The CREX experiment measures the 48Ca neutron radius using an electroweak probe. To run the experiment after PREX-2, which measures the neutron radius of 208Pb with the same electroweak parity violation technique, without a configuration change of the equipment, the septum magnet needs to meet field requirements suitable for CREX. As the magnet is iron-dominated the field response is non-linear at sufficiently high current. This is of particular concern for CREX which is run at a beam energy more than double that of PREX-2 and requires a large septum current to provide the specified bend radius. To determine it's suitability in the experiment the magnet non-linearity must be characterized. To that end, the magnetic flux density was simulated using finite element analysis software and was analyzed in the high-field regions and the zero-field region of the septum. In this thesis I will show evidence of magnetic saturation in the septum magnet at the CREX current and that the requisite field strength can be attained in the magnet gaps. I will also show that the proposed upstream and downstream extensions of the beam pipe shield reduce the maximum fringe field strength by more than 80% and provide a factor of 10 improvement to the maximum field integral inside the beam pipe region.