Energy spectrum features

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$\cdot l$ of H₂ with 0.1 % tritium and 14 T$\cdot l$ of D₂ overlayer in the upstream target, plus 3 T$\cdot l$ 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 $\mu$s, and (3) t>1.5 $\mu$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 $\mu$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 $\mu t$ travelled a separation distance of 18 mm, hence appropriate time cuts can separate these two events. Furthermore, the US D₂ layer had 14 T$\cdot l$ thickness while the DS only 3 T$\cdot l$, resulting in the difference in the peak width. There is indication that we are seeing 3 MeV protons from d$\mu$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 $\mu$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 $\mu t$ hitting Si detectors.