Gross features of Si time spectra, with different energy cuts, are illustrated in Figs. 8.5 and 8.6. Shown in Fig. 8.5 is the Si1 time spectrum for the standard time-of-flight arrangement, whereas that for pure H2 is given in Fig. 8.6. All the histograms have a sharp spike at time zero, which, at least in part, comes from direct muon stops in the Si detector. The low energy part (E<2000 ch) of the spectra, which we saw was dominated by a large background signal (Figs. 8.3, 8.4), has two exponential components, a fast one with the order of 100 ns and a slow one about 2 . This is consistent with muon disappearance rates in heavy elements and hydrogen, respectively, suggesting the signals in this energy region come from muon decay electrons and charged particles from muon capture. Conversion muons from fusion (19 MeV) could also contribute to the long lifetime.
The time spectrum with an energy cut 2001 <E< 4000 ch in Fig. 8.5 exhibits fusion time signals; exponentially decaying in early time ( s) is fusion from the upstream target, while events in s are mostly due to fusion from flying across the drift distance to reach the downstream layer (though the signal is not so clear from the figure due to the unrestrictive energy cut).
Whereas these are obviously absent from the same energy region in Fig. 8.6, comparison between the two figures of the higher energy part ( 4001 <E< 8000 ch) of the spectra indicates excess events in Fig. 8.5. These events, unlikely due to fusion since the maximum energy is about 3.5 MeV and the probability of pile up is very small, are attributed to emitted reaching the Si detector where the muon is transferred to Si and then captured, emitting charged products. In fact, the signal in this region is enhanced when there is no moderating overlayer because of the higher yield of emission into vacuum.