Charged Particle Interactions 22.104 Spring 2002 MIT Department of Nuclear Engineering 22.104 S2002 Types of Particles Z Heavy charged particles – p, d, a, ions Z Electrons MIT Department of Nuclear Engineering 22.104 S2002 Major Heavy Particle Interactions Z We observe: – loss of energy by the particle – deflection of the particle from initial direction Z primarily as a result of: – inelastic collisions with atomic electrons – elastic scattering from nuclei Z but other (less likely) processes are: – Cherenkov radiation – nuclear reactions – bremsstrahlung MIT Department of Nuclear Engineering 22.104 S2002 Collision of Heavy Charged Particle with Atomic Electron MIT Department of Nuclear Engineering 22.104 S2002 Bethe-Bloch formula MIT Department of Nuclear Engineering 22.104 S2002 Bethe-Bloch formula definitions MIT Department of Nuclear Engineering 22.104 S2002 Bethe-Bloch with Corrections MIT Department of Nuclear Engineering 22.104 S2002 dE/dx for various heavy particles MIT Department of Nuclear Engineering 22.104 S2002 Bragg Curve MIT Department of Nuclear Engineering 22.104 S2002 Channeling In Crystals MIT Department of Nuclear Engineering 22.104 S2002 Range Number-Distance Curve MIT Department of Nuclear Engineering 22.104 S2002 Range Curves Heavy Particles MIT Department of Nuclear Engineering 22.104 S2002 Computational Tools Z Generally, exact calculations are difficult due to multiple scatters, low energy effects, etc. Z Use SRIM 2000 as a computational tool for ion interactions in a variety of materials Z Program is on the lab computers in NW13-133 Z Can be obtained on web – http//www.research.ibm.com/ionbeams/home.htm#SRIM MIT Department of Nuclear Engineering 22.104 S2002 SRIM-2000 Input Data MIT Department of Nuclear Engineering 22.104 S2002 Example of SRIM-2000 Output 5.79 MeV a in air MIT Department of Nuclear Engineering 22.104 S2002 Cherenkov Radiation MIT Department of Nuclear Engineering 22.104 S2002 Electron Interactions Z Collisions – Bethe-Bloch modified due to small electron mass Z Bremmstrahlung – small electron mass makes this a major process above a few MeV (note that process is inverse to mass4 !) 2 æ e2 ö s µ r2 = ç ÷ e ç 2 ÷ è mc ø MIT Department of Nuclear Engineering 22.104 S2002 Radiation Loss vs. Collision Loss for Electrons MIT Department of Nuclear Engineering 22.104 S2002 Range Number-distance for Electrons MIT Department of Nuclear Engineering 22.104 S2002 Range Curves for Electrons MIT Department of Nuclear Engineering 22.104 S2002 Absorption Curves for b decay of 185W MIT Department of Nuclear Engineering 22.104 S2002 Example of Multiple Scattering MIT Department of Nuclear Engineering 22.104 S2002 Angular Distribution of Scatter MIT Department of Nuclear Engineering 22.104 S2002 Electron Backscatter MIT Department of Nuclear Engineering 22.104 S2002 Electron Backscatter Coefficients from Light Elements MIT Department of Nuclear Engineering 22.104 S2002 Electron Backscatter from Heavy Elements MIT Department of Nuclear Engineering 22.104 S2002 Radiation Loss vs. Collision Loss for Electrons MIT Department of Nuclear Engineering 22.104 S2002 Practical Example:Electron LINAC Rough rule for LINAC gives R (Rad/min/mtr) as function of accelerator energy in MeV and current in mA. Energy spectrum is flat but photon spectrum shows increase at low energy: 2.67 R = .07iavg E dW = k dE dN dN dW k = = dE dW dE E Thick target correction reduces low energy increase. MIT Department of Nuclear Engineering 22.104 S2002 Bremsstrahlung With Thick Target 9 1 10 8 1 10 7 1 10 M(Eo,Eg) N(Eo,Eg) 6 1 10 5 1 10 4 1 10 0 1 2 3 4 5 6 7 8 9 10 Eg MIT Department of Nuclear Engineering 22.104 S2002.