1.4.2 Britton, et al. (1992,1994)
In 1992 [Bri 92] and again in 1994 [Bri 94], results were reported for another
measurement of the p -> e n ( g ) branching ratio. The
experimental apparatus consisted of a plastic scintillator target, two wire
chambers, a large NaI crystal (TINA), and several other beam counters and
vetos. A diagram of the experimental apparatus can be seen in Figure 1.2. Pions
were stopped in a plastic scintillator target and allowed to decay. Positrons
emitted in the direction of calorimeter passed through two wire chambers and
four thin, plastic scintillator detectors before entering TINA. The TINA
crystal gave extremely good energy resolution as can be seen in Figure 1.3. The
branching ratio was found by fitting the timing spectra for p -> µ
-> e and p -> e n events simultaneously.
Correction for the low energy tail of the p -> e n spectrum
that extended under the spectrum arising from the p -> µ ->
e decay chain was achieved by establishing upper and lower limits on the
tail fraction. The value obtained was (1.2265 ± 0.0078) ×
10-4. The upper limits came from direct-in-beam measurements of the
response function of TINA while the lower limits were derived from a p ->
µ -> e suppressed energy spectrum.
Figure 1.2: The experimental setup from [Bri 92, Bri 94]. The inset shows
details of the target assembly. Events where the decay positrons enter the
large NaI crystal TINA are recorded.