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

tel-00724955, version 1 - 23 Aug 2012
3
72 The EDELWEISS-II experiment
The data are saved in three different formats per Run : some data event files,
a scaler file and a parameter file.
The data files have all necessary information for data analysis, as detailed in the
Table 3.1. The header of an event is made of the variables event number, geometry,
multiplicity and size. The geometry gives a reference to the side(s) of the muon
veto system which got a hit. The multiplicity is the number of TDC with a signal,
plus the TDC of the common stop. For example, an event, which makes an internal
coincidence in a module, has the two TDC of each sides, plus the common stop,
which means a multiplicity multiveto = 3. The size is the size of the data blocks.
After the header is registered the time stamp of the event, which comes from the
bolometer system via a fiber connection. The time stamp is a pattern of 48 bits
of 64-ns width with an additional start bit. As the veto system runs on a 32-bit
processor, the time stamp is split into three times 16 bits and saved into three 32-bit
words. The time stamp has a precision of 10 µs. The local computer time, which
have only a precision of the order of 1 s, is not registered with the event, except in
the very rare case when no time is transferred from the bolometer system. In this
case, the time is registered using all the 32 bits of the first word. Then, are saved
the data blocks, which contain the information about the TDC and ADC entries.
Data files have a 8-hour-length, and there are up to 99 event files in a Run. If more,
a new Run starts (so a new Run does not mean a restart of the DAQ).
The scaler file is used for monitoring and is registered every 15 min, or as specified
into the input file of the DAQ software. If a new Run starts during a continuous
running period, a new scaler file is created in the new Run folder. The parameter
file summarizes the parameters of a running period. It is created at the beginning
of the DAQ program from the input file and is then exported to the new Run folder.
Data acquisition typically runs continuously but is interrupted e.g. when the
HV has to be switched off. HV are switched off for the safety of the people in the
clean room and to protect the veto system from disturbances, when critical works
are being performed or when there are maintenance periods on the bolometers.
On Figure 3.13, one can see long periods with almost continuous data acquisition
interrupted by electric power cuts in the LSM laboratory as well as extensive work
going on in the clean room of the experiment, when the high voltage of the veto
system was switched off for safety reasons. Above the rather constant raw data
rate, there are periods with much higher event rates which could be identified as
maintenance intervals of the cryostat/clean room in which the upper (mobile) veto
modules were moved with the polyethylene shielding, or when radioactive sources
were being manipulated.
Since July 2006, the muon veto system is stable in terms of hardware, we consider
the recorded event data as exploitable since then.
3.3.3 Online monitoring
The monitoring of the muon veto system is made with the data collected through
the scaler cards. A scaler card is a counter. The scaler data are raw data made
of any hit in the system, where no module coincidence is required, and are helpful
to monitor the system. With a rate of 8 kHz for the overall system, which can be
arbitrary changed with the settings of the threshold, these are mainly composed
On the contrary to the bolometers, a Run is made of some files and a continuous period of
data acquisition, i.e. a running period, can stretch over several Runs during some months.