NeuLAND - FAIR

Beamline
Be exit
window
P 6
P 3
P 4
MRPC
124.5 28.5 10
Figure B.3.: Experimental setup at the ELBE facility, with approximate distances in
cm. The MRPC unit is placed on a remote controlled moving table which
allows 50 cm movement both in horizontal and vertical direction. The sizes
of the plastic scintillators are (x/y/z) 20 mm × 20 mm × 5 mm (P3,4), 35 mm
× 35 mm × 5 mm (P6).
of operation of the ELBE accelerator was used, called one electron mode. In this mode,
the gate voltage of the electron gun is reduced much below usual operating parameters.
Then, most accelerated bunches are empty and the rest has only one electron per bunch
[Nau-08]. Two scintillators were used as trigger in coincidence with the RF signal. The
flux of single electrons was 5-20 Hz/cm 2 for the present measurements, much higher than
can be obtained with cosmic rays.
The electron beam first passes through 5 mm thick scintillator (P6) and then a thicker
20 mm scintillator (P3,4). The coincidence requirement defining an electron event is
formed by P6 ∧ P3 ∧ P4 ∧ RF, meaning both photomultipliers of scintillator P3,4 and
scintillator P6 detect a signal above threshold.
In order to determine the time resolution, the time signal from the MRPC under study
is compared with the reference time from the radio frequency signal (hereafter called RF
signal) of the electron source. The high precision RF signal is supplied by the accelerator,
with a jitter of less than 25 ps as measured with a digital oscilloscope. Therefore, no fast
start detector is needed and the time resolution of the scintillators included in the setup
is not critical. For the processing of the timing signal, two options are available:
1. FOPI FEE1-based readout: The RF signal is fed into a 25 ps per channel timeto-digital
converter (TDC, CAEN V1290 2 ). The signals from the readout strips
2 CAEN S.p.A., internet address http://www.caen.it
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