  Northern Illinois University in collaboration with Fermi National Accelerator
Laboratory (FNAL) and Delhi University has been designing and building a proton
CT scanner for applications in proton treatment planning. The Phase II proton
CT scanner consists of eight planes of tracking detectors with two X and two Y
coordinate measurements both before and after the patient. In addition, a range
stack detector consisting of a stack of thin scintillator tiles, arranged in
twelve eight-tile frames, is used to determine the water equivalent path length
(WEPL) of each track through the patient. The X-Y coordinates and WEPL are
required input for image reconstruction software to find the relative (proton)
stopping powers (RSP) value of each voxel in the patient and generate a
corresponding 3D image. In this Note we describe tests conducted in 2015 at the
proton beam at the Central DuPage Hospital in Warrenville, IL, focusing on the
range stack calibration procedure and comparisons with the GEANT~4 range stack
simulation.
