[en] 'The purpose of this program is to deliver pertinent, fundamental information that can be used to make technically defensible decisions on safety issues and processing strategies associated with mixed chemical and radioactive waste cleanup. In particular, an understanding of radiolysis in mixed-phase systems typical of U. Department of Energy (DOE) heterogeneous, radioactive/chemical wastes will be established. This is an important scientific concern with respect to understanding tank waste chemistry issues; it has received relatively little attention. The importance of understanding solid-state radiolysis, secondary electron interactions, charge-transfer dynamics, and the general effect of heterogeneous solids (interface and particulate surface chemistry) on tank waste radiation processes will be demonstrated. In particular, the author will investigate (i) the role of solid-state and interfacial radiolysis in the generation of gases, (ii) the mechanisms of organic compound degradation, (iii) scientific issues underlying safe interim storage, and (iv) the effects of colloid surface-chemical properties on waste chemistry. Controlled radiolysis studies of NaNO₃ solids and SiO₂ particles were carried out using pulsed, low- (5--150 eV) and high- (3 MeV) energy electron-beams at Pacific Northwest National Laboratory (PNNL) and at Argonne National Laboratory (ANL), respectively. The pulsed, low-energy electron beams probe the inelastic scattering and secondary cascading effects produced by high-energy beta and gamma particles. Pulsed radiolysis allows time-resolved measurements of the high-energy processes induced by these particles. Using low-energy (10--75 eV) electron-beam irradiation of nominally dry NaNO₃ solution-grown and melt-grown single crystals, they observed H⁺, Na⁺, O⁺, NO⁺, NO, NO₂, O₂, and O(³P) desorption signals. The threshold measurements and yields indicate that the degradation proceeds mainly via destruction of the nitrate moiety. The H⁺ and Na⁺ yields are primarily related to the presence of water and Na metal, Na hydrides and oxides, or other defect sites on the salt surface. The water is due to diffusion from the bulk of solution-grown crystals and controlled water adsorption on melt-grown crystals. The metallization and/or metal hydride/oxide build-up is a result of the very large electron-beam degradation cross-section (> 10--16 cm²) of NaNO₃. The build-up of alkali-metal colloids during the irradiation of alkali-halide materials is well known and is expected for other alkalai salts such as NaNO₃ . Figure 1 shows the Na⁺ desorption yield as a function of incident electron energy. The signal below the 33 eV is all due to Na buildup and the break seems to be associated with charge build-up, band-bending, and then charge release. Charge trapping and metallization is reduced at temperatures above 420 K, 1,3 a temperature higher than typically found in high-level liquid waste (HLLW) tanks.'

[en] Gas generation, oxidation, corrosion, and other important phenomena accompanying the long-term storage of spent nuclear fuel (SNF) are highly affected by radiation-induced processes at oxide surfaces and interfaces. Oxides covering fuel rods and other surfaces, as well as oxide particles dispersed in the aqueous phase, can promote water radiolysis and decomposition. Surface-enhanced water radiolysis may accelerate oxidation, corrosion, and hydriding of Zircaloy, uranium, and other materials and may affect the amount of H₂ and O₂ produced during SNF storage. Mechanisms of surface-enhanced radiolysis are not well understood. Some proposed mechanisms are based on the transfer of energy initially deposited in the bulk of the oxide phase to the interface. This process depends on the oxide electronic structure and on the energetics and geometric structures of the adsorbed molecules. Typically, mobile electrons, holes, and excitons contribute to the enhancement. Their effective range is directly related to carrier mean free paths and excited state lifetimes. These parameters can be studied with a technique known as laser-stimulated luminescence (LSL). The authors present data on the gamma radiation-induced degradation of water at oxide particle/water interfaces and on time-resolved LSL measurements of ZrO₂ recombination luminescence

[en] The positive ion yield resulting from the interaction of a pulsed 266-nm laser with yttria-stabilized cubic zirconia crystals is investigated. Although the photon energy (4.66 eV) is well below the nominal band-gap energy for ZrO₂ (5.0-5.5 eV), photon stimulated ion desorption (PSD) of Y+, Zr+, YO+ and ZrO+ begins at approx. 2.5 MW/cm². We interpret this as the onset of laser ablation. Cation mass spectra collected using higher laser fluences resemble those obtained via secondary-ion mass spectrometry (SIMS). The similarity between the laser ablation and SIMS data demonstrates the importance of surface electronic structure effects in photon induced degradation of this material

[en] The low-energy (5-15 eV) electron stimulated desorption of D^- and D₂ (¹Σ_g⁺, v=0, J=0 and v=1, J=2) from condensed D₂O films is investigated as a function of substrate temperature. The D^- ions are produced primarily via the ²B₁ dissociative electron attachment resonance. Both the D^- and D₂ yields are enhanced when the substrate temperature increases from 90 to 140 K. The changes in the D^- and D₂ yields with substrate temperature are qualitatively similar. We attribute the increase in yields to thermally induced rotations or breaks in the near-surface hydrogen bonding network. This reduction in co-ordination and coupling reduces the nearest neighbor perturbations and enhances the surface or near surface excited state lifetimes. Production of vibrationally excited D₂ molecules correlates with reactive scattering of D^- at the surface, whereas production of D₂ in the v=0 level also includes molecular elimination from an excited state which is produced by autodetachment

[en] Electron beam induced damage of NaNO₃ single crystals is examined using laser resonance enhanced multiphoton ionization detection of the neutral desorption products, post-irradiation temperature-programmed desorption (TPD), secondary electron emission microscopy (SEEM), and Auger electron spectroscopy (AES). The damage initially involves destruction of the nitrate group and production of O (³P_J) and NO (²II) fragments with nonthermal energy distributions. Specifically, the O (³P_J) J state distribution measured at 100 eV incident electron energy is 5:1.5:0.25 for J = 2:1:0, the NO (²II) vibrational state distribution is 1:0.56:0.35:0.40:0.23 for ν = 0:1:2:3:4, and the NO (²II_1/2,3/2) rotational distribution has a high population of the upper (²II_3/2) spin-orbit component. Production and desorption of these nonthermal fragments are dominated by the decay of {NO₃^-}*. At higher electron fluences, thermalized NO, O₂, and NO₂ are also produced and released, through the NO₂ is a minor product. The authors suggest that the formation and desorption of thermalized NO and O₂ both involve NO₂^- defect states and unimolecular dissociation of NO₃asterisk. This is supported by the observation that the NO and O₂ yields have the same temperature dependence which is well described by the sum of two Maxwell-Boltzmann type equations with activation energies of 0.16 ± 0.03 and 0.010 ± 0.004 eV. O₂ gas is also released in post-irradiation thermal cycling from 110 to 440 K with peaks at approximately260 and approximately340 K. The authors associate the post-irradiation TPD of O₂ with reactions involving O atoms released during thermal decomposition of {NO₂^-hor_ellipsisO} and ONOO^-. The SEEM image shows damage features, and the AES spectra indicate that the irradiated region is depleted in both nitrogen and oxygen relative to Na. The elemental composition shows Na₂O as a final product of the NaNO₃ radiation decomposition. The 100 eV electron beam damage cross section is at least approximately10^-16 cm²

[en] Irradiation of surfaces and interfaces with low-energy (5--150 eV) electrons and ultraviolet photons occurs during the storage of ''mixed'' (chemical/radioactive) waste forms and during processing steps which involve the use of low temperature plasmas. It is well known that electron- and photon-stimulated desorption (ESD and PSD) from wide band-gap materials and interfaces can be initiated by Auger decay of deep valence and shallow core holes. This process consists of hole production, Auger decay, reversal of the Madelung potential, and ion expulsion due to the Coulomb repulsion. ESD and PSD of neutrals also occurs and involves production of electron-hole pairs and excitons. Generally, neutral yields dominate ESD and PSD cross sections, which typically vary between ∼10^-16 and 10^-22 cm². The authors present results on the ESD and PSD of environmentally relevant substrates such as ZrO₂(100), soda-glass, and NaNO₃. The major cation thresholds and yields indicate that ESD and PSD from these complex materials involves Auger stimulated events. In particular, desorption thresholds correlate with ionization of the O(2s), Zr(4p), Si(2p) and Na(2s) levels. The near band-gap threshold energy (∼5--7 eV) for the desorption of neutrals (i.e., atomic oxygen, NO, etc) demonstrate the overall importance of self-trapped and localized excitons in both ESD and PSD of typical ceramics and oxides

[en] The mechanisms of radiation induced degradation of soda glass were investigated via low-energy (5-120 eV) electron-stimulated desorption (ESD) studies. The major ionic desorption products observed are H⁺, Na⁺, O⁺ and Si⁺. The primary thresholds for H⁺, O⁺ and Na⁺ are approximately 25, 30 and 90 eV. These thresholds correlate to deep valence holes which Auger decay. The inter-atomic and intra-atomic Auger processes result in a reversal of the Madelung potential and cation desorption due to Coulomb repulsion. This is one of the main mechanisms of radiation damage of glass surfaces and is typical of wide band-gap materials

[en] Laser resonance enhanced multiphonon ionization spectroscopy was used to measure yields and velocity distributions of the D(²S), O(³P_2,1,0), and O(¹D₂) produced during low-energy (5--120 eV) electron-beam irradiation of amorphous D₂O ice. Electron-stimulated dissociation has a very low energy threshold (∼6--7 eV), and the neutral fragments desorb with low kinetic energies (∼60--85 meV) which are independent of the incident electron energy. The data suggest that desorption of neutral fragments results from dissociation of excited states formed directly or via electron-ion recombination

[en] Using laser resonance-enhanced ionisation spectroscopy, we have studied electron (6-350 eV) simulated dissociation of NO₂ coadsorbed with up to 0.75 monolayer of atomic O on Pt(111). Several dramatic effects on NO₂ dissociation occur due to the presence of O. There is a large (x26) enhancement in the specific dissociation yield, a narrowing of the NO translational energy distributions, and a distinct propensity (>4:1 at low J) for populating the upper Ω=3/2 NO spin-orbit level over the Ω=1/2 level. The spin-orbit state distribution of the O(³P_J) dissociation fragment is (5.0):(2.5):(1.0) for K=2, 1 and 0, which is within experimental error of the statistical (T→∞) 2J+1 limit. The enhanced yield probably results from an increased excited state lifetime due to a reduction in substrate charge-transfer screening. We have also detected O(³P_J=2,1,0) and NO X²Π_3/2,1/2(v=5) above an electron (6-350 eV) beam irradiated Pt(111) surface containing coadsorbed O₂ and NO at 90 K. We conclude that both O(³P_J) and NO(v=5) are laser-induced photodissociation fragments of NO₂ desorbates. The NO₂ is probably the reaction product of a collision between an O atom (created by electron-stimulated dissociation of adsorbed O₂) and an NO(a). We correlate the 10 eV NO₂ production threshold with the dissociative ionization of the 3σ_g molecular bonding orbital of O₂(a). (orig.)

[en] The first observations of dichroic effects in photoelectron angular distributions are reported for photoionization of aligned molecular excited states with circularly polarized light. Photoelectron angular distributions resulting from the two-color, (2+1) REMPI of NO via the A ²summation⁺, v = 0, J = 3/2,5/2 excited states exhibit significant left--right asymmetry. The experimental CD angular distributions are found to be well described by the general theoretical framework recently developed by Dubs, Dixit, and McKoy and are in good qualitative agreement with their calculated REMPI--CD distributions