evaluate the professional exposures of the workers of the nuclear power, the astronauts, or the people living in an environment at the risk. As example, the people living in Western Europe are subjected to an equivalent dose rate from approximately 3 mSv/an.

The concept of absorptive amount is blamed by studies showing that it is necessary to distinguish the internal irraditation from the external irradiation. In this last case, the γ radiation is the principal danger, whereas the α radiations and β are also risk factors in the case of internal-source irradiation.

Conversion of the various subjective units:

1 rem = 0,01 Sv = 10 mSv
1 Sv = 100 rem

The radioactivity originates in mainly the existing Radioisotope S in nature and products during the explosions of the Supernova S. One finds traces of these radioactive elements and their descendants in our Environnement: a rock of Granite contains traces of Uranium which, while disintegrating, emit Radon.

The natural radioactivity of the atoms of our body also results in approximately 8.000 disintegrations a second (8 000 Bq), which make 252 billion per annum on some 1027 atoms our body. The risk of deterioration of an atom is thus of 0,25 × 10-17. The natural exposure to the radioactivity represents 2,5 mSv on the total of 3,5. This amount can vary from 1 to 40 mSv, according to the geological environment and the materials of dwelling.

### Cosmic rays

The Ground is permanently subjected to a flow of primary particles of high energy coming from the space and of the Sun, the cosmic rays. The terrestrial magnetic field deviates the major part of them. The atmosphere absorbing only part of these particles of high energy, a fraction of this one reached the ground, even, crosses the rock layers.

This extraterrestrial radiation involves by a phenomenon of Spallation the production of secondary radioactive particles (voires tertiary) starting from the heavier cores present in the upper atmosphere. This phenomenon is in the beginning, amongst other things, of the production of Carbon-14 on our planet.

Ground is permanently subjected to particles of high energy coming from the space, the cosmic rays. Some of these particles cross the atmosphere. The total effect on the ground is tiny for the moment, but is succeptible to increase with the evolutions of the " nano" technologies; indeed more one electronics component is small, more it approaches dimensions of a particle, and more it is sensitive to the particles. In space, the storms solar and other radiative effects (belts of radiations, ions heavy,…) influence considerably the men and the material, inter alia because of this same reason (for the material). -->

### Radioactivity produced by the Man

However, the major source of potentially dangerous ionizing rays is the result of the human activity: fuels and waste of nuclear industry, repercussions of nuclear tests, medical irradiation (x-rays), research in physics of the particles, environment irradiated following a nuclear accident or a conflict, etc

One speaks sometimes, by abuse language, “natural radioactivity” to indicate the radioactive background noise, due in particular to the cosmic basic radiation and radioactive grounds like the granite.

A contrario, one speaks sometimes about “artificial radioactivity” to indicate the radioactivity due to sources produced by the human activities: synthetic transuranic elements, artificially high radioactive material concentrations, artificial production of gamma ray (in a particle accelerator for example) or x-rays (radiographies).

Physically, it is exactly about the same phenomenon.

### Natural activity

The tectonic radiation due to the rocks (Uranium, Thorium and descendants) is of 0,40 mSv, but it can be ten times more important in granitic areas like the Black Forest in Germany or the Brittany and the Massif Central in France, in particular because of a radioactive gas, the Radon.

The share due to the cosmic radiation represents approximately 0,40 mSv with the sea level, but double to 1.500 m of altitude.

### Health hazard

A radioactive substance must be located by the symbol ☢ (Unicode 2622, UTF-8 E2 98 A2).

The consequences of the radioactivity on health are complex. The health risk depends not only on the intensity on the radiation and the exposure time, but also on the type of fabric concerned - the reproductive bodies are 20 times more significant than the skin. The effects are different according to the vector from the radioactivity:

• exposure to Ionizing rays by a radioactive source remotely
• radioactive Contamination if one swallows or one breathes a radioactive product

The international standards, based on the epidemiologic consequences of the explosion of the bombs of Hiroshima and Nagasaki, leave the principle which the health risk is proportional to the received amount and which any amount of radiation comprises a carcinogenic and genetic risk (CIPR 1990).

The regulation for protection against ionizing radiations is based on three fundamental recommendations:

1. Justification : one should adopt no practice leading to an irradiation unless it does not produce a benefit sufficient for the exposed individuals or the company, compensating for the damage related to this irradiation.
2. Optimization : the irradiation must be on the level low which one can reasonably reach.
3. Limitation of the amount and the risk individual : no individual must receive amounts of irradiation higher than the authorized limits maximum.

Recent studies of IRSN showed that the effects of the chronic radioactive contamination, even with low dose, are not negligible, and could cause various pathologies reaching certain physiological functions (central nervous system, breathing, digestion, reproduction).

Natural environment emits a lower radiation 0,00012 mSv/h. the exposure can become dangerous starting from 0,002 mSv/h, according to the time to which one is subjected there.

The amounts currently tolerated in the various controlled sectors of the French nuclear plants are

• restricted parking zone: from 0,0025 to 0,0075 mSv/h
• green zone: from 0,0075 to 0,02 mSv/h
• yellow zone: from 0,02 to 2 mSv/h
• orange zone: from 2 to 100 mSv/h
• red zone: > 100 mSv/h

See the detailed article radiative Amount '"

### Equivalent amount

The equivalent amount is the measurement of accumulated dose of continuous exposure to ionizing radiations during one year, with factor loadings. Until 1992, the equivalent amounts were not measured in the same way in Europe and with the the United States; today these amounts are standardized.

The accumulated dose of an artificial radioactive source becomes dangerous from 500 mSv (or 50 rem), amount to which one notes the primary symptoms of blood deterioration. In 1992, the maximum equivalent amount (dem) for a person working under ionizing rays was fixed at 15 mSv over the last 12 months in Europe (CERN and England) and at 50 mSv over the last 12 months in the United States. Since August 2003, the dem passed to 20 mSv over the last 12 months.

At the time of a medical scanner, the patient receives an amount of 0,05 mSv with 15 mSv according to the bodies. To avoid any symptom of blood deterioration, one limits oneself to a maximum of three examinations of this type per annum.

See articles Equivalent of amount and effective Equivalent of amount

In France, the regulation fixes the annual limits of radiation at 20 mSv (2 rem) for the workers and at 1 mSv (0,1 rem) for the population.

The factors which protect from radiations are:

• Distance (to avoid putting the head on the source),
• Activity (to reduce the ddd as well as possible),
• Time (to remain the least longest close to the source),
• Screen (to lead, immerse, concrete,… the source).

Certain behaviors are likely to involve an over-exposure with the radioactivity: a patient who carries out 5 radiographies with the X-rays undergoes an amount from approximately 1 mSv; passengers and pilots of the airliners, and the astronauts in orbit, also undergo about 1 mSv at the time of a very intense solar eruption. If they repeat these voyages or carry out missions of long lives, a prolonged exposure increases the risk of irradiation.