Spectroscopy of the neutron-rich actinide nucleus ${}^{240}\mathrm{U}$ following multinucleon-transfer reactions

Background: Nuclear structure information for the neutron-rich actinide nuclei is important since it is the benchmark for theoretical models that provide predictions for the heaviest nuclei.

Purpose: $\gamma$-ray spectroscopy of neutron-rich heavy nuclei in the actinide region.

Method: Multinucleon-transfer reactions in ${}^{70}\text{Zn}+{}^{238}\text{U}$ and ${}^{136}\text{Xe}+{}^{238}\text{U}$ have been measured in two experiments performed at the INFN Legnaro, Italy. In the ${}^{70}\text{Zn}$ experiment the high-resolution HPGe Clover Array (CLARA) coupled to the magnetic spectrometer PRISMA was employed. In the ${}^{136}\text{Xe}$ experiment the high-resolution Advanced Gamma Tracking Array (AGATA) was used in combination with PRISMA and the Detector Array for Multinucleon Transfer Ejectiles (DANTE).

Results: The ground-state band (g.s. band) of ${}^{240}\text{U}$ was measured up to the ${20}^{+}$ level and a tentative assignment was made up to the $\left({24}^{+}\right)$ level. Results from $\gamma \gamma$ coincidence and from particle coincidence analyses are shown. Moments of inertia (MoI) show a clear upbend. Evidence for an extended first negative-parity band of ${}^{240}\text{U}$ is found.

Conclusions: A detailed comparison with latest calculations shows best agreement with cranked relativistic Hartree-Bogoliubov (CRHB) calculations for the g.s. band properties. The negative-parity band shows the characteristics of a $K^{\pi }=0^{-}$ band based on an octupole vibration.

Figure 1

Single $\gamma$-ray spectra for ${}^{68}\text{Zn}$ identified in PRISMA. The corresponding binary partner of the reaction is ${}^{240}\text{U}$. The spectra are Doppler corrected for the targetlike actinide nuclei. The inset shows the TKEL value in arbitrary units divided in three regions: 1, 2, and 3. The color code of the $\gamma$-ray spectra corresponds to the three different TKEL regions.

Figure 2

Particle gated coincidence spectra for ${}^{240}\text{U}$ from the CLARA experiment. Projection on one axis of the $\gamma \gamma$ matrix (a), gate on 162 keV (b), gate on 215 keV (c), gate on 264 keV (d), gate on 307 keV (e), gate on 347 keV (f), gate on 381 keV (g), and the sum of all the shown gated spectra (h).

Figure 3

Two-dimensional (2D) histogram of $\mathrm{\Delta }\mathrm{ToF}$ and TKEL values for all events with ${}^{134}\text{Xe}$ identified in PRISMA. The 2D gate selecting primarily MNT events is plotted as a solid black line.

Figure 4

Doppler-corrected single $\gamma$-ray spectrum for ${}^{238}\text{U}$ gated by ${}^{136}\text{Xe}$ identified in PRISMA. Beside the applied gate for MNT an additional cut on the TKEL value was performed (see black region in inset).

Figure 5

Doppler-corrected single $\gamma$-ray spectrum for ${}^{240}\text{U}$ gated by ${}^{134}\text{Xe}$ identified in PRISMA. Beside the applied gate for MNT (see Fig. 3) an additional cut on the TKEL value was performed (see black region in inset).

Figure 6

Proposed extended level scheme for ${}^{240}\text{U}$. Spin and parity assignments are taken from Ref. [11] or based on $\gamma \gamma$-coincidence relationships. Tentative assignments are given in brackets.

Figure 7

Comparison of experimentally determined level energies with theoretical predictions. (a) Data from this work. (b) Theoretical prediction from cluster model [3], and from a macroscopic-microscopic approach [2] with (c) dynamical coupling or (d) $I(I+1)$ sum rule.

Figure 8

Fits employing the Harris parametrization of $J_{\mathrm{kin}}$ and $J_{\mathrm{dyn}}$ for the U isotopic chain from ${}^{236}\text{U}$ to ${}^{240}\text{U}$. Data for $A=236$ and 238 are taken from Ref. [31].

Figure 9

Kinetic MoI, $J_{\mathrm{kin}}$, from this work (red [gray] points) in comparison to various theoretical predictions. The CRHB + LN(NL1) and CRHB + $\mathrm{LN}(\mathrm{NL}{3}^{*}$) calculations by Afanasjev et al. best reproduce the experimental data. The experimental values for the decays of the $4^{+}$ and $2^{+}$ g.s. band states were taken from the literature [11, 28, 29].

Figure 10

(a) Staggering $S\left(I\right)$ in the three uranium isotopes ${}^{236}\text{U},{}^{238}\text{U,}$ and ${}^{240}\text{U}$. The staggering parameter for ${}^{240}\text{U}$ continues to decrease up to the highest spins while $S\left(I\right)$ saturates in the lighter U isotopes. (b) Ratio of rotational frequencies of the positive- and negative-parity bands as a function of spin. ${}^{236,238}\text{U}$ data taken from Ref. [31].

Figure 11

Experimental data, parametrized as $-ln\mathrm{\Delta }\epsilon \left(I\right)$ vs $I(I+1)/6$ for ${}^{236}\text{U}$ (a), ${}^{238}\text{U}$ (b), and ${}^{240}\text{U}$ (c). Fits with $a=0$ are shown as dashed lines; solid curves include $a$ as a free parameter. ${}^{236,238}\text{U}$ data taken from Ref. [31].