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.
Data Base Structure for SiLi Detector
(ACIS MEMO# 105) TI HLM
CCD Instrument Page
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