Velocity reconstruction in RICH detector AMS experiment

João Roseira Borges
LIP Av. Elias Garcia, 14 1º-andar, 1000-149 Lisboa

Fernando Barão
LIP Av. Elias Garcia, 14 1º-andar, 1000-149 Lisboa

The international AMS (Alpha Magnetic Spectrometer) experiment is designed to study Cosmic Rays (CR) and is expected to extend the knowledge of its spectrum, in terms of nuclei, isotope and particle/antiparticle relative uxes. The statistical amount of data to be produced in the 2-3 years of operation will be of unprecedent paralel on this issues, so that new theoretical advances are expected mainly on propagation models and interactions of Cosmic Rays.
AMS is a large angular acceptance spectrometer composed of different sub-detectors to provide particle identification capabilities. In particular, there will be a RICH (Ring Imaging Cerenkov) detector wich will provide a measurement of the particle velocity.
This unit is composed essentialy by a radiator (see next paragraph), an expansion volume and an array of about 1000 photomultipliers (each one of wich has 16 pixels to increase spatial detection resolution) In the Cerenkov mechanism, photons (usually in UV-visible part of e.m. spectrum) are emitted along a cone of precise aperture angle when charged particles cross an opxienaa-rich-velocity-abstract.dvi.pstical refractive medium ( n > 1 ) - this is the radiator above mentioned - with a larger speed than speed of light (in that medium). This photon’s angle (Cerenkov angle) is well related to the velocity of his mother’s particle.
The geometrical pattern produced by the detected photons is used to reconstruct the Cerenkov angle and therefore the particle velocity. A high degree of accuracy is reached, leading to a velocity resolution of around 0.07 percent for protons.

Grupo de Astronomia da Universidade da Madeira - 2001