[Gate-users] Back-to-Back problem

[Josh Knowland]

All,

I am having a problem with the backtoback helper keyword for pet photons. Perhaps I am doing something wrong you can point out. I have already looked in the user manual and the source code as well.

I have a 1cm3 cube source within a large sensitive detector. I define the source like this:

/gate/source/addSource                                                              src_1cmCentered
/gate/source/src_1cmCentered/gps/particle                       gamma
/gate/source/src_1cmCentered/setType                               backtoback
/gate/source/src_1cmCentered/gps/monoenergy             511 keV
/gate/source/src_1cmCentered/setForcedUnstableFlag true
/gate/source/src_1cmCentered/setForcedHalfLife            6586 s
/gate/source/src_1cmCentered/setActivity                          640000 Bq
/gate/source/src_1cmCentered/gps/type                             Volume
/gate/source/src_1cmCentered/gps/shape                          Para
/gate/source/src_1cmCentered/gps/centre                         0.000 0.000 0.000 mm
/gate/source/src_1cmCentered/gps/halfx                            5.000 mm
/gate/source/src_1cmCentered/gps/halfy                            5.000 mm
/gate/source/src_1cmCentered/gps/halfz                            5.000 mm
/gate/source/src_1cmCentered/gps/angtype                      iso
/gate/source/src_1cmCentered/gps/confine                       1cmCentered_phys

When I run this macro, I get approximately 640,000 in _Run.dat. But wait! I thought it would generate two photons for each!
To test, I deleted the macro line that sets backtoback and reran it. Nothing else changed.

Now I still get approximately 640,000 in _Run.dat!

I compared the hits output from each run and found that with the backtoback flag enabled, I get 55% more hits than I do with it deleted. I would expect 100% more...

Any thoughts? What am I doing wrong? I want to make sure that 2 photons are actually being generated for every 1 Bq I enter. The fact that _Run.dat doesn't change is a small matter if I can be sure the correct number of photons existed.





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