  The MICROMEGAS (MICRO-MEsh GAseous Structure) charge amplification structure
has found wide use in many detection applications, especially as a gain stage
for the charge readout of Time Projection Chambers (TPCs). Here we report on
the behavior of a MICROMEGAS TPC when operated in a high-energy (up to 800 MeV)
neutron beam. It is found that neutron-induced reactions can cause discharges
in some drift gas mixtures that are stable in the absence of the neutron beam.
The discharges result from recoil ions close to the MICROMEGAS that deposit
high specific ionization density and have a limited diffusion time. For a
binary drift gas, increasing the percentage of the molecular component (quench
gas) relative to the noble component and operating at lower pressures generally
improves stability.
