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1、Radioactive Decay and Half Life,Lesson: CRS 1-4,Lesson Objectives,Understand radioactive decay Be able to describe types or radioactive decay Sketch radioactive decay schemes Understand secular and transient equilibrium Illustrate sequential radioactive decay schemes,Overview of Presentation,Definit
2、ions of radioactive decay and half life Types of radioactive decay and decay schemes Derivation of equations for half life and mean life Illustration of sequential decay schemes,Definition of Terms,Radioactivity: denotes spontaneous changes of state in a nucleus that results in the release of energy
3、 in the form of electromagnetic or particle radiations Radioactive decay: denotes spontaneous changes of state in a nucleus that results in the formation of a new isotope Half Life: denotes the time required for one-half of a radioactive species to undergo a nuclear transition,Nuclear Transitions,Nu
4、clear transitions provide mechanisms for a nuclei to obtain their ground states. Nuclear transitions are typically referred to as radioactive decay if a nuclide with a new atomic charge is (Z) produced. In the case of gamma emission, the charge remains the same but the nucleus may reach its ground s
5、tate.,Types of Radioactive Decay and Nuclear Transitions,Alpha decay Beta decay Positron Electron capture Internal conversion Gamma emission,Alpha Decay,In alpha decay an atom with atomic number Z and mass A emits an alpha particle (with Z=2 and A=4) producing a daughter atom with atomic number Z-2
6、and mass number A-4.,Alpha Decay,Note that these are nuclear and not atomic masses. Also note that the disintegration energy is split between the kinetic energies of the reaction products. From two particle kinematics it can be shown that where, m, is the mass of He and, M, is the mass of Rn. Altern
7、atively the Q value can be calculated from the energy of the alpha particle. Radium-226 decays directly to the ground state 95 % of the time and emits a 186 keV gamma 5 % of the time.,Beta Decay,Beta decay includes the processes of b-, b+, and electron capture. b- decay: in b- decay, an antineutrino
8、 and a negative electron are emitted from the nucleus as a result of the transformation of a neutron into a proton. b+ decay: in b+ decay a neutrino and a positron are emitted from the nucleus as a result of a transformation of a proton into a neutron.,Beta Decay,In beta decay, the nucleus emits an
9、electron and an antineutrino. The energy imparted to each particle varies. An example is The Q value is the mass differences between the Co mass minus the Ni and beta masses. The average energy of the beta energy spectrum is about one-third of Q. The maximum energy is the Q value.,Beta Decay of Co-6
10、0,Positron Decay,Nuclei that undergo positron decay emit a positively charged electron (an antielectron) and a neutrino. An example is shown below If Q is expressed in terms of atomic masses, and binding energies are neglected, we obtain For positron decay to be possible, the mass of the parent must
11、 be greater than that of the daughter by 2m (1.02 MeV). Note that electron capture competes with positron decay.,Positron Decay-Example Decay Scheme for Na-22,Orbital Electron Capture,Electron capture decay: an atomic electron is captured by the nucleus, which transforms a proton into a neutron and
12、a neutrino is emitted.,Nuclei which capture an orbital electron, usually a K shell electron, as a decay mode emit a neutrino and decrease the number of protons by one. An example is as shown below The neutrino acquires all the energy associated with the decay. The Q value is given by Since the energ
13、y added to the nucleus is the electron mass minus the binding energy and since Rh has one more electron than Pd, the Q value must be greater than the binding energy of the electron.,Orbital Electron Capture,Electron Capture-Example Decay of Palladium-103,Electron Capture-Example Decay of Potassium-4
14、0,Other decay schemes show a more simplified summary of facts, but more explicit details. Potassium-40 : Electron capture to stable Argon-40 involves the emission of one 1460-keV gamma ray,Example Decay Schemes,The decay scheme from the Table of Isotopes summarizes many facts about the radioactive d
15、ecay of nuclides with the same atomic weight. Potassium-40 : Half life of 1.28 billion years 89% decay to Calcium-40 releases 1312 keV 11% electron capture to Argon-40 releases 1505 keV,Internal Conversion,In the case of internal conversion, energy is transferred directly to an orbital electron (K o
16、r L shell, most likely). A photon is not emitted which then transfers energy to the electron. This process competes with gamma emission. The energy of the ejected electron is the transferred energy minus the binding energy of the orbital electron.,Gamma-ray Emission,Nuclei that decay from an excited state with no change in the number of nucleons are called isomeric. Nuclides in the initial and final states are called isomers. Unlike betas, photons emitted from the nucleus have discrete energies.
