Sergey D. Chemerisov

Radiation and radical chemistry of NO3(-), HNO3, and dialkylphosphoric acids in room-temperature ionic liquids.

Hydrophobic room-temperature ionic liquids (ILs) are considered as possible replacements for molecular diluents for nuclear separations, as well as the basis of new separations processes. Such applications may put the solvents both in high radiation fields and in contact with aqueous raffinate containing 1-6 M HNO(3). In this study, we address the effect of the extracted nitrate and nitric acid on the radiation chemistry of hydrophobic ILs composed of 1-alkyl-3-methylimidazolium cations (and closely related systems). We demonstrate that the nitrate anion competes with the solvent cation as an electron scavenger, with most of the primary radical species converted to NO(3)(•2-) and NO(2)(•) that initiate a complex sequence of radical reactions. In hydrophobic ILs equilibrated with 3 M HNO(3), nearly all electrons released by the ionizing radiation are converted to NO(2)(•). While the reductive pathway is strongly affected by the nitrate and there is also some N-O bond scission via direct excitation, the extent of interference with the oxidative pathway is relatively small; the cation damage is not dramatically affected by the presence of nitrate as most of the detrimental radiolytic products are generated via the oxidative pathway. These results are contrasted with the behavior of dialkylphosphoric acids (a large class of extraction agents for trivalent metal ions). We demonstrate that IL solvents protect these dialkylphosphoric acids against radiation-induced dealkylation.

Electron trapping and hydrogen atoms in oxide glasses

Trapped hydrogen atoms generated in 3 MeV β-radiolysis of B2O3:OH glass below 140 K were studied using electron paramagnetic resonance (EPR). Two types of trapped H atoms were present in this glass; one was an interstitial atom located in a void between several BOB fragments, another was the atom trapped in a cage between two B3O6 (boroxol) rings connected by hydrogen bonds. The geometry of the trapping site was determined using electron spin echo envelope modulation (ESEEM) spectroscopy. Time-resolved pulsed EPR was used to observe mobile H atoms at 300–500 K. The lifetimes (10–100 μs) of the H atoms were controlled by ∼1018 cm−3 of metastable spin centers. The H atoms migrated with diffusion constant of 1.5×107 cm2/s (activation energy of 0.13–0.16 eV), mean residence time at the site of 4–5 ns, and mean jump length of 0.56 nm (at 300 K). This site-to-site migration causes rapid spin relaxation due to modulation of magnetic interactions, such as dipole–dipole interaction of the unpaired electron of the ...

Probing nanoconfined water in zeolite cages: H atom dynamics and spectroscopy

Time-resolved and CW EPR were used to study radiolytically generated H atoms in water/ice nanoclusters in NaA, NaX, NaY, and HY zeolites. H atoms dynamic properties and spectroscopy parameters are sensitive to the structural changes of water due to the nanoconfinement. Transient H atoms in HY and NaY zeolites segregate into two different domains: sodalite and super cages. In NaX zeolite only H atoms from super cages were observed. H atoms are created in both silica phase and adsorbed water by radiolytic processes. The decay of H atoms occurs predominantly via reaction with the radiation-induced defects in silica.

A Solution-Based Approach for Mo-99 Production: Considerations for Nitrate versus Sulfate Media

Molybdenum-99 is the parent of Technetium-99m, which is used in nearly 80% of all nuclear medicine procedures. The medical community has been plagued by Mo-99 shortages due to aging reactors, such as the NRU (National Research Universal) reactor in Canada. There are currently no US producers of Mo-99, and NRU is scheduled for shutdown in 2016, which means that another Mo-99 shortage is imminent unless a potential domestic Mo-99 producer fills the void. Argonne National Laboratory is assisting two potential domestic suppliers of Mo-99 by examining the effects of a uranyl nitrate versus a uranyl sulfate target solution configuration on Mo-99 production. Uranyl nitrate solutions are easier to prepare and do not generate detectable amounts of peroxide upon irradiation, but a high radiation field can lead to a large increase in pH, which can lead to the precipitation of fission products and uranyl hydroxides. Uranyl sulfate solutions are more difficult to prepare, and enough peroxide is generated during irradiation to cause precipitation of uranyl peroxide, but this can be prevented by adding a catalyst to the solution. A titania sorbent can be used to recover Mo-99 from a highly concentrated uranyl nitrate or uranyl sulfate solution; however, different approaches must be taken to prevent precipitation during Mo-99 production.

Manipulation of spatiotemporal photon distribution via chromatic aberration

We demonstrate a spatiotemporal laser-pulse-shaping scheme that exploits the chromatic aberration in a dispersive lens. This normally harmful effect transforms the phase modulation into a beam-size modulation at the focal plane. In combination with the intricate diffraction effect via beam apodization, this method provides a spatiotemporal control of photon distribution with an accuracy of diffraction limit on a time scale of femtoseconds.

Electron spin relaxation and rotational dynamics of tetramethylethylene radical cation in ZSM5

Abstract Electron spin relaxation and methyl rotation dynamics of the tetramethylethylene radical cation in ZSM5 were examined to probe the zeolite–guest and sorbate–sorbate interactions. T 1 and T 2 relaxation times and the intensity of the E lines due to tunneling rotation of methyl groups were measured as a function of temperature, sorbate loading and co-adsorbed argon. The EPR results demonstrate that site heterogeneity and sorbate-induced lattice distortions can affect the measured quantities. Sorbate–sorbate interactions, when they occur, can also be detected.

Electron Linear Accelerator Production and Purification of Scandium-47 from Titanium Dioxide Targets

The photonuclear production of no-carrier-added (NCA) 47Sc from solid NatTiO2 and the subsequent chemical processing and purification have been developed. Scandium-47 was produced by the 48Ti(γ,p)47Sc reaction with Bremsstrahlung photons produced from the braking of electrons in a high-Z (W or Ta) convertor. Production yields were simulated with the PHITS code (Particle and Heavy Ion Transport-code System) and compared to experimental results. Irradiated TiO2 targets were dissolved in fuming H2SO4 in the presence of Na2SO4 and 47Sc was purified using the commercially available Eichrom DGA resin. Typical 47Sc recovery yields were >90% with excellent specific activity for small batches (<185 MBq batches).

Fission Produced 99Mo without a Nuclear Reactor

99Mo, the parent of the widely used medical isotope 99mTc, is currently produced by irradiation of enriched uranium in nuclear reactors. The supply of this isotope is encumbered by the aging of these reactors and concerns about international transportation and nuclear proliferation. Methods: We report results for the production of 99Mo from the accelerator-driven subcritical fission of an aqueous solution containing low enriched uranium. The predominately fast neutrons generated by impinging high-energy electrons onto a tantalum convertor are moderated to thermal energies to increase fission processes. The separation, recovery, and purification of 99Mo were demonstrated using a recycled uranyl sulfate solution. Conclusion: The 99Mo yield and purity were found to be unaffected by reuse of the previously irradiated and processed uranyl sulfate solution. Results from a 51.8-GBq 99Mo production run are presented.

Development of high intensity source of thermal positrons APosS (Argonne Positron Source)

We present an update on the positron-facility development at Argonne National Laboratory. We will discuss advantages of using low-energy electron accelerator, present our latest results on slow positron production simulations, and plans for further development of the facility. We have installed a new converter/moderator assembly that is appropriate for our electron energy that allows increasing the yield about an order of magnitude. We have simulated the relative yields of thermalized positrons as a function of incident positron energy on the moderator. We use these data to calculate positron yields that we compare with our experimental data as well as with available literature data. We will discuss the new design of the next generation positron front end utilization of reflection moderator geometry. We also will discuss planned accelerator upgrades and their impact on APosS.

Micro-SHINE Uranyl Sulfate Irradiations at the Linac

Peroxide formation due to water radiolysis in a uranyl sulfate solution is a concern for the SHINE Medical Technologies process in which Mo-99 is generated from the fission of dissolved low enriched uranium. To investigate the effects of power density and fission on peroxide formation and uranyl-peroxide precipitation, uranyl sulfate solutions were irradiated using a 50 MeV electron linac as part of the micro-SHINE experimental setup. Results are given for uranyl sulfate solutions with both high and low enriched uranium irradiated at different linac powers.