Production and calibration of 9 m2 of bulk-micromegas detectors for the readout of the ND280/TPCs of the T2K experiment

The near detector ND280 of the long baseline neutrino experiment T2K (JPARC, Japan) will contain three large 5 m3 time projection chambers (TPC) for charged particles tracking and identification. These TPCs will be the first large TPCs readout by micro pattern gaseous detectors (MPGD). The bulk-Micromegas MPGD is a micro mesh gaseous structure (MicroMeGaS) in which the woven micromesh is embedded on top of the segmented anode plane of the MPGD by use of standard photolithographic techniques. This MPGD was chosen for its performances in terms of gas gain uniformity, energy resolution and space point resolution, and for its capability to efficiently pave large readout surfaces with minimized dead zones. We developed and improved the bulk-micromegas production methods to be suitable for a uniform, cheap and high quality mass production of 34×36 cm2 bulk-micromegas modules of 1726 pads. Production of 72 bulk-micromegas modules, for an equivalent total readout surface of 9 m2, began in may 2008 at CERN. After the description of a module and its production, we report on the pad per pad calibration with a strong 55Fe X-ray source of the 32 bulk-micromegas produced so far. Gas gain and 5.9 keV energy resolution are presented to illustrate their good uniformity within the whole surface of each module (2.8% and 6% r.m.s dispersion, respectively) and their good reproducibility from one module to another (8% and 3% r.m.s, respectively).