Oxidation State of <sup>229</sup>Th Recoils Implanted into MgF<sub>2</sub> Crystals

Abstract

A solid-state nuclear clock based on the low-lying isomeric state in 229 Th has attracted growing interest. One potential problem for the solid-state nuclear clock approach is the suitability of the doped environment for photon emission of the nuclear isomeric state. Specifically, Th n+ n < 4 ions could open non-radiative decay routes for deexcitation, hindering the photon emission. Here we have used time-resolved photoluminescence (TRPL) and density functional theory (DFT) calculations to characterize MgF 2 crystals that have been implanted with 229 Th recoils via α-decay from a 233 U source with the goal of determining the charge state of the implanted thorium atoms. The DFT calculations predicted Th 4+ to be the lowest energy oxidation state with Th 3+ the next lowest in the MgF 2 crystal environment. The DFT calculations also show Th 4+ :MgF 2 system has a band gap large enough so that the internal electron conversion decay channel is suppressed. Experimentally, we found no evidence for thorium in oxidations state other than +4 using TRPL spectroscopy that has a detection limit for Th n+ n < 4 ions several orders of magnitude smaller than the number of implanted 229 Th recoils. This work shows that the solid-state approach is a viable option for a nuclear clock.

DFT calculation of 229thorium-doped magnesium fluoride for nuclear laser spectroscopy

“…To achieve convergence, SCAN requires a much larger energy cutoff in contrast to LDA, where it was possible to use only 400 eV. In agreement with Barker et al [43], we report that the most favorable configuration is the Mg-vacancy and our calculations also show, that the +2F interstitial charge compensation has almost no energy preference for a linear or bent structure. However, the energy difference between the Mg-vacancy and the +2F configuration is 2.7 eV in our results and thus slightly larger than in the previous study, which reported 2.5 eV.…”

“…Recently, Barker et al [43] performed DFT calculations for thorium-doped MgF 2 . In this work, the preferred charge compensating mechanism was also determined to be the Mgvacancy.…”

“…We calculate energies corresponding to these two doping types using the SCAN functional and we find that the substitutional doping is energetically preferred. Thorium has an oxidation number of +4 [43]. Because a multitude of different possibilities for charge compensation schemes can be Table 1.…”

The $$^{229}$$Th isomer: prospects for a nuclear optical clock

“…Spectral filters can be used to obtain information about the emitted photon energies. Experiments along this line were carried out since 2007 at various institutes around the world [9,53,192,343,413,419,422], however with negative results. A direct detection of the isomeric decay by this method was reported in 2012 [419].…”

“…These crystals are expected to also provide the basis for solid-state nuclear frequency standards [179]. Significant progress has been made in crystal development and theoretical understanding [9,79,85,107,127,150,178,262,287,316,335,337]. Two experimental results using synchrotron radiation have been reported, which were, however, negative: the first one performed by UCLA, using the Advanced Light Source (ALS) of the Lawrence Berkeley National Laboratory [151,152] the second one performed by the Technical University of Vienna, using the Metrology Light Source (MLS) in Berlin, Germany [338].…”