The dE/dx vs. total energy technique for charged particle identification
In a solid-state charged particle telescope such as CRIS or SIS, the
only quantities one has to identify a particle penetrating the
telescope are the energies it deposits in the various detectors (of
known thicknesses) in the stack as it comes to rest. Knowledge of the
angle of incidence of the particle (relative to the stack normal direction)
can be obtained through the use of a hodoscope, a device which
(in one fashion or another) is capable of determining a particle's
trajectory. The hodoscope in CRIS is the SOFT instrument, which uses
a series of scintillating fibers in alternating directions to observe
the trajectory of a particle. Each telescope stack in SIS contains
two "matrix detectors", which are thin detectors with
charged-particle-sensitive strips on both sides; each side's strips
are aligned at 90 degrees to those on the other side, providing event
trajectories.
This figure describes the general idea behind the method. Using two
silicon detectors of known thickness, a particle entering them at a
known (thanks to the hodoscope) angle theta will deposit energy DE in
the top detector and energy E' in the bottom detector. The derivative
dE/dx (energy loss per unit pathlength) can be approximated by the
quantity DE/(DL*sec{theta}), where DL is the thickness of the top
detector at the particle entry point. The total particle energy is
approximated by E', the energy deposited in the bottom detector. This
is a reasonable assumption, as it has been found that charged
particles tend to lose most of their energy near the end of their range