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lecture 3

# lecture 3 - Health Physics Principles EEE 498 EEE511...

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1/12/2012 Lecture 1 1 Health Physics Principles EEE 498, EEE511 Lecture 3 Atomic and Nuclear Structure Spring 2012, ASU Robert Metzger,, GWC 387 Rutherford’s Nuclear Atom The positive charge in an atom is concentrated in a central massive point called the nucleus and that the negative electrons are situated at about one Å from the nucleus. Radius of the nucleus ( A is the atomic mass number): Since the ratio of A/V is constant for all nuclei, the nuclei are similar to liquid drops: all nuclei have the same density regardless of their atomic number. . / 3 4 m 10 2 . 1 3 3 / 1 15 Const V A r V A r Lecture 3

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1/12/2012 Lecture 1 2 Bohr’s Atomic Model How electrons are held outside of the nucleus? What holds the positive charges in the nucleus together? The attractive force between electrons and the nucleus could be balanced by the centrifuge force of the circular motion of the electrons. These orbital can revolve around the nucleus only in certain fixed radii, called stationary states. A photon is emitted only when an electron falls from one orbit to another orbit of lower energy. where f is the frequency of the emitted photon, and E 2 and E 1 are the high- and low-energy orbits, respectively. Bohr Radius : a smallest possible orbital radius of 0.0529 nm. Lecture 3 h E h E f hf E 1 2 ; Bohr’s Atomic Model (Cont.) Example: What is the frequency of a photon emitted when an electron in a hydrogen atom jumps from n = 3 to n = 2? For n=3: E 3 =-12.07 eV For n=2: E 2 =-10.19 eV Lecture 3 Hz 10 55 . 4 s eV 10 136 . 4 eV 07 . 12 s eV 10 136 . 4 eV 19 . 10 14 15 15 f h E h E f 1 2
1/12/2012 Lecture 1 3 The frequency and wavelength of the emitted radiation: Where n f and n i are respectively the orbit numbers for the lower and higher levels, k 0 is Coulomb’s law constant (9.9x10 9 N·m 2 /C 2 ), Z is the atomic number of the atom, e is the electron charge and m is the electron mass. The Bohr’s atomic model has been modified. Bohr’s Atomic Model (Cont.) 2 2 3 4 2 2 0 2 2 2 3 4 2 2 0 2 1 1 2 1 1 1 2 i f i f n n ch e mZ k n n h e mZ k f Lecture 3 Excitation and Ionization Ionization and excitation happens due to interaction between the electric fields of a beta particle and an orbital electron of the absorbing medium. The term "excitation" is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom. Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state.

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