et les types de rayonnement Bases de Physique Nucléaire - 3 Modes de désintégration radioactive et les types de rayonnement Now let’s discuss radioactive decay, including important terms and concepts Jour1 – Leçon 3

Objectif Comprendre les modes de désintégrations radioactives et types de rayonnement Apprendre davantage la structure atomique de base; alpha, bêta et émission gamma; émission de positons; les différences entre les rayons gamma et les rayons X; capture d'électron orbital; et la conversion interne We will discuss the types of radioactive decay and radiations emitted which are important to a health physicist in terms of exposure and dose.

Contenu La structure atomique de base et isotopes Désintégrations alpha et béta émission gamma Spectre de décroissance Différences entre les rayons gamma & rayons- x Emission de Positons Capture de l’électron Orbital Conversion Interne This session will focus on describing the basic structure of the atom; fundamental atomic particles; isotopes; alpha, beta, and gamma modes of disintegration; decay spectra, ; production of x-rays and the essential differences between x-rays and gamma rays; positron emission; orbital electron capture; and internal conversion.

Structure Atomique proton neutron électron The atom consists of neutrons (no electrical charge) and positively charged protons in a central nucleus. Negatively charged electrons circle the atom in various orbitals which represent different energy levels. Protons and neutrons have virtually the same mass. The electron has a mass which is only 1/1820 of that of protons and neutrons. Thus, the nucleus makes up almost all of the mass of an atom. Electrons circle the atom in atomic orbitals which represent different energy states. Electrons closest to the nucleus are the most tightly bound (this orbital is called the K-shell). Going outward from the nucleus, the other orbitals are called the L, M, N, O and P- shells. In the electrically neutral atom, the number of electrons circling the nucleus is equal to the number of protons in the nucleus. The number of protons in the nucleus (called the Z number or atomic number) determines the chemical element.

Numéro Atomique (Z) Hydrogène 1 Carbone 6 Cobalt 27 Sélénium 34 Iridium 77 Uranium 92 This slide shows some examples of the Z number or atomic number of selected chemical elements. This value represents the number of protons in the nucleus.

Isotopes Un isotope d’un élément a: Le même nombre de protons Un nombre de neutrons différent 1H 2H 3H Note that isotopes are the same chemical element, with the same number of protons and electrons. H-3 (tritium) is a radioactive isotope of hydrogen. Some isotopes are stable and some are radioactive (unstable).

Isotopes Le nombre de protons détermine l’élément. Les éléments du même nombre de protons mais un nombre de neutrons différents sont appelés des isotopes. Certains isotopes sont radioactifs. This explains the concept of isotopes and shows atomic nomenclature. “C” stands for the element carbon, the neutron number is shown in the lower right, the Z number (protons) is shown on the lower left, and the atomic mass number A (protons plus neutrons) is shown on the upper left.

Décroissance radioactive Changements spontanés dans le noyau d'un atome instable Résultat est la formation d’un nouveau élément Accompagné par une libération d'énergie, soit sous forme de particule ou de rayonnement électromagnétique ou les deux L’instabilité nucléaire est liée au fait que le rapport N/P est trop élevé ou trop bas Unstable atoms undergo radioactive decay.

Ligne de stabilité N > Z Protons in the nucleus are positively charged and thus tend to repel each other. Were it not for the poorly understood nuclear force, the nucleus would fly apart due to this electrical repulsion. The nuclear force keeps the nucleus together. The dotted line shows where the stable isotopes would fall if the neutron number, N, was always equal to the proton number, Z. However, as Z number increases N becomes greater than Z to offset the electrostatic repulsion of the positively charged protons (see the plot of the stable isotopes in the figure). All the stable isotopes fall on this plot. The unstable (radioactive) isotopes are not on this plot and by their disintegration, they tend to approach stability by adjusting their neutron to proton ratio.

Désintégration Alpha Émission d'un noyau d'hélium très énergique par le noyau d'un atome radioactif Se produit lorsque le rapport N/P est trop faible Résultat d’un produit de désintégration qui a un numéro atomique moins 2 que celui du père et un nombre de masse moins 4 par rapport à celui du père Les particules alpha sont mono-énergétique Alpha decay results in formation of a new chemical element. The decaying nucleus adjusts its neutron to proton ratio to try to get onto the line of stability.

Particule Alpha charge +2 Désintégration Alpha Particule Alpha charge +2 The alpha particle is a helium nucleus with a positive charge of 2 (two protons but no electrons). It is emitted from the nucleus of a decaying atom.

Désintégration Alpha The decay product has an atomic mass number “A” four less than that of the parent and an atomic number “Z” two less than that of the parent. The alpha particle has an atomic mass number of 4 and an atomic number of 2.

Exemple de Désintégration Alpha 226Ra se désintègre par émission alpha Lorsque le 226Ra se désintègre, la masse atomique décroit de 4 et le numéro atomique décroit de 2 Le numéro atomique définit l'élément, donc l'élément change du radium au radon 226Ra  222Rn + 4He 2 86 88 This example shows alpha decay for the familiar radionuclide Ra-226. It decays by alpha emission to a new chemical element Rn-222.

Désintégration Bêta Emission d’un électron par le noyau d’un atome radioactif ( n  p+ + e-1 ) Ce processus se produit lorsque le rapport N/P est trop élevé (c'est à dire, un surplus de neutrons) Les particules bêta sont émises sous forme d’un spectre d'énergies continu (contrairement à des particules alpha) Another type of radioactive disintegration is beta emission. The decaying nucleus adjusts its neutron-to-proton ratio to try to get onto the line of stability.

Désintégration Beta Particule Bêta chargé (-1) The beta particle is emitted from the nucleus and is a negative electron.

Désintégration Bêta The decay product in beta decay has the same atomic mass “A” of the parent but its atomic number “Z” is one greater than that of the parent (a neutron has disappeared but has been replaced by a proton). The antineutrino is emitted to conserve mass and energy but has no significance at all from a health physics standpoint. It produces no dose!

Désintégration Bêta du 99Mo This spectrum shows an example of beta decay of the isotope Mo-99. Part of the time (about 89%) it decays to Tc-99m and part of the time (about 11%) it decays directly to Tc-99, both of which are themselves radioactive. The “m” means metastable, which is an excited energy state of Tc-99. Thus, Tc-99m has excess energy relative to Tc-99 that it must rid itself of by emission of a gamma ray. More will be said about gamma rays in this session.

Spectre Bêta As opposed to alpha particles which are all emitted from the same radionuclide with the same energy, beta particles are emitted with a continuous spectrum of energies from essentially zero up to some maximum energy. The maximum energy available is shared between the beta particle and the anti-neutrino. The maximum energy of the beta spectrum is unique for each beta emitting radioisotope. This situation is analogous to the production of Bremsstrahlung x-radiation.

Règle générale L’énergie moyenne de bêta est le tiers de de son énergie maximale ou: Em = Emax 1 3 Since beta particles are not all emitted with the same energy, a good rule of thumb is that the average energy of a beta spectrum is about 1/3 of the maximum energy of that spectrum. For dose calculation, we would use the average energy, not the maximum.

Emission de Positon (Bêta+) Elle se produit quand le rapport N/P est trop faible ( p+  n + e+ ) Emet un positon (particule bêta dont la charge est positive) Il en résulte l’émission de 2 rayonnements gamma (plus d’info sur ça plus loin) Positrons are positively charged electrons and represent another means by which unstable radionuclides adjust their neutron to proton ratio to get onto the line of stability.

Emission de Positon (Bêta+) In positron emission, the daughter product has the same atomic mass “A” as the parent but its atomic number “Z” is one less than that of the parent. A proton disappears but is replaced by a neutron which has the same mass. In positron emission, a neutrino is also emitted. However, the neutrino has absolutely no significance from a health physics standpoint. It produces no dose.

Désintégration Positon In this example of positron decay, C-11 decays to B-11 with the emission of a positive electron.

Désintégration Positon Another example of positron emission: Mg-23 decays by positron emission to Na-23. The positron has charge but virtually no mass while the neutrino has neither charge nor mass.

Désintégration Positon In this sample spectrum, the positron emitter Na-22 decays to Ne-22. EC stands for orbital Electron Capture which is an alternate mode of disintegration which competes with positron emission. More will be said about electron capture in just a few minutes. In this spectrum, 90% of the decays of Na-22 are by positron emission and 10% of its decays are by electron capture. Also, Na-22 emits a separate positron of a different energy a very small percentage of its decays (0.05% or 5 in 10,000 decays).

Annihilation Positron annihilation is a very important phenomenon associated with positron decay. A positron is antimatter. When antimatter contacts matter, annihilation of both particles occurs (i.e. both particles are completely converted to energy in the form of photons). Matter and antimatter cannot exist together. When a positron at rest encounters an ordinary electron at rest, both annihilate and convert their mass to energy in the form of two photons each with an energy of 511 keV (the rest mass of the electron). The photons are emitted in exactly the opposite direction to satisfy the laws of angular momentum. These photons allow positron emitters to be detected by gamma ray counting. They are also important from a health physics standpoint, since they are relatively high energy, they must be shielded and they produce dose to people.

Capture d’un électron Orbital Appelée aussi capture K Elle se produit lorsque le rapport N/P est trop faible C’est une forme de désintégration en compétition avec l’émission du positon Un électron de la couche orbital est capturé par le noyau: e-1 + p+1  n Résultat est l’émission d’une raie-x caractéristique This is a form of radioactive decay which competes with positron emission (remember the previous slide on Na-22).

Capture d’un électron Orbital In orbital electron capture, the decay product has the same atomic mass as that of the parent but its atomic number “Z” is one less than that of the parent, because a proton has disappeared from the nucleus. The positive proton “captures” a negative electron from the closest orbit (the K shell) and becomes a neutron. An important associated phenomenon is that a photon (called a characteristic x-ray) is emitted as the vacancy in the orbit is filled by electrons transitioning from other orbits.

Capture d’un électron Orbital Here is another example (Be-7) decaying by orbital electron capture.

Ionisation +1 atome ionisé Électron éjecté -1 radiation path Let’s briefly discuss ionization at this point. When an orbital electron is lost from an atom either by radiation interacting with it, an ion pair is produced. The resulting atom has a +1 charge and a hole is left where the electron was. Outer orbital electrons will then fall down or cascade down into the lower orbitals to fill this vacancy. radiation path

Production des rayons-X L’électron remplit Une vacance électron éjecté Raie-x caractéristique When these orbital electrons change orbitals, they change energy states and in the process, they emit photons of specific energies which are “characteristic” of their orbitals for each element. Thus, these photons are called characteristic x-rays. The various chemical elements can be identified by their characteristic x-rays. Note that in orbital electron capture, the characteristic x-rays emitted are characteristic of the daughter product chemical element and not the parent!

Spectre Electromagnétique Rayons x et  Ultra- violet Infra- rouge Visible This figure shows the electromagnetic spectrum. X-rays and gamma rays are photons (i.e. pure energy) and tend to have the highest energy (shortest wavelength) of any radiation on the spectrum. X-rays and gamma rays have enough energy to ionize atoms, whereas ultraviolet, visible, and infrared radiation can excite atoms and molecules but lack sufficient energy to produce ionization. Augmentation de langueur d’onde: décroissance de la fréquence et énergie

Emission du rayonnement Gamma Radiations mono-énergétiques émises par le noyau d'un atome excité qui suit une désintégration radioactive Noyau se débarrassant de son excès d’énergie Possède des énergies caractéristiques qui peuvent être utilisées pour identifier le radionucléide Formes excitées de radionucléides souvent désignées comme «métastables », exemple 99mTc. Ces radionucléides sont appelés aussi “isomères” Note that gamma ray emission does not result in a new chemical element (the number of protons does not change); rather it rids the nucleus of excess energy.

Emission du rayonnement Gamma Gamma Radiation Gamma rays are emitted from the nucleus of an excited atom.

Emission du rayonnement Gamma Here is another slide re-enforcing the point that gamma rays are emitted from the nucleus. They are NOT related to the movement of electrons between orbitals.

Emission du photon Difference entre Rays-X et Rays- Gamma Let’s remember the differences between x-rays and gamma rays. The principal difference is in their point of origin. Gamma rays are emitted from the nucleus of an atom, whereas x-rays are emitted when orbital electrons change orbitals (i.e. change energy states).

Conversion interne Processus alternatif par lequel le noyau excité d'un isotope émet des rayons gamma en se débarrassant de l'énergie d'excitation Le noyau émet un rayonnement gamma qui interagit avec un électron orbital. Cet électron est en suite éjecté de l'atome Les rayons X caractéristiques sont émis quand les électrons orbitaux extérieurs comblent les postes vacants laissés par les électrons de conversion Internal conversion is a competing process with gamma ray emission. In this case, the gamma ray emitted from the nucleus does not make it out of the atom. Instead all of its energy is given to an electron, no gamma is detected even though one was emitted from the nucleus.

Conversion interne Ces rayons-x caractéristiques peuvent eux mêmes être absorbés par les électrons orbitaux éjectés Ces électrons éjectés sont appelés électrons Auger et ils ont une très petite énergie cinétique If the electrons ejected are from the outermost shells (those most loosely bound to the nucleus) they are termed Auger electrons.

Conversion interne In this example, the gamma ray interacts with a K shell electron, ejecting it from its orbital. Although the gamma ray could conceivably interact with an electron in any shell, it is most likely to interact with a K shell electron, since they are the closest to the nucleus.

Résumé du Mécanisme de Décroissance Radioactive Mode dedésintégration Caractéristiques du Radionucléide Père Changement du Numéro Atomique (Z) Changement de la Masse Atomique Commentaires Alpha Pauvre en Neutron -2 -4 Alphas Monoénergetique Beta Riche en Neutron +1 Spèctre d’Energie Beta Positron -1 Spèctre d’Energie Positon Capture Electronique Capture-K; X-rays caractéristique Emitise Gamma Etat d’Energy Excité Aucun Gammas Monoénergétique Conversion Interne Etat d’Energie Excité Ejecte Electrons Orbitaux; x-rays caractéristiques et électron Auger émis This is a summary slide showing the various radioactive decay modes we have discussed in this session.

Résumé Les bases de la structure atomique étaient décrites Les isotopes ont été définis Les modes de désintégration radioactive ont été discutés (y compris alpha, béta, gamma, émission de positon, capture de l’électron orbital, et la conversion interne) L’ionisation a été définie La production de rays-X et la différence entre les rayons gamma et les rayons X ont été décrites

Où obtenir plus d’information Cember, H., Johnson, T. E, Introduction to Health Physics, 4th Edition, McGraw-Hill, New York (2009) International Atomic Energy Agency, Postgraduate Educational Course in Radiation Protection and the Safety of Radiation Sources (PGEC), Training Course Series 18, IAEA, Vienna (2002)