Lesson11a

Lesson11a - Lesson 11a Interactions of Charged Particles...

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Unformatted text preview: Lesson 11a Interactions of Charged Particles with Mat- ter MP200 Radiation Physics - 2010 Duke Medical Physics Graduate Program 1 Introduction Charged particles lose their energy in a manner that is distinctly dif- ferent from that of uncharged radiations ( photons or neutrons). A charged particle is surrounded by its Coulomb electric force field that interacts with orbital electrons (collision loss) and the nucleus (ra- diative loss) of all atoms it encounters as it penetrates into the medium. The energy transfer from the charged particle to matter in each individ- ual interaction is very small. Therefore, a charged particle undergoes a large number of interactions before its kinetic energy is spent. Types of Charged-particle Coulomb force Interactions Charged particle Coulomb force interactions can be divided into three categories depending on the size of the classical impact parameter b compared to the classical atomic radius a . 1. Soft Collisions ( b >>> a ). Coulomb force interaction of the charged particle with orbital elec- tron for b >>> a . The Coulomb force field affects the atom as a whole. It can excite the atom to a higher energy level, and sometimes ionize the atom by ejecting a valence electron. The net effect is the transfer of a small amount of energy to an atom of the absorbing medium. Because large values of b are the most probable than near hits on individual atoms, soft collisions are most numerous type of 2 charged particle interactions. Half of the energy transferred to the medium is due to soft collisions. 2. Hard Collisions ( b a ) Coulomb force interaction of the charged particle with orbital elec- trons, when b is of the order of a atomic radius. Charged particle interacts with atomic electron which is then ejected from the atom with considerable kinetic energy. This electron is called -ray. If this -ray is energetic enough, it can undergo ad- ditional Coulomb interactions, in a separate track (called spur). Hard collisions are very few compared to the soft collisions. The probability for hard collisions depends upon quantum-mechanical spin and exchange effects. Because of that, the effect of hard col- lisions depend on particle type. ( electrons vs. heavy particles ) It should be noted that whenever an inner shell electron is ejected from an atom by a hard collision, characteristic X-rays/Auger electrons will be emitted just as it had been done by photons. 3. Coulomb force interaction of the charged particle with the external nuclear field ( bremsstrahlung production) 3 b << a . When b << a , Coulomb force interaction of charged particle takes place with nucleus. Only light charged particles ( electrons and positrons) experience large amount of energy losses through these interactions....
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This note was uploaded on 10/30/2010 for the course MP 200 taught by Professor Guna during the Fall '10 term at Duke.

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Lesson11a - Lesson 11a Interactions of Charged Particles...

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