Status report on high-pressure TPC

Abstract HELLAZ experiment intends to measure with high precision the low-energy solar neutrino (p–p and 7 Be) spectrum. The method uses the neutrino elastic scattering over the 2×10 30 electrons contained in a 2000 m 3 TPC filled with helium (for its high radio isotopic purity, its low transverse diffusion and its low drift velocity), at a pressure of 20 bar at 300 K. Reconstructing the neutrino energy requires a precise measurement over the angle and energy (between 100 keV and 1 MeV) of the scattered electron. 1% electron energy accuracy is easy to get through an integral charge measurement. An angular accuracy ≈±2° yields a neutrino energy resolution better than 10%, not only enough to separate the p-p and 7 Be spectrum, but which also possibly allows to show the shape variation of p–p spectrum in certain cases of neutrino oscillations. In a TPC the only way to obtain an angular resolution better than 5° is to detect every single ionization electrons of the diffused electron cloud. Hence, we need a very fast detector (collection time ⩽10 ns) and a gain of around 10 6 with a signal over noise ratio near 2 for the single electron detection. The only gas detector that meets the criterion is the MICROMEGAS detector which has a very good separation between electron and ion signal. These conditions are easy to obtain with a gas mixture like He and isoC 4 H 10 at atmospheric pressure. But in the real condition at 20 bar, where we currently use CH 4 as a quencher, we are limited to a gain ⩽4–5×10 4 . We have studied the dependence of the gain with the gas mixture. Several improvements have been done with a new mixing gas He, isoC 4 H 10 and CH 4 . After a simulation with IMONTE45 we did obtain the right mixing gas and high gain up to 5×10 5 with near 50% efficiency for the single electron detection. Recently, a new improvement has been done with a double stage MICROMEGAS. With this set-up we obtained 1.6×10 6 and an efficiency greater than 90% for the single electron detection.