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NE 301 - Introduction to Nuclear Science Spring 2012 Classroom Session 7: Radiation Interaction with Matter
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Reminder Load TurningPoint Reset slides Load List Homework #2 due February 9 2
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Growth of Radioactive Products in a Neutron Flux 3
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4 Notice saturation after 3-5 times T 1/2 radioactive product. Notice saturation after 3-5 times T 1/2 radioactive product. Additional irradiation time does not increase activity. Additional irradiation time does not increase activity.
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Radiation Interaction with Matter 5
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Ionizing Radiation: Electromagnetic Spectrum Each radiation have a characteristic, i.e.: Infrared: Chemical bond vibrations (Raman, IR spectroscopy) Visible: external electron orbitals, plasmas, surface interactions UV: chemical bonds, fluorecense, organic compounds (conjugated bonds) X-rays: internal electron transitions (K-shell) Gamma-rays: nuclear transitions Neutrons (@ mK, can be used to test metal lattices for example) Ionizing Radiation Ionizing
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Radiation Interaction with Matter Five Basic Ways: 1. Ionization 2. Kinetic energy transfer 3. Molecular and atomic excitation 4. Nuclear reactions 5. Radiative processes 7
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1. Ionization Ion pair production Primary (directly by radiation) Secondary (by ions already created) Energy for ion-pair depends on medium For particles Air: 35 eV/ion pair Helium:43 eV/ion pair Xenon:22 eV/ion pair Germanium 2.9 eV/ion pair 8
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2. Kinetic Energy Transfer Energy imparted above the energy required to form the ion-pair 9
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Energy less than needed for ionization Translational Rotational and Vibrational modes As e - fall back to lower energy emits X-rays Auger electrons Eventually dissipated by Bond rupture Luminescence Heat 10 3. Molecular Excitation
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4. Nuclear Reactions Particularly for high energy particles or neutrons Electromagnetic energy is released because of decelerating particles Bremsstrahlung Cerenkov 11 5. Radiative Processes
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Radiation from Decay Processes Charged Directly ionizing (interaction with e - ’s) β’s, α’s, p + ’s, fission fragments, etc. Coulomb interaction – short range of travel Fast moving charged particles It can be completely stopped Uncharged Indirectly ionizing (low prob. of interaction – more penetrating) , X-Rays, UV, neutrons No coulomb interaction – long range of travel Exponential shielding, it cannot be completely stopped 12
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High and Low LET LET: Linear Energy Transfer Concentration of reaction products is proportional to energy lost per unit of travel e.g. 1 MeV ’s – LET=190 eV/nm in water 1 MeV ’s – LET=0.2 eV/nm in water 13
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- Ranges Limited range (strong interaction) Exhibit Bragg peak Cross section of is higher at lower energies Most ionizations at end of path Useful in cancer particle therapy 14 Bragg peak
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Definition of Ranges Extrapolated Range Mean Range R. gives range in g/cm 2 (we’ll see why later) 15
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Ranges in Air Range of particles in air, can be used to find their energies 16 Equation valid for 3 cm < R < 7 cm 3 cm < R < 7 cm (aka. most ’s)
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SRIM/TRIM Montecarlo computer based methods: much better and flexible than equations.
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Put energy 1 MeV=1,000keV Run SRIM-TRIM Use: 18 Select projectile (proton = hydrogen) Select target or find a compound Indicate Target Thickness, such that tracks are visible
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Results Screen 19 Read mean Range and “straggling”
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Calculate and compare the range of a 10 MeV - particle in air using TRIM, plot, and equation. 20
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ranges Ranges are more difficult to compute Electrons get easily scattered Less strongly interacting (range of meters in air) At end near constant Bremsstrahlung radiation. 21
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Examples of formulas: Bethe Formula Berger Method (used in MCNP) 22
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Empirical Equations What is the range of a 5 MeV electron in air?
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