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

4.5 Data analysis summary 99
4.4.3 Spill-in/out
tel-00821629, version 1 - 11 May 2013
Neutrons are captured on Gd once their energy become thermal. The thermalization
process happen through multiple elastic scattering between the
neutron and the nuclei of the atoms of the scintillator, the characteristic
time is of about 30 µs. During the thermalisation process, the neutron is
di↵used in the scintillator. A neutron from ¯⌫ e IBD in the target could reach
the -catcher volume and be captured on H. On the other hand, the neutrino
could interact in the -catcher and the neutron be captured on Gd in
the target volume. Such e↵ects are called spill-out and spill-in respectively.
Such e↵ects have to be taken into account since they do not compensate
precisely, resulting in a net spill-in current which impacts on the normalisation
of the MC simulation [74].
Due to the presence of Gd in the target liquid, the mean live time of a
neutron in the target volume is shorter (⌧ Gd ⇠ 30 µs) than the one in the
-catcher (⌧ H ⇠ 100 µs). So the spill-in probability is expected to be larger
than the spill-out. Thespill-in/out e↵ect is studied with ¯⌫ e MC sample and
the systematics e↵ect due to the MC model (⇠ 0.22 %), the concentration of
Gd in the target (⇠ 0.10 wt. %) and the concentration of H in the -catcher
(negligible e↵ect) are taken into account. Spill-in/out correction is found to
be 1.35 ± 0.29(syst) ± 0.04(stat) % [62].
4.5 Data analysis summary
Tab. 4.2 report a summary of all the necessary input parameters for the
measurement of ✓ 13 . The parameters obtained for the first DC publication
[19] are also reported for comparison. Beyond the increased statistics due
to the longer exposure time (about a factor 2), improvements on the MC
modelling of the low energy neutron physics, energy scale calibration, cosmogenic
background analysis and correlated background analysis allow to
decrease the total systematic uncertainty from ⇠ 3.7 %to⇠ 1.8 %.
4.6 Final fit and measurement of ✓ 13
The measurement of ✓ 13 is performed by a combined fit to the ¯⌫ e rate and
spectral shape, to explicit account the spectral distortion induced by the
oscillation, i.e. energy dependence of the oscillation probability.
The observed ¯⌫ e spectral shape is compared to the expected one, as obtained
by the simulation of the reactor ¯⌫ e flux, and the spectral shape obtained from
background studies. The energy interval between 0.7 MeV and 12.2 MeV
has been divided in 18 non-constant bin to ensure enough statistics per bin.
The expected number of signal and background events is obtained for each