A large liquid scintillator with approximately 80 percent efficiency for detection of neutrons has been used to obtain data on numbers of neutrons emitted per fission for several fissioning nuclides. Reported here are the average number of neutrons per fission, v, and the respective probabilities of 0,1, 2, • • • neutrons per fission, for spontaneous fission of_Pu 240 , Cm 244 , and Cf 252 , and for fission induced in U 233 , U 235 , and Pu 239 by 80-kev neutrons. The values of v for these cases are, respectively, 2.257±0.045, 2.810±0.059, 3.869 ±0.078, 2.585±0.062, 2.47±0.03, and 3.048±0.079. The thermal neutron value for U 235 (2.46=1=0.03), obtained by other methods, was used here as a standard for calibrating the detector efficiency. The efficiency was also measured by another method, involving scattering neutrons into the scintillator, with results in good agreement. The probabilities of 0, 1, 2,---neutrons per fission approach closely a binomial type distribution, with the maximum number of neutrons equal to 5, 6, or 7, depending on p.

Experimental arrangement.the present LIQUID SCINTILLATORS GRAPHITEThe proton angle dp was near 32° measurements. associated with an observed recoil proton was between 0.15 and 0.28 in this experiment, depending on the incident photon energy.Measurements were made with polyethylene and graphite targets and a subtraction made to obtain the cross section in hydrogen. The "carbon background'' was between 15 and 40 percent, depending on the photon energy.The bremsstrahlung spectrum assumed in calculating the cross sections was normalized to give the same total energy in the x-ray beam as measured by a Cornell type ionization chamber with 1-inch copper walls. We are indebted to R. Littauer and the Cornell synchrotron group for both the design of this chamber and its absolute calibration.The measured cross section at 90° is shown as a function of the incident photon energy in Fig. 2, together with the data of Silverman and Stearns 1 at lower energies. The existence of a maximum in this cross section has been the subject of considerable discussion and is predicted, for example, in the theory of Brueckner and Watson. 2 This theory assumes that the TT° photo-Further work is being undertaken to obtain data with somewhat better energy resolution and. to investigate the angular distribution.Because of the importance of the data at 400 and 445 Mev, we have tried to investigate the extent to which these points might be in error because of uncertainties in the upper end of the incident bremsstrahlung spectrum, arising in part from an uncertainty in the synchrotron energy itself. This has been done by making measurements at three different "nominal" synchrotron energies, £o=480, 420, and 370 Mev and taking a weighted average for Fig. 2 in such a way that the internal consistency of the three sets of data is as great as possible.From this analysis, which is given in Fig. 3, it is concluded that the errors introduced by uncertainties in the upper end of the incident bremsstrahlung spectrum are probably less than 10 percent for the 400-Mev point, and 25 percent for the 445-Mev point. 150 200 300 400 MEV INCIDENT PHOTON ENERGY LAB SYSTEM FIG. 2. Differential cross section at 90° in the laboratory system for the photoproduction of neutral pions in hydrogen. The crosses at lower energies are the data of Silverman and Stearns. 1 The indicated errors include estimated instrumental errors in so far as they affect the relative cross section.production involves a resonant interaction between meson and nucleon in a state of isotopic spin and angular momentum f. Whereas this theory is probably not unique in giving a maximum in the cross section, an interpretation in terms of a resonance is suggested by the sharpness of the observed maximum. The observed cross section at 90° falls off above the maximum considerably more rapidly than the curve given by Brueckner and Watson. 2 200 300 400 500 INCIDENT PHOTON ENERGY, LAB (MEV) FIG. 3.Internal consistency of data taken at three different "nominal" synchrotron energies Eo =480, 420, and 370 Mev (ob...

Cross sections for the reactions In 115 (w,w')In 115m and Au l97 (n,n , )A\x l91m have been measured from the reaction thresholds to 5.5 Mev. The lowest mean energy for which the In 115w excitation was observed was 440 kev with an energy spread of ±100 kev; the cross section rises to a peak value of 0.36 barn at 2.5 Mev. The lowest mean energy for which the Au 197w excitation was observed was 420 kev with an energy spread of ±40 kev; the cross section rises to a peak value of 1.3 barns at 2.5 Mev. Both cross sections show irregularities which are evidence of energy levels in the target nuclei.

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