Responsible Geant4 Collaborator: Bernardo Tomè, LIP, Portugal.
The air_shower advanced example is inspired to the configuration of the ULTRA experiment (Nuclear Inst. and Methods in Physics Research, A, Vol.570, pp.22-35, 2007.).
ULTRA (Uv Light Transmission and Reflection in the Atmosphere) was conceived and designed in the framework of the EUSO mission, to provide quantitative measurements of the signal from the Cherenkov light produced by Extensive Air Showers (EAS), that is reflected/diffused when impacts on the Earth surface. The knowledge of such information for different surfaces will test the possibility to detect from Space the Cherenkov light produced by the Ultra High Energy Cosmic Rays.
The ULTRA apparatus is composed by two detectors:
the ETscope, a small EAS array consisting of five scintillator detectors disposed in a centered parallelogram, and
an UV optical device, sensitive to wavelengths in the 300-400 nm range, to detect the Cherenkov light, including:
two narrow field (4.5 FoV) detectors (UVscope), pointing towards the center of the ETscope array;
four wide field (40 deg FoV) detectors (Belenos), two of them pointing to zenith, the other two pointing to nadir.
The ETscope array allows to measure the characteristics of the incoming EAS (core position, arrival direction and size) while the UV devices, operating in coincidence with the ETscope, are used to determine the amount of direct and diffused Cherenkov light reflected on the ground or on the sea surface.
The air_shower example simulates the light detection by the UVscope. The UVscope consists of a Fresnel lens and a photomultiplier located in the focal plane, enclosed in a cylindrical aluminum housing. The lens is 457 mm in diameter and is made of UV transmitting acrylic with 5.6 grooves per mm. The implemented simulation makes use of object parameterisation/replication capabilities to describe the geometry of the Fresnel lens.
The primary vertex generator describes a plane circular source of monochromatic UV optical photons (3eV, about 400 nm). Depending on the user selection the source can be positioned such that the photons are fired to a reflecting surface, mimicking the Cherenkov light reflection, or directly to the UVscope (default case; no reflecting surface is present in this case). Reflection at the surface can be chosen to be specular or diffusive.