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A measurement of the Panofsky ratio in helium-3 Corriveau, François


The Panofsky ratio in ³He, P₃= ω(π⁻³He→³Hπ°)/ω(π⁻³He→³Hγ) , has been measured experimentally for absorption of negative piohs at rest. A 30 MeV π⁻ beam was degraded and stopped in a 1.9 cm thick liquid ³He target. The high-energy photons, from the inflight π°→γγ decay and the radiative capture channels, were detected by a large NaI(Tl) crystal (46 cm ∅ x 51 cm). The large distance (2.8 meters) between the crystal and the helium target provided a good time-of-flight selection of the photons, with a negligible neutron contamination. A 4.5% resolution at 135.8 MeV was achieved by the detector and made possible a good separation between the radiative break-up channels (dnγ + pnnγ) and the peak of interest (³Hγ). About 1.1 x 10⁵ photon events were observed in the data presented in this work. After subtraction of the target empty backgrounds, the theoretical line shapes were folded with the experimental energy resolution and fitted to the data. The Amado model was used to represent the break-up channels and thus extract the Panofsky ratio. A value of P₃ = 2.83 ± 0.07 was determined, not including the inflight corrections and assuming for ³He the internal conversion rate of hydrogen. A general picture of the radiative pion capture processes in nuclei is also given, in which [unclear] is a test case for the impulse approximation and the hypothesis of partial conservation of axial-vector current in nuclei. Moreover, a second ratio, B₃, between the radiative capture processes (ratio of the break-up channels to the elastic channel) has been measured to be B₃ = 1.35 ± 0.11.

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