Tracking stations are outside the decay volume (should be inside)

[yabezsh]

I noticed that our default vacuum in the decay volume only extends to the first tracking station. After that, we have different elements of each tracking station in the vacuum (btw, ultravacuum, 10^-16), but the space between them and between the stations is just "cave" (==air). The tracking stations should be inside the decay volume.

The current configuration probably gives us worse resolution for the vacuum/helium decay volume, so we should not expect worse sensitivity/Bg estimate. However, we need to consult with construction experts to implement the right geometry. In the case of helium, would the tracking stations be outside the decay volume? Where should we place the end cap?