Fission Cross Section Ratios Research Articles

Characterization of 235U, 238U and 239Pu fission ionization chamber foils by [formula omitted] and [formula omitted]-ray spectrometry

The masses of thin, electroplated deposits of 235U, 238U and 239Pu on titanium backings have been determined to better than 1.3% precision by employing three complementary measurement techniques: α spectrometry, 2π gas counting, and γ-ray spectrometry. A dedicated α spectrometer was designed and calibrated to allow for repeatable, high-precision measurements of the absolute α activity of a sample. The isotopic composition of the sample was determined by fitting the peaks in the α spectrum, a technique which was demonstrated to be generally consistent with mass spectrometry. Independently, the total activity of the deposits were measured in a 2π gas flow proportional counter. A detailed study of the effects of α particle straggling in the actinide deposit and α particle recoil off the sample backing material was performed. The masses of these actinide deposits were also measured via γ-ray spectrometry. We found that all three systematically independent techniques produced consistent results, and could be combined into a weighted average with further reduced uncertainty. The mass ratios of these actinide deposits were determined to better than 1% precision via α spectrometry, enabling high-precision fission cross section ratio measurements.

EXFOR-based simultaneous evaluation for fast neutron-induced fission cross section of thorium-232

ABSTRACT The 232Th neutron-induced fission cross section was evaluated from 500 keV to 200 MeV. The experimental 232Th fission cross sections and their ratios to the 235,238U fission cross sections in the EXFOR library were reviewed and analysed by using the least-squares method. The newly published 232Th/235U fission cross-section ratios from the time-of-flight measurements at the CERN n_TOF and CSNS Back-n facilities were compiled in EXFOR. Additional simultaneous evaluation was performed by including the experimental 233,238U and 239,240,241Pu fission cross sections and their ratios. The new evaluation provides the 232Th fission cross section systematically lower than the JENDL-5 cross section. The reduction is 4% in the plateau region between 2 and 6 MeV and more significant in the subthreshold fission region. The present evaluation reduces the 232Th fission cross section averaged over the 252Cf spontaneous fission neutron spectrum from the JENDL-5 evaluation by 4%, which is closer to the other general purpose libraries but underestimates Grundl et al.’s measurement by 11%.

Progress in measurements of fission cross sections and total cross sections at CSNS Back-n white neutron source

The neutron-induced cross sections are of great significance for the design of nuclear devices and advanced reactors for the nuclear energy production. At CSNS Back-n white neutron source, new measurements of the fission cross sections and total cross sections are performed with two sets of Day-one spectrometers based on the multi-cell fast fission ionization chamber (FIC). The neutron-induced 236,238U fission cross sections relative to 235U from the fission threshold energy to 200 MeV were measured by using the TOF method and the Fast Ionization Chamber Spectrometer for Fission Cross Section Measurement (FIXM) in the single/double bunch mode of Back-n. The experimental uncertainties are analyzed in detail, and the results from the two modes are consistent. The measured 236,238U/235U fission cross section ratios are compared with previous experiments and evaluations. The 236,238U(n,f) cross sections are obtained based on the standard 235U fission cross section. The neutron total cross sections of carbon and aluminum in the energy region from 1 eV to 20 MeV have been measured by using the TOF method and transmission method based on the Neutron Total Cross Section Spectrometer (NTOX) in the double bunch mode. The total cross section results after unfolding are in good agreement with the previous measurements as well as the broadening of the ENDF/B-Ⅷ.0 evaluation with Gaussian function within the experimental uncertainty. The present results provide the experimental data for further measurements, relevant evaluations and the design of nuclear system.

Measurement of material isotopics and atom number ratio with [formula omitted]-particle spectroscopy for a NIFFTE fission Time Projection Chamber actinide target

We present the results of a measurement of isotopic concentrations and atomic number ratio of a double-sided actinide target using α-spectroscopy and mass spectrometry. The double-sided actinide target, with predominantly 239Pu on one side and 235U on the other, was used in the fission Time Projection Chamber (fissionTPC) for a measurement of the neutron-induced fission cross-section ratio between the two isotopes. The measured atomic number ratio is needed to extract an absolute measurement fission cross-section ratio. The 239Pu/235U atom number ratio was measured with a combination of mass spectrometry and α-spectroscopy with a planar silicon detector achieving uncertainties of less than 1%. Different strategies for estimating isotopic concentration from the α-spectrum are presented to demonstrate the potential of these methods for non-destructive target assay. We found that a combination of fitting spectra with constraints from mass spectrometry, and summing counts in a region of the spectrum provided the most consistent results with the lowest uncertainty.

Measurement of the U-238/U-235 fission cross section ratio at CSNS – Back-n WNS

Based on the China Spallation Neutron Source (CSNS) – Back-streaming white neutron source (Back-n WNS), the measurement of the U-238/U-235 fission cross section ratio in 1–20 MeV neutron energy region has been carried out by a multi-cell fast fission ionization chamber using the time-of-flight method. The measured resonance peaks of the 235U(n, f) reaction are in good agreement with those of the ENDF/B-8.0 database. The measured six groups of the U-238/U-235 fission cross section ratios in 1–20 MeV neutron energy region agree with those of the ENDF/B-8.0 database in trend, and the average discrepancies between experiment and database are from 1.9% to 2.6%. The relative experimental uncertainties are 2.3%–3.6% (1.4–20 MeV), and the measured data agree with those of database within experimental uncertainties at most energy points. The measured U-238/U-235 fission cross section ratio could provide information and data support in relevant nuclear data evaluation.

Monoenergetic photon-induced fission cross-section ratio measurements for U235 , U238 , and Pu239 from 9.0 to 17.0 MeV

Monoenergetic photon-induced fission cross-section ratio measurements for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi mathvariant="normal">U</mml:mi><mml:mprescripts /><mml:none /><mml:mn>235</mml:mn></mml:mmultiscripts></mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi mathvariant="normal">U</mml:mi><mml:mprescripts /><mml:none /><mml:mn>238</mml:mn></mml:mmultiscripts></mml:math> , and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>Pu</mml:mi><mml:mprescripts /><mml:none /><mml:mn>239</mml:mn></mml:mmultiscripts></mml:math> from 9.0 to 17.0 MeV

Reactor Neutronics: Impact of Fissile Material

Given a wide variety of reactor designs and fuel types, it can be difficult to identify the underlying cause of basic performance differences such as flux level and enrichment requirement. In this paper, using solely the definitions of the core multiplication factor and core power, simple relations have been derived allowing estimates of the flux ratio and fissile material concentration ratio for any reactor concept when 235U is replaced with 239Pu or vice versa. These relations are functions of the neutron nonleakage probability, and one only needs to know the number of neutrons emitted per fission and the fission cross-section ratio between the 235U system and the 239Pu system. It is found that for a reactor concept having significant leakage, the achievable flux level when using 239Pu as fissile material can be up to 45% larger than when using 235U as fissile material, and the required fissile concentration of 239Pu is up to 48% lower than that of 235U to achieve criticality.

Comparison of two fast neutron fluence measurement methods based on Np-237 fission-to-capture ratio measurement (spectral index) and a reverse dark current measurement in a planar silicon detector

The idea of the first method is to search the neutron energy for the ratio of fission cross section to capture cross section of the selected actinide isotope from the nuclear data base that is equal to the measured ratio of the fissioned and captured actinide isotope Np-237.The idea of the second method consists in the measurement of the reverse dark current increase, which is linearly proportional to neutron fluence, induced by the fast neutron irradiation in planar silicon detectors.Np-237 samples and planar silicon detectors were placed inside a subcritical assembly (the Quinta assembly at the Joint Institute for Nuclear Research, Russia) very close to each other assuming that in both samples the same neutron fluence should pass.We concluded that minor actinide samples and planar silicon detectors can be used as neutron fluence detectors especially in the high neutron energy range, where measurements are difficult.Considering the importance of high energy neutron measurement in the ADS (Accelerator Driven System), actinide and silicon detectors could be a very useful tool.

Point-by-point model calculation of the prompt neutron multiplicity distribution ν(A) in the incident neutron energy range of multi-chance fission

Prompt neutron multiplicity distributions ν(A) are required for prompt emission correction of double energy (2E) measurements of fission fragments to determine pre-neutron fragment properties. The lack of experimental ν(A) data especially at incident neutron energies (En) where the multi-chance fission occurs impose the use of ν(A) predicted by models. The Point-by-Point model of prompt emission is able to provide the individual ν(A) of the compound nuclei of the main and secondary nucleus chains undergoing fission at a given En. The total ν(A) is obtained by averaging these individual ν(A) over the probabilities of fission chances (expressed as total and partial fission cross-section ratios). An indirect validation of the total ν(A) results is proposed. At high En, above 70 MeV, the PbP results of individual ν(A) of the first few nuclei of the main and secondary nucleus chains exhibit an almost linear increase. This shape is explained by the damping of shell effects entering the super-fluid expression of the level density parameters. They tend to approach the asymptotic values for most of the fragments. This fact leads to a smooth and almost linear increase of fragment excitation energy with the mass number that is reflected in a smooth and almost linear behaviour of ν(A).

Point-by-Point model calculation of the prompt neutron multiplicity distributionν(A)forU238(n,f)at incident neutron energies ranging from 1 MeV to 80 MeV

Prompt neutron multiplicity distributions \ensuremath{\nu}($A$) are generally required for prompt emission correction of double energy ($2E$) measurements of fission fragments in order to determine pre-neutron fragment properties. The lack of experimental \ensuremath{\nu}($A$) data especially at higher incident neutron energy imposes the use of prompt emission models to predict \ensuremath{\nu}($A$). At incident neutron energies ($\mathit{En}$) where multiple fission chances are involved, the Point-by-Point (PbP) model of prompt emission is able to provide the individual \ensuremath{\nu}($A$) of the compound nuclei of the main and secondary nucleus chains that are undergoing fission at a given $\mathit{En}$. The total \ensuremath{\nu}($A$) are obtained by averaging these individual \ensuremath{\nu}($A$) over the fission chance probabilities (expressed as total and partial fission cross-section ratios). An indirect validation of the total \ensuremath{\nu}($A$) results is proposed. At high $\mathit{En}$ (above 70 MeV) the PbP results of individual \ensuremath{\nu}($A$) of the first few nuclei of the main and secondary nucleus chains exhibit an almost linear increase. This shape is explained by the damping of shell effects entering the superfluid expression of the level-density parameters. They tend to approach the asymptotic values for a great part of the fragments. This fact leads to a smooth and almost linear increase of fragment excitation energy with the fragment mass number that is reflected in a smooth and almost linear behavior of individual \ensuremath{\nu}($A$). The comparison of the present results with those of the GEF code reveals different shapes of \ensuremath{\nu}($A$) as well as different total average neutron multiplicity as a function of the $\mathit{En}$. At high $\mathit{En}$ the PbP calculations definitely reflect the influence of the almost linear shape of individual \ensuremath{\nu}($A$) of the first few nuclei of the U and Pa chains. The differences between the total \ensuremath{\nu}($A$) obtained by averaging the PbP results of individual \ensuremath{\nu}($A$) over fission cross-section ratios of different evaluations are insignificant.

High-accuracy determination of theU238/U235fission cross section ratio up to≈1GeV at n_TOF at CERN

The U238 to U235 fission cross section ratio has been determined at n_TOF up to ≈1 GeV, with two different detection systems, in different geometrical configurations. A total of four datasets has been collected and compared. They are all consistent to each other within the relative systematic uncertainty of 3–4%. The data collected at n_TOF have been suitably combined to yield a unique fission cross section ratio as a function of neutron energy. The result confirms current evaluations up to 200 MeV. Good agreement is also observed with theoretical calculations based on the INCL++/Gemini++ combination up to the highest measured energy. The n_TOF results may help solve a long-standing discrepancy between the two most important experimental datasets available so far above 20 MeV, while extending the neutron energy range for the first time up to ≈1 GeV.1 MoreReceived 27 November 2014DOI:https://doi.org/10.1103/PhysRevC.91.024602This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical Society

Utilizing ( p , d ) and ( p , t ) reactions to obtain ( n , f ) cross sections in uranium nuclei via the surrogate-ratio method

The surrogate ratio method has been tested for ($p,d$) and ($p,t$) reactions on uranium nuclei. ${}^{236}$U and ${}^{238}$U targets were bombarded with 28-MeV protons and the light ion recoils and fission fragments were detected using the Silicon Telescope Array for Reaction Studies detector array at the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory. The ($p$,$df$) reaction channels on ${}^{236}$U and ${}^{238}$U targets were used as a surrogate to determine the $\ensuremath{\sigma}$[${}^{236}$U($n$,$f$)]/$\ensuremath{\sigma}$[${}^{234}$U($n$,$f$)] cross-section ratio. The ($p$,$tf$) reaction channels were also measured with the same targets as a surrogate for the $\ensuremath{\sigma}$[${}^{235}$U($n$,$f$)]/$\ensuremath{\sigma}$[(${}^{233}$U($n$,$f$)] ratio. For the ($p$,$df$) and ($p$,$tf$) surrogate measurements, there is good agreement with accepted ($n$,$f$) values over equivalent neutron energy ranges of ${E}_{n}=0$--7 MeV and ${E}_{n}=0$--5.5 MeV, respectively. An internal surrogate ratio method comparing the ($p,d$) and ($p$,$t$) reaction channels on a single target is also discussed. The $\ensuremath{\sigma}$[${}^{234}$U($n$,$f$)]/$\ensuremath{\sigma}$[${}^{233}$U($n$,$f$)] and $\ensuremath{\sigma}$[${}^{236}$U($n$,$f$)]/$\ensuremath{\sigma}$[${}^{235}$U($n$,$f$)] cross-section ratios are extracted using this method for the ${}^{236}$U and ${}^{238}$U targets, respectively. The resulting fission cross-section ratios show relatively good agreement with accepted values up to ${E}_{n}\ensuremath{\sim}5$ MeV.

Neutron Induced Fission Cross-Sections of 240Pu and 243Am in the Energy Range 1 - 200 MeV

During the last several years, the measurements of neutron-induced fission cross-sections of actinides and stable isotopes have been performed at the GNEIS facility. At present moment, the new series of experiments for measurements of fission cross-section ratios relative to 235U have been began for 240Pu, 243Am and natW in a wide energy range of incident neutrons from 1 MeV to 200 MeV. The measurements were performed using the multiplate ionization chamber and time-of-flight technique. The first results of this measurement in case of 240Pu and 243Am are presented in comparison with the other data.

Symmetric/asymmetric [formula omitted] neutron-induced fission up to 200 MeV

The symmetric SL-mode and lumped asymmetric (S1+S2)-mode fission cross sections of 238 U (n, f) reaction are calculated up to incident neutron energy of 200 MeV within a Hauser–Feshbach statistical model. For each U nuclide, contributing to the observed fission cross section via 238 U (n, xnf) reaction separate outer fission barrier is assumed for the SL-mode fission, while the inner one is assumed to be the same either for the symmetric SL- and lumped asymmetric (S1+S2)-mode fission. Inner saddle is axial asymmetric and mass-symmetric for fissioning U nuclei with number of neutrons N>144, for lower N values axial symmetry is assumed to persist. Axial asymmetry and mass-symmetry are assumed for the outer saddle of the SL-mode, as distinct from asymmetric fission modes, for which axial symmetry and mass-asymmetry are assumed. The SL-mode fission cross section up to emissive threshold is controlled by a rather high outer fission barrier with significant transparency. Damping of collective modes contribution to the level density at intrinsic excitation energies higher than 20–30 MeV for axial symmetric saddle and equilibrium deformations seems to be essential for the 238 U (n, F) observed fission cross section description. Ratio of symmetric to all 238 U neutron-induced fission events is described assuming damping of triaxial collective modes contribution to the level densities at outer saddles relevant for the SL-mode. High sensitivity of the ratio of symmetric and asymmetric fission cross sections to the relative contributions of emissive fission chances with high and low number of pre-fission (n, xnf) neutrons is observed.

Neutron-Induced Fission of 233U, 238U, 232Th, 239Pu, 237Np, natPb and 209Bi Relative to 235U in the Energy Range 1-200 MeV

Fission cross section ratios of 233U, 238U, 232Th, 239Pu, 237Np, natural Pb and 209Bi to 235U have been measured in a wide energy range of incident neutrons from 1 MeV to 200 MeV using a time-of-flight technique at the neutron spectrometer GNEIS based on the 1-GeV proton synchrocyclotron of PNPI. For actinide targets, the threshold cross section method and evaluated data below 14 MeV were used for normalization of the shape measurement data, while the evaluated and recommended fission cross sections of 235U were used to convert the ratio data to absolute fission cross sections. For Pb and Bi targets, an absolute normalization of the measured cross section ratios has been done using the thickness of the targets and detection efficiencies.

Measurement of alpha-decay constant to fission cross section ratio for 237Np, 241Am and alpha to spontaneous fission branching ratio for 243Am using solid state nuclear track detectors

The alpha-decay constant to fission cross section ratio for epi-cadmium neutron induced fission of 237Np, 241Am have been measured using solid state nuclear track detectors (SSNTDs). The measured values were (0.63±0.14).109 s-1.cm-2, (2.31±0.46).1012 s-1.cm-2 for 237Np and 241Am, respectively. The alpha to spontaneous fission branching ratio for 243Am was also measured to be (2.66±0.50).1010 using SSNTDs. These ratios are useful as signatures for the trace level identification of these actinides.

Measurement of alpha-decay constant to fission cross section ratio for actinides using solid state nuclear track detectors

A simple technique based on the measurement of the ratio of alpha-decay constant to neutron induced fission cross section for pure actinides using solid state nuclear track detectors (SSNTDs) is developed for the identification of the actinides in trace levels in pure solutions. The alpha-decay constant to fission cross section ratios for depleted U,238Pu and240Pu have been measured for the epicadmium neutron induced fission of these actinides. The measured values are (6.19±0.34)·106, (6.95±0.26)·1012, (2.12±0.95)·109 and (2.18±1.58)·1011 sec−1·cm−2, respectively. These ratios can be used for the trace level identification of pure actinides.

Thermal and dynamical properties of fusion-fission and fusion-evaporation processes in 20Ne + 159Tb and 20Ne + 169Tm reactions between 8 and 16 MeV/nucleon

We have undertaken a study of the thermal and the dynamical properties of fission and evaporation processes through the observation of neutron and light charged particle (LCP) emission (p, α) in collisions of 20 Ne + 159 Tb and 20 Ne + 169 Tm at 8, 10, 13 and 16 MeV/nucleon using the neutron multidetector DEMON. Both systems lead to the formation of compound nuclei (CN) with initial excitation energies ( E x * ) ranging between 100 and 250 MeV. These CN, formed through complete (CF) or incomplete (IF) fusion reactions, decay through either fission or evaporation residue (ER) formation. Our experimental goal in this study is to establish, as a function of E x * , i) σ ER /σ fission cross-section ratios, ii) pre- and post-scission neutron and LCP multiplicities with a clear cut assignment to CF and IF processes and iii) to try to consistently interpret these data in the framework of the recently modified statistical code GEMINI, including time-dependent fission width probabilities in order to establish the fission time scales.

Measurement of Fast Neutron-Induced Fission Cross Section Ratio of Neptunium-237 relative to Uranium-235 between 6 and 120 keV

To obtain the nuclear data for 237Np which plays an important role in high burn-up reactors and transmutation systems, we measured neutron-induced fission cross section ratios of 237Np relative to 235U between 6 and 120 keV using a pulsed white neutron source produced by the 4.5 MV Dynamitron accelerator of Tohoku University. The fission cross section ratios were measured by two separate runs with the experimental errors of 7% to 25%. The results by two separate runs agreed within the experimental errors. The fission cross section of 237Np was deduced by averaging two results and by using the fission cross section of U taken from JENDL3.2. The present results agreed with the result of Hoffman et al. and was consistent with the newest experimental result by Yamanaka et al. The present result around 10keV agreed with JENDL3.2, while those near 100 keV were close to ENDF/B-VI.

Measurement of fission excitation functions in11B,16O,19F+232Threactions in the extreme sub-barrier energy region

Fission cross sections have been measured for the systems of ${}^{11}\mathrm{B},$ ${}^{16}\mathrm{O},$ ${}^{19}{\mathrm{F}+}^{232}\mathrm{Th}$ in the sub-barrier to extreme-sub-barrier energy (ESBE) region. Two position sensitive avalanche detectors were employed to measure the fission events due to fusion-fission (FF) and transfer fission (TF) processes using the fission fragment folding angle technique. The cross sections for fusion-fission reactions could be well accounted by the coupled channel calculations over a large energy range except in the ESBE region. The transfer induced fission cross sections are observed to be large at extreme sub-barrier energies. The ratio of transfer fission cross section to that of total fission is observed to increase rapidly as the energy is decreased in the sub-barrier region for all the systems. Also in the ESBE region a number of low momentum transfer fission (LMTF) events having folding angle close to $180\ifmmode^\circ\else\textdegree\fi{}$ are observed with a rather weak dependence on the bombarding energy. These events were investigated by several control experiments. Considering the uncertainties involved in the control experiments, it appears difficult to rule out if they originate completely from background neutron induced reactions or from any new process such as Coulomb fission as conjectured in some earlier studies. The results on different components of the fission cross sections have been discussed.