Speaker
Description
The $^{229}$Th nucleus can be excited to a nuclear isomer state with an extremely low excitation energy of $8.28\pm 0.17$ eV. This excited state is within range of current laser technologies, making it an ideal candidate for optical nuclear clock applications. However, the production of such a nuclear clock is hindered by that same low excitation energy, which is of the order of typical electronic shell transitions and larger than the first ionization potential, allowing for internal conversion (IC) to be the dominant decay channel. The implementation of an IC blocking mechanism would therefore enable further studies of the isomer radiative decay and of its exploitation in the context of optical nuclear clock applications. Based on density functional theory calculations, it has been proposed that IC blocking can be achieved by incorporating the $^{229}$Th atoms into a CaF$_2$ crystal, occupying substitutional Ca sites [1]. Since this configuration preserves the large bandgap of CaF$_2$ (12 eV), without formation of gap states, electronic transitions at the isomer energy are forbidden, thus blocking IC.
In order to determine the lattice location of $^{229}$Th, $^{229}$Ac$^+$ ions were implanted (at 30 keV) into a CaF$_2$ single crystal at the EC-SLI set-up. The $\beta^-$ emission channelling patterns from $^{229}$Ac were measured in the vicinity of the CaF$_2$ $\langle 211\rangle$, $\langle 111\rangle$, $\langle 100\rangle$ and $\langle 110\rangle$ directions. Because the $^{229}$Th daughter nuclei are recoiled with an energy of only $2.3$ eV, below typical threshold displacement energies, they are expected to occupy the same lattice sites as those determined for $^{229}$Ac. Preliminary analysis shows that the majority (at least 75 %) of the $^{229}$Ac atoms occupy Ca substitutional sites. In addition, thermal annealing and high temperature implantation are observed to affect the $^{229}$Ac root-mean-square displacement from the ideal Ca substitutional site, which suggests that additional lattice defects (e.g. neighboring F vacancies) may be involved. We will discuss to what extent these high-temperature processes can be exploited to optimize the Ca substitution, towards future studies of the isomer radiative decay and of its exploitation in the context of optical nuclear clock applications.
[1] P. Dessovic et al. , J. Phys.: Condens. Matter 26, 105402 (2014).