NeuLAND - FAIR

HV
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Figure B.1.: Schematic cutout of a neutron detector MRPC module, as seen from one
of the sides where the signals are read out. From top to bottom, a 2 mm
thick stainless steel converter plate (a), a gas volume (b). Subsequently, the
signal cathode formed by copper strips applied on mylar foil (c), and the
high voltage cathode given by mylar foils with one-sided antistatic coating
(d). The 1 mm thick float glass sheets (e) form a symmetrical 2×2 gap
structure. The high voltage anode (f) is from the same material as the high
voltage cathode. The central 4 mm thick signal anode strips (g) also serve
as converter for the subsequent lower half of the MRPC structure. The
gas volumes (b) between signal cathode and outer converter serve to reduce
cross-talk between cathode strips.
Incident neutrons are converted to charged particles in a hadronic shower taking place
in the iron converter. This process takes place either in the 2+2=4 mm thick converter
on top of the 2×2 gap MRPC unit, or in the 4 mm thick converter and central anode
that is placed below the top 2 gaps (figure B.1). Subsequently, the secondary charged
particles are detected in the MRPC structure.
Different from LAND, each 2×2 gap MRPC structure is read out separately, giving a
granularity of 15 mm along the direction of propagation of the beam. Perpendicular to
the beam direction, a granularity of 26.5 mm (25 mm strip width, 1.5 mm gap between
strips) is obtained. Along each MRPC strip length, the spatial resolution, with the
interaction point determined from the time difference between the readout on both sides,
depends on the time resolution of the MRPC.
As each such module (figure B.1) has a low detection efficiency for 400 MeV neutrons,
many modules must be placed one after the other to reach 90% efficiency. For mechanical
reasons, each layer is divided into four modules of 2 m × 0.5 m each, leading to the
preliminary design sketched in figure B.2. In this scheme, the 2 mm thick steel outside
converter plates would also serve as housing and vacuum tight box for the MRPC structure
inside it and provide mechanical stability. In order to service individual modules,
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