Measurement of the absolute fluorescence yield in Nitrogen between 0.5 and 2.3 MeV

International audienceDetecting UHECR showers using fluorescence to determine energy requires a precise knowledge of the fluorescence yield. If the fluorescence mechanism itself is well understood, as well as the number of excited lines in N$_2$ and their precise wavelengths, the yield is known to only about 25\% (even worse for each line). So, different collaborations (Auger, EUSO, HiRes...) decided to create a consortium of about ten experiments, called AirLight, where the fluorescence yield is measured accurately for different electron energies and with different setups. In APC (Paris, France), the EUSO group uses a $^{90}$Sr source (0.5 - 2.3 MeV electron energy, different energies being selected) to produce the fluorescence and a grating spectrophotometer to count the number of photons produced per cm of gas traversed by the electron. Pressure, temperature and gas composition can be varied. The aim is to obtain an accuracy better than 10 \%. A great attention is paid to the determination of the different efficiencies of the setup. In particular, the weak point of most experiments: the PMT efficencies (quantum and collection) will be measured by comparison to a NIST photodiode. This delicate point will be detailed