Research Progress Meeting of February 19, 1948

Much of the cloud chamber work has been directed toward obtaining the angular distribution of protons bombarded by deuterons from the 184-inch cyclotron. The observations have been restricted to those protons arising from neutrons of energy greater than 65 Mev. The cloud chamber used for this work is of 16-inch diruneter and employs a magnetic field of 14000 gauss. The chamber is filled with hydrogen at a pressure of 1/2 atmosphere; the vapor used is water and alchohol. Nearly 800 proton tracks were measured and calculated before analysis of the data indicated that the measuring procedure should be revised. They gave the angular and energy distributions shown in Figures 1 and 2. The discrepencies between the theoretical and experimental energy distribution of the neutrons shows that serious errors were being made. As a first check of possible sources of error, random parts of the data were remeasured. They indicated that errors in the measurements of the angles were being made that gave a mean deviation of about {+-} 2{sup o} in the beam angle and {+-} 4{sup o} in the dip angle. These errors were largest at large scatter angles. In addition, a more serious error occured in the measurements of the curvature. The mean deviation was as large as 15 percent, with many tracks having errors of 50 to 100 percent in the measured curvature. These errors were dUd to the fact that an adequate measuring technique had not been developed, that the original measuring apparatus and projector were difficult to operate, and that the cloud chamber technique had not been perfected. A systematic analysis of the errors and difficulties involved in the experiment made it evident that revisions in the technique were necessary. In view of the inaccuracy of the original measurements it is not possible to make more than a qualitative analysis of the first 800 tracks. The data showed definitely the existence of exchange forces and point to the conclusion that classical forces are slightly dominate in the proton-neutron reaction at these energies. This is indicated by the excess of high angle scattering, with 246 protons scattered at angles greater than 45{sup o} and 183 scattered at angles less than 45{sup o}. It is to be noted that this excess existed despite a selective tendency for the increased errors at large scatter angles to exclude the data from the group having energy greater than 65 Mev. The data taken under revised and improved procedure shows good agreement with theoretical energy distribution as may be seen from Figure 3. The angular distribution given in Table I now shows more definately the asymmetry of the scattering, with 59 protons scattered at angles greater than 45{sup o} and 38 at angles less than 45{sup o}. However, the probable statistical errors are still relatively large, and there is still a possibility that errors are introduced from high energy protons knocked on in the forward direction which may not, ionize heavily enough to be observed. This effect is small since the protons now being observed are knocked on in hydrogen at one and one half atmospheres pressure with the resulting much greater ionization