Fission-fragment mass and energy distributions and mass-versus-energy correlations have been obtained for 252 Cf spontaneous fission and 235 U thermal-neutron-induced fission. Silicon surface-barrier detectors were used in energy correlation measurements; absolute fragment energies were obtained by means of the massdependent energy calibration developed recently at this laboratory. Average total fragment kinetic energies before neutron emission are found to be 186.5±1.2 MeV for 252 Cf and 171.9dbl.4 MeV for 235 U. Detailed experimental results are given and compared with those of other experiments. Observed fine structure in the fragment mass distribution and in the average total fragment kinetic energy as a function of mass is correlated with the energetically preferred even-even nucleon configurations in the fragments. New determinations of the root-mean-square width of the total-kinetic-energy distribution as a function of mass show structure which is also correlated with the energetically preferred even-even fragment configurations. Fission-neutron and gamma-ray data of other experiments are used with the new fragment kinetic energies presented here to examine the total energy balance for fission for the two cases studied.

Fission-fragment mass and energy distributions and mass-versus-energy correlations have been obtained for 239 Pu and 241 Pu thermal-neutron-induced fission. Silicon surface-barrier detectors were used in energycorrelation measurements; absolute fragment energies were obtained by means of a recently developed mass-dependent energy calibration. Average total fragment kinetic energies before neutron emission are found to be 177.7=1=1.8 MeV for ^Pu and 179.6=bl.8 MeV for 241 Pu. Detailed experimental results are given and compared with those of other experiments. Observed fine structure in the fragment mass distribution and in the average total fragment kinetic energy as a function of mass is correlated with the energetically preferred even-even nucleon configurations in the fragments. New determinations of the root-mean-square width of the total-kinetic-energy distribution as a function of fragment mass show structure which also appears to be correlated with the energetically preferred even-even fragment configurations. Fission neutron and gamma-ray data of other experiments are used with the new fragment kinetic energies presented here to examine the total energy balance for fission for the two cases studied. A comparison of the two mass distributions shows the heavy-fragment groups almost superimposed; the light-fragment groups are separated almost uniformly by 2 amu.

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A description is given of a solid-state counter of good resolution, long term stability, and fast rise time which is suitable for heavy charged particles, for example, alpha particles and fission fragments. A simple theoretical model for the counter behavior is presented which is found to describe the observed behavior of the counter very well. The importance of germanium purity in connection with pulse height, rise time, and counter area is discussed. Counters with sensitive areas up to 5 cm2 have been successfully used.

A three-parameter correlation experiment has been performed in which the energies of the two heavy fragments and of the third light particle (usually a long-range alpha particle) have been measured in coincidence. A comparison of the mass distributions thus obtained for particles from binary and ternary fission indicates that the long-range alpha particles are emitted principally at the expense of nucleons in the light fragment for near-symmetric fission and at the expense of nucleons in the heavy fragment for more asymmetric fission. Strong influence of the closed N= 82 and iV= 50 shells is indicated.Surface barrier detectors were used in conjunction with standard low-noise amplifier systems; events were serially recorded by a 128x128x4channel punched-paper-tape correlation recorder. The data were sorted, summed, and correlated through the use of computers, and results have been obtained giving the mass distribution, massenergy correlations, and other kinetic parameters associated with U 235 thermal-neutron-induced three-particle fission.In order to assure high resolution in these measurements, a narrow, highly collimated beam of thermal neutrons from the Oak Ridge Research Reactor was incident on the 50-jiig/cm 2 U 235 deposit, backed by a 70-/Lig/cm 2 self-supporting nickel foil. All detectors were located outside the neutron beam. The two fission fragment detectors, made of 500 ohm-cm silicon, were selected and calibrated with respect to the time-offlight data of Milton and Fraser. 1 The two thirdparticle detectors, made of 3000 ohm-cm silicon, were operated with sensitive depths greater than the range of a 30-MeV alpha particle and were located at about 80° with respect to the axis of the fragment detectors. 2 All four detectors were ~4 cm 2 in area and exhibited resolutions better than 40 keV full width at half maximum for natural alpha particles. The fission fragment detectors exhibited resolutions better than 1.2 MeV for 95-MeV Br 79 and Br 81 ions, as determined in an auxiliary experiment using the Oak Ridge tandem Van de Graaff generator. 3 In the analysis of the data, the simple momentum and mass conservation relations were used: M X E X = M 2 E 2 and M x +M 2 = 235 +1 -4. For binary fission M 1 +M 2 = 235 + 1. It can be shown that recoil from prompt neutron emission by the fragments introduces a dispersion in mass (for any individual mass split) no larger than ~2 amu. For ternary fission the detector geometry is such that recoil from the alpha particles introduces an average dispersion (for any mass split) less than ~2 amu. The dispersion introduced by alphaparticle recoil is thus expected to result in a slightly broadened fragment mass distribution for ternary fission.Among the interesting results obtained from these measurements is the integral mass distribution (summed over all third-particle energies) of the fission products. This distribution is shown in Fig. 1, compared with the mass distribution of fragments from binary fission as obtained during 100 120 140 FRAGMENT MASS (AMU) FIG. 1. Relati...