Ngun TriƯu Tó 8261730 0904505414 Hµnéi , 20-2.2006 M.Planck Einstein N.Bohr M.Curie Radius of observable universe 15000.000.000.light -years Radius of Earth cell Atom Atomic nucleus Energy Wide Range of Radiation Energy and Intensity GeV Accelerator, Cosmic rays MeV Industrial application Medical diagnosis Environment 102 104 Processing Radioisotopes keV 10-2 Medical treatment 106 108 1010 1012 1014 Intensity(s-1) Concentration Bq/g Dose(nGy/h)) interaction of particles and radiation with matter The measurement of nuclear radiation is based on its interaction with the detector In order to understand: 1* The function of nuclear radiation detectors 2* The absorption phenomena in the measurement of the radiation 3* With respect to radiation protection  We shall deal with the most important mechanisms of the interaction between nuclear radiation and matter in their basic features To organize the discussions that follow, it is convenient to arrange the four major categories of radiations into the following matrix: Charged Particulate Radiations Heavy charged particles (characteristic distance ≅10-5m) Fast electrons (characteristic distance ≅10-3m) Uncharged Radiations Neutrons (characteristic length ≅10-1m) X-rays and γ rays (characteristic length 10-1m) Mảnh giấy Lá nhôm Tấm chì Interaction of particles and radiation with matter • Ionization Losses Due to Collisions of Charged Particles Stopping power • Bohr's Formula for Specific Ionization Relativistic Effects and the Density Effect • Dependence of ionization Losses on the Medium • ionization Losses on the Medium Radiation Losses for Electrons Cherenkov Radiation n and γ -radiation Interaction with matter Interaction of Gamma Radiation with Matter Photoelectriceffect Compton scattering Electron-positron Pair production Photoelectric effect M γ-ray e- K Photoelectric effect L ek A.E X A.E :Auger electron X-ray K L M hν Te = Eγ - Ii Photoelectric effect cannot take place for a free electron (not associated with an atom) 10 (2) → Different types of nuclear reactions, viz (n, γ), (n, p), (n, α) and fission reactions; inelastic scattering (n, n'); elastic scattering involving the entry of the neutron into the nucleus, i.e the elastic resonance scattering (n,n)res 76 *→The relative role of each process is defined by the cross section corresponding to it In some materials, the role of elastic scattering is comparatively significant, and in this case a fast neutron loses its energy in a series of successive elastic collisions with the nuclei of these materials (moderation of neutrons) 77 * The process of moderation continues until the kinetic energy of the neutron becomes comparable with the thermal energy of the atoms of the moderating material (moderator) Such neutrons are called thermal neutrons 78 * Subsequent collisions of thermal neutrons with the moderator atoms practically not change the energy of the neutron and result only in their displacement in the material (diffusion of thermal neutrons) which continues until the neutron is absorbed by the nucleus 79 → The interaction of fast and slow neutrons with matter is of utmost importance invarious problems of neutron physics, and in particular in the design of nuclear reactors Some of these problems, for example, the moderation of fast neutrons, should have been considered on the basis of the momentum diagram * 80 * However, a close relation between all the above-mentioned processes involving the interaction of neutrons with matter make it expedient to consider these questions together 81 * Khi ®i qua vËt chất, nơtrôn không trực tiếp iôn hoá nguyên tử Do đó, để ghi nơtrôn, người ta sử dụng hạt tích điện thứ cấp, xuất nơtrôn tương tác với hạt nhân vật chất * Việc ghi nhận nơtrôn chậm tiến hành nhờ hạt tích điện, xuất kết phản ứng hạt nhân, gây nơtrôn Để ghi nơtrôn chậm, người ta thường hay sử dụng phản ứng : B10 + n → He4 + Li7 Ph¶n øng toả lượng 2,78 MeV , đủ lớn để iôn hoấ chất khí BF3 đêtectơ 82 Tiết diện phản ứng tỷ lệ nghịch với tốc độ nơtrôn : ~ 1/ Đối với nơtrôn chậm = 600 barn (1 barn = 10-24cm2) 83 Nơtrôn chậm tương tác với Li6 theo ph¶n øng: Li + n → He4 + H3, toả lượng 4,63 MeV.Như vậy, phản ứng xuất hai hạt tích điện với lượng lớn Một phản ứng khác thường sử dụng để ghi nhận nơtrôn phản ứng phân chia U235 Pu239.Các mảnh phân chia trường hợp có tổng động 160 MeV Trong trường hợp nơtrôn nhanh,để ghi nhận xác định lượng chúng,người ta sử dụng hiệu ứng tán xạ đàn hồi nơtrôn hạt nhân.Giả sử nơtrôn với khối lượng Mn = 1, lượng E0 bị tán xạ góc hạt nhân khối lượng M 84 Năng lượng hạt nhân giật lïi : E = E0.4M cos2θ/(M +1)2 Khi θ = 0, cos = 1, hạt nhân giật lùi thu động cực đại Emax = 4ME0/(M+1)2 Biểu thức hạt nhân giật lùi thu lượng lớn hạt va chạm có khối lượng nhau, nghĩa nơtrôn va chạm với nguyên tử hyđrô Trong thực tế,để ghi nhận xác định lượng nơtrôn nhanh người ta thường hay sử dụng nhiều chất chứa hyđrô Đôi khi, để ghi nơtrôn nhanh người ta sử dụng chất phân chi có ngưỡng tương đối lớn Ví dơ: U238 (ng­ìng ph©n chia1,1MeV), Th232(ng­ìng ph©n chia 1,17 MeV) Np237( ng­ìng ph©n chia 0,4 MeV) 85 1/ Compare the range of electrons, alpha particles and gamma rays in matter , if they have the same energy MeV 2/ Calculate the energy of a MeVgamma-ray photon after Compton scattering through 900 3/ Give a rough estimate of the ratio of the probability per atom for photoelectric absorption in Silicon to that in Germanium 86 4/Indicate which of three major interaction processes (photoelectric absorption,Compton scattering, pair production) is dominant in the following situations: a/ MeV gamma rays in Al, b/ 100keV gamma rays in hydrogen, c/ 10MeV gamma rays in lead 5/A gamma ray photon after Compton scattering through an angle of 900 has an energy of 0,5 MeV Find its energy befor the scattering 6/ Explain the reason why: a/ The process of pair production cannot occur in vacuum b/ The photoelectric effect cannot take place for a free electron 87 Answer by writing one of the heading alphabets into the answer sheet Which statement is wrong? A)α and β -rays impart their energy to matter mainly through ionizations and excitations B) The penetration range ofα - rays and that of fast electrons in a certain substance are of the same order when their energy are the same C) The atomic cross section for photoelectric absorption is less in Ge than in Si at the same photon energy d) The cross section or the probability for Compton scattering increases with incident-photon energy 88 Phương pháp ghi nhận xạ hạt nhân Radiations Radiations Hạt tích điện , Tương tác phôtôn Năng lượng Năng lượng A tương tác tương tác Phản ứng hạt nhân êlectrôn thứ cấp A Prôtôn, hạt Iôn hoá Kích thích tín hiệu dòng/thế A Nơtrôn Photon A Các trình khác Đêtectơ tín hiệu ánh sáng ống nhân quang ®iƯn xư lý tÝn hiƯu Khuch ®¹i Direct current TÝn hiệu Current(voltage) Thiết bị điện Pulse signal Counts Phổ 89 90 ... wavelength for electron 18 1.The wavelength λ of the displaced line increases with the scattering angle θ in such a way that: ∆λ = ∆λ = Λ for for ∆λ = 2Λ for θ = 0, θ = π / 2, and θ =π However,... the expression (5), and for ρ=∞ since the integral is divergent However, it can be shown that the integration limits are not and ∞ but have some finite values ρmin and ρmax Consequently ρmax dT... the Bragg curve for specific ionization of α particles 39 (9) Formula (9) shows that with increasing particle energy, the specific ionization losses first decrease very rapidly (ininverse proportion