Before we go into the detailed quantitative analysis, it may be intructive to take a look at gross qualitative features of our data. Therefore we shall spend this section and the next for that purpose.
Characteristics of the Si energy spectra, for different time cuts, are shown in
Figs. 8.3 and 8.4.
Figure 8.3 is for our standard time-of-flight
measurement arrangement, i.e., 1000 T
of H2 with 0.1 % tritium and
14 T
of D2 overlayer in the upstream target, plus 3 T
D2downstream (target ID = II-9 in Table 4.3 in
page
). Three spectra are plotted with the time cuts of
(1) t>0, (2) t>0.02
s, and (3) t>1.5
s. The histogram (1)
shows a signal from direct beam muon stops in the Si detector, in addition
to a broad fusion signal near 3.5 MeV, predominantly from the US moderating
overlayer, and a strong low energy background peak. The beam signal is very
prompt in time, and a 20 ns delay cut eliminates this very efficiently
(histogram (2)). The histogram (3) with a time cut of t>1.5
s, on
the other hand, shows a much narrower fusion peak mostly coming from the DS
reaction layer. The US fusion takes place typically in the first few 100
ns, while fusion in DS occurs after
travelled a separation distance of
18 mm, hence appropriate time cuts can separate these two
events. Furthermore, the US D2 layer had 14 T
thickness while
the DS only 3 T
,
resulting in the difference in the peak
width. There is indication that we are seeing 3 MeV protons from d
d
fusion, which contributes to the background. This will be treated in
detail in Section 8.2.4.
Comparison of Fig. 8.3 with Fig. 8.4, the latter for pure H2 target with no D2, confirms that the peaks near 3000 - 3500 ch in the former come from fusion in the D2 layers. On the other hand, the peak near 2 MeV persists in Fig. 8.4 (also in the bare target runs), and in fact the peak energy shifted as beam momentum was changed, providing the evidence that this peak is due to muons stopping in the detector.
Other sources of background include: (a) muon decay electrons, (b) charged
particle (proton, deuteron etc.) emission following nuclear muon capture on
heavy elements, (c) muon induced nuclear break up, and (d) scattered beam
electrons. Also the neutrals, such as neutrons and gammas following muon
capture, muonic X rays, or bremsstrahlung from tritium
decay, could
cause background signals in the Si detectors, but probabilities for these
are expected to be small since the detectors had a thickness of only 300
m. Among other possible background processes for targets containing
tritium are conversion muons from muon catalyzed pt fusion:
(19 MeV), and muon capture on Si from
emitted
hitting Si detectors.