in the fluorescence emission spectrum that occur mainly for exciting wavelengths between about 350 nm and 400 nm (figure \[fig:ExEmMap\]). Our modified WLS model predicts a mean number of detected photons that is 17% greater than the Geant4 WLS model.

![The predicted optical photon spectrum in Detector A due to the scintillation (dashed traces) and wavelength-shifting (solid traces) processes, as measured by the upstream PMT for incident 2 GeV protons. The original Geant4 wavelength-shifting model results are represented using red traces, and the results from the modified wavelength-shifting model developed in this work are represented using blue traces.[]{data-label="fig:WLSmodels"}](OPspecAtPMT_2GeV_WbLS_upstream_MyWLSblue_G4WLSred_withLegend.pdf){width="0.8\columnwidth"}

### Scintillation Parameter Optimisation

The scintillation light yields and ionisation quenching factors of all samples were taken as free model parameters. The light yields were estimated by performing a $\chi^2$ optimisation of the simulated photoelecton distribution to the measurement, using data from Detector B when irradiated by 475 MeV protons. The ionisation quenching factor optimisation was performed in a similar manner, using Detector B’s 210 MeV proton irradiation.

![image](NumPE_MeasVsSim_ALL_150813_v2_dashed.pdf){width="\textwidth"}

![Measured (red trace) and simulated (blue trace) photoelectron distribution for a 2 GeV proton beam incident upon water in Detector B, Tub 2. The range over which the minimisation to the measured data 