Using the mass spectrometer and isotope dilution technique, 27 cumulative fission yields from the thermal neutron fission of U233 along with 13 other fission product chains relative to each other have been determined. After normalization of the latter, values are obtained for all but seven fission decay chains whose yields are in excess of 0.5y0. INTRODUCTIONNunlerous studies have been made of the thermal neutron fission yields of Uz3. Katcoff (1) has su~llinarized the worlc preceding 1958, but since this time further mass spectro~lletric studies by Ivanov et al. I n the present worlc, fission yields have been determinecl for isotopes of the elements rubidium, strontium, zirconiuni, molybdeiiuin, ruthenium, cesium, ceriunl, neodymiu~n, and samariuni. These yields, together with the relative yields of the isotopes of krypton and xeiioli previously published froin this laboratory ( 5 ) , tnalte the lnost coinplete study of the fissinli yields of U?3:j that have been made u p to this time. EXPERIMESTALSamples of Us3 diluted with UZ8 were irradiated as U308 in quartz capsules along with cobalt flux monitors in the NRX Chalk River reactor. The U?38 was added a s a diluant to ~ninimize flux depression in the samples. Since the UY38 was a separated isotope obtained from the Oalc Ridge National Laboratories, the U235 present in the samples was found to be less than ly0 of the US3.The cobalt flux monitors, weighing approxilnately 1.14 mg, were made of 0.13 111111 diameter cobalt wire which was sealed in aluminum tubing. After irradiation the activity of the cobalt nloilitors was ineasured with a well-type ionization chamber (I-Iarwell type T.P.A. 1Ik 11) relative to staildards provided by R. C. Hawltins of Chalk River. The evaluatioil of the neutron flux by this method has beell previously described (6). Sample 3 was not monitored in this manner, an approxinzate flux value being obtained from the extent of depletion of Cs13j which results froin neutron absorption during the irradiation. Sample 4 was irradiated in a thermal column a t a flux less than 10" neutron/cm2 sec. This sample was used to obtain the relative yield of the niass 135 and 137 chains, since a t this flus Xel3j removal is small. T h e irradiation data for the various samples is shown in Table I.The 11~11nber of fissions ( F ) in samples 1 and 2 were computetl from the relation where N23:j is the number of U"" atoins in each sample, 2, and 2, are the effective fission and absorption cross sectioils of uranium as defined by Westcott (7), and +t the integrated flux. The effective cross sections for sanlples 1 and 2 have been obtained from the tables of Westcoff (7) assuming the thermal neutro~l cross section values of u, = 534 barns and U , = 589 barns (8), and Westcott's r values of 0.026 and 0.018 which were appropriate to the irradiation positions in the reactor. Values of 6.91 X 1017 and 1.036 X 1017 have been estimated for the n u~n b e r of fissions in samples 1 and 2.'Manuscript