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Lesson7 - Lesson 07 Radioactivity 01 MP200 Radiation...

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Unformatted text preview: Lesson 07 Radioactivity 01 MP200 Radiation Physics - 2010 Duke Medical Physics Graduate Program 1 Introduction In this lesson, we will study, the processes and kinematics of, • α,β and γ decays, • Electron Capture, • Internal Conversion and, • Auger Electron Radioactivity The spontaneous disintegration or rearrangement of the internal structure of an un- stable nucleus by emitting particles or radiation is called ”Radioactivity”. Emitted particles and radiation from radioactivity are collectively called ”rays”. There are three kind of rays in natural occurring radioactivity. • α rays, • β rays (electrons and positrons), • γ rays α rays are helium nucleus and can be blocked by a 0 . 01 mm of lead. β rays are electrons or positrons and can be blocked by a 0.1 mm of lead. γ rays are the most penetrating and can pass through a 100 mm thick lead. In radioactive decay, Mass, energy, electric charge, linear momentum angular momen- tum, and nucleon numbers should be conserved. Example: Initial amount of charge = final amount of charge after decay. Initial nucleus called ” parent ” emits a particle and produces another particle called the ” daughter ”. Daughter may be the same nucleus, in lower energy state or an entirely new nucleus. 2 α decay The process of ejecting an alpha particle from a nucleus is called α decay. The atomic number of the parent P decreases by 2, and the mass number decreases by 4, when the daughter D is produced. A Z P-→ A- 4 Z- 2 D + 4 2 He Example: 226 88 Ra-→ 222 86 Rn + 4 2 He Kinematics of α decay Let us assume, the α decay occurs in a system where the parent is at rest. P-→ D + α From the conservation of energy, M p c 2 = M D c 2 + M α c 2 + K D + K α K D ,K α = Kinetic energies, and M P , M D ,M α = Masses of particles Since, K D + K α ≥ 0, M p ≥ M D + M α Q- value The energy released is the disintegration energy, and it is called the Q- value. This value is equal to the difference between the final and the initial kinetic energies of the system. Q = K D + K α Q = ( M p- M D- M α ) c 2 From conservation of linear momentum and the Q value, 3 Figure 1: α decay M α v = M D V (1) Q = 1 2 M α v 2 + 1 2 M D V 2 (2) Removing V from (1) and (2), Q = 1 2 M α v 2 M D + M α M D Since, K α = 1 2 M α v 2 K α = M D Q M D + M α and, K D = M α Q M D + M α Since, M D + M α A K α = A- 4 A Q Decay Scheme The nuclear transformation from parent to daughter can easily be described by a decay-scheme diagram. This diagram includes the decay mode, branching ratio, and the energy transitions. 4 Conventions: 1. Arrows slanting to the left side indicate decrease in Z. 2. Arrows slanting to the right side indicate the increase in Z....
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Lesson7 - Lesson 07 Radioactivity 01 MP200 Radiation...

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