~ 6 - of the observed nuclide. UCRL-4505 The maxim: of the excitation functions in Fig. 2 are in general agreement with the previous observations. The decrease with increasing energy is ascribed to the increase of the formation probability of less neutron-rich isobars at the higher bombarding energies. The total 109 and 112 mass yields should be nearly the same, for these masses are near the expected peak of the high energy fission-product distribution. Below 75 Mev, the cross sections for formation of pq! 9? and pail are the same (Fig. 2), while above 75 Mev, the pal !4 (n/p = 1.44) formation cross section is lower thar that of pq! 0? (n/p = 1.37). This decrease of the pat 4 formation cross section relative to that of pal 9? can be attributed to the direct formation tyfission of Agtlé in/p = 1.38) by the above hypothesis. This effect was demonstrated Ly isolating a silver fraction from palladium a few minctes after 2a 190-Mev bombardment. This silver fraction was then purified from other fission products by standard radiochemical analyses. After correcting the observed Agi l4 activity for growth from pa’ l4 before separation, the direct formation cross section was found to ke 20 mb at 190 Mev. The difference between the Pd 109 and pai i4 formation cross sections at 190 Mev was about 20 mb. The gr? excitation function also shows a marked decrease with increasing energy (Fig. 2). Attempts were made to measure the direct formation cross section of NE?!, bat the niobium could not be separated from the zirconium present with sufficiert rapidity to afford an unambiguous answer. After correction for the growth from zr?! the upper limit of the Nb’? cross section was set at 8 mk. The maximum of the zr?! cross section is 78 mb, and the cross section at 190 Mew is 72 mb (Fig. 2). This decrease is just outside the experimental error and is almost certainly less than 8 mb. The results of the study of the 97 chain, while not conclu- Sive, are in agreement with the postulate. TV. ACKNOWLEDGMENTS The authors would like to express appreciztion to Dr. Manfred Lindner and Mr. R. N. Osborne for the use of some of their data prior to publication; to Mr. J. T. Vale and the crew of the 184-inch cyclotron, and to Mr. G. B. Ross: and the crew of the 60-1n-h cvelotron for their cooperation in making these bombardments possibile; and to Mrs. E. Read for technical assistance. 90018bb