Performance of a Silicon Drift Detector
Typical spectra from the Mn standard for the 1.6-ms
time constant as a function of indicated input count rate are presented in Figure 4. In a fashion similar to the effect of shorter time constants shown in Figure 2, increased input count rate at the same time constant results in increased spectral artifacts (peak shiſt, peak broadening, high-energy tailing, sum peaks, and other pulse pile-up effects). An additional effect is the indication of apparent X-rays at higher energies than the energy of the incident electrons, which is a physical impossibility but reflects the increased probability of pulse pile-up at high count rates.
Te energy resolution of the Mn Ka peak as a function
Figure 3: Throughput for the Mn standard as a function of indicated input count rate and time constant at 20 kV. Note that the maximum throughput occurs at progressively lower dead times for faster time constants (see text).
to provide. As indicated previously, the input count rate is that indicated by the soſtware package during spectrum acquisition. Tis result is an obvious indication of a non-linearity between indicated input count rate and incident probe current, which will be discussed later.
of time constant and input count rate is given in Figure 5. As would be expected, the best possible energy resolution (that is, smallest full-width at half-maximum [FWHM] at low count rate) improves as the time constant decreases. For the four slower time constants (that is, 1.6–12.8 ms), the FWHM is relatively constant (≤10-eV increase) as a function of input count rate over the entire range of usable count rates (<70 percent dead time). Tough the increase in FWHM as a function of count rate is larger for the fastest two time constants, it remains relatively constant (≤10-eV increase) for input count rates less than ~275 kcps. It should be noted that the last four data points for the 0.5- and 0.8-ms time constants were
Figure 4: Spectra for the Mn standard measured at 20 kV for the 1.6-ms time constant as a function of indicated input count rate. 2011 May •
www.microscopy-today.com 43
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76