NE 301 - Introduction to Nuclear Science Spring 2012

NE 301 - Introduction to Nuclear ScienceSpring 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

Growth of Radioactive Products in a Neutron Flux • Notice saturation after 3-5 times T1/2 radioactive product. • Additional irradiation time does not increase activity.

Radiation Interaction with Matter

Ionizing Radiation: Electromagnetic Spectrum Ionizing Radiation 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 Interaction with Matter • Five Basic Ways: • Ionization • Kinetic energy transfer • Molecular and atomic excitation • Nuclear reactions • Radiative processes

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

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

3. Molecular Excitation • 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

4. Nuclear Reactions • Particularly for high energy particles or neutrons • Electromagnetic energy is released because of decelerating particles • Bremsstrahlung • Cerenkov 5. 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

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

 - Ranges Bragg peak • 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

Definition of Ranges • Extrapolated Range • Mean Range • R. gives range in g/cm2 (we’ll see why later)

Ranges in Air • Range of  particles in air, can be used to find their energies Equation valid for 3 cm < R < 7 cm (aka. most ’s)

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

SRIM-TRIM Use: Put energy 1 MeV=1,000keV Select projectile (proton = hydrogen) Indicate Target Thickness, such that tracks are visible Select target or find a compound Run

Results Screen Read mean Range and “straggling”

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

 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.

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

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

NE 301 - Introduction to Nuclear Science Spring 2012