Preliminary Results on the Production of Mesons by Photons on Carbon and Hydrogen

entering into a delayed coincidence. The stability of the detecting system depends on the sensitivity of the meson counting rate to the minimum amplitudes required of the pulses. Such a plateau curve {counting rate IIs. minimum pulse amplitude) is shown in Flg. 3. With a synchrotron beam intensity of about 10"Mev/sec. and the geometry of Fig. 1, a target of 4 g/cm' of carbon, and 1 in. of aluminum absorber, corresponding to a meson energy of 54 Mev, the meson counting rate is about 15 counts/min. This makes it possible to do experiments more quickly than with photographic emulsions. On the other hand, only positive mesons can be counted in this manner, and absolute cross-section measurements have a large error, since the detection efFiciency is not known with precision. We should like to express our thanks to Professor E. McMillan for his support of this research, and to the operating crew of the synchrotron, especially to Mr. W. Gibbins, for their able cooperation,