Interaction des rayonnements avec la matière- 5

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1 Interaction des rayonnements avec la matière- 5
Neutrons Jour 2 – Presentation 5

2 Objectif Discuter l’interaction des Neutron, la capture radiative, la fission, la section efficace analyse par activation neutronique

3 Contenu Le mécanisme d’interaction des Neutrons
Les catégories d’énergie des Neutrons La capture Radiative L’Émission des particules chargées La Fission La Diffusion élastique et inélastique La Section efficace des neutrons et enlèvement Analyse par activation neutronique

4 Directement ionisants Indirectement ionisants
Interaction des particules chargées Les rayonnements ionisants sont deux types: Directement ionisants alpha & béta Indirectement ionisants photons & neutrons Since neutrons carry no electrical charge, their interaction mechanisms with matter are very different from alpha and beta radiation. Neutrons are said to be “indirectly ionizing” whereas alpha and beta particles are said to be “directly ionizing.” Gamma radiation (photons) are also indirectly ionizing. Gamma rays and neutrons release charged particles in matter which are themselves directly ionizing. Les rayons gamma et les neutrons créent des particules chargées dans la matière qui sont directement ionisants.

5 Interactions des Neutron
Les Neutrons n’ont pas de charges, en effet ils sont indirectement ionisants Toujours un champ «mixte» de neutrons et de rayons gamma Les effets biologiques des neutrons dépendent fortement de l'énergie Les Neutrons sont arbitrairement divisés en deux catégories “lents” (thermiques) ou “rapides” (énergies de 1 MeV et plus) All neutrons are “born fast” and then lose energy as they interact with matter in various ways that we will discuss in this session. The chief interaction mechanisms of neutrons are scattering and capture (followed by emission of a photon or another charged particle from the absorber nucleus). The probability of slow neutron interactions is very dependent on their energy. Neutron dosimetry is particularly challenging since it always involves both neutrons and gamma rays together. One needs to be able to distinguish between the two types of radiation to accurately measure and record radiation dose to humans.

6 Interactions des Neutrons
Touts les neutrons sont «nés rapides" puis perdent de l'énergie car ils interagissent avec la matière Les principaux mécanismes d'interaction des neutrons sont la diffusion et la capture (suivie par l’émission d'un photon ou d'une autre particule chargée à cause de l'absorption du noyau). All neutrons are “born fast” and then lose energy as they interact with matter in various ways that we will discuss in this session. The chief interaction mechanisms of neutrons are scattering and capture (followed by emission of a photon or another charged particle from the absorber nucleus). The probability of slow neutron interactions is very dependent on their energy. Neutron dosimetry is particularly challenging since it always involves both neutrons and gamma rays together. One needs to be able to distinguish between the two types of radiation to accurately measure and record radiation dose to humans.

7 Interaction des Neutrons lents
Capture Radiative 1n + 1H  2H +  1H(n, )2H Cette réaction est importante pour la dosimétrie de neutrons et le blindage. L'hydrogène est un composant du tissu humain, donc cette réaction peut entrainer une dose à l'homme. Ainsi, cette réaction fait des matériaux contenant de l’hydrogène (béton eau, paraffine, polyéthylène, etc.) de bons écrans de protection contre les neutrons. This reaction is important in neutron dosimetry and shielding. Hydrogen is a component of human tissue, so this reaction will produce radiation dose to humans. Also, this reaction makes materials containing hydrogen (concrete, water, parrafin, polyethylene, etc.) good shields for neutrons. H has one proton and one electron. This reaction forms Note that this reaction produces a gamma ray which must be shielded against, since it can also produce dose to people. This gamma ray is fairly energetic, with an energy of 2.23 MeV. It will produce a whole body dose when people are exposed to neutron radiation, by virtue of the neutron interactions with hydrogen in the body. For neutrons of low energy, this interaction contributes much of the dose throughout the human body and for neutron energies up to 2.5 MeV, it contributes a major portion of the dose in deep layers, since the photons can penetrate over considerable distances prior to electron interactions.

8 Interaction des Neutrons lents
Capture Radiative 1n Cd  114Cd +  113Cd(n, ) 114Cd Cette réaction est importante pour produire des écrans de protection contre les neutrons et elle est aussi utilisée comme réaction principale pour certains détecteurs de neutrons This reaction is important in neutron shielding and is also used as the principal reaction for some neutron detectors. Note that this reaction also produces a gamma ray, which can cause dose to people.

9 Emission de Particles chargées
Interaction des Neutrons lents Emission de Particles chargées 1n B  7Li + 4He 10B(n,) 7Li C'est pourquoi le bore est utilisé dans les contrôles des réacteurs nucléaires, car il a tendance à réduire le nombre de neutrons présents et aide à contrôler le processus de fission. This reaction is important for both neutron instrument design (neutron measurement) and neutron shielding. It is also the basis for boron controls used in nuclear power reactors, since it tends to reduce the number of neutrons present and therefore helps control the fission process. Since the alpha particle and the Li-7 ion are very massive particles, they cannot travel very far in matter at all. They are therefore of no concern in terms of neutron shielding.

10 Emission de Particles chargées
Interaction des Neutrons lents Emission de Particles chargées 1n +14N  14C + p 14N(n, p)14C This reaction is important to neutron dosimetry, since N is a component of human tissue. If released in tissue, the proton and C-14 particles can cause dose. The proton and C-14 particles are not significant in terms of neutron shielding, since they will not travel very far in matter. The energy of the proton released in this reaction is 0.6 MeV. For thermal neutrons, this reaction is of particular importance in human tissue. L'énergie du proton libérée dans cette réaction est 0.6 MeV.

11 1n + 235U  produits de fission
Interaction des Neutrons lents La Fission 1n U  produits de fission valable pour plusieurs fissions A very important neutron interaction mechanism is fission, which is the basis for nuclear power reactors. More neutrons are relased in this reaction than are absorbed (the mean number of neutrons released per fission for U-235 is 2.5). This leads to a self-sustaining chain reaction or “critical mass.” Le nombre moyen de neutrons libérés par fission de U-235 est 2.5). Cela conduit à une réaction en chaîne auto-entretenue ou "masse critique".

12 Interaction des Neutrons rapides
Diffusion Elastique – les neutrons interagissent avec des particules de même masse approximativement tels que les protons (balles de billard par exemple) Se produit dans des matériaux riches en hydrogène tels que l’eau, le béton,… Représente environ 80% de la dose de neutrons rapides au niveau des tissus 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.

13 Diffusion élastique In collision with protons, neutrons lose half their energy on average. This reaction makes hydrogenous materials (materials rich in protons) good shields (e.g. concrete, wax, water, and various plastics). This reaction is very important from a health physics standpoint, since it is responsible for most of the tissue dose from fast neutrons. En collision avec les protons, les neutrons perdent en moyenne la moitié de leur énergie.

14 Interaction des Neutrons rapides
Diffusion Inélastique – les neutrons interagissent avec les particules plus lourdes (exemple: le Fer) (analogie des balles de ping-pong frappant la boule de bowling) Pour les neutrons rapides d’énergie de l’ordre de 1 MeV, la diffusion inélastique peut devenir prédominante. La diffusion inélastique se produit principalement avec des absorbeurs de Z élevé. 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. In human tissue, and for fast neutron energies in excess of 10 MeV, inelastic scattering and nuclear reactions (frequently with the emission of several particles) become comparable in frequency with elastic scattering.

15 Diffusion Inélastique
This interaction is best described by the compound nucleus model, in which the neutron is captured, then re-emitted by that target nucleus together with the gamma photon. This is a threshold phenomenon; the neutron energy threshold varies from infinity for hydrogen (I.e. inelastic scattering cannot occur with H) to about 7 MeV for oxygen to less than 1 MeV for U. This reaction is not very significant from a tissue dose standpoint, since tissue is composed of relatively low-Z materials. Iron has a particularly strong probability of fast neutron inelastic scattering. Le neutron est capturé, puis réémis par le noyau cible avec émission d’un photon gamma de faible énergie.

16 Interaction des Neutrons rapides
Pourquoi pensez-vous que l’interaction des neutrons rapides n'est pas importante du point de vue dose dans les tissus? 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. In human tissue, and for fast neutron energies in excess of 10 MeV, inelastic scattering and nuclear reactions (frequently with the emission of several particles) become comparable in frequency with elastic scattering.

17 Section efficace des Neutrons
C’est la probabilité pour qu’un neutron interagisse avec un matériau donné Unité est “barn” où 1 barn = cm2 Neutron cross sections are strongly energy dependent. The barn is an almost legendary unit in the history of neutron physics. The story says that when neutron cross sections were first being measured, the cross section for a given material (maybe boron?) was said to be “as big as a barn!” A “shed” is equal to barns or cm2. The shed is used for particle interactions where the cross section is extremely small, e.g. neutrino interactions.

18 Section efficace microscopique
La section efficace est la somme des différentes sections efficaces pour tous les processus qui peuvent se produire avec un atome donné. L'unité est cm2 total = diffusion + capture + fission Neutrons can be removed from a beam by the absorber material generally in three ways: Scatter Capture Fission The total removal cross section is the sum of the three cross sections for these processes.

19 Section efficace macroscopique
Le produit de la section efficace microscopique par le nombre total d'atomes par cm3 dans le matériau. L’unité est cm-1  total = N total où N = nombre d’atomes/cm3 The macroscopic cross section is used for neutrons, in place of the linear or mass absorption coefficients that are used in photon attenuation. N can be calculated using the material density, atomic weight, and Avogadro’s Number. Noter que la section efficace macroscopique est utilisée pour les neutrons, à la place du coefficient d’atténuation linéique ou massique qui sont utilisés dans le cas des photons.

20 L’élimination d’un neutron par un absorbeur
I = I0 e Où I = neutrons qui passent à travers l’absorbeur I0 = neutrons incidents dans l’absorbeur N total = section efficace macroscopique totale x = épaisseur de l’absorbeur - N totale x Neutron removal is exponential.

21 Analyse par activation neutronique (AAN)
L’analyse par activation neutronique est une technique qui consiste à soumettre l’échantillon à analyser à un flux de neutrons, ce qui conduit à activer un certain nombre d’éléments de cet échantillon. l’AAN est non seulement multi-élémentaire et très sensible mais de plus sa réponse est indépendante de la forme chimique de l’élément. Après l’irradiation, les radio-isotopes ainsi créés peuvent être identifiés avec certitude et quantifiés avec précision . In chemistry, neutron activation analysis (NAA) is a process used for determining the concentrations of elements in a vast amount of materials. NAA allows discrete sampling of elements as it disregards the chemical form of a sample, and focuses solely on its nucleus. The method is based on neutron activation and therefore requires a source of neutrons. The sample is bombarded with neutrons, causing the elements to form radioactive isotopes. The radioactive emissions and radioactive decay paths for each element are well known. Using this information, it is possible to study spectra of the emissions of the radioactive sample, and determine the concentrations of the elements within it. A particular advantage of this technique is that it does not destroy the sample, and thus has been used for analysis of works of art and historical artifacts. NAA can also be used to determine the activity of a radioactive sample

22 Capture de Neutron par 23Na
23Na (n,) 24Na This example illustrates the principle of neutron activation of a stable isotope. In this case, stable Na-23 absorbs a neutron and becomes radioactive Na-24. A gamma ray photon is emitted in this reaction. Na-24 happens to be radioactive, with a half-life of about 15 hours. The short form notation for this capture reaction is shown below the figure. This reaction is known as “radiative capture.”

23 Exemples de l’Importance de l’Activation Neutronique
Production des isotopes (par ex. 60Co, 192Ir, etc.) 5927Co + n → 6027Co → 6028Ni + e + gamma La dosimétrie d’accident (par ex. 24Na dans le sang) La détection de la criminalité dans la médecine légale ( exemple les cheveux de Napoléon) Neutron activation, with its resulting production of radioactive isotopes, is a very important phenomenon in health physics. Some of the important radioisotopes used in industry are made by this process. If a person has been exposed to a criticality accident (which involves a large release of gamma and neutron radiation), their tissues can become activated, which results in residual, induced radioactivity in the body. Na-24 is produced in the person’s blood and this can be measured and related back to the number of neutrons incident on the body. In this manner, neutron dose can be estimated, as well as the total number of neutrons released during the criticality. Acute neutron radiation exposure (e.g., from a nuclear incident) converts some of the stable 23Na in human blood plasma to 24Na. By measuring the concentration of this isotope, the neutron radiation dosage to the victim can be computed. 22Na is a positron emitting isotope with a remarkably long half life By neutron irradiation of objects it is possible to induce radioactivity; this activation of stable isotopes to create radioisotopes is the basis of Neutron activation analysis. One of the most interesting objects which has been studied in this way is the hair of Napolean’s head, which have been examined for their arsenic content. Naturally occurring arsenic is composed of one stable isotope75As. L’Irradiation neutronique des cheveux sur la tête de Napoléon a été examinée pour leur teneur en arsenic. Naturellement l’arsenic se produit et se compose d'un isotope stable de 75As

24 Exemples de l’Importance de l’Activation Neutronique
L’analyse par activation neutronique pour la mesure des éléments traces, Les produits d’activation dans un réacteur est une source d’exposition pour les travailleurs (par ex. 60Co) Ils peuvent exposer le public (par exemple un rayonnement gamma direct de 16N dans la vapeur de BWR est produit par irradiation neutronique de l'eau) Neutron activation analysis is a very sensitive method for quantifying the amounts of trace elements in samples which have been irradiated by neutrons. Co-60, a neutron activation product, is responsible for about 75% of occupational radiation dose to workers in US commercial nuclear power plants. N-16, which has very energetic and highly penetrating gamma ray (7 MeV), is produced by neutron irradiation of water in a commercial BWR reactor core. This N-16 is carried over in the steam into the turbine building. The gamma rays from N-16 can travel far enough to cause exposure to members of the public living near the reactor site. The good news about N-16 is that it has only a 7 sec half-life, so that when the reactor is shut down (and the steam supply stops), the N-16 dose rate decays off very rapidly.

25 Exemples de l’Importance de l’Activation Neutronique
Détermination de la composante des neutrons rapides à Hiroshima Accélérateur de spectrométrie de masse de 63Ni (demi-vie = 100 ans) produit par les neutrons rapides activation de cuivre dans les matériaux de construction La réaction est 63Cu (n, p) 63Ni One final example is the recent effort to better quantify the fast neutron radiation component that was associated with the atomic bomb at Hiroshima, Japan. Neutrons released from the detonation of the atomic bomb irradiated and activated copper in buildings. The copper in the building materials was converted to radioactive Ni-63, which has a 100 yr half-life. Samples of the irradiated copper have been analyzed to measure Ni-63. In this manner, the neutron flux incident on the building can be estimated. Knowing the distance from the building to the hypocenter of the bomb, one can estimate the total number of neutrons released by the bomb. Good estimates of the neutron flux at Hiroshima are needed to determine neutron biological effects and the radiation quality factor for fast neutrons.

26 Summaire Neutron interactions with matter were discussed
Neutron interaction mechanisms, neutron energy categories, the concept of neutron cross section, and neutron removal from a beam and neutron activation were described

27 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)