Gamma-rays from the decay of neutral pions photoproduced from hydrogen by the bremsstrahlung beam of the Caltech synchrotron have been studied with a thallium chloride crystal total absorption spectrometer. The energy spectrum of the decay gamma-rays produced by a range of incident photon energy is obtained by the photon difference method and this spectrum enables a separation of the gamma-rays into two groups: (i) those from the decay of neutral pions produced singly from hydrogen and (ii) those from the decay of neutral pions from multipleproduction reactions. The cross sections for the single-production reaction are in agreement with the recoil proton experiments at Caltech and Cornell. For the multiple-production reactions we measure the cross section for producing neutral pions within a range of kinetic energies: It is shown that all available multiple-production data can be explained in terms of two compound states, one at about 750 Mev and the other at some higher energy. This is in agreement with an analysis of the single-photoproduction data, which is given in an appendix. These two states are, respectively, (T=12, J=12+) and (T=32, J=12+).

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Measurements of the differential cross section for the process γ+p→π0+p have been made at three pion center-of-mass angles: 60°, 90°, and 120°. Values were obtained at intervals of 0.05 BeV (incident laboratory photon energy, k) from approximately 0.6 to 1.2 BeV. Most of the data were obtained by detecting only the recoil protons with a large, wedge-shaped, single-focusing magnetic spectrometer and associated equipment. For θ′π0=60° and k≤0.94 BeV the π0 decays were also required, the decay photons being detected by a lead glass total absorption counter. Although the experimental resolution was considerably narrower than that of most of the previous experiments, its averaging effect was still appreciable in certain regions. Using a six-parameter fit, the data at each angle were unfolded in an effort to eliminate the effects of resolution and to obtain the true cross sections as a function of energy. The results compare reasonably well with those of previous experiments once differences in resolutions and systematic errors are taken into account. The results did not agree with the predictions of a simple resonance model with the resonance quantum numbers suggested by Peierls. The positions and widths of the two cross-section peaks in this energy region are quite similar to those observed in π−p scattering.

The process γ+p→π0+p has been studied by detecting recoil protons from a liquid hydrogen target which was bombarded by the bremsstrahlung beam of the California Institute of Technology electron synchrotron. The angle and momentum of the recoil protons were measured by a magnetic spectrometer-three scintillation counter coincidence system. The process has been studied between photon laboratory energies of 490 and 940 Mev and between pion center-of-mass angles of 31.5° and 147°. Protons which arose from meson pair production were significant at forward laboratory angles. A correction for this contamination is discussed. The results of these measurements show two interesting features. One is that the total cross section, which falls very rapidly above the 32−32 resonance energy near 320 Mev, reaches a minimum at about 600 Mev, and then increases to a broad maximum near 800 or 900 Mev. The other striking feature of the data is that the shape of the angular distribution seems to change rather suddenly near 900 Mev.

The bremsstrahlung beam of the Cornell Bev electron synchrotron has been used to study the reaction γ+p→π0+p over the photon energy range 250 Mev to 1 Bev, and for center-of-mass pion angles between 20° and 70°. The recoil protons, of energies between 10 and 60 Mev, were identified and their energies determined using a range telescope of eight thin plastic scintillators enclosed in a vacuum chamber with the thin liquid hydrogen target. Correlated pulse-height information was obtained by photographing an oscilloscope display and was used to sort out the protons from mesons and electrons. Corrections were made for the background of photoprotons from the Mylar target cup, the energy loss of the protons in the liquid hydrogen, absorption and scattering in the counter telescope, and the variation of beam intensity profile with energy. Compared with previous experiments and extrapolations the results show a somewhat smaller forward differential cross section above 400 Mev. The angular distributions obtained from a least-squares fit to all existing data indicate a d32 assignment for the 760-Mev resonance level. Other implications of the data are also discussed.

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Differential cross-sections have been measured for π0 photoproduction over the energy range 0.8 GeV to 1.4 GeV and at angles between 50° and 90° c.m.

New results are presented on the differential cross-section for the reaction α+p→π0+p, at energies between 600 and 1000 MeV, and c.m. pion angles Θ*π=40° and Θ*π=60°. The present data, together with that at Θ*π=40° already published (11), show an angle-independent position of the second resonance at about 750 MeV. Rather flat angular distributions in the forward c.m. hemisphere are also favoured by these data. On comparing the cross-sections obtained when detecting both the neutral pion and the recoil proton, and when detecting only the latter, estimates of the background of «ghost protons» are obtained, in agreement with the empirical curve proposed in ref. (11).

The polarization of recoiling protons from the photoproduction of π0 mesons on liquid hydrogen has been measured for primary photon energies between 500 and 1000 MeV over a range of π0 c.m. angles from 55° to 130°. The results show structure not observed previously in experiments of less precision. In particular, the polarization at 90° c.m. is close to zero at a primary photon energy of 900 MeV. Also, a strong dependence of polarization on π0 c.m. angle between 600 and 900 MeV was observed. A subsidiary measurement of the polarization of the recoil protons from elastic e−p scattering at 900 MeV and q2=10 F−2 gave a value (1.3±2.0)%.