Silicon-based low energy X-ray detector

X-ray detector is a key component of instruments like XRF, XRD, which are used to characterize a material’s component. Among different x-ray detectors like CsI, Si(Li), SDD, Si-PIN, the later two silicon-based detectors are the most popular ones due to the low production cost, small size, and high spectrum resolution. The XR-100CR is a Si-PIN detector from AmpTek, originally used for XRF analysis. Si-PIN is simply a silicon diode, where the depleted region produces a current pulse due to the ionization-produced electron-hole pair when a radiation comes. This mechanism is similar to that of HPGe. Silicon is used instead of germanium since its characteristic x-ray energy (1.7keV) is much smaller than that of germanium (10-11keV), so it won’t obscure the detection of the sample’s x-ray peaks. Nevertheless, silicon-based detector can only be made very thin (mostly <1mm), so it is only used in low energy x-ray detection (best <100keV).

Here is a diagram of the AmpTek Si-PIN detector structure.

The detector crystal is hold in a vacuum environment, and there is a thermoelectric cooler under it, cooling the crystal about -30ºC.

Unlike most preamplifiers, whose outputs are pulses with exponential tails, the AmpTek detectors output is weird and needed to be shaped by a timing filter amplifier into readable pulses by the MCA. Also, the power supply for this module is complex, requiring several different voltage inputs (see the manual on the AmpTek website for details). Luckily, the company open-sourced the circuit for these parts.

After customized the required power supply board and the amplifier, the unit can be used.

Here are some spectrums:

Smoke detector Am-241 source

Smoke detector Pu-239 source

The detector can also be used to analyze beta sources. I will update them later.