Low Fluence Exposure Research Articles

Effect of heavy ion pre-irradiation on blistering and deuterium retention in tungsten exposed to high-fluence deuterium plasma

Surface blistering and deuterium (D) retention of heavy ion pre-irradiated (1 dpa) tungsten (W) exposed to low-energy (40 eV) and high-flux (1–2 × 1022 D/m2s) D plasma has been investigated with low fluence of 0.1 × 1027 D/m2 and a high fluence of 2.2 × 1027 D/m2. Surface morphology observations show that a large number of blisters are formed on the undamaged W after low-fluence exposure while the area density of the blister will have significantly increased and blister bursting will be triggered in the case of high-fluence exposure. In contrast, the heavy ion pre-irradiation noticeably reduces the area density of blisters for both low- and high-fluence exposures. The thermal desorption spectroscopy (TDS) shows that the total D retention in the pre-damaged W is greater than that of the undamaged sample, and this trend is more significant with increasing fluence/duration of D plasma exposure. The results of positron annihilation Doppler broadening spectrometry (PA-DBS) and TDS indicate that a large number of vacancy-type defects, especially those with a higher trapping energy of D, are induced by the heavy ion pre-irradiation. The increasing defects/D-trap sites may result in two outcomes, the D concentration will disperse at each blister nucleation site and D inward diffusion is enhanced and, therefore, lead to the mitigation of D-induced blistering and increase the D retention. In addition, combined with the enhanced D inward diffusion caused by increasing exposure duration, the D retention is therefore much higher in the pre-damaged W in the case of high-fluence exposure.

Simulated space radiation-induced mutants in the mouse kidney display widespread genomic change.

Exposure to a small number of high-energy heavy charged particles (HZE ions), as found in the deep space environment, could significantly affect astronaut health following prolonged periods of space travel if these ions induce mutations and related cancers. In this study, we used an in vivo mutagenesis assay to define the mutagenic effects of accelerated 56Fe ions (1 GeV/amu, 151 keV/μm) in the mouse kidney epithelium exposed to doses ranging from 0.25 to 2.0 Gy. These doses represent fluences ranging from 1 to 8 particle traversals per cell nucleus. The Aprt locus, located on chromosome 8, was used to select induced and spontaneous mutants. To fully define the mutagenic effects, we used multiple endpoints including mutant frequencies, mutation spectrum for chromosome 8, translocations involving chromosome 8, and mutations affecting non-selected chromosomes. The results demonstrate mutagenic effects that often affect multiple chromosomes for all Fe ion doses tested. For comparison with the most abundant sparsely ionizing particle found in space, we also examined the mutagenic effects of high-energy protons (1 GeV, 0.24 keV/μm) at 0.5 and 1.0 Gy. Similar doses of protons were not as mutagenic as Fe ions for many assays, though genomic effects were detected in Aprt mutants at these doses. Considered as a whole, the data demonstrate that Fe ions are highly mutagenic at the low doses and fluences of relevance to human spaceflight, and that cells with considerable genomic mutations are readily induced by these exposures and persist in the kidney epithelium. The level of genomic change produced by low fluence exposure to heavy ions is reminiscent of the extensive rearrangements seen in tumor genomes suggesting a potential initiation step in radiation carcinogenesis.

Space Radiation Quality Factors and the Delta Ray Dose and Dose-Rate Reduction Effectiveness Factor.

In this paper, the authors recommend that the dose and dose-rate effectiveness factor used for space radiation risk assessments should be based on a comparison of the biological effects of energetic electrons produced along a cosmic ray particles path in low fluence exposures to high dose-rate gamma-ray exposures of doses of about 1 Gy. Methods to implement this approach are described.

Morphological Changes at a Silver Surface Resulting from Exposure to Hyperthermal Atomic Oxygen

Bulk single crystals of Ag(100) and Ag(111) at a sample temperature of 220 °C were exposed to a beam containing hyperthermal atomic oxygen with a nominal translational energy of 5.2 eV. The resultant oxide scales and interface structures were characterized by cross-sectional (scanning) transmission electron microscopy ((S)TEM) and scanning electron microscopy (SEM). The oxide scales formed on Ag(100) and Ag(111) were more than 10 microns thick and contained numerous micropores, microchannels, grain boundaries, and defects (e.g., twins), which can provide pathways for the rapid transport of oxygen atoms through the thick oxide scales to the oxide/Ag interface. Energy dispersive X-ray spectroscopy (EDS) and nanoarea electron diffraction pattern (NA-EDP) investigations revealed that the oxide scales were predominately polycrystalline silver, containing only ∼3−5 atom % of oxygen, which is mainly in the form of Ag2O. Additional low-fluence exposures of thin foils of single-crystal Ag followed by analysis with TEM and NA-EDP identified crystalline grains of Ag, with dimensions of 500−800 nm, imbedded in a porous region of nanometer-sized Ag2O grains during the initial stages of surface transformation by hyperthermal atomic oxygen. A mechanism based on the rapid oxidation of Ag by atomic oxygen followed by thermal reduction of the Ag2O at 220 °C is proposed to explain the formation of the thick and unique polycrystalline silver “oxide” scales that were observed.

Proton irradiation on p-bulk silicon strip detectors using 12 GeV PS at KEK

P-bulk n-strip silicon strip detectors were irradiated with a 12 GeV proton beam at the KEK Proton Synchrotron in order to investigate a radiation damage due to high fluence of high energy protons. Primary 12 GeV protons extracted at the EP1-A beam line was used for the irradiation. The detectors were irradiated with the fluences of 1.1 × 10 14 and 4.3 × 10 13 protons/cm 2 for the high and low fluence exposures, respectively. Bias voltage for achieving the full depletion of the irradiated p-bulk detectors was observed to be significantly higher than that for the n-bulk detectors. The full depletion voltage did not increase monotonically as the fluence increased; it showed little variation up to about 5 × 10 13 p/cm 2 and then started to increase. The behaviour could be explained by assuming a contribution from three processes: effective acceptor creation, persistent acceptor component, and acceptor removal.

Optical absorption of neutron-irradiated silica fibers

Silica-based optical fibers capable of transmitting light in the ultraviolet, visible and infrared regions of the electromagnetic spectrum are gaining widespread use in fusion reactor diagnostic systems. To assess radiation damage in these optical materials, the optical absorption has been measured at room temperature in the interval 250–2000 nm of neutron-irradiated silica fibers containing low or high hydroxyl concentrations. Fiber irradiations were done in either low (1.4 × 1021 n/m2) or high (1.1 × 1023 n/m2) neutron fluence (E > 0.1 MeV) regions of the Los Alamos Spallation Radiation Effects Facility. Attenuation in high-fluence irradiated fibers exceeds 104 dB/km for wavelengths less than about 700 nm, presumably due to radiation-induced absorption of peroxy radical, non-bridging oxygen hole and E′-type centers. Even the low fluence exposures induced considerable fiber absorption in this spectral region. High-OH content fibers exhibit significant radiation-induced absorption above 1500 nm, in addition to the intrinsic absorption due to OH stretching modes.

Heavy ion mutagenesis: linear energy transfer effects and genetic linkage.

We have characterized a series of 69 independent mutants at the endogenous hprt locus of human TK6 lymphoblasts and over 200 independent S1-deficient mutants of the human x hamster hybrid cell line AL arising spontaneously or following low-fluence exposures to densely ionizing Fe ions (600 MeV/amu, linear energy transfer = 190 keV/microns). We find that large deletions are common. The entire hprt gene (> 44 kb) was missing in 19/39 Fe-induced mutants, while only 2/30 spontaneous mutants lost the entire hprt coding sequence. When the gene of interest (S1 locus = M1C1 gene) is located on a nonessential human chromosome 11, multilocus deletions of several million base pairs are observed frequently. The S1 mutation frequency is more than 50-fold greater than the frequency of hprt mutants in the same cells. Taken together, these results suggest that low-fluence exposures to Fe ions are often cytotoxic due to their ability to create multilocus deletions that may often include the loss of essential genes. In addition, the tumorigenic potential of these HZE heavy ions may be due to the high potential for loss of tumor suppressor genes. The relative insensitivity of the hprt locus to mutation is likely due to tight linkage to a gene that is required for viability.

Dark current induced in large CCD arrays by proton-induced elastic reactions and single to multiple-event spallation reactions

Computer simulations of the non-ionizing energy loss deposited in sensitive volumes as a result of proton-induced spallation reactions agree with analytic models for large sensitive volumes exposed to high fluence. They predict unique features for small volumes and low-fluence exposures which are observed in exposures of large arrays of CCD pixels. Calculations of the number of spallation reactions per pixel correlate with the recently reported relative frequency of switching dark-current states.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Perspectives on Fast-Neutron Mutagenesis of Human Lymphoblastoid Cells

The effects of low-fluence exposures to (Pu, Be) neutrons (En = 4.2 MeV) have been studied in a sensitive human B-lymphoblastoid cell line, TK6. Mutations were scored for two genetic loci, hypoxanthine phosphoribosyltransferase (hgprt) and thymidine kinase (tk), as a function of dose and dose rate. For exposures limited to less than one cell cycle, the mutation frequency for the hgprt locus was 1.92 X 10(-7)/cGy. When exposures were protracted over multiple cell generations, mutation yields were increased to 6.07 X 10(-7)/cGy. Similar yields were obtained for the induction of tk-deficient mutants with a normal cell generation time (tk-ng) when exposures were carried out at very low dose rates over multiple cell generations. In the series of data presented here, the results obtained for short-duration neutron exposures are compared with data obtained for monoenergetic heavy charged particles of defined linear energy transfer (LET) produced at the BEVALAC accelerator at Lawrence Berkeley Laboratory. TK6 cells have been exposed to beams ranging in atomic number from 20Ne to 40Ar over an energy range from 330 to 670 MeV/amu. Mutation induction was evaluated for both loci for a subset of these beams. The results obtained with 20Ne ions of 425 MeV/amu (LET = 32 keV/microns) and 28Si ions of 670 MeV/amu (LET = 50 keV/microns) closely resemble the mutation yields obtained for brief exposures to (Pu, Be) neutrons. The nature of alterations in DNA structure induced within the tk locus of tk-ng mutants is reviewed for a series of neutron-induced mutants and a series of mutants induced by exposure to 40Ar ions (470 MeV/amu, LET = 95 keV/microns). The mutational spectra for these two types of mutants were similar and were dominated by allele loss mutations. Multilocus deletions inclusive of the c-erbA1 locus were common among tk-deficient mutants induced by these densely ionizing radiations. For the mutants induced by 40Ar ions, it is likely that the mutations were produced by the traversal of the chromosome by a single particle.

Excimer laser direct etching of GaAs

The direct etching of GaAs substrates by excimer laser light has been studied at wavelengths of 193 nm (ArF). 248 nm (KrF) and 351 nm (XeF). For absorbed laser fluences greater than the direct etching fluence thresholds of 23 mJ cm-2 for ArF, 33 mJ cm-2 for KrF and 80 mJ cm-2 for XeF, the authors observe direct etching in air to depths of order one nanometre per pulse. For low-fluence exposures in either vacuum (10-2 Torr) or N2 (1 atm) environments the etch depth per pulse is less than a quarter of that observed when exposure takes place in air. For higher fluence exposures ( approximately 250 mJ cm-2) the etched regions show signs of melting. Using a line-narrowed KrF excimer laser and holographic interference techniques gratings of period 1.5 mu m were directly etched in GaAs substrates.

Void Formation in Neutron-Irradiated Ti-Al Alloys

Although it is generally thought that Ti1 and Ti-alloys2 are resistant to neutron-irradiation-induced void formation, this conclusion is prinicipally based on relatively low-fluence exposures (&lt;1 X 1022 neutrons/cm2) and limited heavy-ion irradiation experiments3. As part of an investigation of the mechanisms controlling the irradiation responses of Ti and its alloys, two Ti-Al alloys, containing 3 and 6 wt.% Al, respectively, and a pure Ti control material, prepared by the same procedures used for the alloys, have been examined by TEM following neutron irradiation in the EBR-II Reactor to a fluence of 5 X 1022 Neutrons/cm2, E&gt;0.1 MeV (∿35 displacements per atom, or dpa), at a temperature of 500°C.