Si Detectors for Charged Particle Tracking

A big chunk of my expertise is in instrumentation for tracking the path charged particles take through a detector. There are a few different types of detectors, but the focus of this post will be about solid state detectors made using Si technology. Just to mention it, the other main type of detector used for charged particle tracking are those based on gas. Si provide micron level measurements of the position of particles as they traverse the detector volume. As a charged particle traverses the depletion region of a PN junction (diode) the absorbed energy forms electron-hole pairs (negative and positive charge carriers). These charge carriers are liberated within the electric field of the depletion region so they separate and and move towards their respective electrodes, where they induce a signal current.

A signal can also be created by absorbing the energy of a photon with a energy a bit over the bandgap energy of the Si, which is typically a few eV depending a bit on the specifics of how the Si was doped. The induced current incited by the energy absorption from some traversing particle is then guided to an amplification system via conducting material. Designing the amplification system is critical to success because you need to tune the shaping time appropriately to minimize the amount of electronics noise that is contributes. Thermal fluctuations in the lattice of the Si can also contribute noise to the system, so it is critical to cool Si detectors. Personally I have operated one as low as -20 C, which is a challenge because the amplification systems often use a stage that has a statically biased stage of transistors, which produce a considerable amount of heat.

The other critical issue we deal with in particle physics when operating Si detectors is dealing with radiation damage. As the device is exposed to more radiation, you slowly get nuclei in the depletion region of the Si which become dislocated from their lattice site. These dislocations will create states for electrons at energy levels within the band gap that can trap some of the charge liberated from the energy absoroption, which ultimately makes the device less efficient. At some point, this charge inefficiency can become so low that the signal is no longer sufficient to generate a measurable signal. There are several techniques people use, like annealing, to try to get the dislocated nuclei back in place. There is still only a finite lifetime for these kinds of detectors so we typically are constantly working to build new ones so we can replace the old when it becomes to damaged by the radiation it was built to measure.

After we collect the signals for each of the interesting time windows in an experiment, they are fit with models of the paths the charged particles take through the detector volume. Will work on getting a nice post about how these tracking algorithms work sometime soon, but just think of it as a complicated connect the dot game that we need sophisticated software to solve because of the huge combinatorics involved when you have a large number of charged particle tracks in your detector.

The image above was taken from the paper below, which your can learn more about the methods we use to build Si detectors that are more radiation hard so they have a longer serviceable lifetime.

[1] Lindstroem, Gunnar & Fretwurst, E & Kramberger, G & Pintilie, Ioana. (2004). Toward super radiation tolerant semiconductor detectors for future elementary particle research. Journal of Optoelectronics and Advanced Materials. 6. 23-38.