3.2.2.2. Backscattered Electron Imaging

The electron beam can be rastered across the sample by the deflection coils to produce a BSE image that displays variations in average atomic number, Z. The BSE detector system for the MBX microprobe consists of four semiconductors (PN junctions) located at the bottom of the column at a 40º take-off angle (Figure 3.2.2.2a). When backscattered electrons strike these semiconductors a current flows. The size of the current (signal) is proportional to the number of electrons hitting the PN junctions. The BSE detectors are sensitive to light and cannot be used when the sample illumination is on.

	Figure 3.2.2.2a. Bottom of the MBX electron column. The SEM detector is the large brass assembly on the right. The BSE outputs run through the assembly in the middle left with two large bolts. The BSE detectors are located at 90o intervals around the opening for the electron beam (center) on the chrome-colored ring. BSE electronics are housed outside the column (Figure 3.2.2.2b).
Figure 3.2.2.2b. BSE electronics located between spectrometers 2 and 3.

The spatial resolution of BSE images is poor (at best about 0.1 μm) because BSE are produced from the entire upper half of the interaction volume; however, BSE provide valuable information because of their sensitivity to atomic number variations (Figures 3.2.2.2c,d,e). BSE images are displayed on the MAC 3 computer and acquired using the NIH imaging software.

Figure 3.2.2.2c. Backscattered electron image of exsolution of orthopyroxene lamellae in large iron-rich meteoritic pigeonitic pyroxene.