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Chapter 2: Gamma Ray

Interactions with MatterDr. Phillip C. Womble

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Mean Free Path• Consider a radiation withan initial intensity of I0

•  The intensity diminishes

every m it travels in anexponential manner soI(x) is the intensity in xmeters

•  The mean free path is

dened aso vera!e distan"e that a

!amma ray travels beforeintera"tin!

α 

α 

1

)(0

=

=−

 x

e I  x I    x

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Making It Look Like the

Book• Consider a radiation

with an initialintensity of I

0

•   µl is the linear

atten#ation "oe$"ientin "m%&.  

 µ 

 µ 

1

)( 0

=

=−

 x

e I  x I   x

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Photon Interactions• Whether x%ray or γ %

ray' we examine

their absorption•  Two fa"tors of x%ray

absorptiono  The spa"in! between

atoms (density)

o

 The n#mber of ele"tronson ea"h atom

•  This n#mber is "alledthe * n#mber

• Is e+#al to then#mber of protons

,

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Transmission Thru Lead

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Linear AbsorptionCoefficients

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Density is Important

•  The tro#ble with µl is that it does not ta-e mass

into a""o#nt

• ass atten#ation "oe$"iento /atio of µl to ρ (µlρ)

• Density in !"m1 is ρ.

• Cal"#latin! mass atten#ation "oe$"ient of"ompo#ndo

Wei!ht fra"tion 2 per"enta!e of the total wei!ht that an element"omprises' wi

∑=i

iicompound    w µ  µ 

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3 Interactions of GammaRays with Materials

• Photoele"tri" 34e"t

• Compton 5"atterin!

• Pair Prod#"tion

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Photoelectric Effect

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Compton Scattering

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Practical Effect on Spectra

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Pair Production

Must have at least 2 xrest mass of electronenergy or 1.022 MeV

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Filters

Usually used in measurement of Pu gamma rays

ALL Pu samples have241

Am as an indicator

Thin Cd sheet

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Shielding Information