X-Ray Detectors

There are a range of detectors available for energy dispersive X-Ray spectroscopy. These can be split into three main types:

Solid state semiconductor

These are usually based around silicon or germanium chips. The X-Rays enter through a thin beryllium window, and produce electron-hole pairs in the semiconductor region of the detector. The number of electron-hole pairs formed depends on the energy of the incoming X-Ray. The higher the x-ray energy, the larger the number of electron hole pairs. A high voltage is applied to pull the electrons to the back of the detector, where they are collectively recorded as a negative pulse. A multi-channel analyzer (MCA) then counts these pulses, and sorts them into size (which is equivalent to X-Ray energy).

To provide acceptable energy resolution, solid state detectors are typically cooled with liquid nitrogen, resulting in typical resolution of <165eV.

In many cases this cooling is also vital in maintaining correct dopant dispersion throughout the semiconductor, and warming can cause serious damage to the detector. However, new generations of high purity detectors (for example, the Xerophy™ from HORIBA) remove these concerns, and allow the detectors to be repeatedly temperature cycled as required – cooling is only required whilst detecting.

Silicon drift detectors (SDD)

As their name suggests, silicon drift detectors are also based on a silicon detecting element, but the design is somewhat different from the typical solid state detector discussed above. Concentric electrodes are placed on the back surface of the silicon, which are used to steadily pull the electrons (arising from X-Ray absorption) towards the center of the chip. As before, the electrons are collected as a negative pulse, and counted by an MCA.

What is different about the SDD is that very good energy resolutions (~150eV) can be obtained solely with peltier cooling (typically down to -20 to -30 °C). In addition, multiple incident X-Rays can be detected sequentially, since electrons formed by X-Ray absorption in different regions of the detector will have different drift times to the anode. As a result, much higher count rates can be tolerated.

In contrast to the solid state detectors, SDDs show relatively poor sensitivity for high energy X-Rays.

PIN diodes

As its name suggests, a pin diode detector uses a much smaller detector element - however as with the two varieties described above, they are typically based on a silicon semiconductor material for the detection. The small size of the semiconductor element allows peltier cooling, but the reduced thickness compromises sensitivity, particularly for the heavier elements.