1. Introduction to the pb Experiment

3.1.2 DECAY IN FLIGHT

In contrast to previous pion beta experiments, the experiment of McFarlane et al. [McFa 85] at LAMPF used decays in flight of a pion beam. The beam momentum was 522 MeV/c and its intensity was 2·10⁸ p /s. This technique has the advantage of not having to accept all the muons from pion decay. The g -detectors, consisting of a converter array and an array of lead glass blocks (15 X₀) were located outside the 5° cone filled by the intense flux of the muons (Fig. 3.4).

Figure 3.4: Experimental arrangement for the decay in flight experiment. 1: Beam defining collimator; 2: Magnetized collimator; 3: Vacuum tank and decay region; 4: Removable CH₂ target; 5: Veto counter; 6: Converter array, 7: Lead-glass detectors; 8: Beam monitors. The pion beta decay rate R_{p b} was determined by using

, (3.6)

where N_p was the number of beam pions entering the apparatus, N_{p b}the number of pion beta decay events, P² the joint photon conversion probability and T the effective proper time spent by a pion in the decay region. The calibration of the energy scale, the conversion efficiency and the absolute timing of the detectors was done by inserting a CH₂ target and using p ^- and p ⁺ beams which produced p ^₀'s from reaction (3.3). The conversion efficiency P² was found to be 0.5151±0.0062 (1.2%). The total number of pions N_p in the beam was determined by the counting rates of three different monitors which were averaged; the final value was N_p = (2.144±0.0218)·10¹⁹ (1.01%). The quantity T, i.e. the acceptance for the 2 photons to hit the detectors was calculated by a Monte Carlo program. The final value was T=(3.534±0.031)·10^-11s (0.9%), where the error primarily was from the statistics of the Monte Carlo simulation.

From the total of 667000 recorded events, N_{p b}=1223±36.2 events were identified as pion beta events (error 2.9%). Together with the systematic uncertainties the numbers above inserted in equation (3.6) lead to a branching ratio

. (3.7)

This value is the best experimental determination of the pion beta decay rate so far.