4 Electron Int

7/28/2019 4 Electron Int

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PNRA Course onRadiation Safety

Dr. Muhammad Tufail(Izaz-i-Fazeelat)

1

Lecture 4

INTERACTION OF FAST

ELECTRONS


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Interaction of Fast Electrons

Path traversed

Specific energy loss

Electron range and transmission curve

Monoenergetic electron absorption

Beta particle absorption Backscattering

Positron Interactions


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Electrons Path

Follow zigzag path through absorbing

material Interact with orbital electrons of the material

Large fraction of energy is lost in a single

encounter


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4

Specific Energy Loss

Bethe formula

where = v/c

Energy loss is More when electrons pass through the medium of high

Z & N

Less when energy of electrons is more Smaller than that of proton and alphas of same energy

222

22

2

2

2

0

4

11

8

11

1122ln12

ln2

I

Evm

vm

NZe

dx

dE

o


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Specific Energy Loss conti. Lose energy by two ways:

Coulomb interaction Radiative processes

Bremsstrahulung

Linear specific loss by radiative process

3

42ln4

137

1

242

4

cm

E

cm

eZNEZ

dx

dE

oor


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Specific Energy Loss conti.

Total energy lose

Collisional

Radiative

The ratio of radiative loss to collisional loss

is

E energy in MeV

rcdx

dE

dx

dE

dx

dE

700

EZ

dxdE

dxdE

c

r


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Specific Energy Loss conti.

Radiative losses are significant at:

greater electron energies

high Z materials

Coulomb losses are prominent at:

low electron energies through

ionization & excitation


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Electron Transmission

Monoenergetic electrons Range (R)

Extrapolation of the linear portion of the

transmission curve to zero give range

Path length (range) is 100s time greater than the

charged particles of same energy.

Range and material density

low density material, R 2 mm/MeV high density material, R 1 mm/MeV


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Electrons Transmission conti.


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Beta particles

The transmission curve is different from thatfor mono-energetic electrons

For majority of beta spectra curve isnearly exponential

Io counting rate without absorberI counting rate with absorbert absorber thickness g/cm2m absorption coefficient (cm2/g)

t

o

meII


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Relation between m & Em

Energy range relationsE

m 2.5 MeV

Em2.5 MeV

Em

end point energy (MeV); density (g / cm3);

Rm maximum range (cm)

14.1

17

m

m

E

mEmm ER ln095.0265.1412.0

016.053.0 mm

ER


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Backscattering

Re-emergence of electrons after entering inan absorber

Deflection of electrons at large angles

It is prominent for electrons with low incident energy

absorbers with high atomic number


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Backscattering affects the

Response of energy measuring detectors Amount of incident electrons

backscattering at the entrance window of a detector

Re-emission of electrons from the thick backing,

a source


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Positron Interaction

Interactions are similar to electron

interactions except that Coulomb interactions with electrons are

attractive

Annihilation of positron and production of two

photons of energy 0.511 MeV

4 Electron Int

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