On the threshold for ion track formation in CaF₂

Abstract

There is an ongoing debate regarding the mechanism of swift heavy ion (SHI) track formation in CaF 2 . The objective of this study is to shed light on this important topic using a range of complementary experimental techniques. Evidence of the threshold for ion track formation being below 3 keV nm −1 is provided by both transmission electron microscopy (TEM) and Rutherford backscattering spectroscopy in the channelling mode, which has direct consequences for the validity of models describing the response of CaF 2 to SHI irradiation. Furthermore, information about the elemental composition within the ion tracks is obtained using scanning TEM, electron energy loss spectroscopy, and with respect to the stoichiometry of the materials surface by in situ time of flight elastic recoil detection analysis. Advances in the analyses of the experimental data presented here pave the way for a better understanding of the ion track formation.

Monitoring Ion Track Formation Using In Situ RBS/c, ToF-ERDA, and HR-PIXE

“…For in situ RBS/c analysis, it is important that repeated RBS/c measurements at the same spot do not introduce additional damage to the material under investigation. Therefore, we performed test RBS/c measurements using a 2 MeV Li probing beam on the ion radiation sensitive CaF 2 crystal, which contained defects introduced previously by 23 MeV I [ 10 ]. This verified that the RBS/c beam does not introduce additional defects after prolonged exposure, and that RBS/c spectra from irradiated samples (containing disorder) can, therefore, be reliably acquired even after multiple probing beam exposures, as shown in Figure 1 b.…”

“…Ion tracks have been investigated for many years, and the development of this research field has been well documented [ 1 , 2 , 3 , 4 ]. Still, many questions related to the basic understanding of ion track formation are open and subject to vigorous scientific investigations [ 3 , 5 , 6 , 7 , 8 , 9 , 10 ]. Clarification of these intensely debated issues would benefit not only basic research but ion track applications as well [ 11 , 12 ].…”

“…Often it is possible to pinpoint exactly the threshold for ion track formation. Practical considerations like limited availability of the beamtime can also play a significant role at large facilities, thus, it is advantageous for complementary investigations at lower energies to be outsourced elsewhere, as documented in our own research [ 10 , 17 , 18 , 19 , 20 , 21 , 22 ]. SHI applications can benefit from this also, for example, hadron therapy using carbon ions typically requires ~100 MeV beams in order to reach the targeted volume.…”

“…Previously, we have shown that ion track formation in GaN [ 18 ] and TiO 2 [ 20 ] is accompanied by a preferential loss of nitrogen and oxygen, respectively. However, ion track formation in CaF 2 does not result in changes of the surface stoichiometry under the same irradiation conditions [ 10 ]. Here, we present results of similar ToF-ERDA measurements made on other track forming materials, namely quartz SiO 2 , amorphous SiO 2 , SrTiO 3 , and muscovite mica.…”

“…Here, we present results of similar ToF-ERDA measurements made on other track forming materials, namely quartz SiO 2 , amorphous SiO 2 , SrTiO 3 , and muscovite mica. The third case is related to the open question about the origin of the velocity effect, i.e., what is the mechanism behind an increasing damage cross section (ion track size) for low velocity SHI irradiation in insulators [ 7 , 9 , 10 ]? Szenes proposed that the damage cross section increase is related to the activation of the Coulomb explosion mechanism due to the high ionization of the matter along the ion trajectory for slower ions [ 9 ].…”