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the mean free path for gas collisions increases rapidly with
increasing electron energy thus decreasing the frequency of
gas ionizations by the BSE. The contribution of the high
energy BSE to the current amplification cascade is much
less than that of the SE. To make simultaneous use of both
the BSE and SE signals, a detector array such as that shown
in. Fig. 12.13 can be utilized, combining an annular scintil-
lator BSE detector with the GSED. An example of the same
area of the polished Raney nickel alloy simultaneously
obtained with the GSED is shown in. Fig. 12.11b, operat-
ing under VPSEM conditions with water vapor at a pressure
of 600 Pa.
Other variants of the GSED have been developed that
make use of other physical phenomena that occur in the
complex charged particle environment around the beam
impact on the specimen, including the magnetic field induced
by the motion of the charged particles and the cathodolumi-
nescence of certain environmental gases induced by the SE. Fig. 12.10 Uncoated polymer foam imaged under VPSEM conditions at E 0 = 20 keV: (left) large solid angle symmetric BSE detector placed
above the specimen; (bar = 500 μm) (right) same area with induced field SE detector (bar = 500 μm) (Images courtesy J. Mershan, TESCAN)
BSEab 150 mm 150 mmGSED. Fig. 12.11 VPSEM imaging of a polished Raney nickel alloy surface. a backscattered electron detector (BSE). b gaseous secondary electron
detector (GSED). Note the details visible in the shrinkage cavity in the GSED image
12.6 · Detectors for Elevated Pressure Microscopy