Photoproduction of Positive Pions in Hydrogen-Magnetic Spectrometer Method

Abstract

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 $A+Bcos\theta +C{cos}^2\theta$. 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×${10}^{-30\mathrm{}}$ ${\mathrm{cm}}^2$, and falls off above the maximum faster than ${\lambda }^2$.