ACCESS Home What's New ACCESS Science ACCESS Team Why is ACCESS Important? ACCESS Facts ACCESS Instruments

Advanced Cosmic Ray Composition Experiment for the Space Station

TRD

The ACCESS Transition Radiation Detector (TRD) instrument will collect cosmic rays with charges of +4 to +26, or from lithium to iron. The TRD is contained in a large box consuming the bulk of the ACCESS mid-section, and is also referred to as the "velocity" module. It works by taking advantage of a unique physical property associated with the velocity of cosmic rays in this energy and charge range.

The TRD is a new type of technology, stemming from European research at accelerators in the 1960s and 1970s. The detector has been incorporated in only a few cosmic ray missions so far, as opposed to the older but still good methods of charge detectors and energy detectors, which date back nearly 100 years. The TRD proved to be quite useful, though, on NASA's Cosmic Ray Nucleus Experiment (CRNE) mission and subsequent high altitude balloon experiments.

Like the other detectors on ACCESS, the TRD is seemingly both simple and complex. The detector uses thin layers of plastic only 100 microns thick, separated from each other by layers of dead space, or more precisely, a vacuum. The plastic and the vacuum areas constitute different electrical environments for a charged particle. As a cosmic ray moves through one medium and enters the next, its electric field reconfigures to adjust to the environment of that medium. If the cosmic ray is moving fast enough (and if it possesses a certain energy greater than its energy at rest) it will release radiation during this brief "transition" period. That radiation is in the form of an X-ray photon. (X-rays are electromagnetic radiation of very short wavelength and very high-energy; they have shorter wavelengths than ultraviolet light but longer wavelengths than cosmic rays.)

Now, forget about the cosmic ray and concentrate on the photon. This "transition radiation" photon travels through the medium towards a deposit of xenon gas, which is good at absorbing the photon's energy in such a way that we can measure it. The total intensity of all the photons is proportional to the square of the particle's charge, and also increases in a known manner with particle energy. Thus, we can identify the energy and type of cosmic ray that caused that radiation release.

Much can be learned about the structure and evolution of the Milky Way galaxy by studying mid-range cosmic rays from lithium through iron. For example, GCRs are rich in some elements that are quite scarce in our own solar system, such as lithium (Li), beryllium (Be) and boron (B), and the heavier series of the elements scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr) and manganese (Mn). Does this mean that other parts of the Galaxy are compositionally different from our own little region? Or is there some process transforming the cosmic rays as they travel through outer space?