[Gate-users] Divergent photon beam of point source and dose profile along the central axis of water phantom

[Stathis Kamperis]

Dear Gate users,

for once more I will appeal to your expertise. I'd like to simulate a
point source of photons 20 MeV that diverges in space so that when it
hits my water tank it forms a 10x10 cm field. Subsequently, I'd like
to measure the dose depth along the central axis.

I define the water phantom as:

/gate/world/daughters/name                      waterTank
/gate/world/daughters/insert                      box
/gate/waterTank/placement/setTranslation 0 0 25 cm
/gate/waterTank/geometry/setXLength      50 cm
/gate/waterTank/geometry/setYLength      50 cm
/gate/waterTank/geometry/setZLength      50 cm
/gate/waterTank/setMaterial              Water

The above commands place the water phantom centered at (0,0,25). I
then put a source at (0,0,-100cm), i.e. a source coming from the
negative z-axis towards the positive z-axis:

/gate/source/addSource mybeam gps
/gate/source/mybeam/gps/ene 20 MeV
/gate/source/mybeam/gps/particle gamma
/gate/source/mybeam/gps/ene/type   mono

/gate/source/mybeam/gps/position       0 0 -100 cm
/gate/source/mybeam/gps/pos/type       Point
/gate/source/mybeam/gps/ang/type       iso

/gate/source/mybeam/gps/ang/mintheta   177.1376 deg
/gate/source/mybeam/gps/ang/maxtheta   180 deg
/gate/source/mybeam/gps/ang/minphi     0 deg
/gate/source/mybeam/gps/ang/maxphi     180 deg

According to the gps documentation theta = 180 deg corresponds to a
beam travelling towards positive z-axis. Therefore, in order to form a
divergent beam that enters the water phantom as a 10x10cm field, I
need to restrict theta precisly atan(5/100) * 180 / pi = 2.8624 deg.
That is where 177.1376 comes from.

I've looked at my geometry (X-Y, X-Z, Y-Z planes) and it looks ok.

Finally, I put my dose actor at (0,0,0) which is along the central
axis of the phantom since z=0 is the entry point of my beam and the
whole phantom is centered at (0,0,25).

/gate/actor/addActor                               DoseActor  doseDistribution
/gate/actor/doseDistribution/attachTo              waterTank
/gate/actor/doseDistribution/stepHitType           random
/gate/actor/doseDistribution/setPosition           0 0 0 cm
/gate/actor/doseDistribution/setSize               1 1 20 cm
/gate/actor/doseDistribution/setResolution         1 1 2000

(for brevity I skip the commands that are not related to geometry)

The dose actor will output 2000 values that correspond to the 20cm
worth of water phantom (I suppose). The first value corresponding to
depth z=0, the 2nd value for z=0.01cm and so on. Is that correct or do
I have to specify some sort of direction to the actor ?

Also the one million dollar question. The dose profile that I get does
not make sense to me at all. I'm not seeing any build-up region. Here
is the plot:

http://dl.dropbox.com/u/25202446/opengate2.png

Does that make sense to you ?

Thank you for your patience and I promise I'll write up a nice
tutorial for novices like me when I master Gate!

Best regards,
Stathis