Jevgenij Kusakovskij

Dominant stable radicals in irradiated sucrose: g tensors and contribution to the powder electron paramagnetic resonance spectrum.

Ionizing radiation induces a composite, multiline electron paramagnetic resonance (EPR) spectrum in sucrose, that is stable at room temperature and whose intensity is indicative of the radiation dose. Recently, the three radicals which dominate this spectrum were identified and their proton hyperfine tensors were accurately determined. Understanding the powder EPR spectrum of irradiated sucrose, however, also requires an accurate knowledge of the g tensors of these radicals. We extracted these tensors from angular dependent electron nuclear double resonance-induced EPR measurements at 110 K and 34 GHz. Powder spectrum simulations using this completed set of spin Hamiltonian parameters are in good agreement with experimentally recorded spectra in a wide temperature and frequency range. However, as-yet nonidentified radicals also contribute to the EPR spectra of irradiated sucrose in a non-negligible way.

Probing the local structure of the near-infrared emitting persistent phosphor LiGa5O8:Cr3+

Near-infrared emitting persistent phosphors have promising applications in the field of in vivo medical imaging. In this paper, we prepared the persistent phosphor LiGa5O8:Cr3+ (LGO:Cr) which exhibits emission in the tissue transparency window and shows afterglow for multiple hours after excitation. Addition of Si or Ge improves the persistent luminescence. X-ray diffraction and electron microscopy, coupled with elemental analysis, revealed that there is a maximum amount of Si or Ge that can be incorporated in the host. Via X-ray absorption spectroscopy and electron paramagnetic resonance experiments, we studied the local environment of chromium in the LGO spinel host. The presence of both Cr3+ and Cr4+ on octahedral sites in LGO was confirmed. Electron paramagnetic resonance showed that Cr3+ resides in a rhombically distorted octahedral lattice site and that the Cr3+ local environment is sensitive to variation in point defects in the surrounding.

EMR Study and DFT-Assisted Identification of Transient Radicals in X-Irradiated Crystalline Sucrose.

Solid-state sucrose is a well-known dosimetric system, which is capable of reliable dose estimates only at a considerable time after exposure. Immediately after irradiation at room temperature, its electron paramagnetic resonance (EPR) spectrum is dominated by contributions from unstable radicals, which are studied here using continuous-wave EPR and electron-nuclear double resonance (ENDOR) spectroscopy. Four hyperfine tensors of proton couplings were determined, associated with two radical species, and subsequently compared to density functional theory calculation results, which led to the identification of the species with lower abundance (U2) as a radical formed by a H abstraction from C4. The more abundant center (U1) has not been definitively identified yet, but we present compelling evidence that it should be a C6 centered radical. Comparison of the simulated EPR spectra with all available data to the experimental ones suggests that the EPR spectrum of X-irradiated sucrose immediately after irradiation can now be almost entirely understood.

Understanding the dosimetric powder EPR spectrum of sucrose by identification of the stable radiation-induced radicals.

Sucrose, the main component of table sugar, present in nearly every household and quite radiation sensitive, is considered as an interesting emergency dosemeter. Another application of radiation-induced radicals in sugars is the detection of irradiation in sugar-containing foodstuffs. The complexity of electron paramagnetic resonance (EPR) spectra of radicals in these materials, as a result of many hyperfine interactions and the multi-compositeness of the spectra of individual sugars, complicate dose assessment and the improvement of protocols for control and identification of irradiated sugar-containing foodstuffs using EPR. A thorough understanding of the EPR spectrum of individual irradiated sugars is desirable when one wants to reliably use them in a wide variety of dosimetric applications. Recently, the dominant room temperature stable radicals in irradiated sucrose have been thoroughly characterised using EPR, electron nuclear double resonance (ENDOR) and ENDOR-induced EPR. These radicals were structurally identified by comparing their proton hyperfine and g-tensors with the results of Density Functional Theory calculations for test radical structures. In this paper, the authors use the spin Hamiltonian parameters determined in these studies to simulate powder EPR spectra at the standard X-band (9.5 GHz), commonly used in applications, and at higher frequencies, up to J-band (285 GHz), rendering spectra with higher resolution. A few pitfalls in the simulation process are highlighted. The results indicate that the major part of the dosimetric spectrum can be understood in terms of three dominant radicals, but as-yet unidentified radicals also contribute in a non-negligible way.

ENDOR-induced EPR of disordered systems : application to X-irradiated alanine

The electron paramagnetic resonance (EPR) spectra of radiation-induced radicals in organic solids are generally composed of multiple components that largely overlap due to their similar weak g anisotropy and a large number of hyperfine (HF) interactions. Such properties make these systems difficult to study using standard cw EPR spectroscopy even in single crystals. Electron-nuclear double-resonance (ENDOR) spectroscopy is a powerful and widely used complementary technique. In particular, ENDOR-induced EPR (EIE) experiments are useful for separating the overlapping contributions. In the present work, these techniques were employed to study the EPR spectrum of stable radicals in X-irradiated alanine, which is widely used in dosimetric applications. The principal values of all major proton HF interactions of the dominant radicals were determined by analyzing the magnetic field dependence of the ENDOR spectrum at 50 K, where the rotation of methyl groups is frozen. Accurate simulations of the EPR spectrum were performed after the major components were separated using an EIE analysis. As a result, new evidence in favor of the model of the second dominant radical was obtained.