3.3.3 Time Dependence and UV Sensitivity

Early measurements taken in spring 2000 showed a time dependence of the secondary electron yield from certain surfaces. In Figure 3.10 the remarkable decrease by a factor of 4 in the yield is shown for a MgO coated sample over a period of 1/2 hour. The investigated sample was a candidate for a stop surface in the NPD sensor on the Mars Express mission of ESA. Further measurements revealed this behavior to be quiet common. Figure 3.11 shows the decay for an Al on MgF $_{2}$ on Si start surface also for the NPD sensor. Run (i) was the first set of measurements taken immediately after the 80 $^{\circ }$ C/20h bake-out of the chamber. As shown the yield started at quite high values of 0.6 but decreased within 200 minutes of beam exposure to 0.15. This decay could be reproduced by heating the surface again to the bake-out temperature for 20h. Later it was discovered that exposing the surface to UV radiation (from a Xenon lamp, with wavelengths down to 145nm) could `reset' the surface to initial conditions. In Figure 3.11 runs (ii) to (iv) were performed after run (i) without additional heating but after exposure to UV radiation. The decay in the secondary electron yield was well reproduced in run (ii) to (iv) although the initial value was only 0.3. The time constants for the decay depended on the investigated surfaces and also on the primary energy. Figure 3.12 shows the yield obtained for graphite stop surfaces coated with 150nm Al $_{2}$ O $_{3}$ , 150nm MgO, or 150nm MgO on 150nm Al $_{2}$ O $_{3}$ , respectively. Materials with a tungsten top layer behaved differently. Their secondary electron yield did not change significantly with time. Figure 3.13 shows the measured yields for two start surface with a W top layer on MgF $_{2}$ on a Si substrate. The yield was dependent on the incidence angle (see Chapter 3.3.2) and the incidence energy but not on time.

**Figure 3.10:** Early measurement of the time dependent decay of the secondary electron yield on a MgO coated surface. The sample was a candidate for a stop surface and was exposed to air for several weeks.
$\resizebox*{!}{0.35\textheight}{\includegraphics{Probe-4-MgO-Aussenluft-60u.eps.eps}}$

**Figure 3.11:** Secondary electron yield for a 50Å Al on 1400Å MgF $_{2}$ on Si sample at 390eV per atom primary energy. Run (i) was measured immediately after bake out. Runs (ii) to (iv) were taken after a reconditioning of the surface using UV radiation. The results from run (ii) to (iv) are believed to represent the `true' secondary electron yield. On run (ii) a glitch at 140 minutes was removed. The values measured at an angle of incidence of 30 $^{\circ }$ and 82 $^{\circ }$ are plotted for reference. Note the logarithmic scale used for the electron yield.
$\quad$ $\resizebox*{!}{0.36\textheight}{\includegraphics{secelyield_decay_mfal246-9_50A_liz2.eps.eps}}$

**Figure 3.12:** Decay of the secondary electron yield for graphite stop surfaces coated with 150nm Al $_{2}$ O $_{3}$ , 150nm MgO, or 150nm MgO on 150nm Al $_{2}$ O $_{3}$ respectively. For samples with a Al $_{2}$ O $_{3}$ layer the decay time constant was considerably shorter than for the sample with the MgO only overlayer. All measurement were taken at 60 $^{\circ }$ incidence angle. Please note the logarithmic scale used for the electron yield.
$\resizebox*{!}{0.36\textheight}{\includegraphics{secyield_decay_135calmg.eps.eps}}$

**Figure 3.13:** Two start surfaces with a W top layer on MgF $_{2}$ on a Si substrate do not show a significant decay in the secondary electron yield with time. An angle of incidence of 60 $^{\circ }$ was used. The slight increase of the yield was due to not as stable as usual beam and vacuum conditions.
$\resizebox*{!}{0.35\textheight}{\includegraphics{secyield_decay_wmft234.eps.eps}}$