We have used the cadmium difference method to measure thermal-neutron fission cross sections and fission resonance integrals [1, 2] which are of considerable interest for nuclear physics and a number of practical problems such as the optimization of the accumulation of heavy isotopes in reactors.The measurements were made in a horizontal channel of the SM-2 high-flux reactor using 582.2 • 1.3 b and 275 =e 5 b [3] for the 235U thermal-neutron fission cross section and fission resonance integral as reference values. For a cadmium thickness of 1 mm the cadmium ratio for 235U was N40. It was assumed that the epithermal neutron spectrum is described by the 1/E law since a water layer ~40 mm thick takes part in the formation of the neutron spectrum at the exit from the horizontal channel [4, 5].Targets of the isotopes under study and the U standard were attached to aluminum substrates 75 mm in diameter and 0.1 mm thick. The weight of material in a target did not exceed 10 ~g except for 243Am for which it was N300 ~g. The number of nuclei in the uranium and plutonium targets was determined by counting in 2~r geometry, using a grid ionization chamber. The number of nuclei in the americium targetswas determined by comparing the area of the photopeak of the 59.6-keV T line with the area of the photopeak of the same line from a standard spectrometric source calibrated with an accuracy of 3%, and by counting with a small (~10 -4) solid angle counter. The number of nuclei determined by the two methods agreed within 2%. The number of nuclei in the curium targets was determined by the number of spontaneous fissions of 244Cm, 246Cm, and 248Cm. The number of 24~Cf nuclei was determined by ~ counting with the small solid angle counter. The number of nuclei in the targets was determined to within 2.5%. The isotopic compositions of the U, Pu, Am, and Cf targets are shown in Table 1; the curium targets had the same isotopic composition as in [6]. The fission fragments were recorded by a double fission chamber. Targets of the isotope being investigated and the standard (235U) were put together back to back and placed in the fission chamber which was located in the neutron beam.We have taken account of various experimental errors in the present work. The background of neutrons scattered in the room and from the structural members of the chamber was negligible. No depression in the neutron flux was observed. The following corrections were applied to the results of the measurements: 1) the backscattering of ~ particles was taken into account in determining the number of nuclei by (~ counting; 2} the isotopic composition of the samples was taken into account; 3) the temperature of the neutrons of the Maxwell• reactor spectrum was estimated as recommended in [4]; 4) the value of g(T), taking account of the deviation of the fission cross section from the 1/v law for uranium and plutonium, was taken from [7]; for 241Am and 242Am it was determined from the energy dependence of the fission cross section in the thermal region [3, 8],

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