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