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

tel-00724955, version 1 - 23 Aug 2012
6
132 Analysis of Run 10
events in [−1000, −15] ∨ [+5, +1000] ms [−15, +5] ms
expectation 618 ± 3 6.2 ± 0.1
measurement 595 ± 25 6.1 ± 0.2
Table 6.3: Measured and extrapolated rate of accidental coincidences.
tveto − tbolo ∈ [−15, +5] ms low energy events high energy events
ER < 250 keV ER ≥ 250 keV
measured events 10 33
expected accidentals 3.3 ± 0.2 2.8 ± 0.2
excess coincidences 6.7 ± 0.4 30.2 ± 5.5
signal background 2.4 9.1
Table 6.4: Events at ER < 250 keV and ER ≥ 250 keV in the coincidence region.
The time interval defined at high energy is very sharp. The small number of
events may mislead to a too small windows. Different positions of the excess and
different windows are tested in Table 6.2. Note that the exact mean position of the
time interval defined at high energy is 3.30 ms. The number of low energy events
(ER < 250 keV) is the same for ∆t = 0 ± 10 ms and ∆t = −5 ± 10 ms and for
∆t = −5 ms, the maximum of excess events at high energy (ER ≥ 250 keV) is
reached for a window length of 10 ms and is the same as ∆t = −10 ± 10 ms. The
time window is selected to be ∆t = −5 ± 10 ms rather than ∆t = −5 ± 5. The time
window of coincidences for Run 10 is set to
∆tcoinc = −5 ± 10 ms (6.10)
tveto − tbolo ∈ −15, +5 ms. (6.11)
The events in Table 6.2 are excess coincidence candidates after subtraction of the
accidental background. The expected number of accidental events Nacc is calculated
from the bolometer and muon veto rates as in Equation 5.5, with Γ multiveto>3
veto = 5.22
mHz, Γbolo = 10.7 mHz, t = 294.7 4.56 d. On a time window of ∆twin = 2 s
(ie. ±1 s), N exp
acc 624 ± 3. The comparison with the experiment is in Table 6.3.
The calculated rates of accidental events is reproduced by the measurement, they
agree within the statistical uncertainties.
Inside the coincidence time interval, it is interesting to differentiate between
the high recoil energy events ER ≥ 250 keV, which are probably fast neutrons or
even passing through muons, and low energy recoil events ER < 250 keV, which
are more interesting muon-induced phenomena as in the energy region of interest
for the WIMPs. The real number of coincidence candidates has to be extracted
subtracting the accidentals for this region. The results of this calculation is shown
in Table 6.4. The signal over background confirms the clear excess of events inside
the time window of the expected coincidences. The excess coincidences are the
muon-induced candidates and can be compared with the simulated rate from [184].
The simulated rate is extracted from the the simulated energy deposit spectrum
in Ge bolometers in Figure 5.12, as explained previously in Chapter 5. It gives
Γsimu coinc ∼ 0.03 ± 0.01 events·(kg·d) −1 for 1 < ER < 250 keV. However, since the data