Passive neutron multiplicity device for^(240)Pu measurement based on FPGA

A passive neutron multiplicity measurement device,FH-NCM/S1,based on field-programmable gate arrays(FPGAs),is developed specifically for measuring the mass of plutonium-240(^(240)Pu)in mixed oxide fuel.FH-NCM/S1 adopts an inte-grated approach,combining the shift register analysis mode with the pulse-position timestamp mode using an FPGA.The optimal effective length of the^(3)He neutron detector was determined to be 30 cm,and the thickness of the graphite reflector was ascertained to be 15 cm through MCNP simulations.After fabricating the device,calibration measurements were per-formed using a^(252)Cf neutron source;a detection efficiency of 43.07%and detector die-away time of 55.79μs were observed.Nine samples of plutonium oxide were measured under identical conditions using the FH-NCM/S1 in shift register analysis mode and a plutonium waste multiplicity counter.The obtained double rates underwent corrections for detection efficiency(ε)and double gate fraction(f_(d)),resulting in corrected double rates(D_(c)),which were used to validate the accuracy of the shift register analysis mode.Furthermore,the device exhibited fluctuations in the measurement results,and within a single 20 s measurement,these fluctuations remained below 10%.After 30 cycles,the relative error in the mass of^(240)Pu was less than 5%.Finally,correlation calculations confirmed the robust consistency of both measurement modes.This study holds specific significance for the subsequent design and development of neutron multiplicity devices.

Analysis of prompt fission neutron spectrum and multiplicity for ^(237)Np(n, f) in the frame of multi-modal Los Alamos model

The improved version of Los Alamos model with the multi-modal fission approach is used to analyse the prompt fission neutron spectrum and multiplicity for the neutron-induced fission of 237Np. The spectra of neutrons emitted from fragments for the three most dominant fission modes （standard Ⅰ, standard Ⅱ and superlong） are calculated separately and the total spectrum is synthesized. The multi-modal parameters contained in the spectrum model are determined on the basis of experimental data of fission fragment mass distributions. The calculated total prompt fission neutron spectrum and multiplicity are better agreement with the experimental data than those obtained from the conventional treatment of the Los Alamos model.

Role of N/Z in Pre-scission Neutrons as an Observable of Nuclear Friction

A dynamical Langevin model is employed to evaluate the excess of the neutron emission in the fission ofheavy nuclei ^(240)Cf,^(246)Cf,^(254)Cf,^(240)U relative to the standard statistical-model prediction at various saddle-to-scissionfriction strengths.It is shown that when the neutron-to-proton ratio N/Z of the system increases,the sensitivityof the excess to the friction decreases substantially,and it almost disappears for ^(240)U.We suggest that using thosecompound systems with low N/Z favors an accurate determination for the saddle-to-scission friction strength based onthe measurement of the pre-scission neutron multiplicity.

The Method on Deducing Multiplicity Measurement Equations of Neutron/γ

Based on some important domestic and international references,the third γ multiplicity measurement equation is derived,but it is different from the results given in current researches.The neutron multiplicity equation is deduced in this paper,especially the fourth fast-neutron multiplicity equation based on the liquid scintillation detectors,which is more complex than the other multiplicity equations up to the fourth order.The equations given in this paper can be used to verify the validity and availability of principles for the multiplicity measurement up to the fourth order,and extend the application scopes of the neutron multiplicity measurement,such as correcting the additional measurement value to eliminate influences for dead times.It will be the foundation of nuclear researches,if the higher order multiplicity measurement is important for nuclear materials’control and accountability.

Correlations in Nuclear Interactions between ECM/u and Unexplained Experimental Observables

A new concept is introduced for the classification of “unresolved problems” in the understanding of interactions in thick targets irradiated with relativistic ions: The centre-of-mass energy per nucleon of a hypothetical compound nucleus from a primary interaction, ECM/u, is calculated and correlated with experimental observations in thick target irradia- tions. One observes in various reactions of relativistic primary ions with thick targets that there appears to be a thresh- old energy for reactions leading to “unresolved problems” which lies around ECM/u ~ 150 MeV. All “unresolved prob- lems” are exclusively observed above this threshold, whereas below this threshold no “unresolved problems” are found. A similar threshold at 158 ± 3 MeV exists for massive pion production in nuclear interactions. Hagedorn had proposed this threshold decades ago and it is known as the Hagedorn limit. In this paper we will only mention, but not elaborate on Hagedorn’s theoretical concept any further. Some considerations will be presented and further studies in this field are suggested.

Calculation of prompt fission neutron spectra for ^(235)U(n,f )

The prompt fission neutron spectra for the neutron-induced fission of 235U at En 5 MeV are calculated using nuclear evaporation theory with a semi-empirical model, in which the nonconstant and con- stant temperatures related to the Fermi gas model are taken into account. The calculated prompt fission neutron spectra reproduce the experimental data well. For the n（thermal）＋235 U reaction, the average nuclear temperature of the fission fragment, and the probability distribution of the nuclear temperature, are discussed and compared with the Los Alamos model. The energy carried away by γ rays emitted from each fragment is also obtained and the results are in good agreement with the existing experimental data.

Langevin study of neutron emission in the reactions ^(16)O+^(181)Ta and ^(19)F+^(178)Hf

The pre-scission neutrons measured in the reactions ^16O＋^181Ta and ^19F＋^178Hf are studied via a Langevin equation coupled with a statistical decay model. We find that because of the mass asymmetry of different entrance channels, the spin distributions of compound nuclei would be different, consequently, the measured neutrons in these two reactions would also different. This means that the entrance channel will affect the particle emission in the fission process of hot nuclei.

Research on the prompt neutron multiplicity of fission fragments for ^(235)U(n,f)

According to some experimental and evaluated data,the total excitation energy partitioning way between both of the fission fragments was given with a semi-empirical method. With the calculated energy partitioning way,the prompt neutron multiplicity as a function of fragment mass,（A） ,for neutron-induced fission of 235U at En=0.0253 eV,3 MeV,and 5 MeV was calculated. The results are checked with the total average prompt neutron multiplicities and compared with the experimental and evaluated data.

System size effects on probing nuclear dissipation with neutrons

Using a dynamical Langevin equation coupled with a statistical decay model, we calculate the excess of the pre-scission neutron multiplicities over its standard statistical-model values as a function of the nuclear dissipation strength for the three nuclei 19~Os, 2~~Hg, and 21~po which have the same neutron-to-proton ratio N/Z. We find that by decreasing the size of the fissioning nuclei, the effects of nuclear dissipation on the excess of the pre-scission neutron multiplicity are substantially amplified, and that the sensitivity of this excess to the nuclear friction strength is considerably increased as well. We suggest that for those fissioning systems with the same N/Z that are populated in fusion reactions, to obtain a more accurate information of the nuclear dissipation strength by measuring the pre-scission neutron multiplicity, it is best to choose a system with a small size.

Prompt neutron multiplicity distribution for ^(235)U(n,f) at incident energies up to 20 MeV

For the n＋235U fission reaction, the total excitation energy partition of the fission fragments, the average neutron kinetic energy ε （A） and the total average energies Eˉγ（A） removed by γ rays as a function of fission fragment mass are given at incident energies up to 20 MeV. The prompt neutron multiplicity as a function of the fragment mass, ν（A）, for neutron-induced fission of 235U at different incident neutron energies is calculated. The calculated results are checked with the total average prompt neutron multiplicities νˉ and compared with the experimental and evaluated data. Some prompt neutron and γ emission mechanisms are discussed.

Study of different features offission dynamics of ^(224)Th produced in fusion reactions within a stochastic approach

The evaporation residue cross section anisotropy of the fission fragment angular distribution, pre-scission neutron multiplicity and the pre-saddle and post-saddle contributions of the pre-scission neutron multiplicity were analyzed within a stochastic approach based on one-, two- and three-dimensional Langevin equations for the compound nucleus ^224Th formed via a complete fusion. In these calculations, dissipation was generated through the chaos weighted wall and window friction formula. Comparison of the theoretical results with the experimental data showed that three-dimensional Langevin equations with dissipation generated through the chaos weighted wall and window friction formula make it possible to reproduce satisfactorily the above-mentioned experimental data.

Two-dimensional Langevin modeling of fission dynamics of the excited compound nuclei ^(188)Pt,^(227)Pa and ^(251)Es

A stochastic approach based on one-and two-dimensional Langevin equations is applied to calculate the pre-scission neutron multiplicity,fission probability,anisotropy of fission fragment angular distribution,fission cross section and the evaporation cross section for the compound nuclei ^188Pt,^227Pa and ^251Es in an intermediate range of excitation energies.The chaos weighted wall and window friction formula are used in the Langevin equations.The elongation parameter,c,is used as the first dimension and projection of the total spin of the compound nucleus onto the symmetry axis,K,considered as the second dimension in Langevin dynamical calculations.A constant dissipation coefficient of K,γk=0.077（MeV zs）^-1/2）,is used in two-dimensional calculations to reproduce the above mentioned experimental data.Comparison of the theoretical results of the pre-scission neutron multiplicity,fission probability,fission cross section and the evaporation cross section with the experimental data shows that the results of two-dimensional calculations are in better agreement with the experimental data.Furthermore,it is shown that the two-dimensional Langevin equations together with a dissipation coefficient of K,γk=0.077（MeV zs）^-1/2,can satisfactorily reproduce the anisotropy of fission fragment angular distribution for the heavy compound nucleus^251Es.However,a larger value of γk=0.250（MeV zs）^-1/2is needed to reproduce the anisotropy of fission fragment angular distribution for the lighter compound nucleus^227Pa.