The trajectories of muon-decay electrons, reconstructed from the least-squares fit of three wire chamber positions, were extrapolated back to a perpendicular plane bisecting the target, providing an estimate of the position of the muons at the time of decay. The time of electrons was measured by the fast output signal of plastic scintillators. Recalling that the number of muon decays in unit time is proportional to the muon population ( ), the detection of decay electrons provides the measurement of the muon population at a particular position and time.
The evidence for emission is clearly seen in Fig. 7.1. The time and the extrapolated position of muon decay in the z-direction (along the beam) are shown in contour plots for the targets of (a) H2 with 0.1% tritium, and (b) pure H2. The intense region near z=-20 mm in both figures is from muons decaying inside the hydrogen layer and upstream target support, whereas the events near z=20 mm are muons stopping in the bare downstream gold foil where they disappear quickly via nuclear capture. While the pure H2 target (b) shows very few events in the vacuum region between two gold foils (around z=[-10, 10] mm), a strong signal is observed in this region for the H/T target (a), indicating emission of the muonic system into vacuum. The emitted muonic system, when allowed to collide with a separate D2layer, produced dt fusion providing unambiguous identification as .
The information on the velocity distribution of the emitted can be obtained from the correlation between the time and position of muon decay. Since the time for emission is relatively short (100 ns), the slope z/t in the plot roughly corresponds to the longitudinal component of the velocity. Shown in Fig. 7.1 (c) is another measurement, where the standard H/T target was covered by an additional very thin layer of D2 (overlayer). A significant shift in average slope, hence velocity, is observed.
Figure 7.2 compares a series of measurements with varying overlayer thickness of D2 as well as with no overlayer, where the time spectra of muon decay in the vacuum region ( z=[-10, 10] mm) are plotted. The events were normalized to GMU, and the pure H2 run (Fig. 2 (b)) was used for background subtraction. The change in the mean of the time distribution is clear, demonstrating that we can control the velocity of the emitted , an important feature for optimizing the beam energy for the time-of-flight measurements.
Further quantitative analysis requires a comparison with Monte Carlo calculations, which will be discussed in the following section (7.2).