|Message: Re: GPS(GeneralParticleSource) on hemisphere volume||Not Logged In (login)|
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This is a late reply. Hope you have find a solution already. Anyhow you were right that "biasing" is the right approach to use. In GPS you can bias any of the source distributions, position, direction, energy etc. In your case, assuming you want a hemisphere in +Z only, you need to apply biasing to the postion distribution in the z-direction:
/gps/hist/type biasz /gps/hist/point 0. 0. /gps/hist/point 0.5 0. /gps/hist/point 1. 1
Please refer to the online GPS SUM and examples for more help