Levels in ${}^{223}\mathrm{Th}$ populated by $\alpha$ decay of ${}^{227}\mathrm{U}$

Levels in ${}^{223}\mathrm{Th}$ populated by the $\alpha$ decay of ${}^{227}\mathrm{U}$ were investigated using $\alpha \text{−}\gamma$ decay spectroscopy. The ${}^{227}\mathrm{U}$ isotope was produced in the fusion-evaporation reaction ${}^{22}\mathrm{Ne}+{}^{208}\mathrm{Pb}$ at the velocity filter separator for heavy-ion reaction products at Gesellschaft für Schwerionenforschung in Darmstadt (Germany). Several new excited levels and $\gamma$ transitions were identified in ${}^{223}\mathrm{Th}$. An improved $\alpha$-decay scheme of ${}^{227}\mathrm{U}$ was suggested. The experimental $\alpha$-decay energy spectrum of ${}^{227}\mathrm{U}$ was compared with the Monte Carlo simulation performed using the toolkit geant4.

Figure 1

(a) Part of the energy spectrum measured in the PSSD in the reaction ${}^{22}\mathrm{Ne}+{}^{208}\mathrm{Pb}$ at a beam energy of 104 MeV. (b) Events from panel (a) detected as $\alpha 1$ decays in the $\alpha 1\left({}^{227}\mathrm{U}\right)\text{−}\alpha 2\left({}^{223}\mathrm{Th}\right)\text{−}\alpha 3\left({}^{219}\mathrm{Ra}\right)$ correlation search. The search conditions were optimized for $\alpha 1$ decays of ${}^{227}\mathrm{U}$ and suppression of other decays. The peak at $\sim 7010$ keV marked by an asterisk is a so-called artificial peak created by the energy summing of $\alpha$ particles and conversion electrons and does not correspond to an $\alpha$ line of ${}^{227}\mathrm{U}$ (see the decay scheme of ${}^{227}\mathrm{U}$ in Fig. 3). The experimental data (black solid line) are compared with the result of the Monte Carlo simulation performed using geant4 [10] (shaded area).

Figure 2

(a) Energy spectrum of $\gamma$ rays detected within a 5-$\mu \mathrm{s}$ coincidence time after the $\alpha 1$ decays of ${}^{227}\mathrm{U}$ [from the $\alpha 1\text{−}\alpha 2\text{−}\alpha 3$ correlation search; see Fig. 1]. The inset shows the higher-energy part of the spectrum where the number of counts are given in logarithmic scale to better present the weak transitions at 395.5, 437.4, and 488.8 keV (marked by italic font). Their positions were deduced based on the $\alpha \text{−}\gamma$ analysis [see panel (b) and the text for details]. (b) $\alpha \text{−}\gamma$ coincidence spectrum showing pairs of $\alpha$ particles (vertical axis) and $\gamma$ rays from panel (a) (horizontal axis) detected within a coincident time of 5 $\mu \mathrm{s}$.

Figure 3

Suggested $\alpha$-decay scheme of ${}^{227}\mathrm{U}$. Uncertainties of all values are given in the text and in Tables 1 and 2. The lettering of the levels on the left side corresponds to the lettering of the $\alpha \text{−}\gamma$ groups in Fig. 2. Spins and parities of the levels and multipolarities of the transitions are tentative.