Independent Isomeric Yield Ratios Research Articles

Fragment angular momenta in alpha-induced fission of 238U.

Fragment angular momenta are deduced from the radiochemically determined independent isomeric yield ratios of the antimony isotopes $^{126}$,128,130Sb in the alpha-particle-induced fission of $^{238}\mathrm{U}$ at various initial excitation energies (18.3--30.2 MeV) and angular momenta (7.86--13.9\ensuremath{\Elzxh}). It is found that the fragment angular momenta increase with increasing excitation energy and angular momentum of the fissioning nucleus. Calculations of fragment angular momenta are performed based on the Fermi gas approximation for the fissioning nucleus, taking into account the contribution of fissioning nucleus' angular momentum into that of the fragment. The calculated values agree reasonably well with the experimental values indicating that the fragment angular momenta increase with the increasing initial excitation energy and angular momentum of the fissioning nucleus, with the former playing the dominant role through increased fragment moment of inertia and temperature.

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.

Isomeric yield ratio of146La and84Br in the thermal neutron induced fission of235U

The independent isomeric yield ratio of146La and84Br in the thermal neutron fission of235U is reported for the first time with the values of 0.058±0.017 and 0.62±0.20, respectively. the yields have been determined using a fast radiochemical separation technique followed by γ-spectroscopy. The deduced rms angular momentum of84Br is 5.9 ℏ from the statistical model analysis and the rms angular momentum of146La is found too low to be determined.

Influence of collective rotational degrees of freedom in medium energy fission

In the present work rms angular momenta have been deduced for the fission fragments corresponding to131Tem,g and133Tem,g in232Th(α 40 MeV,f) and238U(α 40 MeV,f) systems from the radiochemically determined independent isomeric yield ratios and statistical model based analysis. For131Te and133Te the rms angular momenta deduced are 5.9±1.0 and 7.9±1.2 ħ respectively in232Th(α 40 MeV,f) and 7.2±0.6 and 8.0±0.8 ħ respectively in238U(α 40 MeV,f). Comparison of the present data with the literature data for these fragments in the same compound nuclei236U* and242Pu* at lower excitation energies shows increase in the fragment angular momentum with increasing excitation energy and angular momenta of the fissioning nuclei. Fragment angular momentum deduced theoretically for asymmetric and deformed fragments on the basis of thermal equilibration of the collective rotational degrees e.g., rigid rotation, wriggling, tilting, bending and twisting modes considering the effect of multichance fission, are in good agreement with the experimental observations.

Fragment angular momenta in low and medium energy fission of242Pu

Independent isomeric yield ratios of128Sb were determined radiochemically in the thermal neutron induced fission of241Pu and 34 MeV alpha particle induced fission of238U, both involving the same compound nucleus (242Pu). Fragment angular momenta estimated from the measured isomer ratios using the statistical model analysis showed significantly larger fragment angular momenta in the medium energy fissioning system compared to the low energy fissioning system. This has been attributed to the effect of higher excitation energy and angular momentum in the entrance channel leading to increased fragment temperature, moments of inertia and angular velocity. An attempt was made to calculate the fragment angular momentum in the medium energy fission using the Fermi gas model for the fissioning nucleus, taking into account the multichance fission, saddle shapes of the fissioning nuclei and the angular velocity components of the fissioning nuclei both along and orthogonal to the fission axis. The calculated angular momenta agree well with the experimental results.

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.

Influence of fission fragment nuclear structure on scission configuration in252Cf (S.F.)

With the aim of investigating the influence of fragment nuclear structure e.g. shell closure configuration and odd-even nature on the scission configuration, fragment angular momenta have been deduced from the radiochemically determined independent isomeric yield ratios of the fission products111Pd,131Te,133Te,134I and138Cs in252Cf spontaneous fission. The fragments' angular momenta deduced are 8.8± 1.5, 5.8 ±1.0, 6.1±1.1, 11.5±1.1 and 9.8±1.2 ħ respectively.

Isomeric yield ratio of fission product148Pr in235U(n th,f)

The independent isomeric yield ratio of148Pr in thermal neutron induced fission of235U has been determined experimentally. The fission product148Pr isomers, extracted directly by on-line mass separation technique, have high-spin (J=4) to low-spin (J=1) isomer ratio of 0.14±0.04 using growth and decay analysis. Statistical model calculation of isomeric yeild ratio using constant initial r.m.s. angular momentumJrms can not reproduce either present results or other recent measurements of isomer ratios. TheJrms derived from isomer ratio data in all thermal fissioning systems indicate a wide spread ranging from 2ħ to 13ħ. No clear correlation betweenJrms and isomeric spins or number of neutrons of isomers is found, thus, more model refinements and experimental works should be done in order to evaluate independent isomeric yields correctly.

Independent isomeric yield ratios and primary angular momenta in the photofission ofU235,238with 12-30-MeV bremsstrahlung

The independent isomeric yield ratios for $^{129}\mathrm{Sn}$, $^{130}\mathrm{Sb}$, $^{131}\mathrm{Sn}$, $^{132}\mathrm{Sb}$, $^{133}\mathrm{Te}$, and $^{136}\mathrm{I}$ for the photofission of $^{235}\mathrm{U}$ with 12-30-MeV bremsstrahlung and for $^{126}\mathrm{Sb}$, $^{129}\mathrm{Sn}$, $^{130}\mathrm{Sb}$, $^{131}\mathrm{Sn}$, $^{132}\mathrm{Sb}$, $^{133}\mathrm{Te}$, and $^{136}\mathrm{I}$ for the photofission of $^{238}\mathrm{U}$ have been determined using radiochemical techniques and gamma spectrometry of fission product catcher foils and irradiated uranium samples. The root-mean-square values of the primary angular momenta of the corresponding fission fragments, ${J}_{\mathrm{rms}}$, were calculated with the statistical procedure of Huizenga and Vandenbosch and with the more elaborate deexcitation model of Min and Martinot. Both procedures gave, except for $^{130}\mathrm{Sb}$, almost the same ${J}_{\mathrm{rms}}$ values. Our experimental results show a near independency of ${J}_{\mathrm{rms}}$ on the spin and excitation energy of the compound nucleus. An increase of the ${J}_{\mathrm{rms}}$ values with the excitation energy of the fragments is observed. In addition, a significant proton odd-even effect on the ${J}_{\mathrm{rms}}$ values is present for the photofission of $^{235}\mathrm{U}$ as well as $^{238}\mathrm{U}$. Our experimental photofission results show a qualitative agreement with the theoretical calculations of Dietrich and Zielinska-Pfab\'e.

Dependence of fragment angular momentum on entrance channel inU236fission

In the present work the influence of initial excitation energy and angular momentum of the fissioning system on the angular momenta of fission fragments corresponding to $^{131}\mathrm{Te}$ and $^{133}\mathrm{Te}$ was studied in the $^{232}\mathrm{Th}({\ensuremath{\alpha}}_{30\mathrm{MeV}}, f)$ system. The fragment angular momenta were estimated from the radiochemically determined independent isomeric yield ratios using a statistical model. The obtained data are discussed along with the literature data on the fragment angular momenta in the same fissioning nucleus $^{236}\mathrm{U}$ formed in the $^{235}\mathrm{U}({n}_{\mathrm{th}}, f)$ and $^{232}\mathrm{Th}(\ensuremath{\alpha}, f)$ systems.NUCLEAR REACTIONS $^{232}\mathrm{Th}({\ensuremath{\alpha}}_{30\mathrm{MeV}}, f)$ reaction, fragment angular momentum, independent isomeric yield ratio, initial excitation energy, angular momentum, single particle effect.

Independent isomeric yield ratio ofI134in the photofission ofU235andU238

The independent isomeric yield ratio of $^{134}\mathrm{I}$ has been determined radiochemically for the photofission of $^{235}\mathrm{U}$ and $^{238}\mathrm{U}$ with bremsstrahlung with end-point energies ranging from 12 to 30 MeV. The root-mean-square values of the angular momentum of the corresponding fission fragments, ${J}_{\mathrm{rms}}$, calculated using a statistical deexcitation model show an independence on the compound nucleus angular momentum and excitation energy. This ${J}_{\mathrm{rms}}$ behavior is compared to the results obtained in thermal neutron induced and medium-energy particle induced fission.NUCLEAR REACTIONS, FISSION $^{235,238}\mathrm{U}$($\ensuremath{\gamma}$,$F$), ${E}_{\ensuremath{\gamma}}max=12, 15, 20, 30$ MeV; measured $^{134}\mathrm{I}$ independent isomeric yields; deduced angular momenta.

Fission fragment angular momentum: Ratios of independent yields of isomers ofNb95andI132in thermal-neutron-induced fission ofU233

Independent isomeric yield ratios of the fission products $^{95}\mathrm{Nb}$ and $^{132}\mathrm{I}$ in thermal-neutron-induced fission of $^{233}\mathrm{U}$ have been determined radiochemically. The determined ratios are 0.248 \ifmmode\pm\else\textpm\fi{} 0.029 and 0.268 \ifmmode\pm\else\textpm\fi{} 0.084 for the isomers of $^{95}\mathrm{Nb}$ and of $^{132}\mathrm{I}$, respectively. On the basis of these yield ratios the average angular momenta of the primary fragments, respective of the fission products $^{95}\mathrm{Nb}$ and $^{132}\mathrm{I}$, have been estimated using the statistical model formalism. The estimated values are ${6.35}_{\ensuremath{-}0.95}^{+1.25}\ensuremath{\hbar}$ and $5.7\ifmmode\pm\else\textpm\fi{}1.2 \ensuremath{\hbar}$ for $^{95}\mathrm{Nb}$ and $^{132}\mathrm{I}$, respectively. These data along with other literature data show that fragment angular momenta decrease as proton number deviates from spherical magic shell 50 for given neutron number. Besides, fragment angular momenta do not correlate with prompt neutron number.

Independent isomeric yield ratios of 90Rb and 138Cs in the spontaneous fission of 252Cf

Independent isomeric yield ratios of 90Rb(1−,4−) and 138Cs(3−,6−) in the spontaneous fission of 252Cf were measured by the radiochemical method. A gas-recoil-transport-system was used for the fast and continuous separation of Rb and Cs from their noble gas precusors Kr and Xe, respectively, in the decay chain. The mean angular momentum of the primary fragments was deduced from the isomeric yield ratios. The results are compared with the results of a previous study. The results indicate that the mean angular momentum of fission fragments is nearly independent of fragment mass.

Independent isomeric yield ratio of 148Pm in the thermal-neutron induced fission of 233U

Fractional independent yields of 41·3-d 148m Pm (6-) and 5·37-d 148g Pm (1-) have been determined in the thermal-neutron induced fission of 233U by radiochemical techniques. Calculation based on a statistical theory of the spin distribution of the primary fragment has been made for the isomer ratio. A value of mean angular momentum of the primary fragment, J ̄ = 8·3 ± 2·0 h ̵ , was obtained by comparing the experimental isomer ratio, σ m ( σ m + σ g ) = 0·80±0·06, with the calculated values. The magnitude of the fragment angular momentum agreed well with a predicted value resulting from calculation based on the statistical model using the fragment excitation energy which has been estimated from the experimental data of the prompt neutron and gamma-ray emissions.