Developped and builded by LPSC Grenoble, four MWPC form the tracking
part of LAGRANGE: two plane chambers for the forward direction (theta
< 24 degrees) and two cylindrical chambers for the central angles (16
< theta < 155) with a slight overlap of the two setups. The two
plane chambers are set at a distance of 90 and 130 cm from the target point
and are rotated one with respect to the other by 45 degrees in order
to resolve for ambiguities when more than one charged particle is emitted
at forward angles. Their dimensions are 77 x 77 cm^2 and 96 x 96 cm^2
respectively. The two plane chambers have similar design: tungsten wires
(30 microm diameter for the first chamber with 256 wires, and 35 microm
for the second with 320 wires) are stringed along perpendicular planes;
5 mm gap separates the wires from the cathodes (two external and a common
internal between the two planes) cathodes are 30nm alluminum with 23 nm
mylar for the external ones. The gas used is a mixture of Ar (85%) and
ethane (15%). The efficiency of the plane chambers was measured to be
99.5 %. The central part of the wires is made non sensitive
by wrapping a standerd isolator tape around the wires to allow for the
passage of the beam.
The two internal cylindical chambers, in the central part of the detector,
have wires at 5.0 and 8.5 cm from the beam axis respectively. Having lengths
of 40.0 and 50.5 cm they cover the polar angles from 16 to 160 degrees
in the lab system. They are both slightly uncentered in the along the beam
direction by 3 cm to have a better coverage in the forward direction. Their
support is made of Rohacell, a specific foam of methacrylate sufficiently
rigid but with very low density (0.07 g/cm^3). In the internal part
of each chamber, the wires (20 microm diameter) are stringed horizontally
parallel to the beam axis. A gap of 4 mm separates the wires from the stripped
cathodes, externally glued to the rohacell shell. Cathodes are realised
with very thin copper layers in a kapton support. They are subdivided in
stips of 3.5 mm and spaced by 0.5 mm. They are helicoidally wrapped around
the shell to obtain stereo measurement and improved position resolution.
The same gas as for the plane chambers is used for the cylindrical
ones.