The CREST experiment

The CREST (Cosmic Ray Electron Synchrotron Telescope) instrument will measure the flux of electrons at very high energies, in a region that has not yet been explored.  The graph below shows the current data set and predictions for the flux of cosmic ray electrons.  Keep in mind that these electrons are stripped from their atoms in the violent places of the galaxy, such as supernova remnants (SNRs), and accelerated to near light speeds.  Electrons are special compared to atomic nuclei (what remains of the atom after you strip the electrons off it) because they are relatively light.  Imagine throwing a racquetball and a basketball towards a bunch of hanging strips of cloth. The heavier basketball will penetrate much further than the racquetball, which slows down very quickly, eventually halting. The same is true of electrons compared to nuclei. The electrons are stopped relatively quickly, and fast electrons rapidly become slow electrons.  You can see that effect in the predictions in the graph.  At right about where the data points stop (~3000 GeV in energy) there is a green dashed line that plummets towards zero.  That is a sign of electrons not penetrating very far from their source, and getting absorbed in their journey after acceleration. 

But, you may ask, what is that dashed-dot line doing there at higher energies, over 3000 GeV?  This prediction is for electrons which reach Earth from a particular SNR (Vela), which is close enough to us that electrons accelerated there to these high energies could still reach us.  The detection of electrons at these extremely high energies, which could only come from a nearby source, such as Vela, is a "smoking gun" indication of acceleration in SNRs, since the only nearby candidate sources are SNRs such as Vela.  That's science worth doing.

Note that no data points exist at these extreme energies.  The reason for that is that these energies are very difficult to measure.  The energies are greater than what we can produce on Earth in particle accelerators, such as CERN and Fermilab.  And, furthermore, if you have any familiarity with high energy physics experiments, you know the size of those instruments are huge,  like a big house.  We have to fit our instrument into space on a balloon or rocket. 

CREST principle: detect synchrotron photons from primary electron

CREST design

So how do we detect those extremely high energy electrons?  We don't go for the electron itself directly. Instead, we make use of a process called synchrotron radiation, in which the electron emits photons (particles of x-rays) as it bends in the Earth's magnetic field.  We detect the synchrotron radiation instead of the electron.  The synchrotron process is well understood, and you can take a guess at the emitting electron's energy based on the average energy of emitted x-ray photons. We identify synchrotron x-rays from the background of x-ray photons from the Sun and other sources by looking for them to come at essentially the same time and lie along a line (representing a projection of the electron's path onto the detector from above).  So CREST is an x-ray photon detector, not an electron detector, but by identifying synchrotron x-rays we can learn about the original incoming electron.