Lecture 4

Lecture 4 - 1/12/2012 Health Physics Principles EEE 498,...

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1/12/2012 Lecture 1 1 Health Physics Principles EEE 498, EEE 598 Lecture 4 Radiation Sources: Radioactivity, Transformation Mechanisms Spring 2012, ASU R. Metzger, rlmetzger@asu.edu, BYEN335 Radioactivity Radioactivity: Spontaneous nuclear transformations that result in the formation of new elements, including: Alpha emission, Beta emission, Positron emission, Orbital electron capture. Each transformation may or may not be accompanied by gamma radiation. Type of transformation depends on: The particular type of nuclear stability (whether the isotope is neutron rich or proton rich), The mass-energy relationship among the parent nucleus, daughter nucleus and emitted particle. Lecture 4 2
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1/12/2012 Lecture 1 2 Nuclear Instability Isotopes on the left side of stable isotopes are neutron rich. Isotopes on the right side of stable isotopes are proton rich. Lecture 4 3 Nuclear Instability (Cont.) Unstable isotopes tend to decay toward the stability line. An unstable isotope may have more than one decay mode. Lecture 4 4
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1/12/2012 Lecture 1 3 Alpha Emission Alpha particle is helium nucleus. Its symbol is α or . α is positively charged. Alpha emission happens when the neutron-to-proton ratio is too low. Examples: Lecture 4 5 He 4 U He Pu Th He U Pb He Po 235 92 4 2 239 94 234 90 4 2 238 206 82 4 2 210 84 n/p ratio of the parent nucleus n/p ratio of the daughter nucleus 1.50 1.51 1.59 1.60 1.54 1.55 Alpha Emission (Cont.) Most natural alpha emitters have atomic number more than 82. The exception is Sm-147 with atomic number of 62. Alpha particles are emitted in discrete line energy spectrum. Most of the time, these particles are monoenergetic. The small differences are attributed to differences in the energy level of the daughter nucleus. The nucleus that emits one of the lower energy particles is left in an excited state. The nucleus that emits the highest energy particle is left in the ground state. Lecture 4 6 α-particle energy (MeV) Relative number of particles (%) 4.782 94.6 4.599 5.4 4.340 0.0051 4.194 7X10 -4
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1/12/2012 Lecture 1 4 Lecture 4 7 Alpha Emission (Cont.) Alpha Emission How to calculate alpha Energy Lecture 4 8 e M M d M p M Q 2 ) amu ( Mass of parent Mass of daughter Mass of emitted alpha particle Mass of electron Total energy release associated with the decay
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1/12/2012 Lecture 1 5 where M and m
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Lecture 4 - 1/12/2012 Health Physics Principles EEE 498,...

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