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.

The energy and angular distributions of protons emitted from Ag and Al nuclei when irradiated with the x-rays from a 22-Mev betatron were determined with the use of photographic nuclear emulsions. A maximum x-ray energy of 20.8 Mev was used with Ag and energies of 20.8, 17.1, and 13.9 Mev with Al.With silver the number of neutrons emitted in the same irradiation was determined from the (7, n) induced activity. The ratio of numbers of protons in each Mev interval to the total number of neutrons is compared to the same ratio calculated on the basis of various assumptions as to level density and nuclear reaction cross sections. The comparison suggests that the observed proton spectrum consists of two overlapping components. One is a lower energy group in rough quantitative agreement as to spectrum shape and numbers with calculations from reasonable assumptions as to statistical-model level densities and other nuclear parameters. The second group is a high energy tail (10-14 Mev) of protons definitely outside of the spectrum expected from the statistical model. Moreover, only the high energy group shows angular asymmetry with a preference for emission at 90° to the x-ray beam, which supports the hypothesis that these protons are emitted before the excitation energy is statistically distributed in the nucleus.The ratio of total numbers of protons to neutrons emitted by Ag under 20.8-Mev bremsstrahlung radiation is 0.023±0.008.With Al protons are emitted with spherical symmetry and with a yield, for quanta above about 14 Mev, as large or larger than the (7, n) yield determined by Hirzel and Waffler at 17.6 Mev. The shape of the spectrum and the ratio of (7, p) to (7, n) cross sections are compatible with the assumption of a constant or slowly increasing level density in the residual nucleus, as expected for a light nucleus in the energy range involved. From the maximum energy of emitted protons the proton threshold was determined to be 8.6±0.5 Mev.For use with the theoretical calculation of the proton spectra, the (7, n) cross sections as a function of x-ray quantum energy were determined for Ag 107,109 , Al 27 , and also for Cu 63 . The cross section of Ag 109 was found to have a maximum of 0.32 X 10~2 4 cm 2 at 16.5 Mev, Cu 63 a maximum of O.lOXlO" 24 cm 2 at 17.5 Mev. For Al 27 the cross section is still rising at 22 Mev.

The principal Hugoniot of molybdenum has been determined at a pressure of 2.0 TPa by measuring directly both the shock velocity and the particle velocity behind the shock. Neutrons from an underground nuclear explosion were used to generate a high-pressure shock in a slab of molybdenum by rapidly fission heating an adjacent slab of enriched uranium. A shock velocity of 18.2 mm/μs (±5%) was obtained by determining the transit time of the planar shock between two points in the molybdenum separated by 9.87 mm. A particle velocity of 10.7 mm/μs (±5%) was obtained by observing the Doppler shifts of six neutron resonances in the energy region from 200 to 800 eV in the moving shocked molybdenum. The pressure and density derived from this pair of measurements are 2.0 TPa (20 Mbar) and 24.8 g cm−3, respectively. This experiment represents the first direct determination of a point on the Hugoniot of any material in this pressure region, and the resulting data point is in good agreement with theoretical estimates. This measurement was a successful demonstration that the Doppler-shift technique can be used to obtain particle velocities in this pressure region. It appears that errors in both the shock velocity and the particle velocity can be reduced to approximately ±2% in an improved measurement, resulting in a well-defined Hugoniot for molybdenum, which can be used as a standard in future impedance-matching experiments.

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