favor downward scatter direction in egs++

[kurtsche]

Hello everyone,

Im currently trying to build an Xray chamber in egs++. I have a beam of electrons hitting an anode and I want to favor/follow only the photons that travel in the -z direction, but when looking at the .ptracks file most of the photons are traveling in different directions which is quite inefficient. I tried to work with variance reduction but in all of the examples available it's only used in the scoring plane and I am not sure if that's even a viable option. Can I somehow define that only created photons that are traveling downwards are calculated and all other are ignored.
I mainly need it for dose/fluence scoring where I currently only have around 5 hits in 100000 particles.
Any help is really appreciated!

[rtownson]

We can resume discussion on #1366 to keep it together. I suggest DBS in BEAMnrc because it's still in development for egs++.