Anode wires longer than 1 metre are at intermediary positions supported by
so-called "Wire Locators".
The chosen interval between wire locators guarantees that the sagitta due to
electrostatic forces on a not perfectly centered anode wire remains small.
Typically, these are spaced at intervals of 80 cm.
The precise distance necessarily depends on the
total wire length to be subdivided and thus varies slightly from
module to module.
The largest number of locators per wire section,
i.e. between end-and middle-block, is two.
We experimented with locators of quite different shapes.
The final choice is suitable for production by injection moulding
and was designed by
NIKHEF
in cooperation with
Philips High Tech Plastics.
The design was based on the following criteria:
The locator essentially consists of two disks from which V-shaped sections,
with opposite orientations, have been cut out, the overlap of which
leaves an opening of about 0.050mm diameter for the anode wire.
The geometrical dimensions and tolerances are specified in the sub-assembly drawings of the detector modules.
The Wire Locator design was validated
through the following tests:
At both ends, the straw tubes fir onto cylindrical protrusions of a gas
distribution block. Centring of the sense wire ends requires wire-locator
functions to be incorporated in these "Straw Blocks".
The geometrical dimensions and tolerances are specified in the sub-assembly drawings of the detector modules.
