NE 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 2

Growth of Radioactive Products in a Neutron Flux 3

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.

Radiation Interaction with Matter 5

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

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

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

2. Kinetic Energy Transfer Energy imparted above the energy required to form the ion-pair 9

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 Molecular Excitation

4. Nuclear Reactions Particularly for high energy particles or neutrons Electromagnetic energy is released because of decelerating particles Bremsstrahlung Cerenkov Radiative Processes

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

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

 - 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

Definition of Ranges Extrapolated Range Mean Range R.  gives range in g/cm 2 (we’ll see why later) 15

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)

SRIM/TRIM Montecarlo computer based methods: much better and flexible than equations.

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

Results Screen 19 Read mean Range and “straggling”

Calculate and compare the range of a 10 MeV  - particle in air using TRIM, plot, and equation. 20

 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

Examples of formulas: Bethe Formula Berger Method (used in MCNP) 22

Empirical Equations What is the range of a 5 MeV electron in air?