Coincident Fission Fragments Research Articles

Evidence for a New Compact Symmetric Fission Mode in Light Thorium Isotopes.

Taking benefit of the R3B/SOFIA setup to measure the mass and the nuclear charge of both fission fragments in coincidence with the total prompt-neutron multiplicity, the scission configurations are inferred along the thorium chain, from the asymmetric fission in the heavier isotopes to the symmetric fission in the neutron-deficient thorium. Against all expectations, the symmetric scission in the light thorium isotopes shows a compact configuration, which is in total contrast to what is known in the fission of the heavier thorium isotopes and heavier actinides. This new main symmetric scission mode is characterized by a significant drop in deformation energy of the fission fragments of about 19MeV, compared to the well-known symmetric scission in the uranium-plutonium region.

Calculation of the fission observables in the resolved resonance energy region of the 235U(n,f) reaction

Measurement of the fission fragments in coincidence with the emitted prompt neutrons was undertaken recently, at JRC-Geel institute, for the 235U(n,f) reaction in the resolved resonance energy region, up to 160 eV incident neutron energy. From this experimental work, fluctuations of several fission observables (mass yields, average total kinetic energy T̅K̅E̅, average prompt neutron multiplicity v̅P) were clearly observed. In the present work, these experimental pre-neutron fission fragment mass and kinetic energy distributions were used as input data for the FIFRELIN Monte Carlo code. By adopting the Hauser-Feshbach statistical model, the code simulates the de-excitation of the fission fragments. Four free parameters are available in the code: two of them (called RTmin and RTmax) govern at the scission point the sharing of the total available excitation energy between the two nascent fission fragments, while the two others (called σL and σH) assign the initial fission fragment spins. In this way, fission observables (prompt particles energy spectra and multiplicities, delayed neutrons multiplicity,. . . ) and correlations between them can be predicted and investigated. Here, these four free parameters were tuned in order to reproduce the average prompt neutron multiplicity at the resonance En=19.23 eV, resonance for which the experimental statistical uncertainty on v̅P is the lowest one. Then, the calculations were perfomed for all resonances by keeping the same set of free parameters. We show that the calculated fluctuations of v̅P in the resonances can rather be well reproduced by considering only the fluctuations of the pre-neutron mass yields and kinetic energy. In addition, from our calculation procedure, other fission observables fluctuations can also be predicted.

Experimental study of nuclear fission along the thorium isotopic chain: From asymmetric to symmetric fission

The inverse kinematics technique, applied to radioactive beams and combined to the Coulomb excitation method, is a powerful tool to study low-energy fission. A novel experimental setup was developed within the R3B/SOFIA (Reactions with Relativistic Radioactive Beams/Studies On FIssion with Aladin) collaboration to identify in mass and atomic numbers both fission fragments in coincidence. These new data provide elemental, isobaric, and isotonic yields for the fission along the thorium isotopic chain. Results are also compared to previous measurements using either the same reaction mechanism or thermal-neutron induced fission. This latter comparison permits to probe the influence of the excitation energy in the fission process.

A new detector system for the measurement of high-energy prompt [formula omitted]-rays for low-energy neutron induced fission

We have developed a new setup to measure prompt fission γ-ray spectra in neutron induced fission up to energies sufficient to reveal the structure associated with giant dipole resonances of fission fragments. The setup consists of multi-wire proportional counters, to detect both fission fragments in coincidence, and two large volume (101.6 mm in diameter and 127.0 mm in length) LaBr3(Ce) scintillators, to measure the γ rays. The setup was used to obtain the prompt fission γ-ray spectrum for thermal neutron induced fission of 235U at the PF1B cold-neutron beam facility of the Institut Laue-Langevin, Grenoble, France. We have successfully measured the γ-ray spectrum up to energies of about 20 MeV, what extends the currently known γ-ray spectrum limit to higher energies by approximately a factor of two.

Fission fragment mass distributions from Po210 and At213

Background: The influence of shell effect on the dynamics of the fusion fission process and it's evolution with excitation energy in the pre-actinide Hg-Pb region in general is a matter of intense research in recent years. In particular, a strong ambiguity remains for the neutron shell closed $^{210}$Po nucleus regarding the role of shell effect in fission around $\approx$ 30 - 40 MeV of excitation energy. Purpose: We have measured the fission fragment mass distribution of $^{210}$Po populated using fusion of $^{4}$He + $^{206}$Pb at different excitation energies and compare the result with recent theoretical predictions as well as with our previous measurement for the same nucleus populated through a different entrance channel. Mass distribution in the fission of the neighbouring nuclei $^{213}$At is also studied for comparison. Methods: Two large area Multi-wire Proportional Counters (MWPC) were used for complete kinematical measurement of the coincident fission fragments. The time of flight differences of the coincident fission fragments were used to directly extract the fission fragment mass distributions. Results: The measured fragment mass distribution for the reactions $^{4}$He + $^{206}$Pb and $^{4}$He + $^{209}$Bi were symmetric and the width of the mass distributions were found to increase monotonically with excitation energy above 36.7 MeV and 32.9 MeV, respectively, indicating the absence of shell effects at the saddle. However, in the fission of $^{210}$Po, we find minor deviation from symmetric mass distributions at the lowest excitation energy (30.8 MeV). Conclusion: Persistence of shell effect in fission fragment mass distribution of $^{210}$Po was observed at the excitation energy $\approx$ 31 MeV as predicted by the theory; at higher excitation energy, however, the present study reaffirms the absence of any shell correction in the fission of $^{210}$Po.

The Coincident Fission Fragment Detector (CFFD)

A Parallel Plate Avalanche Counter (PPAC) based fission detector system, called the Coincident Fission Fragment Detector (CFFD), has been developed for the ReA3 re-accelerator facility of the National Superconducting Cyclotron Laboratory (NSCL). Binary reaction kinematics are reconstructed based on position and time-of-flight measurements of fission fragments. Large area PPACs provide 1ns level time resolution and mm level position resolution. The detectors allow measurements of fission product angular and mass distributions of heavy-ion induced fusion reactions. The 30cm by 40cm active area of each PPAC provides large solid angle coverage well suited for measurements of low intensity rare-isotope beams (RIBs).

Accurate isotopic fission yields of electromagnetically induced fission of U238 measured in inverse kinematics at relativistic energies

SOFIA (Studies On Fission with Aladin) is a novel experimental program, dedicated to accurate measurements of fission-fragment isotopic yields. The setup allows us to fully identify, in nuclear charge and mass, both fission fragments in coincidence for the whole fission-fragment range. It was installed at the GSI facility (Darmstadt), to benefit from the relativistic heavy-ion beams available there, and thus to use inverse kinematics. This paper reports on fission yields obtained in electromagnetically induced fission of U238.15 MoreReceived 14 February 2017DOI:https://doi.org/10.1103/PhysRevC.95.054603©2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasFissionLow & intermediate energy heavy-ion reactionsNuclear reactionsPhotonuclear reactionsTechniquesNuclear data analysis & compilationNuclear Physics

Accurate measurements of fission-fragment yields in234,235,236,238U(γ,f) with the SOFIA set-up

SOFIA (Studies On Fission with Aladin) is a new experimental set-up dedicated to accurate measurement of fission-fragments isotopic yields. It is located at GSI, the only place to use inverse kinematics at relativistic energies in order to study the (gamma,f) electromagnetic-induced fission. The SOFIA set-up is a large-acceptance magnetic spectrometer, which allows to fully identify both fission fragments in coincidence on the whole fission-fragment range. This paper will report on fission yields obtained in U-234,U-235,U-236,U-238(gamma,f) reactions.

Studies on fission with ALADIN

A novel technique for fission studies, based on the inverse kinematics approach, is presented. Following pioneering work in the nineties, the SOFIA Collaboration has designed and built an experimental set-up dedicated to the simultaneous measurement of isotopic yields, total kinetic energies and total prompt neutron multiplicities, by fully identifying both fission fragments in coincidence, for the very first time. This experiment, performed at GSI, permits to study the fission of a wide variety of fissioning systems, ranging from mercury to neptunium, possibly far from the valley of stability. A first experiment, performed in 2012, has provided a large array of unprecedented data regarding the nuclear fission process. An excerpt of the results is presented. With this solid starter, further improvements of the experimental set-up are considered, which are consistent with the expected developments at the GSI facility, in order to measure more fission observables in coincidence. The completeness reached in the SOFIA data, permits to scrutinize the correlations between the interesting features of fission, offering a very detailed insight in this still unraveled mechanism.

Constraining the level density using fission of lead projectiles

The nuclear level density is one of the main ingredients for the statistical description of the fission process. In this work, we propose to constrain the description of this parameter by using fission reactions induced by protons and light ions on Pb208 at high kinetic energies. The experiment was performed at GSI (Darmstadt), where the combined use of the inverse kinematics technique with an efficient detection setup allowed us to measure the atomic number of the two fission fragments in coincidence. This measurement permitted us to obtain with high precision the partial fission cross sections and the width of the charge distribution as a function of the atomic number of the fissioning system. These data and others previously measured, covering a large range in fissility, are compared to state-of-the-art calculations. The results reveal that total and partial fission cross sections cannot unambiguously constrain the level density at ground-state and saddle-point deformations and additional observables, such as the width of the charge distribution of the final fission fragments, are required.

A Sample of the Results of the First SOFIA Experiment

The first SOFIA experiment (Studies On FIssion with Aladin) was performed in August 2012 at GSI. The fission of several neutron-deficient actinides and pre-actinides was induced in flight at 700 A MeV by electromagnetic excitation. The use of inverse kinematics provides both high detection and high geometrical efficiency. The complete identification of both fission fragments in coincidence (nuclear charge and mass) was obtained over a broad range of fissioning nuclei. After a short introduction and descrip- tion of the experimental setup, the quality of the charge and mass separation is presented, followed by some results concerning the Coulex-induced fission of 234U and 235U.

Proton-induced fission cross sections onPb208at high kinetic energies

Total fission cross sections of Pb-208 induced by protons have been determined at 370A, 500A, and 650A MeV. The experiment was performed at GSI Darmstadt where the combined use of the inverse kinematics technique with an efficient detection setup allowed us to determine these cross sections with an uncertainty below 6%. This result was achieved by an accurate beam selection and registration of both fission fragments in coincidence which were also clearly distinguished from other reaction channels. These data solve existing discrepancies between previous measurements, providing new values for the Prokofiev systematics. The data also allow us to investigate the fission process at high excitation energies and small deformations. In particular, some fundamental questions about fission dynamics have been addressed, which are related to dissipative and transient time effects.

High-precision measurement of total fission cross sections in spallation reactions of208Pb and238U

Total cross sections for proton- and deuteron-induced-fission of ${}^{208}$Pb and ${}^{238}$U have been determined in the energy range between 500 MeV and 1 GeV. The experiment has been performed in inverse kinematics at GSI Darmstadt, facilitating the counting of the projectiles and the identification of the reaction products. High precision between 5$%$ and 7$%$ has been achieved by individually counting the beam particles and by registering both fission fragments in coincidence with high efficiency and full $Z$ resolution. Fission was clearly distinguished from other reaction channels. The results were found to deviate by up to 30$%$ from Prokofiev's systematics on total fission cross sections. There is good agreement with an elaborate experiment performed in direct kinematics.

Corrections of Prompt-neutron Emission in Fission-fragment Experiments

The number of emitted prompt neutrons in the fission process is strongly dependent on the fragment mass. The deformation of the fragments as well as shell effects give the characteristic “sawtooth” shape. It is well known that the total average neutron emission (ν̄tot) increases as a function of incident-neutron energy. But do these extra emitted neutrons also show a particular dependence on the fragment mass? Some experiments have shown that the additional neutrons are emitted from the heavy fragments only. Recent theoretical studies provided an explanation for this, thus emphasising the validity of these observations. Despite this, in various experiments and calculations an average increase of ν̄ ( A ) was and is still assumed. Experimental data on 234U ( n , f) , have been used in this work to study the influence of different neutron-multiplicity shapes. Based on the double-energy technique one cannot validate the one or the other method, because no prompt neutrons are detected in coincidence with the fission-fragment kinetic energies. However one may investigate the impact of the ν̄ ( A ) choice on the fission observables. Two methods were used in the analysis of the experimental data. In one case a higher ν̄ ( A ) was assumed for all fragment masses and in the other case a higher ν̄ ( A ) was only assumed for the heavy fragments. By comparing the two methods, the choice of ν̄ ( A ) was found to be important in the analysis of fission-fragments with relatively strong implications on the mass- and energy distributions. The results stress the need to determine ν̄ ( A ) by measuring fission fragments in coincidence with prompt-fission neutrons.

Impact of prompt-neutron corrections on final fission-fragment distributions

Background: One important quantity in nuclear fission is the average number of prompt neutrons emitted from the fission fragments, the prompt neutron multiplicity, $\overline{\ensuremath{\nu}}$. The total number of prompt fission neutrons, ${\overline{\ensuremath{\nu}}}_{\text{tot}}$, increases with increasing incident neutron energy. The prompt-neutron multiplicity is also a function of the fragment mass and the total kinetic energy of the fragmentation. Those data are only known in sufficient detail for a few thermal-neutron-induced fission reactions on, for example, ${}^{233,235}$U and ${}^{239}$Pu. The enthralling question has always been asked how the additional excitation energy is shared between the fission fragments. The answer to this question is important in the analysis of fission-fragment data taken with the double-energy technique. Although in the traditional approach the excess neutrons are distributed equally across the mass distribution, a few experiments showed that those neutrons are predominantly emitted by the heavy fragments.Purpose: We investigated the consequences of the $\ensuremath{\nu}(A,\mathrm{TKE},{E}_{\mathrm{n}})$ distribution on the fission fragment observables.Methods: Experimental data obtained for the ${}^{234}$U($n,\phantom{\rule{-0.16em}{0ex}}f$) reaction with a Twin Frisch Grid Ionization Chamber, were analyzed assuming two different methods for the neutron evaporation correction. The effect of the two different methods on the resulting fragment mass and energy distributions is studied.Results: We found that the preneutron mass distributions obtained via the double-energy technique become slightly more symmetric, and that the impact is larger for postneutron fission-fragment distributions. In the most severe cases, a relative yield change up to 20--30$%$ was observed.Conclusions: We conclude that the choice of the prompt-neutron correction method has strong implications on the understanding and modeling of the fission process and encourages new experiments to measure fission fragments in coincidence with prompt fission neutrons. Even more, the correct determination of postneutron fragment yields has an impact on the reliable assessment of the nuclear waste inventory, as well as on the correct prediction of delayed neutron precursor yields.

132Sn+96Zrreaction: A study of fusion enhancement/hindrance

Capture-fission cross sections were measured for the collision of the massive nucleus {sup 132}Sn with {sup 96}Zr at center-of-mass energies ranging from 192.8 to 249.6 MeV in an attempt to study fusion enhancement and hindrance in this reaction involving very neutron-rich nuclei. Coincident fission fragments were detected using silicon detectors. Using angle and energy conditions, deep inelastic scattering events were separated from fission events. Coupled-channels calculations can describe the data if the surface diffuseness parameter, a, is allowed to be 1.10 fm instead of the customary 0.6 fm. The measured capture-fission cross sections agree moderately well with model calculations using the dinuclear system model. If we use this model to predict fusion barrier heights for these reactions, we find the predicted fusion hindrance, as represented by the extra push energy, is greater for the more neutron-rich system, lessening the advantage of the lower interaction barriers with neutron-rich projectiles.

Capture cross sections for the near symmetricSn124+Zr96reaction

Capture-fission cross sections were measured for the near symmetric reaction between the massive nuclei {sup 124}Sn and {sup 96}Zr for center of mass energies from 195 to 265 MeV. Coincident fission fragments were detected and separated from elastic and deep inelastic scattering products by angle/energy/mass conditions. The measured capture cross sections agree quite well with calculations using the dinuclear system (DNS) model. The DNS model also predicts the fusion cross section for this reaction with a fusion barrier height of 208.0 MeV. The deduced extra push energy, corresponding to this barrier height, differs from that deduced from evaporation residue measurements.

Spontaneous fission properties of 104262Rf.

We have measured the mass and kinetic-energy distributions of fragments from the spontaneous fission (SF) of $_{104}^{262}\mathrm{Rf}$. The $_{104}^{262}\mathrm{Rf}$ was produced via the $^{244}\mathrm{Pu}$${(}^{22}$Ne,4n) reaction with a production cross section of \ensuremath{\sim}0.7 nb using 114.4-MeV projectiles. The kinetic energies and times of the coincident fission fragments were measured using our rotating wheel system. From these data the half-life, mass, and kinetic-energy distributions were derived. The total kinetic-energy (TKE) distribution appears to consist of a single component with a most probable pre-neutron-emission TKE of 215\ifmmode\pm\else\textpm\fi{}2 MeV. The mass distribution is symmetric with a full width at half maximum of about 22 mass numbers. These results are consistent with trends observed for other trans-berkelium spontaneously fissioning isotopes. We determined the half-life to be 2.1\ifmmode\pm\else\textpm\fi{}0.2 s by measuring its spontaneous fission decay. We also attempted to observe the alpha decay of $_{104}^{262}\mathrm{Rf}$ by searching for alpha decay correlated in time with SF from the alpha daughter, 1.2-ms $^{258}\mathrm{No}$. We observed no such decays and have set an upper limit of 0.8% (68% confidence level) on the alpha decay branch of $_{104}^{262}\mathrm{Rf}$. \textcopyright{} 1996 The American Physical Society.

Fission probabilities of Cu, Nb, Ag, Ho, Au and Bi nuclei excited by stopped antiprotons

The A dependence of nuclear fission induced by stopped antiprotons has been measured. An unambiguous identification of the binary fission decay mode was provided by a coordinate measurement of complementary fission fragments in coincidence using a large-acceptance fission detector based on low pressure multiwire proportional chambers. A deep fissility minimum was observed nearA=100, in agreement with the general behaviour predicted by the liquid-drop model. An unexpectedly low and high fission probability was found for the Ag and Cu nuclei, respectively.

Electron-capture delayed fission properties ofNp228

Electron-capture delayed fission was positively identified for the first time in $^{228}\mathrm{Np}$ produced via the $^{233}\mathrm{U}$(50-MeV p,6n${)}^{228}$Np reaction. The half-life of the observed fission activity was measured to be 61.4\ifmmode\pm\else\textpm\fi{}1.4 s. The ECDF mode was verified by an x-ray--fission coincidence experiment. Only about 25% of the expected x-ray--fission coincidences were detected which indicates that $^{228}\mathrm{U}$ does not have a fission isomer populated by the electron-capture and that the overall lifetime of the high-energy states populated by the electron-capture is less than about ${10}^{\mathrm{\ensuremath{-}}17}$ s. The fission activity was assigned to ECDF of $^{228}\mathrm{Np}$ based on chemical separation and observation of known progeny activities. The ratio of electron-capture to alpha decay is 1.5\ifmmode\pm\else\textpm\fi{}0.4 for $^{228}\mathrm{Np}$. The production cross section for $^{228}\mathrm{Np}$ is 35\ifmmode\pm\else\textpm\fi{}10 \ensuremath{\mu}b and the delayed fission probability is (2.0\ifmmode\pm\else\textpm\fi{}0.9)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$. The kinetic energies of coincident fission fragments were measured and a pre-neutron emission, most probable total kinetic energy, of 169\ifmmode\pm\else\textpm\fi{}6 MeV was obtained. The mass-yield distribution was found to be predominantly asymmetric.