Silicon Drift Detector
case of a non-controlled environment, SDDs are encapsulated in a TO-8 housing, and an addi- tional external radiation entrance window is implemented, strong enough to withstand atmo- spheric pressure and to ensure a local vacuum around the SDD. Recent external windows are constructed from thin Si3
N4 and SiO2 layers sup-
Figure 6: Energy resolution as a function of the signal shaping time (τ, see text above) and peaking time (time when the pulse reaches the maximum: here: 2.7 × τ) for two different detec- tor designs. The data were taken with an analog shaper. It shows that a 127 eV energy resolution (FWHM) can be obtained with a signal shaping time of 125 ns at – 20°C. Both measurements were performed at 15000 cps. The short shaping time allows for room-temperature operation with an energy resolution of 135 eV (FWHM).
in a short measurement time. In the case of energies below 2 keV, operation in vacuum is mandatory. In this case “window- less” SDDs are usually used. Te internal radiation entrance window of the sensor is oſten a combination of a highly doped rectifying p+
n junction, thin dielectric films, and possibly a
visible-light-absorbing aluminum layer. Te low-energy side of the QE curve in Figure 8 is dominated by signal losses at the internal radiation entrance window of the SDD sensor. In the
ported by a Si grid, which blocks only about 25% of the entering X-rays at 800 eV. As in the case of the internal radiation entrance window of the sensor, a thin layer of aluminum is oſten depos- ited on the outer surface of the external radia- tion entrance window to minimize detection of visible light (from cathodoluminescence) and infra-red (from chamber components). In the case of the internal radiation entrance window without the light-blocking aluminum, the black curve in Figure 8 had a quantum efficiency that was always above 85% (0.85) for X-ray energies between 500 eV and 11 keV for a 450 μm thick detector. Te drop in efficiency above 10 keV is due to the rapid decline in the cross section for the photoelectric processes in silicon, allow- ing many incoming high-energy X-rays to pass through the detector without being absorbed. Te blue and red curves show the QE with 50 nm
and 150 nm Al on top of the internal radiation entrance win- dow to block the visible light. Even for incoming X-rays of 25 keV, the QE is still 20% (0.2). To increase the sensitivity for X-ray energies up to several hundred keV, a scintillator can be combined with an SDD as a photo detector to measure the scintillation light of the converted gamma rays with energies up to more than 1000 keV [15]. Te efficiency shown in Figure 8 is suffi-
cient for most microanalysis tasks because all elements heavier than helium emit K-, L-, or M-shell photons in the range 50 eV to 10 keV. Te figure also shows that a 50 nm Al layer (blue line) allows the detection of Li X-rays at 54 eV, Be at 109 eV, B at 183 eV, and C at 277 eV. A 150 nm (red line) Al layer absorbs nearly all X-rays from Be and B.
Practical Considerations Thermoelectric cooling. The operational temperature range is
detector typically
from -70°C (for microanalysis requiring the best energy
resolution) up to +40°C (for
Figure 7: Output count rate versus input count rate as a function of the processing time for each entering X-ray photon recorded with a standard DPP. As the processing time decreases, the energy resolution on the Mn-Kα peak decreases as well. Thus, 1.5 × 106
incoming X-rays per second can
be detected with an energy resolution of 145 eV, leading to an output count rate into the spectrum of almost 6 × 105
counts/second per SDD channel. At the other extreme, for an energy resolution of 126 eV the output count rate can be 70000 cps. 2020 September •
www.microscopy-today.com
handheld X-ray fluorescence tools). The lower temperatures are required to reduce the ther- mally generated dark current in the sensitive volume, which deteriorates the energy resolu- tion. In current state-of-the-art SDD instru- ments, dark currents of less than 50 pA per cm2 at room temperature are routinely achieved on a 500 μm thick, fully depleted silicon detec- tor. In this case the cooling temperature to achieve Fano-limited
energy resolution approximately –30°C for 60 mm2 is
sensors at a 51
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