PGT Silicon Lithium (SILI) Detector (A Solid State Spectrometer)



X-rays interact with silicon crystals (a semiconductor) to produce electron and hole pairs. The number of such pairs is proprotional to the energy of the interacting radiation. By converting the resulting charge into a voltage pulse whose amplitude is proportional to the X-ray energy, a sequence of detected X-rays can be transformed into a spectrum of counts vs. energy (thereby forming an energy-dispersive spectrometer. Our Si(Li) detector converts incoming X-rays into electron-hole pairs at a rate of one pair for each 3.8 eV of initial energy. The crystal is thick enough to capture virtually 100% of the X-rays in the energy range of interest. The actual quantum efficiency vs. energy may be calculated from X-ray absorption data.


The single crystal measures 3.5 mm thick and has an active diameter of 5.8 mm, yielding an active area of 26.4 mm^2. This crystal is housed in a vacuum enclosure and is cooled to cryogenic temperatures by liquid nitrogen. A 7.5 liter dewar holds enough liquid nitrogen to last a week. A long tube extends out from the dewar and main housing, with the detector located behind a beryllium foil window located on the side of the tube near its end. This arrangement allows the detector to be attached to one of our calibration chambers (also under vacuum). A stepping motor moves the Si(Li) detector in and out of the X-ray beam, providing an absolute calibration of intensity vs. energy. This same beam can then be used to irradiate a CCD for calibration. Apertures of known size restrict the flux of X-rays reaching the Si(Li) detector and the CCD.


A bias voltage of typically 600 V is applied to the Si(Li) crystal to sweep electrons toward a contact at the rear of the crystal. An FET (field-effect transistor) preamp located just behind the crystal converts the charge generated by a detected X-ray photon into a voltage step which is in turn transformed by an Aptek Model 6300 Spectroscopy Amp into a voltage pulse whose amplitude is proportional to the energy of the X-ray. These pulses are received by a Canberra Series 35 Multichannel Analyzer (MCA), which accumulates a series of pulses during an exposure time window to create a pulse-height spectrum of the detected X-rays.
The liquid nitrogen cooling is necessary to lower the noise generated by the FET. In operation, the spectra are virtually free of noise in the energy range of interest.

Related Document

* Data Base Structure for SiLi Detector (ACIS MEMO# 105) TI HLM
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