Practical Electron Microscopy and Database

Almost any materials in the detector window can attenuate the x-ray signals, and thus “windowless” detectors are employed in advanced electron microscopes (EMs). A windowless EDS detector uses no window material to maintain vacuum in the semiconductor detector. In those modern EMs, the vacuum approaches acceptable levels of cleanliness so that windowless detectors have been placed successfully. In practice, thinner vapour barriers are normally used in the so-called `windowless' EDS detector. The atmospheric thin window (ATW) and windowless detectors are the keys for analysis of the K lines of the light elements (4 [Beryllium] < Z < 11 [Sodium]; 0.110 < E_i< 1.04 keV) using EDS technique. Here, Z is atomic number and E_i is ionization energy of atoms.

In modern EMs, Windowless in-column silicon drift detectors have been installed, which provide very high solid angle (e.g. 0.9 sr), which allows high detection efficiency (e.g. 7% of the x-rays emitted from the EM specimen). Figure 4651a shows the schematic illustration of an EDS detector. The “window” can actually be windowless.

Figure 4651a. Schematic illustration of EDS detector.

Furthermore, the comparison of various EDS detectors with windowless, ultra-thin window, atmospheric thin window, and beryllium window is listed on a table on page3923.

Problems can occur on windowless detectors as follows:
        i) When a windowless detector is used in a TEM with an accelerating voltage ≥ 200 keV, the detector can be damaged by backscattered electrons from the TEM specimen.
        ii) When the column of a TEM is exposed to air, a detector shutter is normally closed in advance so that the pressure in the detector does not become atmospheric pressure. Otherwise, frost can form on the surface of the detector, resulting in degradation of EDS measurement. These frost can also originate from moistures brought into the detector during specimen changing.

The X-rays can be absorbed in the detector window. For windowless detectors, the detection performance will extremely be degraded by the formed ice layer on the crystal.

Figure 4651b shows the detection limits attainable for EDS measurements with both Be-window and windowless detectors. For low atomic numbers, the windowless EDS provides higher sensitivity, while the sensitivities for elements with high atomic numbers are the same for both detectors.