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

96 4. Measurement of ✓ 13 with the Double Chooz far detector
The fraction of neutron capture on Gd is found to be 0.8658 ± 0.0015 for
data and 0.8798±0.0007 for MC. The ratio of those quantity is 0.9842 and is
used as MC correction factor, to account for discrepancy between data and
MC. The variation of the selection criteria on the MC reproduces the value
estimated on the data within 0.30 %, which is thus assumed as systematic
uncertainty [54].
tel-00821629, version 1 - 11 May 2013
Figure 4.18: 252 Cf neutron energy spectrum for data and MC (left). Spectral
fit function, used to estimate the fraction on neutron captured on Gd (right).
Due to the 252 Cf neutron multiplicity (3.869 ± 0.078 neutrons per fission
[53]), peaks for the simultaneous captures of two neutrons on Gd and H
(⇠ 10 MeV) and two captures on Gd (⇠ 16 MeV) are also seen.
Time cut e
ciency: ✏ t
The t cut e ciency represents the fraction of neutron captures within the
[2, 100] µs time window. The e ciency is calculated as a ratio between the
events in [2, 100] µs and the events in [0, 200] µs as function of the 252 Cf
source position along the z-axis and the guide tube as shown in Fig. 4.19
and 4.20 (left). The overall e ciency, averaged over the detector volume, is
found to be 96.2 %. The uncertainty are estimated from the discrepancies
between data and MC simulation, preformed with a custom Geant4 code to
properly account for the low energy neutron physics. Standard Geant4 MC,
in fact, assumes neutron capture on free-H, when it is known the H is in a
molecular bound state. The e↵ect is a shorter live time than the observed
ones. The data are found in good agreement with MC as shown in Fig. 4.21,
resulting in a systematic uncertainty of 0.50 % [54]. Data/MC discrepancies
are shown in Fig. 4.19 and 4.20 (right) as a function of the source position.