Micro-Pattern Gaseous Detectors (MPGDs) such as Micromegas or GEM are used in particle physics experiments for their capabilities in particle tracking at high rates. Their excellent position resolutions are well known but their energy characteristics
have been less studied. The energy resolution is mainly affected by the ionisation processes and detector gain fluctuations. This paper presents a method to separetely measure those two contributions to the energy resolution of a Micromegas detector. The method relies on the injection of a controlled number of electrons. The Micromegas has a 1.6-mm drift zone and a 160-$mu$m amplification gap. It is operated in Ne 95%-iC$mathrm{_4}$H$mathrm{_{10}}$ 5% at atmospheric pressure. The electrons are generated by non-linear photoelectric emission issued from the photons of a pulsed 337-nm wavelength laser coupled to a focusing system. The single electron response has been measured at different gains (3.7 10$mathrm{^4}$, 5.0 10$mathrm{^4}$ and 7.0 10$mathrm{^4}$) and is fitted with a good agreement by a Polya distribution. From those fits, a relative gain variance of 0.31$pm$0.02 is deduced. The setup has also been characterised at several voltages by fitting the energy resolution measured as a function of the number of primary electrons, ranging from 5 up to 210. A maximum value of the Fano factor (0.37) has been estimated for a 5.9 keV X-rays interacting in the Ne 95%-iC$mathrm{_4}$H$mathrm{_{10}}$ 5% gas mixture.
The next generation of proton decay and neutrino experiments, the post-SuperKamiokande detectors as those that will take place in megaton size water tanks, will require very large surfaces of photodetection and a large volume of data. Even with large
hemispherical photomultiplier tubes, the expected number of channels should reach hundreds of thousands. A funded R&D program to implement a solution is presented here. The very large surface of photodetection is segmented in macro pixels made of 16 hemispherical (12 inches) photomultiplier tubes connected to an autonomous front-end which works on a triggerless data acquisition mode. The expected data transmission rate is 5 Mb/s per cable, which can be achieved with existing techniques. This architecture allows to reduce considerably the cost and facilitate the industrialization. This document presents the simulations and measurements which define the requirements for the photomultipliers and the electronics. A proto-type of front-end electronics was successfully tested with 16 photomultiplier tubes supplied by a single high voltage, validating the built-in gain adjustment and the calibration principle. The first tests and calculations on the photomultiplier glass led to the study of a new package optimized for a 10 bar pressure in order to sustain the high underwater pressure.
