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

4.3 Backgrounds 89
energy spectrum (right). The time correlation appears flat as expected from
random coincidences. The prompt-like spectrum is dominated by natural
radioactivity and decreases of about 2 order of magnitude above ⇠ 3 MeV.
The accidental background rate is found to be stable over the considered
time period, as shown in Fig. 4.11.
)
-1
Rate (day
0.6
0.5
0.4
0.3
tel-00821629, version 1 - 11 May 2013
Entries / 200 keV
0.2
0 50 100 150 200 250 300 350
Day of data taking
Figure 6: Accidental background rate (day 1 )perdayofdatataking.
Figure 4.11: Accidental background rate as a function of the data taking
day. The accidental background rate is found to be (0.261 ± 0.002) cpd and
it is stable over the considered period.
3000
3
2500
4.3.2 Cosmogenic radioisotopes background
10
2000
The cosmogenic radioisotopes 9 Li and 8 He are produced 10 by cosmic muon
2
1500
spallation on 12 C in the liquid scintillator [17].
1000
10
n-decays with lifetimes of 257 ms for 9 Li and 172 ms for 8 He. The initial
500
0
1
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0 2 4 6 8 10 12
correlated events, which can mimic the E ¯⌫ (MeV) e signal. Due to the long isotopeE (MeV)
6
Entries / 200 keV
Such isotopes undergoes
-decay and later neutron capture produce two temporally and spatially
lifetime, the application of a veto time after each muon interacting in the
Figure 7: Prompt energy spectrum for accidental background events. The
detector wouldsingle leadevents to an spectrum unacceptably is superimposed long in red dead normalized time. to the same number
The 9 Li rate isof obtained entries. by searching for triple fold coincidences between an
¯⌫ e -like candidate and a suitable parent muon in a 20 s long time window
(⇠80 9 Li life time) and fitting the time distribution with an exponential and
a flat components [99].
The 9 Li rate produced by high energy muons (E> 600 MeV, muons crossing
both target and gamma-catcher) is obtained directly by a fit of the t
distribution as shown in Fig. 4.12. A precise rate of (0.95 ± 0.11) cpd is
obtained.
For mid-range energy muons (E in [257, 600] MeV, muons crossing gammacatcher
and a fraction of the target), the 9 Li rate for events near to the muon
track (d < 0.8 m) is obtained directly from the fit of the t distribution