Mass Yield Distribution Research Articles

Observation of fission residues in the16O +181Ta system at Elab≈ 6 MeV/A

Present paper reports on the production cross-section of 24 fission like events (30 ≤ Z ≤ 60) formed via complete fusion-fission and/or incomplete fusion-fission processes in 16 O+ 181 Ta system at energies ≈ 6 MeV/A. Experiments have been performed using the recoil-catcher technique followed by off-line γ-spectroscopy. The measured cross-section of fission-like events is satisfactorily described by a statistical model code. Further, an attempt has been made to study the mass and isotopic yield distributions of fission fragments. The variance of the presently measured isotopic yield distributions has been found to be in agreement with the literature values for some other fissioning systems.

Mass–yield distributions of fission products from photo-fission of natPb induced by 50–70 MeV bremsstrahlung

The mass–yield distributions of various fission products have been determined in the 50-, 60- and 70-MeV end point bremsstrahlung induced fission of natPb using off-line γ-ray spectrometric technique in the electron linac at Pohang Accelerator Laboratory, Korea. The mass–yield distributions are symmetric with average mass of 102.34, 102.25 and 102.03 and FWHM of 21, 22 and 23 mass unit, respectively. From the present data and literature data in the 50–85 MeV bremsstrahlung induced fission of 209Bi the following observations were obtained: (i) The average masses of the yield distributions in the 50–85 MeV bremsstrahlung induced fission of natPb and 209Bi are around 102.25 ± 0.25 and 103 ± 0.5, respectively. (ii) The FWHM of the mass–yield distributions increases from about 21 mass units at 50 MeV to 23 mass units at 70–85 MeV, which is explained from the point of increase in multi-chance fission probability with increasing excitation energy. (iii) Within the bremsstrahlung energy range of 50–85 MeV, the role of nuclear structure effect in the mass–yield distribution was observed in the photo-fission of 209Bi, whereas it was not seen in case of natPb. This may be due to the presence of so many isotopes in natPb unlike mono-isotopic 209Bi.

Measurement of photo-fission yields and photo-neutron cross-sections in 209Bi with 50 and 65 MeV bremsstrahlung

The photo-fission yields and photo-neutron cross-sections of ( $ \gamma$ , 3n) and ( $ \gamma$ , 4n) on 209Bi induced by 50 and 65MeV bremsstrahlung have been measured by using a recoil catcher and an off-line $ \gamma$ -ray spectrometric technique. The mass-yield distribution of fission products in 209Bi induced by bremsstrahlung photons from the present work and literature data in the energy range 28-85MeV is symmetric around 103 mass units. However, the full width at half maximum of the yields distribution increases from 19 mass units at 28-40MeV to 23 mass units at 85MeV. The ( $ \gamma$ , 3n) reaction cross-section in the 50MeV and the ( $ \gamma$ , 4n) reaction cross-section in the 50 and 65MeV bremsstrahlung-induced reaction of 209Bi were determined for the first time.

Product yields for the photo-fission of 209Bi with 2.5 GeV bremsstrahlung

The mass-yield distribution of fission products in the 2.5GeV bremsstrahlung-induced fission of 209Bi have been determined by using the recoil catcher and the off-line γ-spectrometry technique in the high energy electron linac at the Pohang Accelerator Laboratory. The mass-yield determination involves the measurements of cumulative yields for 32 fission products and independent yields of 17 fission products in the photo-fission of 209Bi nuclei. It was found that the mass-yield distribution of fission products in 209Bi is symmetric with an average mass of 95±0.5 and a FWHM of 51±2.0 mass units. Present data at 2.5GeV along with the literature data at 1GeV, 700–600MeV, and 85–28MeV were interpreted from the point of increase of multi-chance fission and multi-nucleon emission probabilities with an increase in excitation energy. It was found that the average mass of the mass-yield distribution of the fission products decreases from 103±0.5 at 28–85MeV to 95±0.5 at 2.5GeV. On the other hand, the FWHM of the mass-yield distribution increases from 19 mass units at 28–40MeV to 51 mass units at 2.5GeV. It was also found that the nuclear structure effect observed at the photo-fission of 209Bi with 28–85MeV bremsstrahlung is washed out at higher energy.

Properties of the reaction 238U [formula omitted] at the vibrational resonances

Recent fission cross-section calculations for the reaction 238U ( n , f ) , based on an extended statistical model, predict a significant change of fission fragment properties, such as the mean mass by Δ A = 1.5 and a notable increase in total kinetic energy in the region of the vibrational resonance at an incident neutron energy E n = 0.9 MeV . This model includes individual fission cross-sections by the asymmetric standard 1 (S1) and standard 2 (S2) as well as the symmetric super-long (SL) mode. In order to verify the model predictions, a dedicated experiment on 238U has been carried out to measure fission-fragment mass yield distributions for incident neutron energies from E n = 2.0 MeV down to 0.9 MeV, where the fission characteristics at the vibrational resonance at E n = 0.9 MeV were investigated for the first time. The previously reported distinct structure in the angular anisotropy around E n = 1.2 and 1.6 MeV was observed at E n = 0.9 MeV as well. The predicted large changes in fission fragment mass yield and total kinetic energy could not be confirmed. In the resonance the mean total kinetic energy is only about 0.5 MeV higher than at E n = 1.8 MeV . At the same time, a slight decrease of the mean heavy fragment mass was observed, probably indicating a slightly increased contribution of the S1 fission mode.

Post-Neutron Mass Yield Distribution and Photo-Neutron Cross-Section Measurements in 209Bi with 65-MeV Bremsstrahlung

Post-Neutron Mass Yield Distribution and Photo-Neutron Cross-Section Measurements in 209Bi with 65-MeV Bremsstrahlung

Laser ablation of a platinum target in water. II. Ablation rate and nanoparticle size distributions

This is the second in a series of three papers examining nanomaterial formation in laser ablation in liquids (LAL). Here we study the effect of the laser wavelength and fluence on the mass yield and size distribution of nanoparticles prepared by laser ablation of a platinum target immersed in water. For all wavelengths tested, laser fluences in the range of 10–70J∕cm2 resulted in spheroidal, nonagglomerated platinum nanoparticles with sizes ranging from 1to30nm. Nanoparticle size distributions are found to be composed of two modes that are attributed to thermal vaporization and explosive boiling mechanisms. The peak of the smaller size mode remains nearly constant at 3nm for all laser conditions, which is suggested to be due to the strong confinement of the vapor plume by the liquid. The larger size mode peaks in the range of 5–15nm with a population that is strongly dependent on the laser parameters. It is concluded that changes in the mean size reported in many earlier studies on LAL of metal targets are a result of the relative quantity of nanoparticles from each mechanism rather than direct control over the ablation process. Additionally, it was observed that the yield of platinum nanoparticles was significantly larger for 1064nm wavelength at fluences greater than 10J∕cm2. The maximum ablation rate was approximately 4.4mg∕h, with an estimated ablation and collection efficiency of 0.9μg∕J. Dependence of the mass yield on wavelength and fluence is seen to be dependent primarily on the extent of the explosive mechanism.

06/00629 Prediction of fission mass-yield distributions based on cross section calculations: Hambsch, F.-J. et al. Annals of Nuclear Energy, 2005, 32, (12), 1297–1304.

The statistical model for fission cross-section evaluation is extended by the concept of multi-modality of the fission process. The three most dominant fission modes, the two asymmetric standard I (S 1) and standard I1 (S2) modes and the symmetric superlong (SL) mode are taken into account. Deconvoluted fission cross-sections for S1, S2 and SL modes for 5~238U(n, f) and 237Np(n, f), based on experimental branching ratios, were calculated for the first time in the incident neutron energy range from 0.01 to 5.5 MeV. Good agreement was found with the experimental fission cross-section data. Branching ratios were deduced for the different modes over the whole incident neutron energy region. Fission fragment mass yield distributions have been calculated at incident neutron energies, where no experimental data exist.

Isobaric production cross sections from 0.6 GeV proton irradiation of neptunium and thorium using mass spectrometry

A high-fluence proton irradiation of neptunium was the last experiment in PSIs programme ATHENA related to accelerator-based transmutation. The principal aim of the programme has been to provide experimental data for the validation of theoretical models in nucleon–meson transport codes, with emphasis on the mass yield distribution of fission and spallation products. An improved mass spectrometry method has allowed the direct derivation of isobaric production cross sections with only minor corrections and an estimate of the fission cross section by integration in the fission hump. In a second sample position of the irradiation head, a repetition of the previous ATHENA experiment with thorium was possible, profiting from the improved mass spectrometry technique. The experimental results are better predicted by the code FUSSPOT than by HETC/RAL, both used at PSI.

Prediction of fission mass-yield distributions based on cross section calculations

For the first time, fission mass-yield distributions have been predicted based on an extended statistical model for fission cross section calculations. In this model, the concept of the multi-modality of the fission process has been incorporated. The three most dominant fission modes, the two asymmetric standard I (S1) and standard II (S2) modes and the symmetric superlong (SL) mode are taken into account. De-convoluted fission cross sections for S1, S2 and SL modes for 235,238U(n, f) and 237Np(n, f), based on experimental branching ratios, were calculated for the first time in the incident neutron energy range from 0.01 to 5.5 MeV providing good agreement with the experimental fission cross section data. The branching ratios obtained from the modal fission cross section calculations have been used to deduce the corresponding fission yield distributions, including mean values also for incident neutron energies hitherto not accessible to experiment.

Primary fragment mass-yield distributions for asymmetric fission path of heavy nuclei

The primary fragment mass-yield distribution for the asymmetric fission path in heavy nuclei, 233Pa, 239Np, 245Am and 249Bk at the excitation energy of ~20 MeV are experimentally constructed based on the intensities of total kinetic energies for individual mass splits. The results revealed an interesting phenomenon: in all the studied fissioning systems, the inner wings of the mass-yield distributions in the asymmetric fission path appear along the same mass-wall of A = 130 fragment mass. The asymmetric mass-yield distribution indicates the strong effect of structural shells in fragments on the final mass division process of the asymmetric fission path.

Photofission of 197Au and 209Bi at intermediate energies

Recoil properties and yields of radionuclides formed in the photofission of 197Au and 209Bi by bremsstrahlung of end-point energies (E 0) from 300 to 1100 MeV have been investigated using the thick-target thick-catcher method. The kinetic energies T of the residual nuclei were deduced based on the two-step vector model and discussed by comparing with the reported results on protoninduced reactions as well as those on photospallation. The charge distribution was reproduced by a Gaussian function with the most probable charge Zp expressed by a linera function of the product mass number A and with the A-independent width FWHM CD. Based on the charge distribution parameters, the symmetric mass yield distribution with the most probable mass A p of 92 m.u. and the width FWHM MD of 39 m.u. was obtained for 197Au at E 0≥600 MeV. The A p value for 209Bi was larger by 4 m.u. than that for 197Au and the FWHM MD was smaller by 6 m.u. A comparison with the calculations using the Photon-induced Intranuclear Cascade Analysis 3 code combined with the Generalized Evaporation Model code (PICA3/GEM) was also performed.

Shapes of Fragment Mass-Yield Distributions and Shapes of Scissioning Nuclei in Actinides

Three fimdamental types (named A-, B- and O-type) in the fragment mass-yield distribution and three independent types of the scission deformation (β~1.65, 1.53 and 1.33) for actinide nuclei in fission process are experimentally observed. The correlations between the final shapes of fissioning nuclei and the shapes of the fragment mass-yield distributions are hence revealed and presented. A-type: deformation path with β~1.65 resulting in a typical FWHM of >20 u; B-type: deformation path with β~1.53 leading to a typical FWHM of ~15 u; O-type: deformation path with β~1.33 producing a FWHM of ~9 u.

Projectile dependence of radioactive spallation products induced in copper by high-energy heavy ions

Irradiation experiments were performed at the HIMAC (heavy-ion medical accelerator in Chiba) facility, National Institute of Radiological Sciences. The radioactive spallation products in a thick Cu target were obtained for 230 and 100 MeV/nucleon Ne, C, He, and p ions. The gamma-ray spectra from irradiated Cu pieces inserted into the composite Cu target were measured with a high-purity Ge detector. From the gamma-ray spectra, we obtained the variation of radioactive yields with depth in the Cu target and the mass-yield distribution of nuclides produced on the surface. The results show that the cross sections strongly depend on the projectile mass and, consequently, on the mass number difference between Cu and the produced nuclides. The most important feature of the present data is the extraction of the energy dependence of the cross sections. The measured cross sections were fitted to a modified form of Rudstam's semiempirical formula in order to evaluate the cross sections of unmeasured nuclides. These estimated cross sections were added to the experimental yields for each mass number and the total mass yield and isobaric charge distributions, and the slope of the mass-yield distribution were obtained.

Measurement of Mass Yield Distributions in Proton-Induced Fission of Minor Actinides

Fission product yields of low-energy proton-induced fission of minor actinides, 237Np, 241Am, 243Am and 248Cm, were measured by γ-ray spectrometry in order to construct the mass yield distributions. Enriched targets of the minor actinides were irradiated by proton beams with the energies of 25 and 30 MeV for several hours at the JAERI tandem accelerator. About 40 fission products were measured and their formation cross sections were determined. Systematic trends of the mass yield distributions in the fission of the minor actinides are discussed quantitatively.

Projectile Dependency of Radioactivities of Spallation Products Induced in Copper

The radioactivities of spallation products in a Cu target were obtained by bombarding 230 and 100 MeV/nucleon Ne, C, He, p ions. Irradiation experiments were performed at the HIMAC (Heavy Ion Medical Accelerator in Chiba) facility, National Institute of Radiological Sciences. The gamma-ray spectra from irradiated samples were measured with a HPGe detector. From the gamma-ray spectra, we obtained the variation of radioactivities of nuclides produced in Cu sample with Cu target thickness and the mass-yield distribution of nuclides produced in Cu sample on the surface of Cu target. The results showed that the dependence of the cross sections on the projectile mass strongly depends on the mass number difference between Cu and produced nuclide.

Photofission of209Bi at intermediate energies

SummaryThe reaction yields of 63 radionuclides with the mass numberA= 56 − 135 produced in the photofission of209Bi by bremsstrahlung of end-point energies (E0) from 450 to 1100 MeV have been measured using a catcher foil technique with the aid of intensive chemical separation. The charge distribution was well reproduced by a Gaussian function with the most probable charge (Zp) expressed by a linear function ofA,i.e.,Zp=RA+S, and with theA-independent full width at half maximum (FWHMCD). The charge distribution parametersR,Sand FWHMCDwere independent ofE0above 600 MeV, reflecting the resonance nature in photonuclear reactions at intermediate energies. The weighted mean values atE0≥ 600 MeV wereR= 0.421 ± 0.001,S= 0.6 ± 0.1 and FWHMCD= 2.1 ± 0.1 charge unit (c.u.). The numbers of pre- and post-fission neutrons were deduced to be νpre= 12 ± 1 and νpost= 1.4 ± 0.3, respectively, assuming the Unchanged Charge Distribution (UCD). Based on the charge distribution parameters, the symmetric mass yield distributions with the most probable massApof 96 ± 1 mass unit (m.u.) and the width FWHMMDof 33 ± 1 m.u. were also obtained. The characteristics of the charge and mass yield distributions are discussed by referring to those for197Au based on the results of calculations using the Photon-induced Intranuclear Cascade Analysis code combined with the Generalized Evaporation Model (PICA3/GEM).

Strangeness suppression in proton-proton collisions

Can the yield of a strange particle serve as the smoking gun for the creation of a quark-gluon plasma? In order to answer this question one has first to understand strange particle production in elementary pp reactions. Here one observes a big difference between BNL RHIC and CERN SPS energies. The mass yield distribution is much steeper at SPS than at RHIC. Surprisingly the form of the mass yield for ${(e}^{+}{e}^{\ensuremath{-}})$ annihilation, which is almost energy independent, agrees perfectly with pp reactions at RHIC. After having verified that the recently advanced NEXUS approach reproduces well the existing data, we interpret the results: strangeness is suppressed in proton-proton collisions at SPS energy as compared to electron-positron ${(e}^{+}{e}^{\ensuremath{-}})$ annihilation due to the limited masses of the strings produced in the reaction.

Mass and charge distributions in the very asymmetric mass region of the neutron induced fission of [formula omitted

The mass-separator Lohengrin was used to measure the yields of the light fission products with A=74−85 and their nuclear charge and kinetic energy distributions from the odd- Z compound nucleus 239 Np ∗ formed by double capture of thermal neutrons. The mass yield distribution reveals an influence of the fragment shell with N=50 affecting also the nuclear charge and kinetic energy distributions. An odd–even effect for protons is found in the very asymmetric mass division, increasing from 4% to 35% with increasing fission asymmetry. This is in contrast to findings in normal asymmetric fission (region of high fission yields) where no odd–even effect for protons was observed. An odd–even effect for neutrons is also found comparable in size with that for protons. The latter effect exists also in normal asymmetric fission and is at least partly attributed to prompt neutron emission from the fragments. Some information on the number of prompt neutrons emitted is also obtained. From this information and from the energy dependence of the odd–even effect for protons it is concluded that the very light fragments originate from a cold and nearly undeformed light sphere of a dumbbell scission configuration characterised by a double shell closure with 28 protons and about 50 neutrons. In contrast to the double shell closure at mass 132 ( Z=50, N≈82) the two shell closures in the light sphere do not coincide at one mass but are realised at A=70 and 80, respectively. This leads to a layered structure of the light sphere of the dumbbell scission configuration.

Study of the 237 Np(n,f)-reaction at MeV neutron energies

The fission fragment properties of the reaction 237 Np(n, f ) have been investigated at IRMM in the neutron energy range E n=0.3 to 5.5 MeV. Above E n=0.5 MeV, the TKE averaged over all fission fragment masses shows a continuously decreasing trend. The observed variations in the TKE as a function of the compound nuclear excitation energy have been attributed to a combination of the changes in the mass yield and TKE (A) distributions. A description of the two-dimensional mass–TKE distributions has been obtained in terms of fission modes in the frame of the multimodal random neck-rupture model. The inclusion of the superlong symmetric mode is necessary to explain the observed dip in the experimental TKE close to symmetry.