Total Fission Cross Sections Research Articles

Investigating the fission process at high excitation energies through proton induced reactions on181Ta

In this work we have investigated total fission cross section of 181 Ta+ 1 Ha t FRS (FRagment Separator -GSI) at 1, 0.8, 0.5 and 0.3 GeV with an specific setup, pro- viding high accuracy measurements of the cross section values. We compare the results obtained in this experiment, with several calculations performed with the intra-nuclear cascade model (INCL v4.1) coupled to de-excitation code (ABLAv3p), according to two different models describing fission process at high-excitation energies: statistical model of Bohr and Wheeler and the dynamical description of the fission process. The compar- ison with data of previous experiments is also discussed in order to address the existing discrepancies with this new results.

Fission studies with 140 MeVαparticles

Binary fission induced by 140 MeV $\ensuremath{\alpha}$ particles has been measured for $^{\mathrm{nat}}\mathrm{Ag}$, $^{139}\mathrm{La}$, $^{165}\mathrm{Ho}$, and $^{197}\mathrm{Au}$ targets. The measured quantities are the total kinetic energies, fragment masses, and fission cross sections. The results are compared with other data and systematics. A minimum of the fission probability in the vicinity ${Z}^{2}/A=24$ is observed.

Fission studies in the reaction of 19F with 209Bi at 99.2 MeV

Fission product yield studies in the reaction of 99.2 MeV 19F with 209Bi have been carried out for the first time using gamma-ray spectrometry. The cross sections for the production of fission products have been determined. The yield distribution of fission products was found to be symmetric and broad with FWHM around 22 mass units and peak near mass 111. The average number of neutrons emitted per fission has been found to be around 6.7. The comparison of the fission products yield distribution of 209Bi using projectiles like 4He, 12C, 16O, and 19F have shown that the mass of symmetric peak increases as the mass of the compound nucleus increases. The high fission yield around mass 112 has been attributed to the presence of deformed neutron shells. The total fission cross section and width of the mass distribution have been found to be low in case of 16O induced fission as compared to the 4He, 12C, and 19F induced fission of 209Bi.

Proton-induced fission on actinide nuclei at energies 27 and 63 MeV

Measurements of proton-induced fission on 232Th, 238U, 237Np, 239Pu and 241Am nuclei at proton energies of 26.5 MeV and 62.9 MeV were performed at the Louvain-la-Neuve cyclotron facility. Neutrons were detected with the multidetector DEMON. Mass and energy distributions of fission fragments, neutron angular distribution and multiplicities and total fission cross sections were determined. The relative contributions of pre-equilibrium neutron emission and compound nucleus evaporation, as well as of post-scission evaporation from the fission fragments were analyzed with moving multisource fits and were also compared with the predictions of the nuclear-reaction model code TALYS. Detailed comparisons with the data indicate that there is still room for improvement in this code.

Energy dependence of proton induced fission cross sections for heavy nuclei in the energy range 200–1000 MeV

Total cross sections for proton induced fission of ${}^{\mathrm{nat}}\mathrm{Pb}$, $^{209}\mathrm{Bi}$, $^{232}\mathrm{Th}$, $^{233}\mathrm{U}$, $^{235}\mathrm{U}$, $^{238}\mathrm{U}$, $^{237}\mathrm{Np}$, and $^{239}\mathrm{Pu}$ nuclei were measured in the range 200--1000 MeV with an energy step of 100 MeV. The experiment was carried out at 1 GeV in a PNPI synchrocyclotron. Complementary fragments of the binary fission from a thin target were detected in coincidence by two parallel plate avalanche counters (PPAC) located close to the target. An assembly of the two PPACs and with the target in between them was placed directly into the beam, which provided a large solid angle acceptance. The beam monitoring system employed a scintillation counter telescope that was used for direct proton counting as well as for registration of the elastic $\mathit{pp}$-scattering events from the auxiliary (${\mathrm{CH}}_{2}){}_{n}$ target. The measured energy dependence of the total fission cross sections is presented. The results are compared with other available experimental data as well as with calculations in the frame of the cascade-evaporation model.

Conditions for the manifestation of transient effects in fission

The conditions for the manifestation of transient effects in fission are carefully examined by analysing experimental data where fission is induced by peripheral heavy-ion collisions at relativistic energies. Experimental total nuclear fission cross sections of 238U at 1 A GeV on gold and uranium targets are compared with the predictions of a nuclear-reaction code, where transient effects in fission are modeled using different approximations to the numerical time-dependent fission-decay width: a new analytical description based on the solution of the Fokker–Planck equation and two widely used but less realistic descriptions, a step function and an exponential-like function. The experimental data are only reproduced when dissipation is considered. The influence of transient effects on the fission process, as well as the deduced value of the dissipation strength β, depends strongly on the approximation applied for the time-dependent fission-decay width. In particular, a meticulous analysis sheds severe doubts on the use of the exponential-like in-growth function. Finally, we investigate which should be the characteristics of experimental observables to be most sensitive to transient effects in fission. The pertinence of observables related to the excitation energy at saddle is discussed.

Nuclide cross-sections of fission fragments in the reaction 208Pb + p at 500 A MeV

The isotopic distributions and recoil velocities of the fission fragments produced in the spallation reaction 208Pb + p at 500 A MeV have been measured using the inverse-kinematics technique, a lead beam onto a liquid-hydrogen target, and the high-resolution spectrometer FRS at GSI. The shapes of the different distributions are found in good agreement with previously published data while the deduced total fission cross-section is higher than expected from existing systematics and some previous measurements. From the experimental data, the characteristics of the average fissioning system can be reconstructed in charge, mass and excitation energy, and the average number of post-fission neutrons can be inferred. The results are also compared to different models describing the spallation reaction. The intranuclear cascade code INCL4 followed by the de-excitation code ABLA is shown to describe reasonably well the evolution of the isotopic distribution shapes between 500 and 1000 A MeV.

Method of Measurement of Cross Sections of Heavy Nuclei Fission Induced by Intermediate Energy Protons

A method of measurement of total fission cross sections and an experimental set-up are described. Experiments are carried out at the 1 GeV synchrocyclotron of PNPI with an updated beam system which allows one to obtain proton beams of various energies from 200 to 1000 MeV with intensity up to 107 protons/s. The proposed measurement method is based on the registration in coincidence of both fission fragments by two parallel plate avalanche counters. The counters together with the target to be investigated are placed directly on the proton beam which provides a large solid angle acceptance. The beam monitoring is performed by scintillation telescopes. The first preliminary results on energy dependences of total fission cross sections of 238U, 235U, 209Bi and natPb are presented.

Measurement of Cross Sections of Heavy Nuclei Fission Induced by Intermediate Energy Protons in the Energy Range 200 - 1000 MeV

A method of measurement of total fission cross sections and experimental set-up are described. Experiments are carried out at 1 GeV synchrocyclotron of PNPI with an updated beam system which allows one to obtain proton beams of various energies from 200 to 1000 MeV with intensity about 106 protons/s. The proposed measurement method is based on the registration in coincidence of both fission fragments by two parallel plate avalanche counters. The counters together with the target to be investigated are placed directly on the proton beam which provides a large solid angle acceptance. The beam monitoring is performed by scintillation telescopes. The first results on total fission cross sections of 238U and 209Bi at some proton energies in the range from 200 MeV to 1000 MeV are presented.

Signatures of fission dynamics in highly excited nuclei produced in 197Au(800 A MeV) on proton collisions

197Au(800 A MeV)-on-proton collisions are used to investigate the fission dynamics at high excitation energy. The kinematic properties together with the isotopic identification of the fission fragments allow to determine the mass, charge and excitation energy of the fissioning nucleus at saddle. The comparison of these observables and the measured total fission cross section with model calculations evidences a clear hindrance of fission at high excitation energy that can be explained in terms of nuclear dissipation. Assuming a statistical evaporation for other de-excitation channels than fission, an estimated value of the transient time of fission of (3±1)×10 −21 s is obtained.

Mass Distribution in 12C Induced Fission of 232Th

Abstract Formation cross sections of several fission products have been determined using recoil catcher technique followed by gamma-ray spectrometry in 12C induced fission of 232Th at E lab = 72 MeV, corresponding to E cm just above the Coulomb barrier. The measured formation cross sections were used to get the mass distribution by using known charge distribution systematic. Critical data analysis was carried out to look for the signatures of transfer induced fission. However, within the experimental uncertainty of about 10%, no clear indication of transfer induced fission could be seen at this energy level. The mass distribution shows a single peaked broad Gaussian distribution with the most probable mass of 119.5±1.1 and FWHM of 40.6 mass units. The total fission cross section computed from the mass distribution curve is 771±50 mb.

Mass distribution in11Binduced fission of232Th

Formation cross sections of several fission products have been determined using recoil catcher technique followed by \ensuremath{\gamma}-ray spectrometry in ${}^{11}\mathrm{B}$ induced fission of ${}^{232}\mathrm{Th}$ at ${E}_{\mathrm{lab}}=72,$ 60, and 55 MeV. The data show significant admixture of fission from compound nucleus formed by complete fusion as well as targetlike nuclei formed by transfer reaction. Mass distributions for both the fissioning systems have been obtained using the systematics of charge distribution in low and medium energy fission. The mass distribution for complete fusion fission is broad, Gaussian whereas it is asymmetric for transfer induced fission. The evaporation residue cross sections of targetlike nuclei formed in ${}^{232}{\mathrm{T}\mathrm{h}(}^{11}{\mathrm{B},\mathrm{}}^{10}{\mathrm{B}\mathrm{e})}^{233}\mathrm{Pa}$ reaction were also measured. The measured evaporation residue cross sections and the decay probabilities of targetlike nucleus ${}^{233}\mathrm{Pa},$ calculated by the PACE2 code have been used to estimate the proton transfer fission cross sections which were found to be negligible compared to the total transfer cross section for all the projectile energies used. The transfer fission cross section is dominated by alpha transfer fission as inferred by measured forward to backward ratios for several fission products as well as ${Q}_{\mathrm{gg}}$ systematics for the probable transfer reactions. The proportion of transfer fission cross section to the total fission cross section was 15, 17, and 22 % at projectile energies of 72, 60, and 55 MeV, respectively.

A high-fluence 0.6 GEV proton irradiation experiment with thin uranium and thorium targets

High-fluence proton irradiation eperiments with thin actinide targets are part of PSI's programme related to accelerator-based transmutation studies. The aim is to provide experimental data for the validation of theoretical models in nucleon-meson transport codes, with emphasis on the mass yield distribution of spallation and fission products. The paper presents results of an irradiation experiment carried out with 238UO 2 and 232ThO 2. Based on mass spectrometric measurements, isobaric production cross-sections for fission and spallation products, as well as total fission cross-sections of the target nuclides, are reported. The experimentally deduced total fission cross-sections for both target nuclides are found to be approximately twice as large as values reported in certain earlier experiments, as well as with respect to calculational results obtained using the HETC code and the RAL high-energy fission model.

Total and nuclear fission cross sections ofU238at relativistic energies

Total fission cross sections of U238 projectiles were measured at bombarding energies of 0.6 and 1 GeV per nucleon for seven different targets (Be, C, Al, Cu, In, Au, and U). It is found that all data points fall onto one curve, independent of bombarding energy, once the electromagnetic contribution to the total fission cross sections is subtracted. The abrasion-ablation model predicts a significantly weaker target dependence than observed, and underestimates the nuclear fission cross sections for the heavier targets. © 1996 The American Physical Society. Received 1 March 1996DOI:https://doi.org/10.1103/PhysRevC.53.3143©1996 American Physical Society

Mass distribution in 19F induced fission of 232Th.

Formation cross sections of several fission products have been determined using the recoil catcher technique followed by {gamma}-ray spectrometry in {sup 19}F induced fission of {sup 232}Th at {ital E}{sub lab}=95 and 112 MeV. The data show significant admixture of fission from compound nuclei formed by complete fusion as well as targetlike nuclei formed by transfer reactions. Mass distributions for both the fissioning systems have been obtained using the systematics of charge distribution in low and medium energy fission. Mass distribution for complete fusion fission is broad Gaussian whereas it is asymmetric for transfer induced fission. At 95 MeV the transfer fission constitutes about 28{percent} of total fission cross section while at 112 MeV it is about 14{percent}, showing that the transfer fission fraction decreases with increasing projectile energy across the barrier. The evaporation residue cross sections of the targetlike nucleus formed in the {sup 232}Th ({sup 19}F, {sup 18}O) {sup 233}Pa reaction were also measured. The evaporation residue cross sections and the calculated decay probabilities of the targetlike nucleus {sup 233}Pa by PACE2 have been used to estimate the fraction of proton and {alpha} transfer fission in the total transfer fission cross section. {copyright} {ital 1996 The American Physicalmore » Society.}« less

Analysis of dissipative effects in nuclear fission observed in the fragmentation of 238U projectiles

The abrasion-ablation model widely used previously to describe peripheral nuclear reactions at relativistic energies has been generalized to be applicable to heavy reaction partners for which it is important to account for nuclear fission. It is shown that the changes of the fission widths and the transient times of the competing decay modes caused by the nuclear viscosity must be included to consistently describe the available experimental data. The model predictions are in good agreement with measured total fission cross sections for high-energy 238U projectiles impinging on different targets. Furthermore, recently measured production cross sections of neutron-deficient U and Pa isotopes in the fragmentation of 238U projectiles at 950A MeV are well described by the model, if a nuclear viscosity parameter of β = 1 × 1021 s−1 is chosen. The model predicts rather high production cross sections for a wide range of fissile exotic nuclei which may be produced by the fragmentation of 238U.

Total cross section measurements of 16O+232Th incomplete fusion followed by fission at 140 MeV.

Total cross sections for incomplete fusion followed by fission have been measured for the reaction [sup 16]O+[sup 232]Th at 140 MeV. In-plane and out-of-plane measurements were made of cross sections for beamlike fragments in coincidence with fission fragments. The beamlike fragments were detected with a Bragg curve spectrometer which is position sensitive in the polar direction. The beamlike particles observed in coincidence with fission fragments were He, Li, Be, B, C, N, and O. One fission fragment from each reaction was detected by one of three surface barrier detectors using time of flight for particle identification. The measured cross section due to fission following incomplete fusion is 747[plus minus]112 mb, or 42% of the total fission cross section. The strongest incomplete fusion channels were the He and C channels. The average transferred angular momentum for each incomplete fusion channel was predicted using the [ital Q][sub opt] model of Wilczynski so that the [ital K] distribution could be estimated from the rotating liquid drop model. The experimental angular distributions were fitted using the saddle point transition state model, with the transferred angular momentum [ital J] and the dealignment factor [alpha][sub 0] as free parameters. The saddle-point transition state model is foundmore » to adequately describe the observed angular correlations for fission following incomplete fusion. The fitted parameter [ital J] showed systematic deviations from the [ital Q][sub opt] model predictions, suggesting that the average impact parameter is larger than expected for heavy projectile fragments.« less

Mass Distribution in 16O-induced Fission of 232Th

Formation cross sections of several fission products have been determined using recoil catcher technique followed by gamma-ray spectrometry in 12C induced fission of 232Th at E lab = 72 MeV, corresponding to E cm just above the Coulomb barrier. The measured formation cross sections were used to get the mass distribution by using known charge distribution systematic. Critical data analysis was carried out to look for the signatures of transfer induced fission. However, within the experimental uncertainty of about 10%, no clear indication of transfer induced fission could be seen at this energy level. The mass distribution shows a single peaked broad Gaussian distribution with the most probable mass of 119.5±1.1 and FWHM of 40.6 mass units. The total fission cross section computed from the mass distribution curve is 771±50 mb.

Peripheral interactions in the collision of 16O+238U at 110.4 MeV bombarding energy.

Inclusive and semiexclusive elemental production cross sections of projectilelike fragments emerging in the collision [sup 16]O+[sup 238]U at 110.4 MeV have been investigated. Energy spectra and angular distributions of projectile residues with charge from [ital Z]=3 to [ital Z]=8 have been measured. It has been found that, as expected, the dominant reaction mechanism in the peripheral collisions is a mass transfer from projectile to target mainly dominated by a transfer of an [alpha] particle. The in-plane coincidences between projectilelike and fission fragments suggest that 14% of the total fission cross section is associated with the fission of a targetlike nucleus.

Cross Sections for Formation of Fission Products in the 238U(p, f) Reaction at Proton Energies between 12 and 30 MeV

In this paper cumulative {sup 238}U(p,f) cross sections are given for 37 nuclides in the energy region 12.5 to 17.5 MeV and for four nuclides up to 30 MeV. Also, 11 independent cross sections are reported. A theoretical model for calculating such quantities is described. The calculated cumulative cross sections are in relatively good agreement with the experimental values, except in the region of symmetric fission, where the calculated values are smaller. The measured total fission cross sections are in good agreement with previous results. From the independent cross sections, the influence of the nuclear shell Z = 50 on the charge distribution of fission fragments can be seen clearly.