Interaction des rayonnements avec la matière - 6 Blindage des Neutrons Jour 2 – Leçon 6

Objectif Apprendre à propos du blindage des neutrons (ainsi que des rayons gamma associés) Discuter de l’importance de la radioprotection des sources de neutrons et de leur blindage

Contenu Catégories des énergies des neutrons Principes du blindage des Neutrons Matériaux du blindage Emission gamma à partir des blindages pour les neutrons Les sources de neutrons

Catégories des énergies des neutrons Nom des Neutron /Titre Energie (eV) Neutrons froids 0 < 0.025 Neutrons thermiques 0.025 Neutrons Epythermiques 0.025 < 0.4 Neutrons Cadmium 0.4 < 0.6 Neutrons Epicadmium 0.6 < 1 Neutrons lents 1 < 10 Neutrons Resonance 10 < 300 Neutrons Intermedières 300 < 1,000,000 Neutrons rapides 1,000,000 < 20,000,000 Neutrons Relativistes >20,000,000 Thermal neutrons are neutrons which have the same energy and move at the same velocity as a gas molecule does at a temperature of 20° C. The velocity of a thermal neutron is 2200 m/sec. As indicated previously, fast neutrons are those neutrons with energies of 1 MeV or greater. For purposes of this discussion, “very fast neutrons” will be defined as neutrons with energies greater than 10 MeV.

Blindage des Neutron Compliqué et pas simple Le choix du blindage dépend fortement de l’énergie des neutrons Le blindage des neutrons est combiné avec les matériaux absorbeurs de Z élevé et de Z faible Dans tous les cas, les matériaux de blindage doivent tenir compte des rayons gamma induits Neutron shielding is more challenging than shielding for other kinds of radiation such as gamma and beta. Remember that “all neutrons are born fast.” That is, they have a lot of energy when they are emitted. Also, there is no such thing as a pure neutron radiation field. Gamma rays and even x-rays will also be present and will present exposure potential, as well as shielding issues. Having no electric charge the neutron cannot interact with material directly or via the coulomb force, instead relying mainly on its strong force with nuclei. There are fewer reactions due to the short range of the force, and the probability of a reaction is reduced further by the fact that a neutron must pass within 10-13 cm of the nucleus before an interaction can take place. The distance that a neutron can travel through matter without an interaction is known as its Mean Free Path. There are many types of neutron interactions depending upon the energy: highly energetic neutrons can produce many varied secondary particles, and these particles will change in character as the energy reduces.

Blindage des Neutron Pour les neutrons très rapides avec les énergies > 10 MeV utiliser la diffusion inélastique (Matériaux de Z élevé) Pour les neutrons rapides avec des énergies > 1 MeV, utiliser la diffusion élastique (modération avec H) Pour les neutrons lents utiliser les absorbeurs Elastic scattering is the most likely interaction between fast neutrons and low-atomic numbered absorbers. This interaction is a “billiard ball” type collision, in which kinetic energy and momentum are conserved. Up to neutron energies of the order of 10 MeV, the most important interaction of fast neutrons with matter is elastic scattering. In inelastic scattering, kinetic energy and momentum are not conserved. Rather, some of the kinetic energy is transferred to the target nucleus which excites the nucleus. The excitation energy is then emitted as a gamma-ray photon. Inelastic scattering occurs primarily with high-Z absorbers. For fast neutrons of energies of about 1 MeV, inelastic scattering can become appreciable.

Blindage des neutrons très rapides Utiliser les matériaux de blindage dans cet ordre: Z élevé (par ex. Pb, Fe ou béton) – la diffusion inélastique pour le ralentissement initial Suivit de Z faible, Z (H) (par ex. polymères, eau, béton) – diffusion élastique For neutrons with energies greater than 10 MeV, use inelastic scattering for very energetic neutrons first and then slow them down further by elastic scattering with hydrogenous materials.

Blindage des neutrons rapides Utiliser les matériaux de faible-Z (avec H) ( par exemple polymères, eau, béton) A l’avantage de la diffusion élastique avec de l'hydrogène Elastic scattering interaction predominates for neutrons with energies less than 10 MeV.

Blindage des neutrons rapides Des noyaux lourds Fast neutrons are not efficiently slowed down by scattering off heavy nuclei such as iron. Little energy is lost by the neutron in this type of interaction.

Diffusion des Neutrons rapides Par les noyaux d’hydrogène Fast neutrons give up a lot of energy when they collide with hydrogen nuclei (i.e. protons) and thus are slowed down very efficiently.

Blindage pour les neutrons lents De préférence utiliser les absorbeurs pour capturer les neutrons lents sans avoir d’émission gamma Utiliser les absorbeurs tel que le B ou Li qui utilise la réaction (n,) sans émission de gamma de capture Second choix est les matériaux hydrogénés utilisant la réaction 1H (n,) 2H (émet un gamma de très haute énergie de 2.23 MeV) dont il faut se protéger Note that a gamma ray is emitted from an excited state of Li-7 following the (n, ) reaction with B-10. However, its energy (0.478 MeV) is less than that of the gamma ray emitted from the capture reaction with H (I.e. 2.23 MeV). The 10B (n, ) 7Li reaction does not of itself emit a gamma ray. Neither does the 6Li (n, ) 3H reaction. Thermal Neutrons can be virtually eliminated by the presence of high thermal neutron cross section materials such as Boron, Lithium, or Cadmium.

Blindage pour les Neutrons d’énergies mixtes Ralentir les neutrons puis les absorber Utiliser les matériaux de faible Z – diffusion élastique pour modérer les énergies de neutrons Utiliser des matériaux absorbeurs pour éliminer les neutrons modérés Utiliser des matériaux de Z élevé pour se protéger contre les rayons X et gamma induits The principle here is to slow down the neutrons, capture them, and shield against the associated gamma rays which are emitted.

Rayons Gamma Produits par le blindage des Neutrons Les rayons gamma secondaires proviennent principalement de la capture de neutrons thermiques La diffusion inélastique des neutrons contribue également peu Les rayons gamma secondaires sont moins importants pour les neutrons d’énergies plus élevées

Rayons Gamma de capture de Neutrons pour les matériaux choisis Cible Neutron themiques Section efficace (barns) Rayons gamma de forte énergie (MeV) Al 0.235 7.724 B-10 3837 0.478 Cd 2450 9.046 C-12 0.0034 4.95 H 0.332 2.23 Si 0.160 10.599 N-14 0.075 10.833 The 0.478 MeV gamma ray from B-10 neutron capture comes from emission from an excited state of Li-7 following the 10B(n,)7Li reaction. Some relatively high energy gamma rays are emitted by thermal neutron capture, I.e. (n,) reactions. Recall that secondary gamma rays are less important for higher neutron energies.

Composition des matériaux pour le blindage commun des neutrons Eléments Contenus Atomes/cm3 (x10-21) Polyethylene Boré (8% B4C en poids) H C B-10 B-11 76.8 39.2 0.658 2.67 Eau O 66.9 33.45 Béton Al Si 13.75 45.87 1.743 20.15 The density of borated polyethylene (I.e. 0.97 g/cc) is very close to that of water (I.e. 1 g/cc). One material, polyethylene, has proven to be an especially useful neutron shield. Polyethylene is an exceptional base material for neutron shielding because of its outstanding nuclear, physical and chemical properties. Of all practical materials, it contains more hydrogen atoms in a given volume than any other substance. It also has excellent machining and fabrication characteristics. It is chemically inert, and can be obtained in a very pure form. Its physical properties permit uniform distribution of various additives throughout the material. These additives can be varied over a wide range to meet specific needs. Other base materials include: - Epoxy, Silicone, Urethane, Hydrocarbons, refractories, and cementitious materials. Additives include: - Boron, B-10, Lead, Li-6, Tungsten, Gadolinium, and Cadmium.

Energie moyenne des neutrons (MeV) Sources de Neutrons Source Réaction Energie moyenne des neutrons (MeV) Réacteur Fission Spèctre de Fission 24Na + Be (,n) 0.83 Ra + Be (,n) 5.0 Po + Be 4.0 252Cf Fission spontanée Pu + Be This slide shows some common neutron sources. Neutrons are produced in these sources by fission, photoneutron production (,n), or by alpha capture reactions (,n). Neutrons are also produced by accelerators. Some of the sources listed above produce neutrons of a single energy (I.e. monoenergetic sources) and some (e.g. fission sources and Cf-252) produce neutrons with a spectrum of energies.

Blindage pour les neutrons de Po-Be Matériau Demi-épaisseur (cm) Paraffine 6.6 Eau 5.4 12% Borax dans l’eau 5.3 Laiton 4.9 Acier (laminage à froid) Plomb 6.8 Aluminum 7.8 Note that PoBe neutron sources produce neutrons with an average energy of 4.0 MeV. The half-thickness of the material is that thickness required to reduce the initial neutron intensity by a factor of 2. Based solely on amount of absorber material required, for these neutrons, brass or steel appear to be the best shields.

Résumé Les principes du blinde des neutrons a été discuté Les participants ont compris les différentes catégories des énergies de neutrons, les principes de blindage, des matériaux de blindage, comment utiliser les matériaux de blindage commun pour les sources de rayonnement gamma associées à des sources de neutrons

Où trouver 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)