Neutron Induced Fission Of 235U Research Articles

Future research program on prompt $\gamma$ -ray emission in nuclear fission

In recent years the measurement of prompt fission \(\gamma\)-ray spectra (PFGS) has gained renewed interest, after about forty years since the first comprehensive studies of the reactions 235U(nth, f), 239Pu(nth,f) and 252Cf(sf). The renaissance was initiated by requests for new values especially for \(\gamma\)-ray multiplicity and average total energy release per fission in neutron-induced fission of 235U and 239Pu. Both isotopes are considered the most important ones with respect to the modeling of innovative cores required for the Generation-IV reactors, the majority working with fast neutrons. During the last 5 years we have conducted a systematic study of spectral data for thermal-neutron-induced fission on 235U and 241Pu as well as for the spontaneous fission of 252Cf with unprecedented accuracy. From the new data we conclude that those reactions do not considerably contribute to the observed heat excess and suspect other reactions playing a significant role. Possible contributions may originate from fast-neutron-induced reactions on 238U, which is largely present in the fuel, or from \(\gamma\)-induced fission from neutron capture in the construction material. A first experiment campaign on prompt \(\gamma\)-ray emission from fast-neutron-induced fission on 235,238U was successfully performed in order to test our assumptions. In the following we attempt to summarize, what has been done in the field to date, and to motivate future measurement campaigns exploiting dedicated neutron and photon beams as well as upcoming highly efficient detector assemblies.

Measurement of fission product yields in the quasi-mono-energetic neutron-induced fission of 238U

The cumulative yields of various fission products in the 6.35, 8.53, 9.35 and 12.52 MeV quasi-mono-energetic neutron induced fission of 238U have been determined by using the off-line γ-ray spectrometric technique. The mass chain yields were obtained from the fission product yields by using charge distribution correction. From the mass yield data, the peak-to-valley (P/V) ratio, the average value of light mass (〈AL〉), heavy mass (〈AH〉) and thus the average number of neutrons (〈υ〉expt) were obtained in the 238U(n,f) reaction for the above mentioned four neutron energies. The present and literature data in the 238U(n,f) and 238U (γ,f) reactions at various energies were compared to arrive at the following conclusions. (i) The yields of fission products for A=133–134, A=138–140, and A=143–144 and their complementary products in the 238U(n,f) and 238U(γ,f) reactions are higher than other fission products, which has been explained from the point of even–odd effect and standard I and standard II asymmetric mode of fission. (ii) The yields of symmetric products increase and thus the peak-to-valley (P/V) ratios decrease with excitation energy, whereas the 〈ν〉expt values increase with excitation energy. (iii) The variation of 〈AL〉,〈AH〉 and 〈υ〉expt values with excitation energy behave differently in between 238U(n,f) and 238U(γ,f) reactions, which may be due to the different types of reaction mechanism for the neutron and photon with 238U.

Measurements of fission product yield in the neutron-induced fission of 238U with average energies of 9.35 MeV and 12.52 MeV

The yields of various fission products in the neutron-induced fission of 238U with the flux-weightedaveraged neutron energies of 9.35 MeV and 12.52 MeV were determined by using an off-line gammaray spectroscopic technique. The neutrons were generated using the 7Li(p, n) reaction at Bhabha Atomic Research Centre-Tata Institute of Fundamental Research Pelletron facility, Mumbai. The gamma- ray activities of the fission products were counted in a highly-shielded HPGe detector over a period of several weeks to identify the decaying fission products. At both the neutron energies, the fission-yield values are reported for twelve fission product. The results obtained from the present work have been compared with the similar data for mono-energetic neutrons of comparable energy from the literature and are found to be in good agreement. The peak-to-valley (P/V) ratios were calculated from the fission-yield data and were found to decreases for neutron energy from 9.35 to 12.52 MeV, which indicates the role of excitation energy. The effect of the nuclear structure on the fission product-yield is discussed.

Experimental Studies of Prompt Fission Neutron Energy Spectra

Abstract Prompt fission neutron spectra were measured in the reactions 238U(n,f), 235U(n,f) and 237Np(n,f) at different incident neutron energies. The neutrons were detected using a coaxial doped p-terphenyl scintillation detector in coincidence with fission fragments and their time-of-flight was recorded. The properties of the neutron detector were determined and the results are presented in this work. A preliminary neutron detection efficiency was applied to data from the neutron-induced fission of 238U at En = 5.2 MeV, leading to encouraging results.

Prompt energy distribution of 235U(n,f)[formula omitted] at bombarding energies of 1–20 MeV

The distributions of prompt γ rays from the spontaneous fission of 252Cf and neutron-induced fission of 235U were measured up to ∼4MeV using a liquid scintillator array. The unfolding of measured fission γ rays is presented using the Single Value Decomposition and iterative Bayesian methods. General agreement was found with comparisons made with previous measurements. The energy dependence of the prompt γ-ray distributions for the spontaneous fission of 252Cf and the neutron-induced fission of 235U from bombarding energies of 1-2, 5-10, and 10–20MeV were found to be almost identical in the γ-ray energy region 1–4MeV.

Regression analysis for a bottom-up approach to analyzing semi-prompt fission gamma yields

We present an empirical model that describes the yield of gamma rays emitted by fission in the time interval from 20 to 958ns following a fission event. The analysis is based on experimental data from neutron-induced fission of 235U and 239Pu. The model is devised by first using regression analysis to identify likely patterns in the data and to choose plausible fitting functions. We provide statistical and physical arguments in support of time and energy independence. The intensity of the emitted gamma rays can be described as a bivariate distribution that is the product of independent variates for energy and time. We test several plausible distribution families for the energy and time variates and use maximum likelihood and minimum χ2 to estimate distribution parameters. Because of the uncertainty in the experimental data, multiple combinations of variate pairs give rise to a surface that plausibly well fits the observations well. The best-fit variate turns out to be lognormal in energy and F in time. The findings illustrated in this paper can be used to simulate gamma ray de-excitation from fission in Monte Carlo codes.

Simulations and developments of the Low Energy Neutron detector Array LENA

Abstract Prototypes of the Low Energy Neutron detector Array (LENA) have been tested and compared with detailed GEANT simulations. LENA will consist of plastic scintillation bars with the dimensions 1000×45×10 mm3. The tests have been performed with γ - ray sources and neutrons originating from the neutron-induced fission of 235U. The simulations agreed very well with the measured response and were therefore used to simulate the response to mono-energetic neutrons with different detection thresholds. LENA will be used to detect low-energy neutrons from (p,n)-type reactions with low momentum transfer foreseen at the R3B and EXL setups at FAIR, Darmstadt.

Calculation of Prompt Fission Product Average Cross Sections for Neutron-Induced Fission of 235U and 239Pu

Yield-weighted average cross sections of neutron radiative capture, (n,2n), and (n,3n) reactions over prompt fission products (FPs) from 235U and 239Pu are calculated. The prompt fission production yields are taken from the ENDF/B-VII.0 library. The FPs for each fissile material exist over a range of approximately 1000 neutron-rich nuclides. Several nuclear reaction codes are utilized for calculating the cross sections on each individual fission product—EMPIRE-2.19, TALYS-1.0, GNASH, and CoH. The influence of the FP isomers on the average cross sections is examined with TALYS. We investigate the dependence of the average cross sections on the number of FPs taken for averaging. It is shown that the average capture cross section is much more sensitive to the number of FPs included, compared with the (n,2n) and (n,3n) reactions. An intercomparison of the calculated cross sections with the different reaction codes is carried out. In the capture reaction, EMPIRE predicted lower cross section than TALYS and CoH owing to different default assumptions used in the γ-ray strength function modeling. Moreover, the preequilibrium models implemented in each code give different predictions for the neutron-emission reactions, although the differences are relatively small. We also discuss a difference between the macroscopic and microscopic calculation options in TALYS for the pre-equilibrium model, optical potential model, and γ-ray strength function. The predictive capability of the reaction codes for the capture reaction is examined by comparing their calculations with the ENDF data, which are based on measurements. Compared with the historic Foster and Arthur's evaluation, our new (n,2n) predictions are similar, although our capture predictions are almost an order of magnitude higher. Recommended cross sections for use in applications have been tabulated in ENDF-formatted files.

Mean energies of prompt fission neutrons in the neutron-induced fission of 235U at primary energies in the range E n < 20 MeV

The spectra of neutrons accompanying the induced fission of 235U target nuclei are described theoretically. It is confirmed that a third neutron source must be introduced in order to reproduce the shape of experimental distributions at high energies of primary neutrons (previously, a third source was used in describing the spectra of neutrons emitted in 232Th and 238U fission). On the basis of experimental results and their analysis, the mean energy of fission neutrons is estimated as a function of the bombarding-neutron energy up to En = 20 MeV.

A method and setup for studying the energy dependence of delayed neutron characteristics in nuclear fission induced by neutrons from the T(p, n), D(d, n), and T(d, n) reactions

A method and a setup based on this method are described, with which it has for the first time become possible to measure the energy dependence of the absolute total and relative yields of delayed neutrons and the half-fives of their precursors from neutron-induced fission of heavy nuclei in the course of one experiment. T(p, n)3He, D(d, n)3He, and T(d, n)4He nuclear reactions induced by high-energy charged-particle beams from the KΓ-2.5 electrostatic accelerator at the Institute of Physics and Power Engineering are used as sources of monoenergetic neutrons. The measured total delayed neutron yields from neutron-induced fission of 233U and 239Pu nuclei in the energy range of 0.37–4.7 MeV, as well as the relative yields of delayed neutrons and the half-lives of their precursors from neutron-induced fission of 239Pu in the range of 15.8–17.9 MeV, are presented as an illustration of the method. The uncertainties of the data obtained by means of this method are shown to be significantly lower than the uncertainties of similar data measured using other techniques.

Total prompt energy release in the neutron-induced fission of 235U, 238U, and 239Pu

This study addresses, for the first time, the total prompt energy release and its components for the fission of 235U, 238U, and 239Pu as a function of the kinetic energy of the neutron inducing the fission. The components are extracted from experimental measurements, where they exist, together with model-dependent calculation, interpolation, and extrapolation. While the components display clear dependencies upon the incident neutron energy, their sums display only weak, yet definite, energy dependencies. Also addressed is the total prompt energy deposition in fission for the same three systems. Results are presented in equation form. New measurements are recommended as a consequence of this study.

Characteristics of a lead slowing-down spectrometer coupled to the LANSCE accelerator

A description is given of a lead slowing-down spectrometer (LSDS) installed at the 800-MeV proton accelerator of the Los Alamos Neutron Science Center (LANSCE). The LSDS is designed to study neutron-induced fission on actinides that can only be obtained or used in small quantities. The characteristics of this LSDS (energy–time relation, energy resolution, neutron flux) are presented through simulations with MCNPX and measurements with several different methods. Results on neutron-induced fission of 235U and 239Pu with tens of micrograms and tens of nanograms, respectively, are presented. Finally, additional MCNPX calculations have been performed to simulate the measurement of the cross-section for U 235 m ( n , f ) using different target quantities and different initial isomer-to-ground state compositions.

First observation of isomeric transition from 148m Pr and parity assignment for excited levels in 148Pr

Internal conversion electrons from the decay of 148Ce and 148mPr have been measured with a Si(Li) detector. The radioactive sources were produced by the thermal neutron-induced fission of 235U, followed by an on-line mass separation. K- and L-conversion electrons of the isomeric transition from 148mPr were newly observed at 34.6 and 70.8 keV, respectively. The M3 multipolarity was assigned to this transition from the experimental conversion coefficients, and, consequently, the 76.8 keV isomeric level was found to be a 4- state. Parities of some excited levels in 148Pr were also deduced.

Conversion electron measurement in the β--decay of 151Pr

The β--decay of 151Pr produced by the thermal neutron-induced fission of 235U has been studied using an on-line isotope separator. From an internal-conversion electron measurement with a Si(Li) detector, K-conversion coefficients were obtained for 20 γ-transitions. Spins and parities of 6 excited levels in 151Nd were newly determined from the deduced multipolarities: even parities for the 543 and 627 keV levels, odd parities for 250 and 599 keV, (3/2, 5/2)+ for 685 keV, and (1/2, 3/2)+ for 880 keV. The level structure was compared with the rotation-vibration coupling Nilsson model.

Decay Studies on Neutron-Rich Isotope 151Ce

The β− decay of 151Ce produced by the thermal neutron-induced fission of 235U has been studied using an isotope separator on-line. From γ-ray singles, γ-γ coincidence and internal conversion electron measurements, the decay scheme of 151Ce with a half-life of 1.72(5) s was proposed for the first time.

Energy dependence of relative abundances and periods of delayed neutrons from neutron-induced fission of 235U, 238U, 239Pu in 6- and 8-group model representation

The energy dependence of the relative abundances and periods has been measured for neutron induced fission of the main fuel nuclides, 235U and 239Pu in the energy range from epi-thermal to 5 MeV and 238U in the energy range from 1 to 5 MeV. The efforts undertaken in improving the experimental and data processing procedures made it possible to improve the accuracy of the delayed neutron parameters determining their time-dependent behavior. In terms of the average half-life of delayed neutron precursors — the value that unequivocally characterizes the particular fissioning system — the real scale of the changes in the relative abundances and periods of delayed neutrons were determined.

Fission-mode calculations for 239U, a revision of the multi-modal random neck-rupture model

In the course of an experimental study of the fragment characteristics after neutron-induced fission of 238U with incident neutron energies between 1.2 and 5.8 MeV fission-mode calculations in the frame of the multi-modal random neck-rupture model have been performed. During these calculations some technical parts of the model have been revised. The identification of fission modes is now based on unequivocal and reproducible criteria. The Rayleigh criterion has consequently been applied to determine each possible scission configuration. The most remarkable new results are that all physically relevant fission modes branch off in the second potential minimum exhibiting different outer barriers, and that the total kinetic energy distribution of the fission fragments is a direct consequence of the Rayleigh criterion. The fragment characteristics as the mean mass, the mean total kinetic energy and the corresponding width obtained from the fission-mode calculations compare reasonably well with the experimental findings. For the first time the weighted fission cross sections through each particular fission mode have been analyzed simultaneously using a Hill-Wheeler type expression for the transmission through a double-humped fission barrier. The results support the picture of individual outer barriers with slightly different penetrabilities and a slightly lower inner barrier.

Possible exit channel effect on isomer yield ratios

Isomer yield ratio measurements in fission are important in understanding the fission process. With the development of new instrumental techniques, a large number of yield data are now available. The experimental data on isomer yield ratios in the thermal neutron induced fission of235U are compared with those calculated from the simple statistical model byMadland andEngland. The method of calculation has been extended to the isotopes having more than one isomeric state. The results may be explained according to the multi-exit-channel model of fission.

Measurements of Delayed Neutron Decay Constants and Fission Yields from 235U, 237Np, 241Am, and 243Am

Delayed neutron yields and decay constants for 235U, 237Np, 241Am, and 243Am were measured at the Texas A&M University TRIG A reactor using a fast pneumatic transfer system. The detection system consisted of an array of BF3 proportional counters embedded in a polyethylene cylinder. The measured values of the total delayed neutron yield per 100 fissions from thermal neutron-induced fission of 235U, 237Np, 241Am, and 243Am were determined to be 1.59 ± 0.04, 1.29 ± 0.04, 0.49 ± 0.02, and 0.84 ± 0.04, respectively. The newly measured values of delayed neutron group parameters and total yields were compared with other values recommended by Keepin; Waldo, Karam, and Meyer; and Tuttle. Very good agreement was obtained, especially for 235U.

Determination of pre-scission kinetic energy in low energy fission using isotonic charge distribution model

The parameters of Isotonic Charge Distribution Model have been calculated for several isotones of three low energy fissioning systems. The global charge even-odd effects are determined as 0.27±0.04 for thermal neutron induced fission of233U(U233T), 0.27±0.05 for U235T and 0.13±0.05 for PU239T. The pre-scission kinetic energies for U235T and PU239T are estimated and found to be ∼16 MeV.