Measurement Of Fission Cross Section Research Articles

Designing an upgrade of the Medley setup for light-ion production and fission cross-section measurements

Measurements of neutron-induced fission cross-sections and light-ion production are planned in the energy range 1–40MeV at the upcoming Neutrons For Science (NFS) facility. In order to prepare our detector setup for the neutron beam with continuous energy spectrum, a simulation software was written using the Geant4 toolkit for both measurement situations. The neutron energy range around 20MeV is troublesome when it comes to the cross-sections used by Geant4 since data-driven cross-sections are only available below 20MeV but not above, where they are based on semi-empirical models. Several customisations were made to the standard classes in Geant4 in order to produce consistent results over the whole simulated energy range.Expected uncertainties are reported for both types of measurements. The simulations have shown that a simultaneous precision measurement of the three standard cross-sections H(n,n), 235U(n,f) and 238U(n,f) relative to each other is feasible using a triple layered target. As high resolution timing detectors for fission fragments we plan to use Parallel Plate Avalanche Counters (PPACs). The simulation results have put some restrictions on the design of these detectors as well as on the target design. This study suggests a fissile target no thicker than 2μm (1.7mg/cm2) and a PPAC foil thickness preferably less than 1μm. We also comment on the usability of Geant4 for simulation studies of neutron reactions in this energy range.

Neutron science facility for neutron time-of-flight and fission cross-section measurements at RAON

In the middle of 2018, a heavy-ion accelerator complex that will be built in South Korea plans to provide the first primary beams into the neutron science facility (NSF) for producing fast neutrons. Deuteron with a maximum energy of 53 MeV and protons with a maximum energy of 88 MeV accelerated by superconducting driver linac (SCL1) will be delivered into the target hall at the NSF. A pulsed neutron beam will be provided for neutron time-of-flight (TOF) and neutron-induced reaction cross-section measurements. At the NSF, white and mono-energetic fast neutrons will be produced when either a deuteron or a proton beam bombards a light nuclei target such as C and Li. Preliminary thermal calculations have been performed with a rotating C (graphite) target and its maximum temperature was about 530 °C, much less than its melting point. For neutron TOF measurements, two flight paths of 5 m and 20 m are considered for high-flux and low-energy neutron beams, respectively. Basically, 0° and 30° neutron collimators will be considered to obtain various neutron energies for the neutron TOF measurements. A clearing magnet is used to deflect the proton beam to a beam dump when it crosses a thin target. In addition, the neutron beam dump will be designed to generate a background due to back-scattered neutrons and photons that should be as low as possible in the TOF area. Fission cross-section measurements with a few percent uncertainties are set to be a short-term ultimate goal after building the neutron TOF facility at the NSF. In order to achieve few-percent fission cross-section measurements at the NSF, we plan to employ a time projection chamber (TPC). It can measure charged particle trajectories in the active volume in three dimensions, as well as the energy deposition, and it can significantly improve the accuracies of the fission cross-section measurements.

Uncertainty Quantification in Fission Cross Section Measurements at LANSCE

Neutron-induced fission cross sections have been measured for several isotopes of uranium and plutonium at the Los Alamos Neutron Science Center (LANSCE) over a wide range of incident neutron energies. The total uncertainties in these measurements are in the range 3–5% above 100 keV of incident neutron energy, which results from uncertainties in the target, neutron source, and detector system. The individual sources of uncertainties are assumed to be uncorrelated, however correlation in the cross section across neutron energy bins are considered. The quantification of the uncertainty contributions will be described here.

Quantitative molecular plating of large-area 242Pu targets with improved layer properties

For measurements of the neutron-induced fission cross section of 242Pu, large-area (42cm2) 242Pu targets were prepared on Ti-coated Si wafers by means of constant current density molecular plating. Radiochemical separations were performed prior to the platings. Quantitative deposition yields (>95%) were determined for all targets by means of alpha-particle spectroscopy. Layer densities in the range of 100–150μg/cm2 were obtained. The homogeneity of the targets was studied by radiographic imaging.A comparative study between the quality of the layers produced on the Ti-coated Si wafers and the quality of layers grown on normal Ti foils was carried out by applying scanning electron microscopy and energy dispersive X-ray spectroscopy. Ti-coated Si wafers resulted clearly superior to Ti foils in the production of homogeneous 242Pu layers with minimum defectivity.

Fast Neutron–Induced Fission Cross Sections of 233, 234, 236, 238U up to 200 MeV

The 233, 234, 236, 238U fission cross sections have been measured relative to 235U(n, f) for incident neutron energies from 200 KeV to 200 MeV using neutron time-of-flight at the Los Alamos Neutron Science Center. The results are generally consistent with the current ENDF/B-VII evaluation, but some discrepancies with previous measurements above 20 to 30 MeV are observed. These measurements are part of a campaign to measure fission cross sections with high precision in support of fast reactor technology.

A time projection chamber for high accuracy and precision fission cross-section measurements

The fission Time Projection Chamber (fissionTPC) is a compact (15cm diameter) two-chamber MICROMEGAS TPC designed to make precision cross-section measurements of neutron-induced fission. The actinide targets are placed on the central cathode and irradiated with a neutron beam that passes axially through the TPC inducing fission in the target. The 4π acceptance for fission fragments and complete charged particle track reconstruction are powerful features of the fissionTPC which will be used to measure fission cross-sections and examine the associated systematic errors. This paper provides a detailed description of the design requirements, the design solutions, and the initial performance of the fissionTPC.

Measuring Light-ion Production and Fission Cross Sections Normalised to H(n,p) Scattering at the Upcoming NFS Facility

The Medley detector setup is planned to be moved to and used at the new neutron facility NFS where measurements of light-ion production and fission cross-sections are planned at 1-40 MeV. Medley has eight detector telescopes providing ΔE-ΔE-E data, each consisting of two silicon detectors and a CsI(Tl) detector at the back. The telescope setup can be rotated and arranged to cover any angle. Medley has previously been used in many measurements at The Svedberg Laboratory (TSL) in Uppsala mainly with a quasi-mono-energetic neutron beam at 96 and 175 MeV. To be able to do measurements at NFS, which will have a white neutron beam, Medley needs to detect the reaction products with a high timing resolution providing the ToF of the primary neutron. In this paper we discuss the design of the Medley upgrade along with simulations of the setup. We explore the use of Parallel Plate Avalanche Counters (PPACs) which work very well for detecting fission fragments but require more consideration for detecting deeply penetrating particles.

Development of an experimental set-up for the measurement of neutron-induced fission and capture cross sections of highly radioactive fissile nuclei

The measurement of neutron-capture cross sections of many actinides is complicated by the difficulty in separating capture γ-rays from the large fission-fragment prompt γ-ray background. For example, current estimates of the capture cross section of 233U show large discrepancies, with differences of more than 20%. To improve the accuracy of data, a new experimental set-up for the simultaneous measurement of the neutron-induced capture and fission cross sections was designed, assembled and optimized. The measurements will be performed at the GEel LINear Accelerator (GELINA) neutron time-of-flight facility in Belgium, where neutron cross sections can be measured over a wide energy range with high energy resolution. The fission detector consists of a dedicated multi-plate high-efficiency ionization chamber (IC). The γ-rays produced in capture reaction are detected by an array of C6D6 scintillators. Fission γ–rays are distinguished from capture γ–rays by the anticoincidence signals from the IC and the C6D6 detectors. For the undetected fission events a correction has to be applied based on the efficiency of the IC that should be high and known with a high accuracy. The performance of the IC during dedicated test experiments is presented, focusing on the determination of the detection efficiency.

Investigation of the performance of a MicroMegas detector suitable for neutron-induced fission cross-section measurements

The performance of a state-of-the-art MicroMegas detector, suitable for neutron- induced fission cross-section measurements has been examined. Special emphasis was put on the study of the gain and the resolution function.

Development of an experimental set-up for the measurement of neutron-induced fission and capture cross sections of radioactive fissile nuclei

A new experimental set-up for a simultaneous measurement of neutron induced capture and fission cross sections was designed, assembled and optimized. The measurements will be performed at GEel LINear Accelerator (GELINA) neutron time- of-flight facility in Belgium, where neutron cross sections can be measured over a wide energy range with high energy resolution. The fission events detector consists of a dedicated multi-plate high efficiency fission ionization chamber (IC). The -rays produced in capture reaction are detected by an efficient array of C6D6 scintillators. Fission -rays events are distinguished from capture events by the anticoincidence signals from the IC and the C6D6 detectors. For the undetected fission events a correction has to be applied with respect to the efficiency of the IC that should be high and known with a high precision. Another important issue is the good separation between fission-fragment (FF) and the high alpha pile-up. The performances of the IC during test experiments are presented, focusing in particular on the detection efficiency.

Measurement of the neutron-induced fission cross-section of 241Am at the time-of-flight facility n_TOF

The neutron-induced fission cross-section of 241Am has been measured relative to the standard fission cross-section of 235U between 0.5 and 20MeV. The experiment was performed at the CERN n_TOF facility. Fission fragments were detected by a fast ionization chamber by discriminating against the \(\alpha\)-particles from the high radioactivity of the samples. The high instantaneous neutron flux and the low background of the n_TOF facility enabled us to obtain uncertainties of \( \approx\) 5%. With the present results it was possible to resolve discrepancies between previous data sets and to confirm current evaluations, thus providing important information for design studies of future reactors with improved fuel burn-up.

Characterization of actinide targets for fission cross section measurements

In this work the characterization of thin actinide targets ( 237Np, 238U, 235U), man- ufactured for measurements of fission cross section, will be presented. The determi- nation of the mass of the actinide content of each target has been performed via alpha spectrometry, utilizing two SSB detectors, with 50mm2 and 3000mm2 active surface, in order to combine good resolution and good statistics respectively and obtain a smaller error in the final mass result. The impurities of the targets were quantitatively estimated with the same technique. The homogeneity of the samples was examined with two independent methods. Firstly, RBS measurements were per- formed using a proton beam of 2 MeV, provided by the 5.5 MV Tandem Van de Graaff accelerator of the N.C.S.R. ”Demokritos”. The thickness of each target was quantitatively estimated at various points. Secondly, CR-39 plastic track detectors were used to measure the alpha tracks from the surface of the targets and provide an image of the activity of the surface.

Fission cross-section measurements of 237Np, 242m Am, and 245Cm with lead slowing-down neutron spectrometer

The cross sections for neutron-induced fission of 237Np, Am, and 245Cm have been measured in the energy range from 0.1 eV to 2 keV using the lead slowing-down neutron spectrometer at the Research Reactor Institute, Kyoto University. The fission cross sections were deduced relative to those for 235U(n,f) and 10B(n,a). The cross section for 237Np(n,f) was obtained with the experimental uncertainty of about 20% in the energy region below 25 eV and typically 13% above this energy. The experimental uncertainties for 242m Am and 245Cm were, respectively, of the order of 14% and 9% in the entire energy region.

Neutron-induced fission cross section measurement of 233U, 241Am and 243Am in the energy range 0.5 MeV ⩽ En ⩽ 20 MeV at n_TOF at CERN

Neutron-induced fission cross section measurements of 233U, 243Am and 241Am relative to 235U have been carried out at the neutron time-of-flight facility n_TOF at CERN. A fast ionization chamber has been employed. All samples were located in the same detector; therefore the studied elements and the reference 235U target are subject to the same neutron beam.

Measurement of fission cross section for 232Th(n,x)89Rb reaction induced by neutrons around 14 MeV

In order to investigate the fission process in more detail, and to compare with the measurement of cumulative fission yields, the fission cross section of the 232Th(n,x)89Rb reaction induced by 14MeV neutron was measured using the activation technique. In our measurement the neutron flux was determined using the monitor 27Al(n,α)24Na reaction, and the neutron energies were measured by the method of cross-section ratios of 90Zr(n,2n)89Zr to 93Nb(n,2n)92mNb reactions. The cross sections were deduced as 14.0±0.9mb at En=14.7±0.3MeV and 13.2±1.0mb at En=14.1±0.3MeV.

Criticality experiments for validation of cross sections: the237Np case

237 Np is one of the actinides which is most abundantly produced in present reactors, and it could be a good candidate for incineration due to its long life time. However this requires a good knowledge of neutron cross sections. The n_TOF collaboration has carried out a fission cross section measurement at the n_TOF neutron facility at CERN, spanning a very broad energy range, from a fraction of eV to GeV. When compared to previous measurements the n_TOF fission cross section appears to be higher by 5-7% beyond the first fission threshold. We show however that several previous measurements, although consistent with each other, are dependent through re-normalization procedures. To check the relevance of n_TOF data, we simulate an experiment, performed at Los Alamos, with a sphere of 237 Np of 6 kg surrounded by enriched uranium 235 U, so as to reach criticity with fast neutrons. When introducing the n_TOF data for the fission cross section of 237 Np, the simulation predicts a multiplicating factor k eff in better agreement with the experiment as the deviation of 750 pcm is reduced to 250 pcm. We explore also the hypothesis of possible deficiencies of the inelastic cross section in 235 U which has been invoked by some authors to explain the deviation of 750 pcm. However, the large distortion that should be applied to reconcile the critical experiment with its simulation discredits this option. These outcomes support the hypothesis of a higher fission cross section of 237 Np.

Measurement of neutron capture and fission cross sections of233U in the resonance region

In the framework of studies concerning new fuel cycles and nuclear wastes incineration experimental data of the α ratio between capture and fission cross sections of 233U reactions play an important role in the Th/U cycle. The safety evaluation and the detailed performance assessment for the generation IV nuclear-energy system based on 232Th cycle strongly depend on this ratio. Since the current data are scarce and sometimes contradictory, new experimental studies are required. The measurement will take place at the neutron time-of-flight facility GELINA at Geel, designed to perform neutron cross section measurements with high incident neutron-energy resolution. A dedicated high efficiency fission ionization chamber (IC) as fission fragment detector and six C6D6 liquid scintilators sensitive to γ-rays and neutrons will be used. The method, based on the IC energy response study, allowing to distinguish between gammas originating from fission and capture, in the resonance region, will be presented.

Measurement of the neutron-induced fission cross-section of 243Am relative to 235U from 0.5 to 20 MeV

The ratio of the neutron-induced fission cross-sections of 243Am and 235U was measured in the energy range from 0.5 to 20 MeV with uncertainties of ≈ 4%. The experiment was performed at the CERN n_TOF facility using a fast ionization chamber. With the good counting statistics that could be achieved thanks to the high instantaneous flux and the low backgrounds, the present results are useful for resolving discrepancies in previous data sets and are important for future reactors with improved fuel burn-up.

Unified Description of Fission Probability for the Intermediate-energy Nuclear Data Evaluation

The systematics of fission probability with unified description of neutron-, protonand photoninduced reactions has been obtained by using its simple behaviour as following. Experimental fission probabilities were deduced from measured fission cross sections and calculated totalreaction cross sections. We assumed a functional form which is designed to describe the maximum excitation-energy dependence of fission probability. According to our preliminary research, the Z/A-dependence was seen for the saturation probability (one of the fitting parameters). Furthermore, some correlations were observed among the other fitting parameters. The present results can describe the fission cross sections in the intermediate energy region within about 10% accuracy in the energy region from several tens of MeV to 3 GeV.

Fission Physics and Cross Section Measurements with a Lead Slowing down Spectrometer

energy range for the LSDS is 0.1 eV to 10 keV with energy resolution of about 35%. Two new LSDS applications were recently developed at RPI; the rst enables simultaneous measurements of the ssion cross section and ssion fragment mass and energy distributions as a function of the incident neutron energy. The second enables measurements of the (n, ) and (n;p) cross sections for materials with a positive Q value for these reactions. Fission measurements of 252 Cf, 235 U, and 239 Pu were completed and provide information on ssion fragment and energy distributions in resonance clusters. Measurements of the (n, ) cross section for 147;149 Sm were completed and compared to previously measured data. The new data indicate that the existing evaluations need to be adjusted.