ne 301 - introduction to nuclear science spring 2012
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NE 301 - Introduction to Nuclear Science Spring 2012. Classroom Session 7: Radiation Interaction with Matter. Reminder. Load TurningPoint Reset slides Load List Homework #2 due February 9. Growth of Radioactive Products in a Neutron Flux. - PowerPoint PPT PresentationTRANSCRIPT
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NE 301 - Introduction to Nuclear ScienceSpring 2012
Classroom Session 7:
•Radiation Interaction with Matter
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ReminderLoad TurningPoint Reset slides Load List Homework #2 due February 9
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Growth of Radioactive Products in a Neutron Flux
B B
B
BB
N NB
N 0 0B
formation rate: R= n n is number of atoms
dN n
dNn
B
t t
tB
notice
Ndt
dtN
BA( , )n B C
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Growth of Radioactive Products in a Neutron Flux
BB B BA =N n (1 )te
• Notice saturation after 3-5 times T1/2 radioactive product.
• Additional irradiation time does not increase activity.
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Radiation Interaction with Matter
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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 interactionsUV: chemical bonds, fluorecense, organic compounds (conjugated bonds)
X-rays: internal electron transitions (K-shell)Gamma-rays: nuclear transitionsNeutrons (@ mK, can be used to test metal lattices for example)
Ionizing RadiationIo
nizin
g
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Radiation Interaction with MatterFive Basic Ways:1. Ionization2. Kinetic energy transfer3. Molecular and atomic excitation4. Nuclear reactions5. Radiative processes
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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
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2. Kinetic Energy TransferEnergy imparted above the energy required to form the ion-pair
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Energy less than needed for ionization Translational Rotational and Vibrational modesAs e- fall back to lower energy emits X-rays Auger electronsEventually dissipated by Bond rupture Luminescence Heat
3. Molecular Excitation
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4. Nuclear ReactionsParticularly for high energy particles or neutrons
Electromagnetic energy is released because of decelerating particles Bremsstrahlung Cerenkov
5. Radiative Processes
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Radiation from Decay ProcessesCharged 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
stopped12
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High and Low LETLET: Linear Energy TransferConcentration 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
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- RangesLimited range (strong interaction)Exhibit Bragg peakCross section of is higher at lower energies
Most ionizations at end of pathUseful in cancer particle therapy
Bragg peak
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Definition of RangesExtrapolated Range
Mean Range
R. givesrange in g/cm2
(we’ll see why later)
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Ranges in AirRange of particles in air, can be used to find their energies
3/2( ) 0.318 E (E in MeV)R cm
Equation valid for 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:
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Select projectile (proton = hydrogen)
Select target or find a compound
Indicate Target Thickness, such that tracks are
visible
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Results Screen
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Readmean
Range and “straggling”
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Calculate and compare the range of a 10 MeV -particle in air using TRIM, plot, and equation.
3/2( ) 0.318 E (E in MeV)R cm
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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.
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Examples of formulas:Bethe Formula
Berger Method (used in MCNP)
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Empirical EquationsWhat is the range of a 5 MeV electron in air?
210 /
102
cmgRMeVEELogx
R cxbxa