Etude de bruit de fond induit par les muons dans l'expérience ...

tel-00724955, version 1 - 23 Aug 2012
5.4 Coincidences between the muon veto and the bolometers 109
hits/crystal [1/keV/day]
Energy deposit in bolometers/hit [keV]
Figure 5.12: Geant-4 simulation of muon-induced Germanium hits. The total energy
deposit (blue) can be assigned to electron recoils by electrons and photons (green) and
nuclear recoils mainly by neutrons (purple). Figure from [184].
Even if we decide to change the time interval and to go for e.g. 23 ± 10 ms, the
number of correlated excess events remains almost unchanged.
The simulated rate is extracted from Figure 5.12. In Figure 5.12 is shown the
simulated energy deposit spectrum in Ge bolometers as a result of muon-induced
electromagnetic and hadronic showers. The contribution of nuclear recoil clearly
dominates below 50 keV, which is also a feature of the measured coincidences, cf.
Figure 5.13. However, there is a small threshold of 15 keV on the bolometer heat
channel, which allows to disregard a part of the exponential contribution and allows
to estimate the expected rate in the following way. There are 4 · 10 −5 hits per
crystal (1 keV·day) at 250 keV, this corresponds to Γ simu
coinc = 4 · 10 −5 × 250 0.320
events·(kg·d) −1 . For 1 < ER < 250 keV, the coincidence rates are
Γ simu
coinc ∼ 0.03 ± 0.01 events·(kg·d) −1
Γ meas
coinc = 0.043 ± 0.015 events·(kg·d) −1
(5.7)
(5.8)
The measurement is well reproduced by the expectation, but note that the simulation
is made for the full range of energy deposit below 250 keV and for the compact
geometry of all 120 bolometers. For ER ≥ 250 keV, the measured coincidence rate
is 0.09 ± 0.02 events·(kg·d) −1 .
5.4.2 Identification of the coincidence events
It is not possible to define which of the 44 events are accidental events and which
are the muon-induced events, but it is possible to visualize them on a Q-plot to
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