dirty secrets about absorber thickness

SRIM results for 5.486 MeV He passing through S2 or LEDA dead layer:
S2: 0.8 um Al, average angle 10', average energy loss 126 keV.
LEDA: 0.4 um Al, average angle 30', average energy loss 72 keV.

The java code gives 233 keV loss through the S2, for alphas of the same energy. This is something I've run into before: the parameters used to calculate energy losses (from Ziegler's tables--I think they're the same as in the DEDX code) are usually pretty close to SRIM, but sometimes differences do show up. An inelegant way around this problem is to enter a fake thickness for the dead layer, chosen so that the java code's energy loss for that thickness matches SRIM's energy loss for the real thickness.

So far I've been using 108 ug/cm2 = 0.4 um for LEDA's dead layer thickness, and twice that for the S2. The values that give sensible answers are 88 ug/cm2=0.33 um for LEDA and 106 ug/cm2 = 0.39 um for S2. These are what I used in the alpha calibration simulation.