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 $\gamma$-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 ${{}^{230}\mathrm{Th}}^{\ast }$, ${{}^{234}\mathrm{U}}^{\ast }$, ${{}^{236}\mathrm{U}}^{\ast }$, ${{}^{240}\mathrm{Pu}}^{\ast }$, ${{}^{242}\mathrm{Pu}}^{\ast }$, ${}^{244}\mathrm{Cm}$(SF), ${{}^{246}\mathrm{Cm}}^{\ast }$, ${{}^{250}\mathrm{Cf}}^{\ast }$, 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 (${\delta }_p$) which decreases significantly with the increase of Coulomb parameter ($Z_F^2/A_F^{1/3}$).

Figure 1

The plot of yield-weighted average fragment angular momenta as a function of the atomic number of various elements in fifteen different even-$Z$ fissioning systems.

Figure 2

Effect of the spherical 82-neutron shell on the fragment angular momentum in various fissioning systems.

Figure 3

Effect of deformed 88- and 66-neutron shell on the fragment angular momentum in ${}^{252}\mathrm{Cf}$(SF). The arrows indicate the corresponding axis.

Figure 4

Plot of fragment angular momentum and average neutron number emitted as a function of mass number of different fission products in ${}^{235}\mathrm{U}$($n_{\mathrm{th}},f$ ) and ${}^{252}\mathrm{Cf}$(SF). (The dotted line represents the neutron emission curve, and the other symbols are for the fragment angular momentum. In ${}^{235}\mathrm{U}$($n_{\mathrm{th}},f$) the triangles joined with the solid line are for ${}^{95-98}\mathrm{Y}$ and ${}^{123-129}\mathrm{In}$. The filled triangles are for ${}^{128}\mathrm{In}$ and ${}^{128}\mathrm{Sb}$. The open squares are for ${}^{130}\mathrm{Sn}$ and ${}^{130}\mathrm{Sb}$. The filled squares are for ${}^{132}\mathrm{Sb}$, ${}^{132}\mathrm{Te}$, and ${}^{132}\mathrm{I}$. The inverted triangles are for ${}^{133}\mathrm{Te}$ and ${}^{133}\mathrm{Xe}$. In ${}^{252}\mathrm{Cf}$(SF) the filled circles are for ${}^{132}\mathrm{Sb}$ and ${}^{132}\mathrm{I}$).

Figure 5

Expanded portion of the plot of the fragment angular momentum as a function of mass number in the mass region 120–140 for the ${}^{235}\mathrm{U}$($n_{\mathrm{th}},f$) reaction.

Figure 6

Correlation of the yield-weighted average fragment angular momenta and the elemental yields in fifteen different even-$Z$ fissioning systems. (The dashed line is for yield-weighted average fragment angular momenta and the solid line is for elemental yields.)