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One of the major scientific interests in the behaviour of He in diamond is due to the belief that the amount of $^{4}$He and the $^{3}$He /$^{4}$He ratio found within the material or its inclusions can be used to date terrestrial diamonds [1,2] or learn about the origins of meteoritic nanodiamonds [3]. Recently, He implantation has also been found to create colour centers in diamond that act as single photon emitters [4]. Among the issues of interest are the lattice location of He, which also concerns the one following ion implantation, since part of He is introduced into the material due to the alpha decay from the U and Th decay chains, as well as its diffusion behaviour. The latter is of particular relevance since possible out-diffusion of He at elevated temperatures (typically above 900°C) and on geological time scales (10$^{9}$ years) could alter the outcome of dating experiments.
In this contribution we report on the lattice location of the short-lived ion implanted nuclear probe $^{6}$He (t$_{1/2}$=807 ms), which was performed using the beta emission channeling method at CERN’s ISOLDE facility. $^{6}$He was implanted into an artificial diamond sample with 30 keV at room temperature and up to 800°C. By means of comparing the measured emission channeling patterns along different crystallographic directions with simulated yields for a variety of possible lattice sites, we conclude that all of the implanted $^{6}$He occupies tetrahedral (T) interstitial sites, in agreement with theoretical predictions that T sites should be the preferred positions of He in diamond [5-8]. Implantation at 800°C resulted in a drop in the tetrahedral interstitial fraction by ~20%, which we interpret as the onset of diffusion of $^{6}$He, thus being able to reach the surface of the sample or escaping to the bulk during its lifetime. From this we can estimate that the activation energy for interstitial migration of He is around 1.73 eV, which roughly agrees with theoretical predictions of 2.35 eV [5] and 1.97 eV [6]. Activation energies around 2 eV would mean that simple interstitial He cannot be stable in diamond on geological time scales, thus to remain inside it should be bound to some defect in the material or exist in another form such as within inclusions of other minerals or liquids, or possibly small He bubbles.
[1] S. Basu, et al, “An overview of noble gas (He, Ne, Ar, Xe) contents and isotope signals in terrestrial diamond”, Earth-Science Reviews 126 (2013) 235.
[2] Y. Weiss, et al, “Helium in diamonds unravels over a billion years of craton metasomatism”, Nat. Comm. 12 (2021) 2667.
[3] A.P. Koscheev, et al, “History of trace gases in presolar diamonds inferred from ion-implantation experiments”, Nature 412 (2001) 615.
[4] G. Prestopino, et al, “Photo-physical properties of He-related color centers in diamond”, Appl. Phys. Lett. 111 (2017) 111105.
[5] J.P. Goss, et al, “Density functional simulations of noble-gas impurities in diamond”, Phys. Rev. B 80 (2009) 085204.
[5] D.J. Cherniak et al, “Diffusion of helium, hydrogen and deuterium in diamond: Experiment, theory and geochemical applications”, Geochimica et Cosmochimica Acta 232 (2018) 206.
[7] A. Aghajamali, et al, “Molecular Dynamics Approach for Predicting Release Temperatures of Noble Gases in Presolar Nanodiamonds” Astrophys. J. 916 (2021) 85.
[8] R.A. Beck, et al, X. Li, “Electronic Structures and Spectroscopic Signatures of Noble-Gas-Doped Nanodiamonds”, ACS Phys. Chem. Au 3 (2023) 299.
