Experimental study of nuclear fission along the thorium isotopic chain: From asymmetric to symmetric fission

The inverse kinematics technique, applied to radioactive beams and combined to the Coulomb excitation method, is a powerful tool to study low-energy fission. A novel experimental setup was developed within the R3B/SOFIA (Reactions with Relativistic Radioactive Beams/Studies On FIssion with Aladin) collaboration to identify in mass and atomic numbers both fission fragments in coincidence. These new data provide elemental, isobaric, and isotonic yields for the fission along the thorium isotopic chain. Results are also compared to previous measurements using either the same reaction mechanism or thermal-neutron induced fission. This latter comparison permits to probe the influence of the excitation energy in the fission process.

Figure 1

Schematic view of the FRS and of the first part of the setup, dedicated to the identification of the secondary beams.

Figure 2

Correlation plot between the two ionic charge-state measurements of the secondary beam, obtained from the FRS setting centered around ${}^{226}\mathrm{Th}$. The three lines represent the three main combinations of charge states in both MUSIC detectors.

Figure 3

Identification plot of the cocktail beams centered around ${}^{222,226,230}\mathrm{Th}$. Only events leading to fission in the active target (anodes or cathodes) or in the exit window of the second MUSIC are selected. Other events are rejected by the trigger (if no fission occurs) or by the analysis.

Figure 4

The total electromagnetic differential cross section (first raw), the fission probability (second raw), and the excitation function (third raw) are plotted as function of the excitation energy for the following fissioning nuclei: ${}^{220}\mathrm{Ac}$ (first column), ${}^{222}\mathrm{Th}$ (second column), ${}^{226}\mathrm{Th}$ (third column), and ${}^{228}\mathrm{Pa}$ (fourth column). The total electromagnetic differential cross sections are calculated for a beam at the energy ${\langle E\rangle }_{MOT}$ given in Table 1 on a ${}^{238}\mathrm{U}$ target with the parametrizations, from [19] (red full line), and from the hydrodynamic model (blue dashed line). The fission probabilities are calculated by GEF. The neutron separation energy ($S_n$) is indicated by an arrow.

Figure 5

Schematic view of the R3B/SOFIA setup dedicated to the identification of both fission fragments in coincidence.

Figure 6

Variation of the efficiency with the nuclear charge. Error bars originate from the statistical uncertainty of the data.

Figure 7

Elemental distributions measured for ${}^{226}\mathrm{Th}(\gamma ,f)$.

Figure 8

Isobaric (a) and isotopic (b) distributions measured for ${}^{226}\mathrm{Th}(\gamma ,f)$.

Figure 9

Illustration of the analysis to infer the isotonic yields from the electromagnetically induced fission of the ${}^{226}\mathrm{Th}$. Rebinned data, normalized to 200% are represented in red. Results from the multi-Gaussian fit in black. (a) relative uncertainties: the red full squares represent the statistical, whereas the black open squares the systematic extracted from the fit. (b) Isotonic yields.

Figure 10

Comparison of the elemental yields for the thorium isotopes measured in this work (red full lines) and those from Ref. [1] (blue dashed line).

Figure 11

Isobaric yields for the thorium isotopes with statistical errors (vertical black lines) and systematic uncertainties (blue shaded areas) due to the fit procedure.

Figure 12

Isotonic yields for the thorium isotopes with statistical errors (vertical black lines) and the systematic uncertainties (blue shaded areas) due to the fit procedure.

Figure 13

Comparison of the local even-odd staggering (a) and elemental yields (b) for the fission of the ${}^{230}\mathrm{Th}$ fissioning nucleus. Represented by a red full line, ${}^{230}\mathrm{Th}$ is populated via the $(\gamma ,f)$ reaction in inverse kinematics (this work). The purple dotted [10] and blue dashed lines [11], represent fission of ${}^{230}\mathrm{Th}$ populated by the ${}^{229}\mathrm{Th}(n_{th},f)$ reaction in direct kinematics.

Figure 14

Comparison of isobaric yields for the fission of the ${}^{230}\mathrm{Th}$ fissioning nucleus. For red full line, ${}^{230}\mathrm{Th}$ is populated by the $(\gamma ,f)$ reaction in inverse kinematics (this work). For blue dashed line [11], ${}^{230}\mathrm{Th}$ is populated by the ${}^{229}\mathrm{Th}(n_{th},f)$ reaction in direct kinematics.

Figure 15

Isotopic distribution of the light fission fragments produced in ${}^{230}\mathrm{Th}(\gamma ,f)$.

Figure 16

Measured elemental distributions of the fission fragments from electromagnetic-induced fission of ${}^{219,220,223,227}\mathrm{Ac}$, ${}^{221,222,223,225,226,229,230}\mathrm{Th}$, and ${}^{228,229}\mathrm{Pa}$.

Figure 17

Comparison of the elemental yields measured in this work (in red) and in Ref. [1] (in blue dotted line).

Figure 18

Measured isobaric distributions of the fission fragments from electromagnetic-induced fission of ${}^{221,222,223,225,226,229,230}\mathrm{Th}$ and ${}^{228,229}\mathrm{Pa}$.

Figure 19

Measured isotonic distributions of the fission fragments from electromagnetic-induced fission of ${}^{221,222,223,225,226,229,230}\mathrm{Th}$ and ${}^{228,229}\mathrm{Pa}$.

Figure 20

Isobaric yields (left column) and isotonic yields (right column) for the ${}^{221,222,225,226,229,230}\mathrm{Th}$ and ${}^{228}\mathrm{Pa}$ isotopes with the statistical error bars (black errors bars) and the systematic uncertainty (light blue interval) due to the fit procedure.