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DOE OSTI2022

Compact automated apparatus for rapid astatine recovery from nitric acid media: Design, application, and impurity characterization

We report automation of irradiated bismuth target dissolution and astatine recovery from nitric acid media has been achieved at Texas A&M University. This process can be controlled remotely; it does not require any chemical treatment or evaporation of dissolution solution prior to the separation of above-mentioned elements on an extraction chromatography column, making the final product ready within 20 min. The accompanying radio-impurities have been identified and successfully separated from astatine, leading to greater than 99% radiopurity of the desired fractions. The system consists of only one pump and a set of LabVIEW controlled valves, and allows the connection of up to 10 different columns, providing the opportunity to prepare a variety of air dry columns, which can be different column geometries, containing astatine ready for further chemistry or shipment to desired facilities.

42 ENGINEERING↗

NASA NTRS1992

Geochemistry of HASP, VLT, and other glasses from double drive tube 79001/2

The Apollo 17 double drive tube 79001/2 (station 9, Van Serg Crater) is distinctive because of its extreme maturity, abundance, and variety of glass clasts. It contains mare glasses of both high Ti and very low Ti (VLT) compositions, and highland glasses of all compositions common in lunar regolith samples: highland basalt (feldspathic; Al2O3 greater than 23 wt percent), KREEP (Al2O3 less than 23 wt percent, K2O greater than 0.25 wt percent), and low-K Fra Mauro (LKFM; Al2O3 less than 23 wt percent, K2O less than 0.25 wt percent). It also contains rare specimens of high-alumina, silica-poor (HASP), and ultra Mg glasses. HASP glasses contain insufficient SiO2 to permit the calculation of a standard norm, and are thought to be the product of volatilization during impact melting. They have been studied by electron microprobe major-element analysis techniques but have not previously been analyzed for trace elements. The samples analyzed for this study were polished grain mounts of the 90-160 micron fraction of four sieved samples from the 79001/2 core (depth range 2.3-11.5 cm). A total of 80 glasses were analyzed by SEM/EDS and electron microprobe, and a subset of 33 of the glasses, representing a wide range of compositional types, was chosen for high-sensitivity INAA. A microdrilling device removed disks (mostly 50-100 micron diameter, weighing approx. 0.1-0.5 micro-g) for INAA. Preliminary data reported here are based only on short counts done within two weeks of irradiation.

Lindstrom, D. J.↗

DOE OSTI2025

Analysis of Radiological Release From Fueled Irradiation Experiments During Manual Handling (Slides)

Irradiation experiments are manually handled at the Advanced Test Reactor (ATR) by qualified operators using long handled tools. Standard handling evolutions include insertion and removal from the reactor vessel, transfer to/from a storage location or cask in the ATR canal, and handling/reconfigurations at a canal working tray. Such routine handling has the potential to result in mechanical damage to the experiment boundary which is credited to retain fission products. Since damage can occur due to operator error, this is an anticipated occurrence. This work determines the radiological consequences to receptors inside the ATR facility, as well as public receptors. Given the wide variety of fuel types tested at the ATR, a generic approach to the analysis is taken. The radiological inventory is determined to bound a variety of fuel types (e.g., ceramic and molten fuel matrices) and fission powers on the U-235 enrichment spectrum. The source term analysis considers different release fractions to bound different fuel types and burnups. The postulated handling events occur underwater; thus, retention of the released isotopic content is considered within the canal water column. Retention of radionuclides in the water column is determined using a modified approach of United States (US) Nuclear Regulatory Commission (NRC) Regulatory Guide (RG) 1.183. Radiological dose to the facility receptor is determined using a compartment release model. Dose to the public receptor is determined using atmospheric dispersion models using site specific atmospheric conditions with the use of the Radiological Safety Analysis Computer (RSAC) program version 7.2.

21 - SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLAN↗

NASA NTRS2004

Environment Partitioning and Reactivity of Polybrominated Diphenylethers

Polybrominated diphenyl ethers (PBDEs) are an important class of flame retardants. Annual global demand for these compounds was over 67,000 metric tons in 2001. PBDEs have recently been extensively investigated as environmental contaminants because they have been detected in air, sediment, and tissue samples from urban and remote areas. Important issues include quantifying PBDE partitioning in various environmental compartments, and elucidating transformation pathways. The partitioning of PBDE congeners to aerosols was estimated for 16 sites in the United States, Canada, and Mexico. The aerosol particles were PM2.5, the total suspended particle (TSP) concentration varied between 3.0 - 55.4 micro g/cubic meter, and the organic fraction ranged from 11 - 41%; these data are published values for each site. It is estimated that the largest fraction of each PBDE associated with the aerosol particles occurs in Mexico City, and the smallest fraction in Colorado Plateau. Although the organic fraction in Mexico City is about 60% of that observed in the Colorado Plateau, the TSP is larger by a factor of about 18.5, and it is the difference in TSP that strongly influences the fraction of particle-bound PBDE in this case. PBDE partitioning to PM2.5 particles also varies seasonally because of temperature variations. For the less brominated congeners the percentage that is particle-bound is relatively low, regardless of air temperature. In contrast, the heavier congeners exhibit a significant temperature dependence: as the temperature decreases (fall, winter) the percentage of PBDE that is particle-bound increases. The partitioning calculations complement experimental data indicating that decabromodiphenyl ether (DBDE) dissolved in hexane transforms very rapidly when irradiated with solar light. DBDE is the most highly brominated PBDE congener (10 bromine atoms) and occurs in the commercial formulation which is subject to the largest global demand.

Hua, Inez↗

DOE OSTI2022

Nondestructive Photo-Cross-Linking of Microphase-Separated Diblock Polymers through Coumarin Dimerization

The effect of long-wavelength ultraviolet photocrosslinking on microphase-separated coumarin-containing block polymers was studied by photo-rheometry and small-angle X-ray scattering. This model system consisted of three photocrosslinkable diblock polymers of poly(methoxyethyl acrylate)-b-poly(hexyl methacrylate-co-coumarin methacrylate) with different volume fractions of the crosslinkable coumarin-containing block, which microphase separated into lamellar and cylindrical morphologies. All polymers stiffened upon exposure to 365 nm light, with much greater relative increases in moduli recorded for lamellae-forming polymers (ca. 3200% increase) compared to the cylinder-forming polymer (ca. 550% increase). Disordering transitions that were evident in uncrosslinked samples were no longer observed after crosslinking in the ordered state, and domain sizes were found to remain stable to heating. The photocrosslinking reaction only proceeded under active irradiation, indicating a high degree of spatiotemporal control over curing in this system. Finally, at constant concentration of couamarin within the crosslinkable block, the cure rate was largely independent of polymer composition, suggesting a constant local concentration of coumarin moieties within the segregated crosslinkable domains. Lastly, these findings establish a set of specific structure-property relationships governing the phase-selective photocrosslinking of diblock polymers that can guide the design of robust nanostructured materials.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗

DOE OSTI2025

Sensitivity of Simulated Polar Climate to Improved Partitioning of Spectral Solar Irradiance Between Visible and Near‐Infrared Bands

The solar radiative processes that contribute to Earth's surface and atmospheric energy budgets are strongly dependent on wavelength. For example, snow and water vapor become more absorptive as incident radiation shifts from visible (VIS) to near-infrared (NIR) wavelengths. Some earth system models (ESMs) aggregate solar radiation into just two bands (VIS and NIR) partitioned at 0.7 μm for transmission between the atmospheric and surface components. In the widely used radiative transfer model RRTMG_SW this partition is near the center of the overlap spectral band spanning 0.625–0.778 μm, whose flux is often approximated as being evenly divided between the VIS and NIR surface bands. Using a hyperspectral radiative transfer model, we show that the fractional downwelling surface flux within the overlap band is usually distributed about 55.5%:44.5% VIS:NIR. This improved approximation shifts as much as 4.27 W m −2 from the NIR to the VIS band, leading to an instantaneous decrease in surface absorption of up to 0.91 W m −2 over snow-covered surfaces. Century-long fully coupled ESM simulations show surface absorption over snow decreases by over 1.6 W m −2 . The coupled response in sea ice regions amplifies the initial forcing ten-fold, and increases seasonal sea ice area by up to 4.9%. These results highlight the importance of accurately representing the spectral distribution of solar radiation in the cryosphere.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2023

Demonstration of a 4.32 μ m cutoff InAsSbBi n B n photodetector, a lattice-matched random alloy III–V solution for mid-wave infrared sensing

InAsSbBi nBn photodetectors are demonstrated that are lattice-matched to the underlying GaSb substrate with a 4.32 μm wavelength cutoff at 150 K that is 0.3 μm longer than that of lattice-matched InAsSb at this temperature reflecting a 0.5% Bi mole fraction in the InAsSbBi active region. A low growth temperature was utilized to facilitate the incorporation of Bi, resulting in a minority carrier lifetime on the order of 24 ns in the InAsSbBi active region. Nevertheless, the detectors exhibit a quantum efficiency of 17% at 3.3 μm wavelength with a dark current density of 50 μA/cm2 at 150 K and −0.4 V bias and the strong photoresponse turn-on characteristic of a random alloy at 4.32 μm wavelength and 150 K. A shot noise-equivalent irradiance analysis indicates that this detectors' dark-current-limited noise-equivalent irradiance of 1012 cm−2 s−1 is two orders of magnitude greater than the Rule 07 expectation for this cutoff, and dark-current-limited shot noise-equivalent irradiance performance transitions to photon-limited at 1.7 × 1015 photons/cm2 s.

Physics↗

DOE OSTI2022

Understanding Fission Gas Bubble Distribution and Zirconium Redistribution in Neutron-irradiated U-Zr Metallic Fuel Using Machine Learning

U-10wt.% Zr (U-10Zr) based metallic fuel is the leading candidate for next-generation sodium cooled fast reactor in United States. Currently, Idaho National Laboratory (INL) has been the leading national laboratory for research, development, and demonstration (RD&D) on metallic fuel. Advanced post-irradiation characterization will help to understand fuel microstructure and property change during irradiation, benefiting fuel qualification for commercial application. Characterization capabilities ranging from sub-nanometer to micrometer, such as scanning electron microscopy (SEM), focused ion beam (FIB) sampling, transmission electron microscopy (TEM) characterization, and local thermal conductivity microscopy (TCM), have been utilized recently on irradiated U-10Zr fuel samples to gain a better understanding of nuclear fuel microstructure and property evolution inside a reactor. The FIB/SEM coupled with energy dispersive X-ray spectroscopy (EDS) can capture the essential information to achieve better understanding of fuel behaviors. Inside a nuclear reactor, the phase and microstructure of U-10Zr is constantly changing under neutron bombardment. For example, the gaseous fission product atoms have a limited solubility inside fuel matrix and tend to precipitate out in bubble form, which not only contribute to fuel thermal conductivity degradation but also provide a shortcut for movement of fission products, i.e. lanthanides. The resultant deposition of lanthanides at the cladding inner surface will potentially trigger a chemical reaction/interaction between nuclear fuel and cladding at reactor operational conditions, threatening fuel integrity and safety. FIB/SEM coupled with EDS can provide the fission bubble information as well as probe into phase separation or Zr redistribution, which is fundamental to predict the fuel performance. With high velocity image data generating method, such as FIB/SEM, an automatic way to extract the microstructural information quantitively can better serve the needs from post irradiation characterization. A trained machine learning model, named Decision Tree, is employed to generate a bubble classifier and to categorize bubbles into three categories: isolated bubble, connected without lanthanides, and connected with lanthanides bubbles[3]. This work presents a showcase of this approach on six regions of a fuel cross-section along the radial temperature gradient. We obtained distributions of bubble categories and porosity rates along the six regions. Moreover, a secondary phase U-Zr2 was determined and found on regions 5 and 6. The secondary phase fraction was increasing from 15.61% in region 5 to 34.79% in region 6 based on this approach . This quantitative data offers insights into the lanthanide migration and potentially thermal conductivity degradation. This information from machine learning will be fed into fuel design code for better prediction of fuel performance.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

DOE OSTI2020

Comparison of total emitted solar-induced chlorophyll fluorescence (SIF) and top-of-canopy (TOC) SIF in estimating photosynthesis

We report many studies have shown that solar-induced fluorescence (SIF) has a good potential to predict gross primary production (GPP) of vegetation. What we measured by remote sensing or near-surface platforms is top-of-canopy (TOC) SIF (SIF toc ), which is not necessarily equal or proportional to total emitted SIF (SIF tot ) from the entire canopy due to the (re)absorption and scattering effects. However, photosynthesis, the process that plants use to fix carbon from the atmosphere, occurs at the entire vertical canopy. Here, by using the recollision theory, we calculated SIF tot at 760 nm from the measured SIF toc , hyperspectral reflectance (R), canopy interception (i) and leaf albedo (ω l ). Among them, both SIF toc and R can be obtained from concurrent TOC spectral measurements; i and ω l in the near-infrared region can be estimated from the open access datasets with a good accuracy. Our result confirms that the measured SIF toc only accounts for a small fraction of SIF tot : the above-the-canopy sensor can only “see” on average 22.9% of SIF tot at Harvard Forest. SIF tot has the following advantages over SIF toc in estimating GPP: (1) SIF tot improves the diurnal estimate of canopy GPP, especially capable to capture the midday depression of photosynthesis which may cause the large discrepancies between SIF toc and GPP on a diurnal basis, (2) SIF tot produces a stronger correlation with GPP from plants with complex canopy structure or under sky conditions with more diffuse irradiance, and (3) the SIF tot -GPP relationship shows a stronger resilience to environmental stresses. The fluorescence escape ratio (f esc ), the ratio between SIF toc multiplied by π and SIF tot , is mostly determined by the sun-canopy-sensor geometry and leaf inclination distribution. The effect of LAI and the leaf chlorophyll concentration on f esc is marginal at the 760 nm wavelength. Our results suggest that converting SIF toc into SIF tot provides a better tool to understand and estimate GPP.

54 ENVIRONMENTAL SCIENCES↗

NASA NTRS2017

Effects of Ionizing Radiation on Murine Gene Expression in Skin and Bone

Long duration spaceflight causes a negative calcium balance and reduces bone density in astronauts. The potential for exposure to space radiation to contribute to lasting decrements in bone mass is not yet understood. Sustained changes to bone mass have a relatively long latency for development, however skin is a radiation sensitive organ and changes in skin gene expression may serve as an early radiation biomarker of exposures and may correlate with adverse effects on skeletal tissue. Previous studies have shown that FGF18 gene expression levels of hair follicles collected from astronauts on the ISS rose over time. In the hair follicle, FGF18 signaling mediates radioresistance in the telogen by arresting the cell cycle, and FGF18 has the potential to function as a radioprotector. In bone, FGF18 appears to regulate cell proliferation and differentiation positively during osteogenesis and negatively during chondrogenesis. Cellular defense responses to radiation are shared by a variety of organs, hence in this study, we examined whether radiation induced gene expression changes in skin may be predictive of the responses of skeletal tissue to radiation exposure. We have examined oxidative stress and growth arrest pathways in mouse skin and long bones by measuring gene expression levels via quantitative polymerase chain reaction (qPCR) after exposure to total body irradiation (TBI). To investigate the effects of irradiation on gene expression, we used skin and femora (cortical shaft) from the following treatment groups: control (normally loaded, sham-irradiated), and TBI (0.5 Gy Fe-56 600 MeV/n and 0.5 Gy H-1 150 MeV/n). Animals were euthanized one and 11 days post-IR. Statistical analysis was performed via a Student's ttest. In skin samples one day after IR, skin expression of FGF18 was significantly greater (3.8X) than sham-irradiated controls (3.8X), but did not differ 11 days post TBI. Expression levels of other radiation related genes (Nfe2l2, Trp53, Cdkn1a, FoxO3, Gadd45g, SOD1), was not different due to TBI at either time point. In bone (femora) TBI significantly increased (3.8X) expression of the pro-bone resorption cytokine, MCP-1, one day after TBI. FGF18 expression in skin and MCP- 1 expression in bone were found to be positively correlated (P less than 0.002, r=0.8779). Further, microcomputed tomography analysis of tibae from these animals showed reduced fractional cancellous bone volume (-21.7%) at 11 days post exposure. These results suggest that early radiation induced changes in FGF18 gene expression in skin may have value for predicting subsequent loss of cancellous bone mass.

space radiation↗

DOE OSTI2023

MCNP Neutronic Design Optimization of an Accelerator-Driven Subcritical Assembly for Mo-99 Production

Molybdenum-99 (99Mo) has been in short supply in the recent decades. The most common productionmechanism, irradiation in a reactor fueled by highly enriched uranium (HEU), raisesproliferation concerns. Novel efforts are being pursued to meet the demand for 99Mo withoutHEU irradiation, relying on the processing of low-enriched uranium or natural uranium. Onesuch concept is to use an electron linear accelerator to generate bremsstrahlung that, through aphotonuclear reaction, can help multiply neutrons in a subcritical assembly. The goal of this workwas to neutronically optimize the subcritical assembly for keff and 99Mo production. This optimizationis unique for The design was optimized in terms of reflector type, pitch, and LEU andNU mass. Results of the paper show the effects of fuel mass, pitch, and reflector material on keffand the NU fission fraction. keff was found to be mostly driven by LEU mass, and the NU fissionfraction was dependent on several parameters, primarily pitch and mass.

Conant, Andrew↗

DOE OSTI2024

Nanostructured Alumina Forming Austenitic Alloy (NAFA) Production Using Mechanical Alloying and High-Temperature Consolidation

Alumina-forming austenitic (AFA) alloys are a promising class of nuclear materials because of their high-temperature oxidation/corrosion resistance and mechanical properties. Unfortunately, these alloys are limited in use for core material applications, and they are specifically limited for use as nuclear fuel cladding because of their high Ni-transmutation and helium generation rate in-service. Improving these alloys through a fine dispersion of oxide precipitates and thus increasing the effective irradiation sink strength of the alloy system may mitigate many of the degradation phenomena expected during alloy deployment. These phenomena include high-temperature helium embrittlement and cavity swelling for lead-cooled fast reactor applications. This work effort uses combination of conventional and advanced manufacturing approaches are being used to fabricate nanostructured AFA (NAFA) materials. As the first objective of this initiative, a conventional AFA was modified using mechanical alloying and extrusion to alter the precipitation characteristics to include a fine dispersion of nanoscale oxides intended to serve as traps for irradiation-induced point defects and transmuted He within the lattice. This analysis compared the efficacy of the conventional mechanical alloying and extrusion approach with the unalloyed AFA consolidated approach using hot isostatic pressing (HIP). It was found that, although the mechanical alloying approach is successful in producing a fine distribution of oxides within the first nanostructured AFA (NAFA-1), the distribution is heterogeneous because of the mild milling parameters used to prevent cold welding of powder to the spherical milling media. The additional dispersion of oxides, coupled with a higher volume fraction of other secondary phases in the NAFA-1, produces higher alloy strengths that range up to 600°C in comparison to the unalloyed HIP AFA, thus exceeding the operating temperature of lead-cooled fast reactors. However, the strength of the NAFA-1 is lower than that of the HIP AFA at 800°C, which is presumed to be a function of increased secondary phases from the nonoptimized AFA chemistry. Future work is planned on a newly procured NAFA-2 chemistry that is more suitable for advanced reactor applications exploring new manufacturing routes.

36 MATERIALS SCIENCE↗

NASA NTRS1996

DNA damage and repair in oncogenic transformation by heavy ion radiation

Energetic heavy ions are present in galactic cosmic rays and solar particle events. One of the most important late effects in risk assessment is carcinogenesis. We have studied the carcinogenic effects of heavy ions at the cellular and molecular levels and have obtained quantitative data on dose-response curves and on the repair of oncogenic lesions for heavy particles with various charges and energies. Studies with repair inhibitors and restriction endonucleases indicated that for oncogenic transformation DNA is the primary target. Results from heavy ion experiments showed that the cross section increased with LET and reached a maximum value of about 0.02 micrometer2 at about 500 keV/micrometer. This limited size of cross section suggests that only a fraction of cellular genomic DNA is important in radiogenic transformation. Free radical scavengers, such as DMSO, do not give any effect on induction of oncogenic transformation by 600 MeV/u iron particles, suggesting most oncogenic damage induced by high-LET heavy ions is through direct action. Repair studies with stationary phase cells showed that the amount of reparable oncogenic lesions decreased with an increase of LET and that heavy ions with LET greater than 200 keV/micrometer produced only irreparable oncogenic damage. An enhancement effect for oncogenic transformation was observed in cells irradiated by low-dose-rate argon ions (400 MeV/u; 120 keV/micrometer). Chromosomal aberrations, such as translocation and deletion, but not sister chromatid exchange, are essential for heavy-ion-induced oncogenic transformation. The basic mechanism(s) of misrepair of DNA damage, which form oncogenic lesions, is unknown.

NASA Discipline Number 45-10↗

DOE OSTI2021

Development of an Activation Analysis Methodology to Support the Disposal of the High Flux Isotope Reactor Original Reflector Container

The original beryllium reflector container was installed prior to the first ever High Flux Isotope Reactor (HFIR) cycle and was removed at the end of cycle 382. The original beryllium reflector was part of every HFIR irradiation cycle from September of 1966 to October of 2000. After its service lifetime, the beryllium reflector container has been interim stored at the fuel pool storage, which is located adjacent to the reactor. Due to space limitations in the pool fuel storage, it is necessary to remove the beryllium reflector container and send it to a location that can be stored permanently. Prior to shipping the beryllium reflector container, the radiological activity must be known. The challenge in determining the overall activity as well as the isotopic inventory and volumetric material fractions lays on the fact that some of the original reflector container stainless steel quantities are not known. Therefore, a methodology to determine a conservative activation scenario was developed.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗

NASA NTRS2002

Analysis of unrejoined chromosomal breakage in human fibroblast cells exposed to low- and high-LET radiation

Reported studies of DNA breakage induced by radiation of various qualities have generally shown a higher fraction of unrejoined residual breaks after high-LET exposure. This observation is supported by the argument that high-LET radiation induced DNA breaks that are more complex in nature and, thus, less likely to be repaired. In most cases the doses used in these studies were very high. We have studied unrejoined chromosome breaks by analyzing chromosome aberrations using a fluorescence in situ hybridization (FISH) technique with a combination of whole chromosome specific probes and probes specific for the telomere region of the chromosomes. Confluent human fibroblast cells (AG1522) were irradiated with gamma rays, 490 MeV/nucleon Si, or with Fe ions at either 200 and 500 MeV/nucleon, and were allowed to repair at 37 degrees C for 24 hours after exposure. A chemically induced premature chromosome condensation (PCC) technique was used to condense chromosomes in the G2 phase of the cell cycle. Results showed that the frequency of unrejoined chromosome breaks was higher after high-LET radiation, and the ratio of unrejoined to misrejoined chromosome breaks increased steadily with LET up a peak value at 440 keV/microm.

NASA Discipline Radiation Health↗

NASA NTRS2005

Single Particle Damage Events in Candidate Star Camera Sensors

Si charge coupled devices (CCDs) are currently the preeminent detector in star cameras as well as in the near ultraviolet (uv) to visible wavelength region for astronomical observations in space and in earth-observing space missions. Unfortunately, the performance of CCDs is permanently degraded by total ionizing dose (TID) and displacement damage effects. TID produces threshold voltage shifts on the CCD gates and displacement damage reduces the charge transfer efficiency (CTE), increases the dark current, produces dark current nonuniformities and creates random telegraph noise in individual pixels. In addition to these long term effects, cosmic ray and trapped proton transients also interfere with device operation on orbit. In the present paper, we investigate the dark current behavior of CCDs - in particular the formation and annealing of hot pixels. Such pixels degrade the ability of a CCD to perform science and also can present problems to the performance of star camera functions (especially if their numbers are not correctly anticipated). To date, most dark current radiation studies have been performed by irradiating the CCDs at room temperature but this can result in a significantly optimistic picture of the hot pixel count. We know from the Hubble Space Telescope (HST) that high dark current pixels (so-called hot pixels or hot spikes) accumulate as a function of time on orbit. For example, the HST Advanced Camera for Surveys/Wide Field Camera instrument performs monthly anneals despite the loss of observational time, in order to partially anneal the hot pixels. Note that the fact that significant reduction in hot pixel populations occurs for room temperature anneals is not presently understood since none of the commonly expected defects in Si (e.g. divacancy, E center, and A-center) anneal at such a low temperature. A HST Wide Field Camera 3 (WFC3) CCD manufactured by E2V was irradiated while operating at -83C and the dark current studied as a function of temperature while the CCD was warmed to a sequence of temperatures up to a maximum of +30C. The device was then cooled back down to -83 and re-measured. Hot pixel populations were tracked during the warm-up and cool-down. Hot pixel annealing began below 40C and the anneal process was largely completed before the detector reached +3OC. There was no apparent sharp temperature dependence in the annealing. Although a large fraction of the hot pixels fell below the threshold to be counted as a hot pixel, they nevertheless remained warmer than the remaining population. The details of the mechanism for the formation and annealing of hot pixels is not presently understood, but it appears likely that hot pixels are associated with displacement damage occurring in high electric field regions.

Marshall, Paul↗

NASA NTRS2011

Dose Response for Chromosome Aberrations in Human Lymphocytes and Fibroblasts After Exposure to Very Low Dose of High Let Radiation

The relationship between biological effects and low doses of absorbed radiation is still uncertain, especially for high LET radiation exposure. Estimates of risks from low-dose and low-dose-rates are often extrapolated using data from Japanese atomic bomb survivor with either linear or linear quadratic models of fit. In this study, chromosome aberrations were measured in human peripheral blood lymphocytes and normal skin fibroblasts cells after exposure to very low dose (0.01 - 0.20 Gy) of 170 MeV/u Si-28 ions or 600 MeV/u Fe-56 ions, including doses where on average less than one direct ion traversal per cell nucleus occurs. Chromosomes were analyzed using the whole-chromosome fluorescence in situ hybridization (FISH) technique during the first cell division after irradiation, and chromosome aberrations were identified as either simple exchanges (translocations and dicentrics) or complex exchanges (involving >2 breaks in 2 or more chromosomes). The responses for doses above 0.1 Gy (more than one ion traverses a cell) showed linear dose responses. However, for doses less than 0.1 Gy, both Si-28 ions and Fe-56 ions showed a dose independent response above background chromosome aberrations frequencies. Possible explanations for our results are non-targeted effects due to aberrant cell signaling [1], or delta-ray dose fluctuations [2] where a fraction of cells receive significant delta-ray doses due to the contributions of multiple ion tracks that do not directly traverse cell nuclei where chromosome aberrations are scored.

Hada, M.↗

DOE OSTI2021

Near-surface modification of defective KTaO 3 by ionizing ion irradiation

The synergistic effect of nuclear (S n ) and electronic (S e ) energy loss observed in some ABO 3 perovskites has attracted considerable attention due to the real possibility to modify various near-surface properties, such as the electronic and optical properties, by patterning ion tracks in the defective near-surface regions. In this study, we show that low-energy ion-induced disordering in conjunction with ionizing ion irradiation (18 MeV Si, 21 MeV Ni and 91.6 MeV Xe) is a promising approach for tailoring ion tracks in the near-surface of defective KTaO 3 . Experimental characterization and computer simulations reveal that the size of these latent ion tracks increases with S e and level of pre-existing damage. These results further reveal that the threshold S e value (S e th ) for track creation increases with decreasing pre-damage level. The values of S e th increase from 5.02 keV nm -1 , for a pre-existing fractional disorder of 0.53 in KTaO 3 , to 10.81 keV nm -1 for pristine KTaO 3 . Above these thresholds, amorphous latent tracks are produced due local melting and rapid quenching. Below a disorder fraction of 0.08 and S e ≤ 6.68 keV nm -1 , the synergistic effect is not active, and damage accumulation is suppressed due to a competing ionization-induced damage annealing process. These results indicate that, depending on S e and the amount of pre-existing damage, highly ionizing ions can either enhance or suppress damage accumulation in KTaO 3 , thus providing a pathway to tailoring defects states. Comprehending the conflicting roles of highly ionizing ions in defective ABO 3 oxides is vital for understanding and predictive modeling of ion-solid interactions in complex oxides, as well as for achieving control over ion track size in the near-surface of defective KTaO 3 .

36 MATERIALS SCIENCE↗