Etude et impact du bruit de fond corrélé pour la mesure de l'angle ...

68 3. The Double Chooz experiment
breakdown of the di↵erent contribution to the prediction of the reactor ¯⌫ e
rate uncertainty is shown in Fig. 3.20.
3.6 Detector simulation
tel-00821629, version 1 - 11 May 2013
The detector response is modeled through a detailed Geant4 [4] simulation
with improvements on the scintillation process, photocathode optical surface
model and thermal neutron model.
The custom scintillation process implements detailed light waveforms, spectra,
re-emission and Birks law quenching. The photocathode model is based
on a standard model of a thin, semitransparent surface with absorption and
refractive index, including also the photoelectron collection e ciency as a
function of the position of the emission on the photocathode. The custom
neutron thermalization process implements molecular elastic scattering for
neutrons below 4 eV and a radiative capture model with improved final state
gamma modeling.
The detector geometry is modeled to a fine level of details, with a particular
regard to the geometry of the PMTs, the mu-metal shield and all the materials
near the active volume such as tank walls and supports. The dimension
of the tank walls, of the supports and the position and orientation of the
PMTs were checked during installation and verified by photographic survey
with sub-mm accuracy.
The optical parameters used in the detector model are based on dedicated
measurements [8] and tuned with calibration data. The scintillator emission
spectrum, light attenuation and quenching were measured on scintillator
sample with dedicated laboratory setup [21].
The ¯⌫ e events are generated in correspondence with data taking runs, with
fluxes and rates obtained through the reactor simulations. The radioactive
decays from calibration sources were simulated using detailed models of nuclear
levels for each source, taking into account branching ratios and energy
spectra.
3.7 Read-out system simulation
The Geant4-based detector simulation outputs the deposited charge and the
time at which each PE strikes the photocathode of each PMT. The read out
system simulation (RoSS) converts these informations into an equivalent
waveform as digitised by the FADC.
RoSS accounts for the response of elements associated with detector readout
as the PMTs, FEE, FADC, trigger system and DAQ. The simulation
relies on the measured probability distribution function (PDF) to empirically
characterise the response to each single PE as measured by the full read-out
chain.