Radiation/Medical Physics Problem -- Scattered and Transmitted photons at detector

[Graham87]

Homework Statement

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Relevant Equations

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I managed to calculate the fluence of the scattered photons. However, not the transferred photons. In the solution sheet the fluence rate has an l₂² in the numerator in the end of the solution sheet. Where does that come from?

 

[TSny]

I managed to calculate the fluence of the scattered photons. However, not the transferred photons. In the solution sheet the fluence rate has an l₂² in the numerator in the end of the solution sheet. Where does that come from?

Note that ##\dot \varphi_0## is defined to be the fluence rate at the position of the graphite cylinder, which is at a distance ##l_1 = l_2## from the source. Thus, what would be the fluence rate at a distance of ##l_3## from the source, expressed in terms of ##\dot \varphi_0##, ##l_2##, and ##l_3##, assuming no lead shield?