Journal Articles

Browse

Now showing 1 - 8 of 8

14C analyses at the ANTARES AMS Centre: dating the log coffins of northwest Thailand
(Elsevier, 1994-06-03) Hotchkis, MAC; Fink, D; Jacobsen, GE; Lawson, EM; Shying, ME; Smith, AM; Tuniz, C; Barbetti, M; Grave, P; Quan, HM; Head, J
Recent results of 14C analyses at the ANTARES AMS Centre are presented. Test measurements of 14C blanks demonstrate an ultimate sensitivity of the order of 10−15 (14C/12C ratio). Measurements of unknowns have been made with a precision in the range 1–1.5% using a “slow cycling” mode of operation where the injection magnet field is changed to inject 14C and 13C alternately. Results are presented for a series of log coffins from cave burials in NW Thailand. © 1994 Elsevier B.V.
Accelerator mass spectrometry analyses of environmental radionuclides: sensitivity, precision and standardisation
(Elsevier, 2000-10-01) Hotchkis, MAC; Fink, D; Tuniz, C; Vogt, S
Accelerator Mass Spectrometry (AMS) is the analytical technique of choice for the detection of long-lived radionuclides which cannot be practically analysed with decay counting or conventional mass spectrometry. AMS allows an isotopic sensitivity as low as one part in 1015 for 14C (5.73 ka), 10Be (1.6 Ma), 26Al (720 ka), 36Cl (301 ka), 41Ca (104 ka), 129I (16 Ma) and other long-lived radionuclides occurring in nature at ultra-trace levels. These radionuclides can be used as tracers and chronometers in many disciplines: geology, archaeology, astrophysics, biomedicine and materials science. Low-level decay counting techniques have been developed in the last 40–50 years to detect the concentration of cosmogenic, radiogenic and anthropogenic radionuclides in a variety of specimens. Radioactivity measurements for long-lived radionuclides are made difficult by low counting rates and in some cases the need for complicated radiochemistry procedures and efficient detectors of soft β-particles and low energy x-rays. The sensitivity of AMS is unaffected by the half-life of the isotope being measured, since the atoms not the radiations that result from their decay, are counted directly. Hence, the efficiency of AMS in the detection of long-lived radionuclides is 106–109 times higher than decay counting and the size of the sample required for analysis is reduced accordingly. For example, 14C is being analysed in samples containing as little as 20 μg carbon. There is also a world-wide effort to use AMS for the analysis of rare nuclides of heavy mass, such as actinides, with important applications in safeguards and nuclear waste disposal. Finally, AMS microprobes are being developed for the in-situ analysis of stable isotopes in geological samples, semiconductors and other materials. Unfortunately, the use of AMS is limited by the expensive accelerator technology required, but there are several attempts to develop compact AMS spectrometers at low (⩽0.5 MV) terminal voltages. Recent advances in AMS will be reviewed with highlights from the scientific programs at Lucas Heights and other AMS centres. © 2000 Elsevier Science Ltd.
Equipment and methodology for high precision, high throughput 14C AMS analyses at ANTARES
(Elsevier, 1994-06-03) Smith, AM; Fink, D; Hotchkis, MAC; Jacobsen, GE; Lawson, EM; Shying, ME; Tuniz, C; Watt, GC; Fallon, J; Ellis, PJ
The original Rutgers FN tandem accelerator has been modified for high-precision high-throughput AMS analyses at the ANTARES AMS Center. The status of this work and future plans are discussed, with emphasis on 14C. Isotopic ratios are measured at ANTARES by sequentially injecting stable isotopes and the radioisotope, and the systems developed for this purpose are described. Progress with a new injection platform and high intensity, multi-sample ion source is also given. © 1994 Elsevier B.V.
Impact of extraneous mispositioned events on motion-corrected brain SPECT images of freely moving animals
(American Association of Physicists in Medicine, 2014-08-18) Angelis, GI; Ryder, WJ; Bashar, R; Fulton, RR; Meikle, SR
Purpose: Single photon emission computed tomography (SPECT) brain imaging of freely moving small animals would allow a wide range of important neurological processes and behaviors to be studied, which are normally inhibited by anesthetic drugs or precluded due to the animal being restrained. While rigid body motion of the head can be tracked and accounted for in the reconstruction, activity in the torso may confound brain measurements, especially since motion of the torso is more complex (i.e., nonrigid) and not well correlated with that of the head. The authors investigated the impact of mispositioned events and attenuation due to the torso on the accuracy of motion corrected brain images of freely moving mice. Methods: Monte Carlo simulations of a realistic voxelized mouse phantom and a dual compartment phantom were performed. Each phantom comprised a target and an extraneous compartment which were able to move independently of each other. Motion correction was performed based on the known motion of the target compartment only. Two SPECT camera geometries were investigated: a rotating single head detector and a stationary full ring detector. The effects of motion, detector geometry, and energy of the emitted photons (hence, attenuation) on bias and noise in reconstructed brain regions were evaluated. Results: The authors observed two main sources of bias: (a) motion-related inconsistencies in the projection data and (b) the mismatch between attenuation and emission. Both effects are caused by the assumption that the orientation of the torso is difficult to track and model, and therefore cannot be conveniently corrected for. The motion induced bias in some regions was up to 12% when no attenuation effects were considered, while it reached 40% when also combined with attenuation related inconsistencies. The detector geometry (i.e., rotating vs full ring) has a big impact on the accuracy of the reconstructed images, with the full ring detector being more advantageous. Conclusions: Motion-induced inconsistencies in the projection data and attenuation/emission mismatch are the two main causes of bias in reconstructed brain images when there is complex motion. It appears that these two factors have a synergistic effect on the qualitative and quantitative accuracy of the reconstructed images. © 2014 American Association of Physicists in Medicine.
Integration of ice-core, marine and terrestrial records for the Australian Last Glacial Maximum and Termination: a contribution from the OZ INTIMATE group
(Wiley, 2006-10) Turney, CSM; Haberle, SG; Fink, D; Kershaw, AP; Barbetti, M; Barrows, TT; Black, M; Cohen, TJ; Corrège, T; Hesse, PP; Hua, Q; Johnston, R; Morgan, VI; Moss, PT; Nanson, GC; van Ommen, TD; Rule, S; Williams, NJ; Zhao, JX; D'Costa, D; Feng, YX; Gagan, MK; Mooney, SD; Xia, Q
The degree to which Southern Hemisphere climatic changes during the end of the last glacial period and early Holocene (30-8 ka) were influenced or initiated by events occurring in the high latitudes of the Northern Hemisphere is a complex issue. There is conflicting evidence for the degree of hemispheric ‘teleconnection’ and an unresolved debate as to the principle forcing mechanism(s). The available hypotheses are difficult to test robustly, however, because the few detailed palaeoclimatic records in the Southern Hemisphere are widely dispersed and lack duplication. Here we present climatic and environmental reconstructions from across Australia, a key region of the Southern Hemisphere because of the range of environments it covers and the potentially important role regional atmospheric and oceanic controls play in global climate change. We identify a general scheme of events for the end of the last glacial period and early Holocene but a detailed reconstruction proved problematic. Significant progress in climate quantification and geochronological control is now urgently required to robustly investigate change through this period. © 2006 John Wiley & Sons, Ltd.
Piloting Rasch model scoring of the national eye institute visual function questionnaire in uveitis
(Springer Nature, 2024-04-16) Gittel, L; Li, JQ; Dell, J; Wintergerst, MWM; Heinz, C; Finger, RP; Terheyden, JH
Introduction The National Eye Institute Visual Function Questionnaire (NEI VFQ) is a common patient-reported outcome measure (PROM) in uveitis trials. Its psychometric properties using state-of-the-art scoring based on Rasch models, a latent trait model that improves accuracy of PROMs assessment, has not yet been investigated. Methods The study participants were recruited online from uveitis patient organizations, where individuals self-reported their uveitis diagnosis and visual acuity level. These participants then completed the NEI VFQ-25. The visual function (VF) and socioemotional (SE) subscales were psychometrically analysed in terms of item fit, targeting, internal consistency, dimensionality, and differential item functioning (DIF), using Rasch models. Criterion validity was examined based on associations between NEI VFQ person measures and recent visual acuity (VA) levels. Results Ninety-nine participants recruited online from uveitis patient organizations (68 women, 31 men; mean age 50 ± 15 years; 46.5% self-reported receiving systematic therapy for uveitis, 0.6% NEI VFQ-25 missing data) were included. The mean difficulty of items was lower than the average person ability. None of the items demonstrated misfit to an extent that would induce noise into the measurement. The consistency metrics person reliability and person separation index of the subscales were 0.85 and 2.34 (NEI VFQ-VF), 0.86 and 2.52 (NEI VFQ-SE), respectively. There was no evidence of multidimensionality and none of the items showed DIF by gender. The differences between item and person measures were 1.44 (NEI VFQ-VF) and 1.03 (NEI VFQ-SE). NEI VFQ-25 person measures were significantly lower in participants with visual impairment (all p values ≤ 0.007). Conclusion Rasch model-based scoring of the re-engineered NEI VFQ-25 demonstrates acceptable internal consistency, item fit and construct validity for assessing two key domains of quality of life in individuals self-reporting uveitis. The PROM was targeted at a higher level of difficulty than present in our heterogeneous sample. © 2024 © The Author(s) 2024. Open Access This article is licensed under a Creative Commons Attribution 4.0 International Licence.
Triclinic apatites
(International Union of Crystallography, 2007-04) Baikie, T; Mercier, PHJ; Elcombe, MM; Kim, JY; Le Page, Y; Mitchell, LD; White, TJ; Whitfield, PS
Apatites commonly adopt P63/m hexagonal symmetry. More rarely, monoclinic chemical analogues have been recognized, including the biologically significant hydroxyapatite, Ca10(PO4)6(OH)2, but the driving force towards lower symmetry has not been systematically examined. A combination of diffraction observations and ab initio calculations for Ca10(AsO4)6F2 and Ca10(VO4)6F2 show these materials are triclinic apatites in which the AsO4 and VO4 tetrahedra tilt to relieve stress at the metal and metalloid sites to yield reasonable bond-valence sums. An analysis of the triclinic non-stoichiometric apatites La10 − x(GeO4)6O3 − 1.5x and Ca10(PO4)6(OH)2 − xOx/2 confirms this scheme of tetrahedral rotations, while Cd10(PO4)6F2 and Ca10(CrO4)6F2 are predicted to be isostructural. These distortions are in contrast to the better known P1121/b monoclinic dimorphs of chloroapatite and hydroxyapatite, where the impetus for symmetry reduction is ordered anion (OH− and Cl−) displacements which are necessary to obtain acceptable bond lengths. These results are important for designing apatites with specific structural and crystal-chemical characteristics. © 2007, International Union of Crystallography
X-ray mass attenuation coefficients and imaginary components of the atomic form factor of zinc over the energy range of 7.2-15.2 keV
(American Physical Society, 2010-02) Rae, NA; Chantler, CT; Barnea, Z; de Jonge, MD; Tran, CQ; Hester, JR
The x-ray mass attenuation coefficients of zinc are measured in a high-accuracy experiment between 7.2 and 15.2 keV with an absolute accuracy of 0.044% and 0.197%. This is the most accurate determination of any attenuation coefficient on a bending-magnet beamline and reduces the absolute uncertainty by a factor of 3 compared to earlier work by advances in integrated column density determination and the full-foil mapping technique described herein. We define a relative accuracy of 0.006%, which is not the same as either the precision or the absolute accuracy. Relative accuracy is the appropriate parameter for standard implementation of analysis of near-edge spectra. Values of the imaginary components f″ of the x-ray form factor of zinc are derived. Observed differences between the measured mass attenuation coefficients and various theoretical calculations reach a maximum of about 5% at the absorption edge and up to 2% further than 1 keV away from the edge. The measurements invite improvements in the theoretical calculations of mass attenuation coefficients of zinc. © 2010, American Physical Society

Browse

Browsing Journal Articles by Subject "Accuracy"

Results Per Page

Sort Options