EGS4 results for 1TeV NLC shows 5-10 hits/mm2/train at r=1cm and less than 1 hit/mm2 at r=2cm, however 10 times more hits by backscattered particles to be reduced by a low-Z mask. 30K photons/train penetrate the CDC at r=30cm even at 4T!, which may not be tolerable for a drift chamber. Most of photons come from the interaction of the pairs with the beam pipe and from backscattered from the surfaces which are closes in z to the IP, namely the luminosity monitor at z=190cm, the front face of the quadrupole doublet at 2m, and the septum of the rf shield, currently at 189cm. Synchrotron radiation backgrounds: at r=30cm, z=100cm is 167K for 2T and 110K for 4T per train-crossing!
Anyway, we need a performance study on CDC and VTX under a condition of backgrounds by ABEL/JIM simulation.