Particles going through the rohacell cages could completely miss the material in the cages, they could go through up to 2.5 cm of rohacell, or anything in between. The following table summarizes the amount of material in the MVD and the approximate energy loss in each material. Mine is not perfect, and should slightly overestimate the counting rate because it produces too many higher energy beta's.
Details are are here .
A pisa simulation of the MVD showed that a minimum energy of about 1 MeV was needed go through all of the material and then to trigger the scintillator in this model. The back of the envelope calculation above gives an estimated average energy loss of 0.4 MeV in the material. For details on the pisa code used, click here . This should be a significant overestimate. The following two plots show the initial kinetic energy distribution of a betas in the simulation followed by the initial kinetic energy distribution for those betas which triggered the scintillator.