T. Datta

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.

Nature of the superconducting transition in high Tc Tl-Ca-Ba-Cu-O compounds: Positron annihilation studies

Abstract Doppler broadened annihilation radiation (DBAR) spectra have been recorded as a function of temperature for three TlCaBaCuO superconducting samples, namely A: mixed phase (2223 + 2122 with T c = 125 K, B: single phase (2122) with T c = 108 K, C: argon annealed analogue of B with T c = 95 K. The two component analyses of these spectra bring out certain interesting features. For all the samples the temperature dependent parameters T B (broad component width) and I N (narrow component intensity) were found to show a sharp maximum at respective T c s, indicating an increase in the oxygen valence electron momentum and increased electron density at oxygen vacancy/defect sites. On cooling below T c these parameters were found to decrease monotonically except for the T B of the mixed phase sample A which shows another maximum at 100 K - a manifestation of the presence of the 2122 phase. The observation of higher I N values for sample A compared to sample B and C is probably due to additional CuO, CaO planes present in the former providing a higher number of oxygen vacancies/defect sites. The results are discussed.

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.

Positron annihilation spectroscopy in YBa2Cu3O7−x

Abstract Doppler-broadened annihilation radiation (DBAR) spectra have been measured in the high-Tc orthorhombic YBa2Cu3O7−x (superconductor) and a similar tetragonal YBCO compound (non-superconductor) as a function of temperature between 77 K and 300 K. Analyses of these spectra bring out certain interesting features. Two components are present in both the cases. A narrow component of near constant width ∼2.4 keV for the superconductor and ∼1.9 keV for the nonsuperconductor are seen to arise due to trapped positrons in defects and oxygen vacancies. In addition, a broad component of width ∼3.6 keV arises due to annihilation with 2s+2p electrons of oxygen ions in the non-superconductor at all temperatures and in the superconductor above 105 K. In the superconducting state two drastic changes occur: (a) width of the broad component shows a sharp increase on cooling from 105 K to 77 K, showing an increase in momentum of the oxygen ion electrons; (b) the intensity of the narrow component increases with decreasing temperature, probably due to a structural change leading to enhancement of trapping sites and/or increase in electron density at the vacancy sites. These changes seem to be closely associated with the superconducting phase transition.

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.

Positron annihilation studies of Bi2CaSr2Cu2Ox and Bi1.6Pb0.4Ca2Sr2Cu3Oy in the region of the superconducting transition

Abstract Doppler broadened annihilation radiation (DBAR) spectral parameters have been reported- for the first time- between 77 K and 300 K, for several Bi-based oxide superconductors, viz. A: single phase (2122) Bi2CaSr2Cu2Ox with Tc=85 K (R=0), B: a mixed phase lead doped sample containing both 2122 and 2223 with a nominal composition Bi1.6Pb0.4Ca2Sr2Cu3Oy, and, C: another 2122+2223 sample with same nominal composition as that of B but synthesised under a different heat-treatment schedule so as to yield a Tc=85 K (R=0). Analyses of these spectra using PAACFIT program yielded two components, of which the intensity of the narrow component, IN, and, the width of the broad component, TB, were seen to be the only temperature dependent parameters. At the onset of superconducting transition both TB and IN were seen to increase to a maximum value and decrease on further cooling. A double peak structure in TB vs temperature profile were observed in sample B and C, similar to one reported by us in Tl-Ca-Ba-Cu-O systems. In addition, presence of a magnetic field (1 KG) yielded no significant change in the DBAR spectral parameters. The results are discussed.

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.

Mass asymmetry dependence of angular distribution in237Np(α29 and 44 MeV,f): single particle effect

Angular distributions of fission products have been measured as a function of mass asymmetry in the odd-Z237Np(α29 and 44 MeV,f) system using a recoil-catcher technique and off-line gamma spectrometry. Higher angular anisotropies were observed for the asymmetric mode products compared to the symmetric mode products at both energies. Average anisotropies for individual modes are lower than those for neighbouring even-even fissioning nucleus242Pu due to odd-nucleon spin effect. Present data have been analysed according to the transition state model assuming two modes of fission with characteristic saddle-shapes, barriers and multichance fission probabilities. It is seen that angular distributions for the symmetric and asymmetric modes are decided at and well past the corresponding saddle points respectively. Odd-nucleon spin contribution (〈k2〉) to the tilting mode variance have been deduced. For (241)Am fission, 〈k2〉 values for the asymmetric and symmetric modes are ≤ 14 and > 14 ħ2 respectively. The 〈k2〉 value averaged over several nuclei from preactinide (201Ti) to actinide (248Cf) is 11.5 ± 4.2 ħ2. Average 〈k2〉 value is in close agreement with the theoretical estimate.

Doppler-broadened positron annihilation studies in Y-Ba-Cu-O, Tl-Ca-Ba-Cu-O and Bi-Ca-Sr-Cu-O superconductors

Doppler-broadened annihilation radiation spectra have been measured as a function of temperature from 77 K to 300 K, for several high temperature oxide superconductors viz single-phase YBCO, single- and mixed-phase Tl-Ca-Ba-Cu-O and Bi-Ca-Sr-Cu-O compounds. The temperature-dependent parameters extracted respond to a change at the onset of superconducting transition. The observations point to involvement of oxygen valence electrons at the onset of superconducting transition. Also a possible structural change and/or increase in electron density at the oxygen vacancy/defect sites seem to accompany the transition. In addition, the parameters derived are seen to be sensitive to the presence of more than one superconducting phases in mixed phase samples.