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Observation of two-photon emission after pion capture in carbon-12 Mazzucato, Eddy

Abstract

The ¹²C(π⁻,2γ)X reaction for stopped pions has been investigated at TRIUMF using two large Nal crystals (TINA and MINA) and two lead glass Cerenkov counters. A 20 MeV pion beam from the stopped π /μ channel (M9) was used. The incoming pion beam was defined by a 3-counter telescope (S₁,S₂,S₃) and was stopped in a 1.25 cm thick carbon target. The last telescope counter (S₃) determined the time of arrival of the pions. The four γ-detectors were located at +55°, +105°, -55°, and -135°, with respect to the incident pion beam. Scintillation counters were placed in front of each γ-detector in order to identify charged particles. For the Nal crystals, time-of-flight information was used to separate neutrons and gammas originating from the (π⁻,nn) and (π⁻,nγ) reactions. Accidental coincidences from simultaneous (π⁻,γ) reactions generated by multiple pion stops in the target were rejected by measuring the energy deposited by the incoming pions in S₃. A total of 2.5x10¹¹ pions were stopped, and after background subtraction, about 500 good events were observed at 6 different opening angles (θ[sub γγ] = 50°, 80°, 110°, 120°, 160°, 170°). The total branching ratio for the ¹²C(π⁻,2γ) reaction was measured to be (1.2 ± 0.2)x0⁻⁵; this value was found to be very sensitive to the low-energy threshold of the Y~detectors. The good energy resolution (=10%) of the Nal crystals permitted the investigation of the energy-sharing between the two photons at θ[sub γγ] = 120°. The sum energy spectrum of the two photons peaks at 120 MeV i and has a width of about 40 MeV. Its shape indicates that most of the available energy is carried off by the two photons and that the residual nucleus, most likely ¹²B, is not highly excited. Detailed theoretical calculations made by other authors predict a total 2γ rate approximately 30% larger than our measured value. No evidence for pion condensates or precondensate effects in ¹²C was found.

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