Low-energy Fission Of Actinides Research Articles

Isotopic yield distributions of transfer- and fusion-induced fission from238U+12C reactions in inverse kinematics

A novel method to access the complete identification in atomic number Z and mass A of fragments produced in low-energy fission of actinides is presented. This method, based on the use of multi- nucleon transfer and fusion reactions in inverse kinematics, is applied in this work to reactions between a 238U beam and a 12C target to produce and induce fission of moderately excited actinides. The fission fragments are detected and fully identified with the VAMOS spectrometer of GANIL, allowing the measurement of fragment yields of several hundreds of isotopes in a range between A ~ 80 and ~ 160, and from Z ~ 30 to ~ 64. For the first time, complete isotopic yield distributions of fragments from well-defined fissioning systems are available. Together with the precise measurement of the fragment emission angles and velocities, this technique gives further insight into the nuclear-fission process.

Two-mode fission – experimental verification and characterization of two fission-modes

AbstractExperimental verification and characterization of the two-mode fission are reviewed. The presence of two independent deformation-paths in low energy fission of actinides is demonstrated by studying correlation among saddle-point configurations, scission-point configurations, and mass-yield distributions; the elongated scission configuration is related with the fission process that goes over a higher threshold energy and results in a symmetric mass-division mode, while the compact scission configuration with the process that experiences a lower threshold ends up with an asymmetric mass-division mode. Based on an extensive systematic analysis of scission properties in a wide range of actinide fission, the bimodal fission observed in the spontaneous fission of the heavy actinides is interpreted as the result of the presence of two fission paths, namely, the ordinary asymmetric fission path and a strongly shell-influenced symmetric mode.

Odd-even effect in fragment angular momentum in low-energy fission of actinides

Quantitative explanation for the odd-even effect on fragment angular momenta in the low-energy fission of actinides have been provided by taking into account the single particle spin of the odd proton at the fragment’s scission point deformation in the case of odd-Z fragments along with the contribution from the population of angular momentum bearing collective vibrations of the fissioning nucleus at scission point. The calculated fragment angular momenta have been found to be in very good agreement with the experimental data for fragments in the mass number region of 130–140. The odd-even effect observed in the fragment angular momenta in the low-energy fission of actinides has been explained quantitatively for the first time.

Angular momentum of fission fragments in low energy fission of actinides

Independent isomeric yield ratios (IYR) of $^{128}\mathrm{Sb}$, $^{130}\mathrm{Sb}$, $^{132}\mathrm{Sb}$, $^{131}\mathrm{Te}$, $^{133}\mathrm{Te}$, $^{132}\mathrm{I}$, $^{134}\mathrm{I}$, $^{136}\mathrm{I}$, $^{135}\mathrm{Xe}$, and $^{138}\mathrm{Cs}$ have been determined in fast neutron induced fission of $^{232}\mathrm{Th}$, $^{238}\mathrm{U}$, $^{240}\mathrm{Pu}$, and $^{244}\mathrm{Cm}$ as well as in thermal neutron induced fission of $^{232}\mathrm{U}$ and $^{238}\mathrm{Pu}$ using radiochemical and offline $\ensuremath{\gamma}$-ray spectrometric techniques. From the IYR, fragment angular momenta (${J}_{\mathrm{rms}}$) have been deduced using a spin-dependent statistical model analysis. These data along with the literature data for ${^{230}\mathrm{Th}}^{\ensuremath{\ast}}$, ${^{234}\mathrm{U}}^{\ensuremath{\ast}}$, ${^{236}\mathrm{U}}^{\ensuremath{\ast}}$, ${^{240}\mathrm{Pu}}^{\ensuremath{\ast}}$, ${^{242}\mathrm{Pu}}^{\ensuremath{\ast}}$, $^{244}\mathrm{Cm}$(SF), ${^{246}\mathrm{Cm}}^{\ensuremath{\ast}}$, ${^{250}\mathrm{Cf}}^{\ensuremath{\ast}}$, and $^{252}\mathrm{Cf}$(SF) for fifteen even-$Z$ fissioning systems show the following important features: (i) The ${J}_{\mathrm{rms}}$ of the odd-$Z$ fission products are higher than those of the even-$Z$ fission products, indicating the odd-even effect. For both the odd-$Z$ and even-$Z$ fission products, the ${J}_{\mathrm{rms}}$ increase with ${Z}_{F}^{2}/{A}_{F}$. (ii) The ${J}_{\mathrm{rms}}$ of fragments with spherical 50-$p$ and 82-$n$ shells are lower compared to those of fragments outside the spherical shell, indicating the effect of shell closure proximity. (iii) The ${J}_{\mathrm{rms}}$ of the fission products increase with mass number in spite of fluctuations in shell closure proximity and odd-even effects but do not show any correlation with the neutron emission curve. (iv) In all fifteen even-$Z$ fissioning systems from Th to Cf, the yield-weighted ${J}_{\mathrm{rms}}$ values show an anticorrelation with the elemental yields. (v) The odd-even fluctuation on ${J}_{\mathrm{rms}}$ does not change drastically from Th to Cf, unlike the proton odd-even effect (${\ensuremath{\delta}}_{p}$) which decreases significantly with the increase of Coulomb parameter (${Z}_{F}^{2}/{A}_{F}^{1/3}$).

Charge distribution in low energy fission of actinides

Charge distribution studies were carried out for heavy mass fission products in the thermal neutron induced fission of 232U and 238Pu as well as in the spontaneous fission of 244Cm using off-line gamma ray spectrometry. The width parameter (σZ/σA), the most probable charge/mass (Zp/Ap) and the charge polarization (ΔZ) as a function of fragment mass were deduced. The slope of charge polarization with mass asymmetry (δ(ΔZ)/δA′), average charge dispersion parameter (⟨σZ⟩) and proton odd–even effect (δp) were also obtained for these fissioning systems. The δ(ΔZ)/δA′ and ⟨σZ⟩ for the above three fissioning systems are determined for the first time. The δp value in 244Cm(SF) is also determined for the first time. These data along with the literature data for other even-Z fissioning systems such as 230Th*, 232Th*, 233Th*, 234U*, 236U*, 238U*, 239U*, 240Pu*, 241Pu*, 242Pu*, 245Cm*, 246Cm*, 250Cf* and 252Cf(SF) are discussed in terms of nuclear structure effect and dynamics of descent from saddle to the scission point. The role of excitation energy in low energy fission is also discussed by comparing the data of spontaneous, thermal and fast neutron induced fission of actinides.

Systematics of charge distribution studies in low-energy fission of actinides

Systematics of charge distribution studies in low-energy fission of actinides

Systematics of charge distribution studies in low-energy fission of actinides

Charge distribution studies were carried out in the thermal neutron-induced fission of 229Th, 241Pu and 245Cm as well as in the spontaneous fission of 252Cf. The width parameter ( σ z / σ A ), the most probable charge/mass ( Z p/ A p) and the charge polarization (ΔZ) as a function of fragment mass were deduced. The slope of charge polarization as a function of fragment mass [ δ( ΔZ)/ δA′], average charge dispersion parameter (〈 σ z 〉) and proton odd-even effect ( δ p) were also obtained for these fissioning systems. These data along with the literature data for even- Z fissioning systems such as 233Th ∗, 233U ∗, 234U ∗, 236U ∗, 239U ∗, 239Pu ∗, 240Pu ∗ and 250Cf ∗ are discussed in terms of nuclear-structure effects and dynamics of descent from saddle point to the point of neck formation and from the formation of the neck to the scission point.

Systematics of fragment angular momentum in low-energy fission of actinides

Independent isomeric yield ratios for 128Sb, 130Sb, 132Sb, 131Te, 133Te, 132I, 134I, 136I, 135Xe and 138Cs in 229Th(nth, f), for 136I in 233U(nth, f) and 239Pu(nth, f), for 138Cs in 235U(nth, f), for 130Sb, 136I and 135Xe in 241Pu(nth, f), for 128Sb, 130Sb, 130Sb, 132Sb, 131Te, 133Te, 134I, 136I, 135Xe and 138Cs in 245Cm(nth, f) and for 128Sb, 130Sb, 132Sb, 136I and 135Xe in 252Cf(S.F.) have been determined using radiochemical and gamma-ray spectrometric techniques. From the isomeric yield ratios, fragment angular momenta (Jrms) have been deduced using spin-dependent statistical-model analysis. These data along with the literature data in the above fissioning systems as well as in 249Cf(nth, f) show several important features. These features are: (i) Angular momenta for fragments with spherical 50-proton shell, 82-neutron shell and even-Z products are lower compared to the fragments with deformed 88-neutron shell, no shells and odd-Z products indicating the nuclear-structure effects. (ii) Fission fragment Jrms has a nearly inverse correlation with elemental yield in fissioning systems from 230Th∗ to 252Cf possibly due to coupling between the collective and intrinsic degrees of freedom. (iii) Although the percentage odd-even effect in the elemental yield decreases from 230Th∗ to 250Cf∗ and 252Cf, the odd-even fluctuation on fragment Jrms remains nearly the same in spite of the inverse correlation. This possibly indicates the effect of fragment deformation. (iv) Fission-product elemental yield as well as angular momentum have no definite correlation with fissionability since both are decided near the scission point.

二重モード核分裂

二重モード核分裂

Charge Distribution in the Mass Region 128 —134 in Low Energy Fission of Actinides

Charge Distribution in the Mass Region 128 —134 in Low Energy Fission of Actinides

Systematic behavior of mass-yield curves in low-energy fission of actinides

Mass yield curves of spontaneous fissions, thermal-neutron induced fissions, monochromatic-neutron and proton induced fissions of actinides are reviewed. Mean masses of light and heavy asymmetric mass yield peaks are listed. The subtle change of the shape of the asymmetric heavy peak is expressed in terms of the parameters of two Gaussians that fit the observed data well. One Gaussian is narrow in width and always peaks at A=133–136 while the other is broader and peaks at A=140−144. The FWHM of the heavy asymmetric peak becomes minimum for the fissioning mass A=240−245. An analysis of the projectile energy dependence of mass yield curves indicated, at least, two distinctively different behaviors, one for asymmetric products and the other for symmetric products. From the energy dependence of the peak-to-valley rations, the “extra-energy” required for symmetric mass division, or the difference in the fission barrier heights, in the nomeuclature of the two-mode hypothesis, is deduced for a wide range of fission nuclides.

Systematic analysis of mass yield curves in low-energy fission of actinides.

Mass yield curves in low-energy proton-induced fissions of $^{233}\mathrm{U}$, $^{235}\mathrm{U}$, $^{238}\mathrm{U}$, $^{237}\mathrm{Np}$, $^{239}\mathrm{Pu}$, $^{242}\mathrm{Pu}$, $^{244}\mathrm{Pu}$, $^{241}\mathrm{Am}$, and $^{243}\mathrm{Am}$ were measured. The full widths at half maximum (FWHM) of the heavier asymmetric peak of the mass yield curves showed a sudden dip in the region of ${A}_{f}$=240--245. The correlation between the P/V ratio and the FWHM of the asymmetric mass yield peak was obtained as a function of proton energy. The FWHM values were independent of the incident proton energy as long as the P/V ratio was larger than 10 for all the fissioning systems studied. The excitation energy dependence of the mass yield curves was investigated. It was found that the excitation energy dependence of symmetrically divided fission products was apparently different from that of asymmetrically divided ones. With a simple assumption of the two-mode hypothesis, the observed mass yield curves could be decomposed into two mass yield curves. Asymmetric mass yield peaks could be best represented by the sum of two Gaussians, and their variation could be expressed by the variation of Gaussian parameters. The peak positions of the two Gaussians were always A=133--135 and A=140--142 for all the investigated fissioning nuclides. The preference of those fragment masses is discussed in terms of both the scission-point model and the saddle-point model.

Recent radiochemical studies of the fission process

This article summarizes the recent radiochemical investigations on mass, charge kinetic energy and fragment angular distributions in low energy fission of actinides.