The x-rays from a 22-Mev betatron were used to observe the thresholds for 23 photo-nuclear reactions which are listed in Table I. The observed thresholds are compared with the mass data for the lighter elements and with those obtained from an empirical treatment of the masses for the heavier elements.The excitation curves were extended up to 21 Mev in a few cases. These curves were very much alike except that for the Ta(Y, n) reaction. This curve showed an abrupt decrease in slope at about 18 Mev which indicates a much reduced cross section for photons above this energy.

The yields of the two reactions U m (y,n)U 2 * 7 and U 238 (7, fission) produced by bremsstrahlung have been measured as a function of the maximum energy of the x-ray spectrum. The ratio of the number of fissions to the number of U 237 atoms produced decreased from 0.390 at 8 Mev to 0.275 at 12 Mev and then increased to 0.465 at 22 Mev. These data are interpreted to show that for maximum x-ray energies in excess of 12 Mev, a fraction of the U 237 atoms formed by the (y,n) reaction are left with sufficient excitation energy that fission or secondary neutron emission can occur. The cross sections for the reactions as a function of gammaray energy have been calculated from the yield data.The relativity precession of the asteroid Icarus is computed to be 10.05" per century. This value is about one-quarter that of Mercury and exceeds that of any major planet excluding Mercury. Consideration of a figure of merit, which determines the possible precision in measurement of the perihelion advance, indicates that the motion of Icarus can yield a further test of the precessional formula of general relativity.

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