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Advanced Cosmic Ray Composition Experiment for the Space Station

Calorimeter

Measuring cosmic rays in the 10¹²-10¹⁵ eV energy range is the primary objective of the ACCESS mission. Consider a hydrogen cosmic ray traveling through space with an energy of 10¹⁵ eV (electron volts). At rest, that proton (a hydrogen cosmic ray is simply a proton) would be 10⁹ eV -- a million times less energetic. The calorimeter is designed to measure such ultra-high energies, and for this reason it is called the "energy module" on ACCESS. Astrophysicists have known for decades that particles below 10¹⁴ eV have different characteristics than particles above 10¹⁶ eV. On a plotted graph, the slope of the energy spectra of particles above 10¹⁶ eV is slightly steeper than those below 10¹⁴ eV. The energy 10¹⁵ eV seems to be a "knee" in this curve. ACCESS will zoom in on this "knee" to find how it is related to the mechanisms of cosmic ray acceleration and propagation -- one of the major questions in particle astrophysics today.

Whereas other detectors on ACCESS use ultra-thin layers of materials to measure properties of cosmic rays, the calorimeter relies on thickness. High-energy protons and helium cosmic rays blow right through the other ACCESS detectors. But the calorimeter is nearly one meter deep. On top of the calorimeter is a sheet of silicon. This helps determine the position and charge of the incoming cosmic ray. Next, there are four sheets of graphite, each 10 cm thick. Cosmic rays hit these hard carbon layers and create a shower of secondary particles. The secondary particles then cascade through 12 layers of bismuth germanate crystals, each 2.5 cm thick. The crystals radiate light proportionally with the energies of the particles. The energy of the initial cosmic ray is more or less the sum of the energies of these secondary particles, as detected by the crystals. The ATIC balloon instrument, now in development, utilizes these same techniques.

Ideally, we would like an infinitely thick layer of crystals for the secondary particles to cascade through. But size and weight constraints on the International Space Station require the ACCESS calorimeter to have a relatively thin design compared to thicker ground-based calorimeters. Thus, some secondary particles will pass through all the layers of crystals, and exit from the crystals carrying away energy. This lost energy won't be added to the total sum seen in the crystals. The calorimeter on ACCESS solves this problem through a series of complicated calculations and clever tinkering with the first few interactions between cosmic rays and secondary particles.

Over the past decade, theorists have developed a convincing explanation of how a supernova shock wave accounts for the observed features in most high-energy (relativistic) particles. ACCESS will provide the first direct measurements to test this theory.