Process for preparing ethylene copolymers in the presence of free-radical polymerization initiator at pressures in the range of from 160 MPa to 350 MPa and temperatures in the range of from 100\xb0 C. A system may include a stage configured to support a specimen and coupled to a measurement device. A substantially transparent cone optically coupled to the photo conductor cell, and a first layer of phosphor material coated onto the transparent cone. The first strap is secured by both ends to the base portion of the sleeve in a location that is opposite to the cutout.