The Advanced Cosmic ray Composition Experiment for the Space Station (ACCESS) is a mission to address many of the interesting questions of comic ray origin and their lifetime in our galaxy. It measures elemental composition over the entire range of the periodic table. The more abundant nuclei, lighter than iron, are measured to high energies (~10^15 eV) to directly explore the `knee' region of the cosmic ray energy spectrum. ACCESS is also designed to detect fluxes of ultra-heavy (more massive than iron)nuclei with high charge resolution. This will allow some definitive measurements on the nucleosythesis origins of elements for Z<83. A further goal of this experiment is to measure electron fluxes to >1TeV, where some significant anisotropy in the arrival directions might be expected, caused by local sources. The mission will consist of a large detector (several m^2) which would be launched onboard the Space Shuttle and attached to the International Space Station ) for an exposure of 3 years. The instrumentation is completely electronic and provides data via the Station telemetry links for recording and later analysis. The less severe pointing requirements for this payload compared with other astronomical facilities make ACCESS a natural match to the space exposure facilities provided by ISS. This instrument shown schematically below, has three main detector components. The upper module contains the element identifier. The middle section consists of of a system for measuring very high, relativisitic velocities. These are large relatively low mass counter systems which are designed to measure low flux components of cosmic rays. The energy identifier at the bottom is a more massive component which is sensitive to the total particle energy.
Click here for the ACCESS study document released February 2001
Click here for the NASA Midex AO
Click here for the internal site (registered users only)
Cosmic rays form a major component of the energy density of the interstellar medium. They consist of such a wide range of particles and energies that it is sometimes difficult to believe these can be the result of a single source. In fact there are only a few candidate energy sources which can provide the enormous amounts of power needed to sustain the flux of these particles in our galaxy.
The flux of cosmic ray particles at earth is measured to decline quickly with increasing particle energy. The spectrum is a power law of nearly constant slope over a widerange from 10^8 eV to beyond 10^20 eV. Measurements above ~10^10 eV reflect the population of particles in the interstellar medium, unaffected by solar magnetic fields. The spectral slope steepens significantly in the region around ~3x10^15 eV producing a `knee' in the cosmic ray energy spectrum. The presence of this `knee' in an otherwise featureless spectrum is made more remarkable by its coincidence with the predicted maximum energies that can be produced by shock acceleration of charged particles in supernova blast waves. The study of the chemical composition in the `knee' region will provide a firm observational base for these ideas and point the way to the mechanisms of production of cosmic rays at even higher energies -- which are at present unknown.
At the highest energies single protons carrying many Joules of energy have been detected, the most energetic particles ever found. The mechanism which produces this extreme concentration of energy in a single particle remains unidentified. Since charged cosmic rays are deflected in the magnetic fields of the galaxy their arrival directions cannot generally be used to directly infer their origins. However, both the population of energetic particles and the magnetic fields are a significant dynamic component of our galaxy and influence its evolution and origin. The average behavior of cosmic rays in our galaxy is known from measurements of the fraction of secondary nuclei and the surviving fraction of Be clock nuclei at low energies. Observations also show that higher energy particles apparently escape more readily from this system than those at lower energy. However the real extent and population of the confinement volume remains unknown and the source spectrum at energies near the `knee' at 10^15 eV is at present masked by our lack of knowledge of the energy dependence of the escape at high energy.
The origin of cosmic rays even at low energies is still uncertain. Although it seems likely from energetics that they must fuelled by energy from supernovae explosions the original material which is accelerated is unclear. One idea is that there is an existing population of ionized particles in the interstellar medium, largely from stellar winds, which are acclerated by the supernova shock waves. Another is that the material is originally bound up in grains in the interstellar environment which are hit by the shock waves. Accurate measurements of elemental abundances, particularly for elements heavier than iron, should be able to resolve between these ideas.
A clearer understanding of cosmic rays at present and some idea of earlier epochs is needed to interpret galactic chemical evolution for the rarer light elements since these are predominantly produced by cosmic ray spallation. For example, cosmic ray spallation has a direct effect on the inferred amount of Li produced in big bang nucleosynthesis. A recurrent theme in cosmic ray research is to investigate properties which can lead to an understanding of the source regions, acceleration mechanisms and transport in our galaxy, progress has been made but there still remain many unanswered questions. Many of these scientific issues are directly addressed by the ACCESS experiment.
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