Backscatter scanner simulation

Threw together this MCNP input file to work out the depth dose curve for a very simplified back scatter scanner. Since I don't have enough details to model exactly what's going on, I made a lot of simplifications regarding the geometry.

I'm not terribly concerned about creating a detailed model of the scanner. This is all just back of the envelope stuff. I just want to get some order of magnitude estimates that will give me an idea of how the dose would be distributed.

It's not complete yet. I still need to work on the source definition and set up an appropriate x-ray spectrum.


Backscatter screener simulation
c Simulation to look at the depth dose curve for one of those
c airport backscatter screening machines
c The "passenger" is modeled as a rectangular slab of water
c 1.8m tall by 0.5m wide x 0.3m thick
c Souce is modeled as a planar source 1.8m tall by 0.5m wide
c based on the assumption that these backscatter machines
c operate using a scanning line source (which seems to be
c a reasonable assumption)
c --------- Cell cards -------------------------------
1 2 -1.0 -1 imp:p=1 $ "Passenger"
2 1 -1.205e-3 1 -2 imp:p=1 $ Air around passenger

c --------- Surface cards ----------------------------
1 rpp -30 30 -15 15 -85 85 $ "Passenger" 60cm x 30cm x 170cm
2 rpp -250 250 -250 250 -250 250 $ universe, 500 * 500 * 500 cm^3

c --------- Data cards -------------------------------
mode p
rand gen=2 $ Use l'Ecuyer RNG, larger random number space
c --------- Source Definition ------------------------
c Define a planar source emitting 50 kVp photons located 150 cm away from
c the water slab
sdef par=2 x=d1 y=150 z=d2 erg=d3
vec=-1 0 0 dir=1
si1 h -30 30
sp1 d 0 1
si2 h -85 85
sp2 d 0 1
# SI3 SP3
c 50 kVp spectrum goes here
nps 1e9
c --------- Tallies ----------------------------------
c Mesh tally inside water slab
fmesh4:p geom=xyz origin=-30 -15 -85
imesh=60 iints=60
jmesh=30 jints=30
kmesh=170 jints=170
fm14 1.60651e-5 2 -5 -6 $ mesh tally in uGy per source photon
fc14 Water slab dose in uGy/source photon
c --------- Materials --------------------------------
c Dry air at sea level - Mass density = 0.001205 g/cm^3
m1 6000 -0.000124
7000 -0.755268
8000 -0.231781
18000 -0.012827
c Water - Density 1 g/cm^3
m2 1000 -0.111898
8000 -0.888102