DENSITY EFFECT IN RELATIVISTIC K-SHELL IONIZATION

Using a grazing emission method, we have made measurements of 22.6-GeV electron impact ionization cross sections at the front and back faces of approximately 70-mum thick Al and Cu foils. These give, respective ly, the cross sections without and with a density effect predicted by Fermi. For Cu, the back cross section ratio at 22.6 and 5.1 GeV (previ ously measured) is 1.00 +/- 0.04, confirming the predicted energy inde pendence of this cross section. The absolute cross sections agree over all with theoretical calculations.