We propose deep polarimetric exposures with CAM of three fields in
 Cas A to image the MIR synchrotron emission separately from the
 (dominant) warm dust component. Because synchrotron loss time scales
 are predicted to be comparable to dynamical time scales in the ISOCAM
 wavelength range, information will be gained on the sites of
 acceleration of the relativistic electrons, their diffusion
 timescales, and the magnetic field strength in the supernova remnant.
 In this way we will test different hypotheses for the acceleration
 mechanism for the electrons and may gain empirical insight into the
 injection mechanism.