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

tel-00724955, version 1 - 23 Aug 2012
4 Muon detection
Contents
4.1 Muon veto conventions
4.2 Life time of the muon veto
4.3 Muon rate
4.4 Mean muon energy deposit per module
4.5 Tracking muon candidates
Who ordered that?
I. I. Rabi.
Muons are the only component of the primary cosmic ray interactions in the atmosphere
that survives underground beyond the ∼ 10 mwe scale. And though
there is a reduction of the cosmic muon flux by the rock overburden in LSM and
equivalent laboratories of more than 106 compared to sea level, neutrons produced
in deep inelastic scattering (DIS) of cosmic muons are the most prominent background
in the upcoming second generation experiments searching directly for Dark
Matter. The identification of muons in the vicinity of the Germanium detectors allows
a significant suppression of this background source and hence an improvement
in the experimental sensitivity.
The muon veto consists of 42 plastic scintillator modules, read out at both ends
with an electronics and a data read-out fully independent from the bolometer system,
except for a fiber connection transferring the overall experiment synchronization as
well as some restricted information about muon hits, as detailed in Section 3.3. The
muon veto is the extreme outside layer of EDELWEISS-II, surrounding almost hermetically
the experimental setup with a total surface of ∼ 100 m2 , see Figures 3.10
and 3.11, page 69. The muon veto has been installed since May 2005 at the underground
laboratory LSM in Modane, ready for data taking since September 2005 and
stable in terms of long term measuring conditions since July 2006.
4.1 Muon veto conventions
The muon veto registers two kinds of data files: The scaler files made of raw data
and the event data file.
The raw data from the scaler cards correspond to any signal collected by the
photomultipliers at the end of a side of a module and above the threshold of the
discriminator cards. It is the number of hits per channel per 15 min, with no other
information. They are used to monitor the system, cf. Section 3.3.3. With a rate
81
4