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

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

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)

SRIM/TRIM

Montecarlo computer based methods: much better and flexible than

equations.

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

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)

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

210 /

102

cmgRMeVEELogx

R cxbxa