S. P. Dange

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

Abstract Independent isomeric yield ratios for 128 Sb, 130 Sb, 132 Sb, 131 Te, 133 Te, 132 I, 134 I, 136 I, 135 Xe and 138 Cs in 229 Th(n th , f), for 136 I in 233 U(n th , f) and 239 Pu(n th , f), for 138 Cs in 235 U(n th , f), for 130 Sb, 136 I and 135 Xe in 241 Pu(n th , f), for 128 Sb, 130 Sb, 130 Sb, 132 Sb, 131 Te, 133 Te, 134 I, 136 I, 135 Xe and 138 Cs in 245 Cm(n th , f) and for 128 Sb, 130 Sb, 132 Sb, 136 I and 135 Xe in 252 Cf(S.F.) have been determined using radiochemical and gamma-ray spectrometric techniques. From the isomeric yield ratios, fragment angular momenta ( J rms ) 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 249 Cf(n th , 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 J rms has a nearly inverse correlation with elemental yield in fissioning systems from 230 Th ∗ to 252 Cf 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 230 Th ∗ to 250 Cf ∗ and 252 Cf, the odd-even fluctuation on fragment J rms 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.

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

Abstract 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 (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 as a function of fragment mass [δ(ΔZ)/δA′], average charge dispersion parameter (〈σz〉) and proton odd-even effect (δp) 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.

Correlations of fission fragment angular momentum with collective and intrinsic degrees of freedom

In the present work angular momenta of the fragments corresponding to132Im,g have been deduced from the radiochemically determined independent isomeric yield ratios and statistical model based analysis in neutron induced fission of235U,239Pu and245Cm and spontaneous fission of252Cf. These data along with similar data on134I, reported earlier from this laboratory, bring out the effects of deformed 66n and spherical 82n shells on fragment angular momentum showing also an inverse correlation of the latter with elemental yields. Quantitative estimates of fragment scission point deformation and the coefficient of change of fragment angular momentum with kinetic/excitation energy have been deduced and are seen to be in good agreement with the expected theoretical estimates.

Effect of shell closure proximity on fragment angular momenta in241Pu/nth'f/

Fission fragment angular momenta in thermal neutron induced fission of241Pu have been deduced for the first time from radiochemically determined independent isomeric yield ratios of the corresponding fission products131, 133Te and132I. The values obtained are 5.8±1.1 ħ, 5.95±1.2 ħ and 8.85±0.65 ħ for131, 133Te and132I, resp. The present data along with the literature data are discussed in the light of influence of fragment nuclear structure, such as the presence of odd proton in the fragment and spherical and/or deformed neutron shell configuration.

Kinetic energy distribution in the thermal neutron fission of 233U and 239Pu

Recoil ranges of fission products in the thermal neutron fission of 233U and 239Pu were determined using radiochemical techniques as well as direct counting with a Ge(Li) detector. The recoil ranges were converted into kinetic energies using different range-energy equations and the relative merits of these equations are discussed. The kinetic energy deficit was calculated from the kinetic energy distribution. From the kinetic energy distribution the average kinetic energy was calculated using the mass-yield data available in literature. The average kinetic energy was found to vary linearly with the fissionability parameter. Deformations of fission fragments and the variation of the deformation as a function of fragment mass were calculated using the kinetic energy data.

Some important aspects of fragment angular momentum in medium energy fission of 238U

Abstract Independent isomeric yield ratios of 131 Te, 133 T and 134 I hava been determined at five different energies in the range of 25–44 MeV alpha particle induced fission of 238 U using radiochemical and gamma spectrometric techniques. From the independent isomeric yield ratios, fragment angular momenta ( J rms ) have been deduced using a statistical model analysis. The J rms were also calculated theoretically based on thermal equilibration of various collective modes after considering the occurrence of multichance fission. These data and the literature data for various fragments in the mass region 126–136 in 238 U(α,f), 238 U(p,f) and 238 U(γ,f) show the following important features: (i) Both the entrance channel excitation energy and input angular momentum affect the fragment angular momentum in the exit channel. (ii) There are two groups of fission products from the point of view of change of fragment angular momentum with increase in excitation energy and input angular momentum. (iii) Fragment angular momentum depends on nuclear structure effect such as shell closure proximity and odd-even effect. (iv) The fragment angular momentum calculated theoretically based on statistical equilibration of various collective modes are in good agreement with the experimental values indicating the validity of such an assumption.

Dependence of the Angular Momenta of Fission Fragments on their Nuclear Structure

In order to investigate the influence of the nuclear structure of a fission fragment on the scission configuration, angular momenta of 13 1 fragment have been deduced from the radiochemically determined independent isomeric yield ratios of 1 in the fission of 2 3 U, 2 3 U, 1 3 , Pu and 3 , 1 Pu induced by thermal neutrons. An odd-even effect observed in the angular momentum of the fragment indicates a greater extent of deformation in the odd-Z fragments; the angular momentum of the 1 3 I fragment decreases when the neutron number of the complementary fragment approaches the 66 η deformed shell configuration.

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

Fractional cumulative yields (FCY) of I 2 Sn, Sb, Te and Te have been determined in thermal neutron induced fission of 2 3 U, 2 3 U, Pu, 2 4 Pu and 2 4 Cm using direct gamma spectrometric and radiochemical techniques. Charge distribution systematics e.g., the width parameter (σζ), most probable charge (ZP) and magnitude of charge polarization (ΔΖ) have been deduced in all the four mass chains in these fissioning systems as well as in Cf(n lh,f) on the basis of the present and literature data. It is observed that the parameters σζ and Δ Ζ show the effect of 50p and 82 η spherical shells. Systematic increase of σ ζ values with increase in fissility parameter has been observed in all the four mass chains and is the consequence of dynamical effects. Variation of ΔΖ for a fixed heavy mass with increase of fissibility parameter on the other hand is interpreted from the point of mass asymmetry.

Recoil ranges and kinetic energy distribution in the reactor neutron induced fission of

Recoil ranges in aluminium have been determined for seventeen fission products, from reactor neutron induced fission of Am. From the recoil range data, kinetic energy distribution as a function of fragment mass has been obtained, using three different range-energy equations. I t has been found tha t both in magnitude as well as the shape of the kinetic energy distribution is dependent on the range-energy equations. Using the kinetic energy data obtained in the present work, deformation of fission fragments has been calculated.

sup 241

Fractional cumulative yields (FCY) of various fission products in the light mass region have been determined in the thermal neutron induced fission of Th, Pu and Cm as well as in the spontaneous fission of Cf using the direct gamma spectrometric technique. These data and similar data from the literature were used to deduce the charge distribution parameters such as the width parameter (σζ), the most probable charge (ZP) and the charge polarization (A Ζ = ZP—ZUCD) as a function of the fragment mass for these four fissioning systems. Results of the present work along with our earlier work for heavy mass chains show smaller value of σζ for NP = 50 (A = 84—90) and ZP = 50, NF = 82 (A = 128-136) indicating an effect of shell closure proximity where as the oscillating nature of σζ and ΔΖ in the interval of five mass units indicates a proton pairing effect. Besides the oscillating nature, the ΔΖ values decrease systematically with the approach of symmetric split which is characteristic of low energy fission.