The photoproduction of neutral pions from hydrogen has been studied by counting the recoil protons with a magnetic spectrometer and scintillation counters. The process has been studied between photon energies of 260 and 450 Mev and between center-of-momentum pion angles of 70° and 153°. The excitation functions show a resonance type shape with maxima at about 320 Mev. Angular distributions are analyzed in the form A+Bcosθ+Ccos2θ in the center-of-momentum system. The coefficient B, which gives the front-back asymmetry, is small at all energies; and the ratio −AC is 1.22±0.10 at all energies between 295 and 450 Mev. The maximum cross section at 90° in the c.m. system is 26×10−30 cm2/steradian for 320-Mev photons. The total cross section divided by the square of the c.m. photon wavelength has a maximum near 340 Mev, and drops by nearly a factor of two at 450 Mev. These results are consistent with magnetic dipole and electric quadrupole absorption leading to a resonant state of the pion-nucleon system of angular momentum 32 and isotopic spin 32.
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This paper reports measurements of the differential cross section for photoproduction of neutral pions in hydrogen at energies 300, 400, and 450 Mev, at center-of-momentum angles of 70° to 150°. One decay photon from the neutral pion is observed in coincidence with the recoil proton, whose energy and angle are measured to define the photon energy. The results obtained by this method are in good agreement with more accurate measurements obtained recently by the method of observing only the recoil proton.
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