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