ACCESS - Advanced Cosmic Ray Composition Experiment for the Space Station


     ACCESS is a mission to address several of the most important questions currently open in cosmic ray physics. It will measure the energy spectra of individual elements from hydrogen to iron in cosmic radiation up to energies of approximately 1015 eV. The data will permit a measurement of the elemental composition of cosmic radiation close to the theoretically derived limits for efficient supernova acceleration (~ Z x 1014 eV). It will also allow a measurement of the energy dependence of the flux ratio of the secondary products of nuclear interactions to primary cosmic ray elements, giving us clues as to the energy dependence of the loss of galactic cosmic rays, allowing better modeling of the initial fluxes. These results will provide a test for supernova shockwaves as a possible accelerator mechanism for nuclei. By measuring the flux of rare heavy nuclei (up to Uranium) ACCESS will also provide clues as to the source of cosmic nuclei (e.g. existing ionized particles from stellar winds or interstellar material bound up in grains). A possible additional benefit would be if some directional dependence could be found for electrons at energies in the TeV range. ACCESS will be mounted on the International Space Station (ISS) as an attached payload for a three year exposure. The ACCESS baseline is composed of three modules: (1) an Ultra Heavy (UH) nuclei module, (2) A Transition Radiation Detector (TRD) module, and (3) a calorimeter. The UMD ACCESS group is to simulate and optimize a calorimeter for ACCESS. The ACCESS calorimeter will need to measure the energy of an incident particle with adequate resolution. It should have as large a geometric acceptance as possible to collect the largest data sample possible. A 'tracking' capability will aid in charge measurement by a silicon array or other charge measurement device. An experimental model based on GEANT code is being developed at UMD to trace cosmic ray particles and their ensuing cascade through the full Calorimeter system. A simulated event display shows the cascade (View 1 and View 2) and the detector response simulated in this process.