Which of the two is correct way of incorporating radioactive decays?

Particles, Track, Event, Run and Biasing
1

I have a small box filled with radioactive gas; the box also contains detector. I am confused which one of the two is correct way to ensure radioactive decays taken into account:

First:

/process/had/rdm/noVolumes
/process/had/rdm/selectVolume gas_chamber

Second:

/process/had/rdm/selectVolume gas_chamber

I get different output with these two; with relatively more events in my detector with second approach. I expected the same output as First option basically doesn’t select any volume and then just chooses the ‘gas_chamber’ where decays will be applied. Which is what Second option must be doing? How are these approaches different and which is correct?