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The excitation functions for positive pion production from hydrogen have been obtained in the energy region from 230 Mev to 450 Mev and at laboratory pion angles of 24°, 38°, 53°, 73°, 93°, 115°, 140°, and 160°. The pions are detected and identified by measuring their range and ionization in a scintillation counter telescope. The above data are analyzed to give the angular distributions in the center-of-momentum system, and a least-squares analysis made to determine coefficients in σ(θ)=A+Bcosθ+Ccos2θ. The total cross section shows a peak at 300 Mev of magnitude 2.20×10−28 cm2. The coefficient B passes through a maximum negative value at 250 Mev and then passes through zero at 325 Mev and remains positive up to the highest energy measured.
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Positive pions produced in a cold, high-pressure hydrogen gas target by the 500-Mev bremsstrahlung of the CalTech synchrotron, have been analyzed by a large magnetic spectrometer. The photoproduction cross section has been measured as a function of photon energy at laboratory angles of 12.5°, 30°, 51°, 73°, 104°, 140°, and 180°. The energy region covered depends somewhat on the angle, but is typically from 200 to 470 Mev. From these excitation curves the angular distribution of the photopions in the center of momentum system is obtained for various photon energies, and these angular distributions are analyzed in the form A+Bcosθ+Ccos2θ. The angular distribution has a backward maximum at low energies and a forward maximum at high energies, the coefficient B changing sign at about 340 Mev. The total cross section shows a striking maximum near 290 Mev, of magnitude 205×10−30 cm2, and falls off above the maximum faster than λ2.
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The ratio of the yields of negative and positive pions photoproduced in deuterium has been measured at six photon energies between 500 and 1000 Mev and at seven angles between 20° and 160° in the center-of-momentum system of the photon and target nucleon. Pions were selected with a magnetic spectrometer and identified using momentum and specific ionization in a scintillation counter telescope. The spectator model of the deuteron was used to identify the photon energy. Statistical errors assigned to the π−π+ ratio range between five and fifteen percent. The results of the present experiment join smoothly with the low-energy π−π+ ratios obtained by Sands et al. At high energies the π−π+ ratio varies from 0.5 at forward angles and energies near 900 Mev to 2.5 at 160° c.m. and energies 600 to 800 Mev. The cross sections for π− photo-production from neutrons have been derived from the π−π+ ratio and the CalTech π+ photoproduction data. The angular distributions for π− production are considerably different from those for π+; there is, for example, a systematic increase at the most backward angles. The energy dependence of the total cross section for π− is similar to that for π+, although the second resonance peak occurs at a slightly lower energy, and at 900 and 1000 Mev the π− cross section is smaller by a factor 1.6. A comparison is made of the cross sections for π+ photoproduction from hydrogen and deuterium, although the accuracy of this comparison is not high.
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Measurements have been made on the ratio of pion-production cross sections at right angles to and along the photon electric-field vector. The positive and negative pions were first momentum-analyzed and counted by means of a counter telescope. Data have been taken at 45, 90, and 135° in the c.m. system, and at proton energies of 225, 330, and 450 MeV. A comparison of the data is made with the dispersion-relation calculation of McKinley.
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The azimuthal asymmetry Σ=(σ⊥−σII)(σ⊥+σII) in π+ photoproduction by linearly polarized bremsstrahlung was measured at photon energies from 475 to 750 MeV at 90° and 135° in the center-of-mass system. The experimental results show that even in this energy region, π+ are produced predominantly in the plane of the magnetic vector.
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The differential cross section for the reaction γ+p→π++n was measured using the Caltech 1.5-GeV electron synchrotron. The positive pions were detected and momentum analyzed in a multichannel magnetic spectrometer and the data were recorded in the memory of a pulse-height analyzer. The energy resolution was improved over previous experiments and an attempt was made to minimize systematic errors. The data are presented in the form of energy distributions at 12 lab angles from 34° to 155°, and the range of lab proton energies extended from 500 to 1350 MeV. Data were not taken at all energies for each angle, since the maximum useful momentum of the spectrometer, 600 MeVc, restricted the maximum energy for lab angles less than or equal to 74°.
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The differential cross section for the reaction γ+p→π+n was measured at 19 photon energies between 300 and 750 MeV in the laboratory frame, for pion angles between 0° and 130° in the c.m. system. The pions were analyzed in angle and momentum with a magnetic spectrometer and detected by a counter telescope. The 0° measurements could be achieved, in spite of the excessive positron rate, owing to a mass-spectrometer arrangement. No direct indication for the electromagnetic excitation of the P11 resonance (1466 MeV) was found. Comparison is made with theoretical calculations of π+ photoproduction.
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