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CsI has been selected as the photocathode material
in order to enhance the bare MCP quantum efficiency by about a factor of
three in the ROSAT energy range (0.1 to 2.4 keV).
The quantum efficiency was measured at a set of discrete energies and at
the average angle of incidence for the ROSAT X-ray optics. These data
are shown plotted in Figure 4.10 
and listed in
Table 4.4 . The features in the plot are based on fitting the
measured data points and the major dip around 0.6 keV is due
to absorption edges in both Cs and I near this energy.
| Energy (keV) | quantum efficiency |
| 0.3 | 0.22 |
| 0.7 | 0.18 |
| 0.8 | 0.25 |
| 1.5 | 0.28 |
| 1.7 | 0.23 |