The major source of beam positrons above 55 MeV / c produced at 500 MeV is from the decay π 0 → γγ promptly followed by γ -ray conversion to electron- positron pairs in the target material. The positron mo- mentum distribution is nearly flat as shown in Ref.[4]. These positrons are prompt with respect to the proton beam burst. We searched for delayed positrons coming from the beam line by varying the beam channel momentum be- tween 50 MeV / c (below the µ + → e $^{+}$νν edge) and 80 MeV / c. Tight cuts on energy loss in the beam counters suppressed the pion and muon contaminations to a neg- ligible level. By selecting events with the beam energy in the NaI detector, pions and muons were further sup- pressed as well as the background from π + → µ + → e + decays coming from stopped pions near the detector. At this point, events originated from π 0 were dominant. The π + → e $^{+}$ν component was enhanced by select- ing delayed events using the TOF. Fig.4a shows yields of delayed positrons normalized to the total positrons with the beam channel momentum. The edge around 52 MeV / c is from µ + → e $^{+}$νν decays from the production target. The rate of delayed π + → e $^{+}$ν events was 0.6 % of the total positrons at the same beam momentum. The peak momentum shift of 1.3 MeV was consistent with the energy loss of positrons in the production tar- get (0.5–1 MeV) and the uncertainty in the calibration of 1 %. Fig.4b shows a time spectrum for 68.5 MeV / c positrons with respect to the proton burst (the RF sig- nal). The delayed component has a decay constant of 24.6 ± 2.8 ns, which is consistent with the pion lifetime [9]. It is worthwhile to mention that these positrons are expected to be ∼ 100 % circular-polarized. By flipping the beam channel polarity to negative, we also searched for delayed 50 MeV / c electrons from µ − → e $^{−}$νν decays in the production target as a potential high-intensity source of stopped negative muons. The ratio of delayed and prompt electrons was measured to be (3 . 5 ± 0 . 4) × 10 − $^{3}$, which is consistent with an estimate of 6 × 10 − 3 based on the product of the ratio of delayed and prompt positrons (2.9), the π $^{−}$/π + production ratio in this energy region (1 / 5), and the fraction of decay-in-flight of pions in which muons stop in the target (1 %) [10]. the requirements of the PIENU experiment, including a pion rate of 100 k / s and positron suppression. A new calibration source from π + → e $^{+}$ν decays in the target was identified for low-momentum beam channels. The authors wish to thank C. Ballard, N. Khan, R. Kokke, D. Evans, K. Reiniger, and the beam line group for the design and installation work. This work was supported by the Natural Science and Engineering Research Council and the National Research Coun- cil of Canada. One of the authors (MB) has been supported by US National Science Foundation grant Phy-0553611. The TRIUMF M13 channel was modified to achieve a pion / positron ratio of > 50 using a di ff erential energy- loss method. The performance of the extension satisfies 4 5. Acknowledgment 4. Conclusion Figure 4: (Top:a) Fraction of delayed positrons with the beam mo- mentum and (Bottom:b) time spectrum of 68.5 MeV / c positrons with respect to the proton burst. The fit of the delayed component t o an exponential curve is shown in the bold line. Momentum (MeV/c) 50 60 70 80 Ratio -3 10$^{-2}$ 10$^{-1}$ 10 1 ν +  e → + π +  e + Time (ns) 0 10 20 30 40 COUNTS 10 2 10$^{3}$ 10$^{4}$ 10$^{5}$ 10 2.8 ns  = 24.6 