Mass Distribution and Kinetics ofU235Thermal-Neutron-Induced Three-Particle Fission

Abstract

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...

The Periodic Table Possible Coincided with an Unfolded Shape of Atomic Nuclei

“…At leftmost bottom sides of a double peak curve, A ~ 80 and ~ 130, are two vanishing points of A L , A H (Unik et al, 1973), neutron (Bowman, Milton, Thompson, & Swiatecki, 1963), and α-particle (Schmitt, Neiler, Walter, & Chetham-Strode, 1962), which both point to a sector where its 2 edges are enclosed by long 2 a axes, i.e., 2 (Bowman, Milton, Thompson, & Swiatecki, 1963), suggesting that maximal neutron yield is from a sector of short 2 c axis edges. In (Gindler, Flynn, Glendenin, & Sjoblom, 1977).…”

XV.—Separation of the light particle in the “ternary” fission process

“…Also in text- fig. 1 (curve (&)) the binary-fragment yield curve of Schmitt et al (1962) is given for comparison. It will be seen that B tt (F') + V a (F) varies very little, about a mean value of some 17*5 MeV, over the range of A(L') for which the yield is significant, whereas, over the corresponding range of ^f(H'), it varies considerably-from about 23 MeV when A(H') = i2g, through values less than 15-5 MeV for i36<yi(H')< 139, to more than 19 MeV when A(H') = 151.…”

Sur l'origine des alphas de scission

“…2.1 Si le deficit de quatre unites est corrige, la distribution des masses dans la fission a, except6 le fait qu'elle est plus 6troite, coincide dans tous ses aspects avec celle de la fission ordinaire [1,2].…”