6 research outputs found

    Development of optimised tissue-equivalent materials for proton therapy

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    OBJECTIVE: In proton therapy there is a need for proton optimised tissue-equivalent materials as existing phantom materials can produce large uncertainties in the determination of absorbed dose and range measurements. The aim of this work is to develop and characterise optimised tissue-equivalent materials for proton therapy. APPROACH: A mathematical model was developed to enable the formulation of epoxy-resin based tissue-equivalent materials that are optimised for all relevant interactions of protons with matter, as well as photon interactions, which play a role in the acquisition of CT numbers. This model developed formulations for vertebra bone- and skeletal muscle-equivalent plastic materials. The tissue equivalence of these new materials and commercial bone- and muscle-equivalent plastic materials were theoretical compared against biological tissue compositions. The new materials were manufactured and characterised by their mass density, relative stopping power (RSP) measurements, and CT scans to evaluate their tissue-equivalence. MAIN RESULTS: Results showed that existing tissue-equivalent materials can produce large uncertainties in proton therapy dosimetry. In particular commercial bone materials showed to have a relative difference up to 8 % for range. On the contrary, the best optimised formulations were shown to mimic their target human tissues within 1-2 % for the mass density and RSP. Furthermore, their CT-predicted RSP agreed within 1-2 % of the experimental RSP, confirming their suitability as clinical phantom materials. SIGNIFICANCE: We have developed a tool for the formulation of tissue-equivalent materials optimised for proton dosimetry. Our model has enabled the development of proton optimised tissue-equivalent materials which perform better than existing tissue-equivalent materials. These new materials will enable the advancement of clinical proton phantoms for accurate proton dosimetry

    Reactivation of tectonics, crustal underplating, and uplift after 60 Myr of passive subsidence, Raukumara Basin, Hikurangi-Kermadec fore arc, New Zealand: implications for global growth and recycling of continents

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    We use seismic reflection and refraction data to determine crustal structure, to map a fore-arc basin containing 12 km of sediment, and to image the subduction thrust at 35 km depth. Seismic reflection megasequences within the basin are correlated with onshore geology: megasequence X, Late Cretaceous and Paleogene marine passive margin sediments; megasequence Y, a similar to 10,000 km(3) submarine landslide emplaced during subduction initiation at 22 Ma; and megasequence Z, a Neogene subduction margin megasequence. The Moho lies at 17 km beneath the basin center and at 35 km at the southern margin. Beneath the western basin margin, we interpret reflective units as deformed Gondwana fore-arc sediment that was thrust in Cretaceous time over oceanic crust 7 km thick. Raukumara Basin has normal faults at its western margin and is uplifted along its eastern and southern margins. Raukumara Basin represents a rigid fore-arc block > 150 km long, which contrasts with widespread faulting and large Neogene vertical axis rotations farther south. Taper of the western edge of allochthonous unit Y and westward thickening and downlap of immediately overlying strata suggest westward or northwestward paleoslope and emplacement direction rather than southwestward, as proposed for the correlative onshore allochthon. Spatial correlation between rock uplift of the eastern and southern basin margins with the intersection between Moho and subduction thrust leads us to suggest that crustal underplating is modulated by fore-arc crustal thickness. The trench slope has many small extensional faults and lacks coherent internal reflections, suggesting collapse of indurated rock, rather than accretion of > 1 km of sediment from the downgoing plate. The lack of volcanic intrusion east of the active arc, and stratigraphic evidence for the broadening of East Cape Ridge with time, suggests net fore-arc accretion since 22 Ma. We propose a cyclical fore-arc kinematic: rock moves down a subduction channel to near the base of the crust, where underplating drives rock uplift, oversteepens the trench slope, and causes collapse toward the trench and subduction channel. Cyclical rock particle paths led to persistent trench slope subsidence during net accretion. Existing global estimates of fore-arc loss are systematically too high because they assume vertical particle paths. Citation: Sutherland, R., et al. (2009), Reactivation of tectonics, crustal underplating, and uplift after 60 Myr of passive subsidence, Raukumara Basin, Hikurangi-Kermadec fore arc, New Zealand: Implications for global growth and recycling of continents, Tectonics, 28, TC5017, doi: 10.1029/2008TC002356

    Factors influencing the success of hand-reared juvenile brushtail possums (Trichosurus vulpecula) released into the wild

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    Context: Wildlife is ubiquitous in urban environments, resulting in frequent interactions with humans and human infrastructure. The result of these interactions is often negative, in the form of road injury, orphaning of dependent young or eviction from natural home ranges. Wildlife rehabilitation programmes are devised to counter these negative interactions. However, the success of current management strategies is rarely assessed. Aims: We aimed to determine whether short-term survival of juvenile hand-reared common brushtail possums (Trichosurus vulpecula) released in the wild was a function of (1) intrinsic factors such as sex, individual personality and level of human habituation, and (2) extrinsic factors such as release practice (soft vs hard-release) or location (urban vs rural). We also evaluated the relationship between habituation to humans and time spent in care by possums, if presence of conspecifics during the rehabilitation process influenced the development of individual personality, and if this differed in hand-raised animals compared with wild individuals. Methods: We radio-tracked and monitored 20 hand-reared juvenile possums (10 females and 10 males) for up to 40 days after release in the wild. Key results: Eight possums (40%) survived until the end of the study, nine possums (45%) were killed by foxes or had to be returned into care and three possums (15%) had unknown fates (i.e. lost VHF signal). We found that more exploratory individuals and those less human-habituated were more likely to be successful in the wild in both rural and urban areas, whether or not they were hard or soft released. Conclusions: Our results suggest that personality is a key criterion to consider when evaluating the success of rehabilitation programmes. Behavioural traits of hand-reared brushtail possums differed significantly from those of wild individuals, showing that captive conditions can affect the development of personality. Hand-reared possums that spent more time in care were also more likely to display highly human-habituated behaviours. Implications: By demonstrating which factors influence success of hand-reared wildlife after release in the wild, our results help in evaluating current rearing and release practices, and improve the evidence base for developing best practice wildlife rehabilitation guidelines
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