link to lecture transcript - UT-H GSBS Medical Physics Class Site

First, let’s recall that the attenuation coefficient is the fraction attenuated per unit
absorber b b thickness. hi k How many photons h are attenuated? d? We could ld use the h
exponential approximation but when we’re estimating let’s use the thin absorber
approximation. The number of photons attenuated is the number of incident
photons times the attenuation coefficient times the absorber thickness.
How many incident photons do we have? We have been told that the number of
incident photons is 104 cdetp otoss 0. . WWhat at is s the t e Compton Co pto attenuation atte uat o coefficient coe c e t for o ½ MeV eV
photons. We look that up in Table A2a in Johns & Cunningham and find that to be
0.2892 10-28 m2 per electron.
What’s the thickness of the absorber? The thickness of the absorber is the thickness
of the bone times the density of bone. We multiply the thickness of bone, 0.6 cm,
by the density of bone, 1.650 kg/m3 , which we get from the Table 5.3. Next we
multiply by 10-2 multiply by 10 to convert the bone thickness from meters to cm
2 to convert the bone thickness from meters to cm.
Let’s convert that into centimeters times the electron density. Remember we have
to be able to calculate the electron density to be Z over A times Avogadro’s number.
Z over A is about a half. So electron density is always going to be roughly half of
Avogadro’s number. That’s a very good approximation for low to moderate Z
material, such as bone. Electron density comes out to be 3.19 10 23 electrons per
kilogram. So the electron areal density is 3.16 10 27 electrons per meter squared.
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