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Lesson11c_1 - Lesson 11c Interactions of Electrons with...

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Lesson 11c Interactions of Electrons with Matter MP200 Radiation Physics - 2010 Duke Medical Physics Graduate Program 1
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Interactions of electrons with Matter In this lesson, electrons and positrons are grouped together and called ”beta particles”. Since all forms of ionizing radiation eventually results in electrons, the interactions of electrons with matter are of importance in radiation physics. Beta particles deposit energy through two kinds of mechanisms : Collisional losses - excitation and ionization of atoms Radiative losses - Bremsstrahlung Collisional Losses Fast moving electrons lose energy, from interactions from the orbital electrons in the medium. 2
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These interactions can excite and ionize the atom. Losses of energy through these mechanisms are called ”collisional losses”. Not like heavy particles, an electron losses a large fraction of its energy in a single head-on collision with an electron. The maximum energy transfer from a charged particle to an electron is(non relativistic) given by, Q max = 4 m e M ( M + m e ) 2 E when, M = m e , Q max = E Since the electron, collides with an identical particle (same mass), large scattering angles are possible. When, two electrons collide, the electron with lower energy is consid- ered as the struck particle. Radiative Losses Radiation is emitted, when a charged particle is accelerated in an elec- tromagnetic field. The acceleration of a heavy charged particle, by the electromagnetic force of a nucleus in an atomic collision is small. An electron, having a small mass, can be accelerated strongly by the same electromagnetic force within the atom, and hence it can emit ra- diation, and this radiation is called bremsstrahlung. If the motion of the charged particle is such that its velocity( β ) and acceleration are parallel (collinear), the pattern of radiation emission is 3
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as shown is figure. θ max is the angle for which the intensity is maximum, and when, β -→ 1 θ max = 1 2 1 - β 2 = 1 2 γ When the instantaneous velocity( v ) and acceleration( a ) are perpendic- ular to each other, radiation is intense in the forward direction. An example of this type of radiation occurs in circular accelerators like synchrotron or cyclotron. When the electron passes a nucleus, it is deflected by the Coulomb field of the nucleus, and it emits bremsstrahlung radiation. Emitted radiation has a continuous energy distribution from zero to maximum kinetic energy of the incoming electron.
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