Random Neck Rupture Research Articles

Measurement of Fragment Mass Dependent Kinetic Energy and Neutron Multiplicity for Thermal Neutron Induced Fission of Plutonium-239.

Kinetic energy and neutron multiplicity as a function of fragment mass were measured for the thermal neutron induced fission of 239Pu. By measuring the velocities and energies of two fission fragments simultaneously, both of the pre-neutron emission fragment mass m* and the post-neutron mass m were obtained. The fragment mass dependent neutron multiplicity ν(m*) was deduced by subtracting m from m*. The fragment mass dependent total kinetic energy TKE(m*) was also obtained from this data. The fragment velocity was measured by time-of-flight (TOF) method, for which the start signal was triggered by a very thin plastic scintillation film detector (TFD) and the stop signal was obtained by a silicon surface barrier detector (SSBD) which was also used for the measurement of [he fragment kinetic energy. The present result of ν(m*) is in good agreement with that of Apalin et al. and that in the light fragment region of Fraser et al. The obtained TKE(m*) agrees well with the data of Wagemans et al. A calculation was carried out with the model proposed by Brosa et al. who assumed multichannel fission paths and a random neck rupture. It is seen that the calculated results of both fragment mass dependent kinetic energy and neutron multiplicity represent experimental data well.

Fragment characteristics for the photofission of 238U with 6.1–13.1 MeV bremsstrahhmg

The photofission of 238U has been studied experimentally for seven bremsstrahlung endpoint energies between 6.1 and 13.1 MeV. The postneutron yields of the fission products were obtained with γ-spectrometry techniques. Provisional mass and kinetic energy distributions were measured using a double energy detection technique. From the combination of both methods average neutron emission curves and preneutron mass-energy distributions were deduced. The systematic trends of the fragment characteristics have been studied as a function of the compound nucleus excitation energy. The results are compared with expectations from the energy partition model of Ruben et al. They are also discussed in the framework of the scission point model of Wilkins et al., and of the multimode fission with random neck rupture model of Brosa et al. The mass and kinetic energy data can be represented well by the superposition of two dominant mass asymmetric fission modes (standard I and standard II), and one relatively weak mass-symmetric fission mode (superlong). The standard I mode yield diminishes slightly with increasing compound-nucleus excitation energy. Up to an excitation energy of 7.9 MeV, i.e. the fission barrier plus pairing gap, the average fission-fragment total kinetic energy increases, and the average total ‘asymptotic’ excitation energy and the charge odd-even effect in the element yields are about constant ( δ P = (29 ± 2)%). These results hint to low energy dissipation. At higher compound nucleus excitation energies the fragment kinetic energy decreases, their excitation energy increases, and the proton odd-even effect decreases exponentially.

Comparative study of the fragments' mass and energy characteristics in the spontaneous fussion of 238Pu, 240Pu and 242Pu and in the thermal-neutron-induced fission of 239Pu

The energy and mass distribution and their correlations have been studied for the spontaneous fission of 238, 240, 242Pu and for the thermal-neutron-induced fission of 239Pu. A comparison of 240Pu(s.f.) and 239Pu(nth,f) shows that the increase in excitation energy mainly results in an increase of the intrinsic excitation energy. A comparison of the results for 238Pu, 240Pu and 242Pu(s.f.) demonstrates the occurence of different fission modes with varying relative probability. These results are discussed in terms of the scission point model as well as in terms of the fission channel model with random neck-rupture.

Nuclear scission

Nuclear scission is described by random neck rupture and random neck rupture is traced back to a series of instabilities. The paper contains a concise collection of formulas for the computation of the prescission shape and for the derivation of the mass distribution, the total kinetic energy, and the neutron multiplicity from that prescission shape. Evidence for the two main items of random neck rupture, namely rupture and randomness, is produced. Multichannel fission means that several prescission shapes occur.

Mass-split dependence of the pre- and post-scission neutron multiplicities for fission of 251Es

Pre- and post-scission neutron multiplicities have been measured as a function of fission mass-split for the reactions of 105 and 120 MeV 19F with 232Th. The post-scission multiplicities show no evidence for persistence of the sawtooth yield observed for spontaneous fission of 252Cf; the dependence on mass-split is not consistent with the random neck rupture model. The pre-scission multiplicities show no evidence of quasi-fission for asymmetric mass-splits. It is proposed that pre-scission neutron multiplicity measurements should be a useful tool in studying quasi-fission produced by reactions with heavier projectiles.

Ar+Mo. giant fluctuations but small drift

We study recent measurements of Ar+Mo reactions. Transport models do not agree with the data for large energy losses; they predict too small fluctuations and too much drift. Random neck rupture is however in keeping with experiments. We explain random neck rupture by very simple analysis and show that its basic ingredient, the critical ratio of total length over neck radius, is largely independent of angular momentum. We propose experiments to examine random rupture, one of them, a neutron multiplicity experiment, with a prediction of a saw-tooth structure.

Random neck rupture in deep-inelastic collisions

Neck formation and fluctuating neck scission in deep-inelastic collisions (DIC) of heavy nuclei are studied. Assuming the applicability of hydrodynamics to gross nuclear reactions, the authors find that a hydrodynamic instability governs the exit channel of the reaction. This suggests a new model, random neck rupture, for DIC. It explains the observation of almost no mean mass shift despite the extremely large variances of the fragments produced. Random neck rupture relates the mass variances to the energy losses and thus establishes an extended Viola systematics. The origin of the lead-uranium anomaly is discussed. They propose the use of alpha -particle multiplicity experiments to measure the size of the neck.