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The linear accelerator is again in operation after a shutdown fore repairs and minor changes. Radiofrequency checks of the instrument were made while the Van de Graaff was open. The vacuum of the system is now very good, the base pressure being 1.7 x 10{sup -6} mm Hg with a rate of rise of 800 s/{micro}s. Some preliminary results have been obtained on the activation of carbon with protons. The experiments were done using 10 mil, 50 mg/cm{sup 2} polystyrene discs. The activation curve obtained is shown. The break shown in the curve was found on two separate runs and it yet unexplained. In another experiment a proton induced activity in nickel was found. This had a 3.3 second half-life. Positive mesons have been detected on plates exposed in the 184-inch cyclotron with an arrangement shown schematically. The beam ws allowed to strike a 1/16 inch carbon target, and the mesons, which are bent around by the magnetic field, were detected with photographic plaes placed beneath the circulating beam. A series of experiments were done in which the plates were placed at distances varying from 1-1/2 to 3/4 inch from the beam between 1/4 inch plates of copper. The plates obtained at 1 inch and 3/4 inch were so blackened as to be unusable. From the plates secuired at 1-1/4 inch Lattes has found a total of 30 positive meson tracks, of which 20 end in the emulsion. Eight of these were found to give rise to seconaries, none of which stopped in the emulsion. This work was done with 50 micron plates and will be extended using 100 micron plates. The data will be further analyzed to see if all of the positive mesons and in secondaries. The number of positive mesons produced seems to be comparable to the number of negative ones. Several other arrangements for placing the plates have been suggested and some tried. For example, plates were mounted directly under the target on the theory that mesons circling through an angular distance of 360{sup o} would strike the plates mounted on back of the target. There were too many neutrons present to make this scheme practical so it was abandoned. Various methods have been devised to cut down the neutron background. One of these involved the use of an elaborate copper shield, which was tried unsuccessfully. This shield actually increased the neutron background. A system of defining vanes will be tried, the purpose of which is to achieve better definition of the lower edge of the beam. Mr. Horning discussed the efforts that have been made to predict the cross section for meson production on the basis of the present meson theories. He emphasized that at the present state of knowledge no definite predictions were possible, although the theory is sufficient to give some order of magnitude results

Research Progress Meeting for March 11,1948