In astroparticle physics, an Ultra-High-Energy Cosmic Ray (UHECR) is a cosmic ray particle with an extremely high kinetic energy, far beyond its rest mass and the energies typical of other cosmic ray particles. These particles are significant for astrophysics and fundamental physics theory, because they have energies comparable to the Greisen-Zatsepin-Kuzmin (GZK) limit, which occurs at about 5*10^19 eV (i.e., about 8 J).

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NIRFE: trying a new way to detect UHECRs

The atmosphere emits fluorescence light in the Near Infrared (NIR) region, in an analogous way to the well-known fluorescence emission in the UltraViolet (UV). The main features are:
    - in the wavelength range 0.8-1.7 um, the emission shows many atomic lines and molecular bands. The main bands are at 1045 nm, 1230 nm, and 1320 nm.
    - the light yield of the NIR fluorescence in that range is about a factor of 5 lower than the yield of the UV fluorescence in the 300-400 nm range.

The transmission of the atmosphere in the UV and the NIR region are quite different. In the UV, the light is absorbed mainly by ozone, and suffers diffusion because of the large Rayleigh scattering cross section. All these factors reflect into an UV light attenuation length of a few tens km. In the NIR region, the Rayleigh scattering is negligible (it scales as 1/lambda^4) and the light transmission is ruled mainly by the water molecule. There are however many windows where absorption is null and the atmosphere is completely transparent. Between 1 and 2 um, those windows are centered at about 1.0 um (infrared Y-band), 1.2 um (infrared J-band) and 1.6 um (infrared H-band). It is a lucky feature that main emission of the NIR fluorescence lays in the Y and J bands, and partially in the H band.

The main progress of the NIR fluorescence on the detection of UHECRs comes from the possibility to observe events at very long distance. If R is the light attenuation length, then the rate of observable events by a fluorescence detector is proportional to R^2. It is clear therefore that, since R is longer for the NIR light than for the UV light, there could be a great enhancement of the rate of observable events.
The NIR fluorescence technique is at a too young stage of development to be implemented as a real detector. We think that there are the numbers to make a small test prototype which has the main purpose to demonstrate that the NIR fluorescence technique works and it is capable of detect UHECRs at very long distance, much longer than with the UV fluorescence.