NE 301 - Introduction to Nuclear ScienceSpring 2012

Classroom Session 7:

•Radiation Interaction with Matter

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Reminder

Load 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

dNn

dN

n

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 Radiation

Ion

izin

g

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Radiation Interaction with Matter

Five 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 Transfer

Energy imparted above the energy required to form the ion-pair

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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

3. Molecular Excitation

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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

stopped12

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High and Low LET

LET: 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 Ranges

Extrapolated Range

Mean Range

R. givesrange in g/cm2

(we’ll see why later)

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Ranges in Air

Range 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