Lesson10b

Lesson10b - Lesson 10b Interaction of Photons with Matter...

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Unformatted text preview: Lesson 10b Interaction of Photons with Matter MP200 Radiation Physics - 2010 Duke Medical Physics Graduate Program 1 Photoelectric Effect The emission of electrons from a metal surface as a result of light ab- sorption is called the ” Photoelectric effect ”. It is of significance in physics, since it shows the particle nature of light. Light is considered as a discrete quanta(photon) of energy E = hν, , where h = Planks constant ν = frequency of light . The kinetic energy of the ejected electron, T = hν- E b E b = energy needed to remove electron from metal When E b is minimum, T is maximum, and then E b is called the work function of metal. ( i.e. energy of the most loosely bound electron ). 2 In the process, the energy of the photon is completely absorbed by the atom. The scattering of the ejected electrons is shown in figure. The figure shows distribution of the angle of ejected photoelectrons vs incident photon energy. Electrons are ejected approx. at right angles when low-energy photons interact photo-electrically. The interaction cross section per atom ( a τ ) The probability of ejecting a photoelectron, when a photon strikes an atom is the interaction cross section per atom. It depends on Z and hν , and the relation is given by, a τ ≈ k Z n ( hν ) m cm 2 .atom- 1 3 Here, n ≈ 4 at hν = 0 . 1 MeV and gradually rising to 4.6 at 3 MeV . m ≈ 3 at hν = 0 . 1 MeV and gradually decreasing to 1 at 5 MeV . Photoelectric event is important in energy hν ≈ . 1 MeV and below. a τ ∝ Z 4 hν 3 cm 2 .atom- 1 Photoelectric Mass attenuation coefficient The photoelectric mass attenuation coefficient is also defined as the cross section per unit mass and it is given by, τ ρ = N A A a τ where, N A = 6 . 023 × 10 23 atoms.mole- 1 = Avogadro’s number A = atomic weight in gram Also, A = cZ where c = Constant Simplifying we get, τ ρ !...
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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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Lesson10b - Lesson 10b Interaction of Photons with Matter...

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