Restricted energy loss of ultrarelativistic particles in thin targets – A search for deviations from constancy

Experimental results for the restricted energy loss of ultrarelativistic electrons, with Lorentz factors up to 3×1053×105, in a 535μm thin silicon detector are presented. The combination of high Lorentz factors and thin targets, opens for the possibility to study two mutually excluding effects, both based upon theory and on scarce experimental observations. One is a disappearance of the density effect when the transit time becomes so short that the effect of plasma-oscillations becomes negligible. This would result in an increased energy loss as the Lorentz factor increases. The other is an effect based on the existence of a coherence length over which there are contributions to the energy loss. This effect leads to a substantially decreased energy loss for thin targets as the Lorentz factor increases. The latter is shown to be incompatible with our measurements, whereas the former may be present with a correction of a few percent to an otherwise constant energy loss. Furthermore, we point to significant effects related to synchrotron and transition radiation, that must be carefully considered in a possible future experiment, and may as well have been of significance for the conclusion in earlier experiments.