Using additives: Turn off CERFAC!

Users should be warned that if one is using WLS or scintillator, the trick of prescaling the number of photons by the quantum-efficiency of the phototube as a function of wavelength (or CERFAC) no longer works. In the case of the scintillation photons, this optimization could in principle be done, but as soon as a wavelength shifter is introduced to the system, all bets are off. This is because the MC has no way of knowing at the time of creation of a photon what it's destiny will be: absoption, hitting a PMT, or getting absorbed and re-emitted by the WLS. If the latter, the wavelength changes, and therefore any initial weighting is rendered incorrect.

To run using additives, users should ensure that they are using only the simplest CERFAC strategy:

 $mode_cerfac  1
 $cerfac                0.28
 $cerfac_ii             0.337

Because of this (and because scintillation and WLS tend to generate a lot more light) running addtives is SLOW. This is an unavoidable consequence of the sheer volume of photons that the system needs to simulate.