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The results of some preliminary measurements on the elastic scattering of photons by protons are reported. In the region of the (3,3) resonance and immediately above, our measurements join smoothly with those previously reported at lower energies. The scattering cross section shows a peak in the vicinity of 300 Mev. There is indication of a cos 2 0 term in the center-of-mass angular distribution. W E report the results of some observations on the proton Compton effect, i.e., the elastic scattering of photons by protons:y+p->p'+y'.(1) This reaction appears to be adequately described by a modified Klein-Nishina formula 1 below the threshold for meson production; above this threshold, however, mesonic processes dominate the scattering. By making measurements in this region, it can be hoped that important information about the meson-nucleon system can be obtained. 2,3 Observation of reaction (1) above the meson threshold is made more difficult by the presence of neutral photopions, y+P -> p'+Tfi -> p'+y'+y",whose production kinematics resemble those of reaction (1), and whose decay photons can have energies as high as that of the elastically scattered quantum. Reaction (2) has a cross section about two orders higher than that of (1). EXPERIMENTAL METHODThe photons to be scattered were obtained in the form of a beam of bremsstrahlung from the Cornell 1.2-Bev electron synchrotron. The machine energy was adjusted so as to produce a photon spectrum with a selected upper limit Wo for each run. Discrimination in favor of reaction (1) relied on two factors: First, measurements were carried out at such angles that the recoil protons from (1) were of longer range than any that could be produced by (2), given the maximum incident energy Wo; second, the angular correlation between the scattered photon and the recoil proton was studied. This correlation is exact (determined by experimental geometric factors) for reaction (1), but is smeared out by the decay kinematics of the neutral pions from reaction (2).Our apparatus is shown in Fig. 1. A beam of brems-* Supported by joint contract of Office of Naval Research and the U. S. Atomic Energy Commission.

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The neutral decay modes of the r) are observed by detecting the resulting y's in a leadplate spark-chamber array. The T)'s are produced in the reaction TT +p-~n +TJ, at 707 MeV/c. The neutron is detected with scintillators and its energy measured by a timeof flight technique. Our results for the neutral branching ratios are 2y = (57.9± 5.2) %, 7r°yy = (24.4±5.0)%, 37r° = (17.7±3.5)%.

We report the observation of an s-wave 7rV enhancement at 715 MeV of width 150 MeV by the study of the reaction ir~+p-+n +7r° + 7r 0 in which the momentum of the neutron and the angles of the four gamma rays are all measured,,