Multiple Particle Positron Emission Particle Tracking and its Application to Flows in Porous Media

Positron emission particle tracking (PEPT) is a method for flow interrogation capable of measurement in opaque systems. In this work a novel method for PEPT is introduced that allows for simultaneous tracking of multiple tracers. This method (M-PEPT) is adapted from optical particle tracking techniques and is designed to track an arbitrary number of positron-emitting tracer-particles entering and leaving the field of view of a detector array. M-PEPT is described, and its applicability is demonstrated for a number of measurements ranging from turbulent shear flow interrogation to cell migration. It is found that this method can locate over 80 particles simultaneously with spatial resolution of order 0.2 mm at tracking frequency of 10 Hz and, at lower particle number densities, can achieve similar spatial resolution at tracking frequency 1000 Hz. The method is limited in its ability to resolve particles approaching close to one another, and suggestions for future improvements are made.M-PEPT is used to study flow in porous media constructed from packing of glass beads of different diameters. Anomalous (i.e. non-Fickian) dispersion of tracers is studied in these systems under the continuous time random walk (CTRW) paradigm. Pore-length transition time distributions are measured, and it is found that in all cases, these distributions indicate the presence of long waiting times between transitions, confirming the central assumption of the CTRW model. All systems demonstrate non-Fickian spreading of tracers at early and intermediate times with a late time recovery of Fickian dispersion, but a clear link between transition time distributions and tracer spreading is not made. Velocity increment statistics are examined, and it is found that temporal velocity increments in the mean-flow direction show a universal scaling. Spatial velocity increments also appear to collapse to a similar form, but there is insufficient data to determine the presence of universal scaling

The role of dredge-up in double white dwarf mergers

We present the results of an investigation of the dredge-up and mixing during the merger of two white dwarfs with different chemical compositions by conducting hydrodynamic simulations of binary mergers for three representative mass ratios. In all the simulations, the total mass of the two white dwarfs is $\lesssim1.0~{\rm M_\odot}$. Mergers involving a CO and a He white dwarf have been suggested as a possible formation channel for R Coronae Borealis type stars, and we are interested in testing if such mergers lead to conditions and outcomes in agreement with observations. Even if the conditions during the merger and subsequent nucleosynthesis favor the production of $^{18}{\mathrm O}$, the merger must avoid dredging up large amounts of $^{16}{\mathrm O}$, or else it will be difficult to produce sufficient $^{18}{\mathrm O}$ to explain the oxygen ratio observed to be of order unity. We performed a total of 9 simulations using two different grid-based hydrodynamics codes using fixed and adaptive meshes, and one smooth particle hydrodynamics (SPH) code. We find that in most of the simulations, $>10^{-2}~{\rm M_\odot}$ of $^{16}{\mathrm O}$ is indeed dredged up during the merger. However, in SPH simulations where the accretor is a hybrid He/CO white dwarf with a $\sim 0.1~{\rm M_\odot}$ layer of helium on top, we find that no $^{16}{\mathrm O}$ is being dredged up, while in the $q=0.8$ simulation $<10^{-4}~{\rm M_\odot}$ of $^{16}{\mathrm O}$ has been brought up, making a WD binary consisting of a hybrid CO/He WD and a companion He WD an excellent candidate for the progenitor of RCB stars.Comment: Accepted for publication in Ap

Why health visiting? Examining the potential public health benefits from health visiting practice within a universal service: A narrative review of the literature

INTRODUCTION: There is increasing international interest in universal, health promoting services for pregnancy and the first three years of life and the concept of proportionate universalism. Drawing on a narrative review of literature, this paper explores mechanisms by which such services might contribute to health improvement and reducing health inequalities. OBJECTIVES: Through a narrative review of empirical literature, to identify: DESIGN: The paper draws upon a scoping study and narrative review. REVIEW METHODS: We used three complementary approaches to search the widely dispersed literature: Our key inclusion criterion was information about health visiting practice. We included empirical papers from United Kingdom (UK) from 2004 to February 2012 and older seminal papers identified in search (3), identifying a total of 348 papers for inclusion. A thematic content analysis compared the older (up to 2003) with more recent research (2004 onwards). RESULTS: The analysis revealed health visiting practice as potentially characterized by a particular 'orientation to practice.' This embodied the values, skills and attitudes needed to deliver universal health visiting services through salutogenesis (health creation), person-centredness (human valuing) and viewing the person in situation (human ecology). Research about health visiting actions focuses on home visiting, needs assessment and parent-health visitor relationships. The detailed description of health visitors' skills, attitudes, values, and their application in practice, provides an explanation of how universal provision can potentially help to promote health and shift the social gradient of health inequalities. CONCLUSIONS: Identification of needs across an undifferentiated, universal caseload, combined with an outreach style that enhances uptake of needed services and appropriate health or parenting information, creates opportunities for parents who may otherwise have remained unaware of, or unwilling to engage with such provision. There is a lack of evaluative research about health visiting practice, service organization or universal health visiting as potential mechanisms for promoting health and reducing health inequalities. This paper offers a potential foundation for such research in future

The SPIRITS Sample of Luminous Infrared Transients: Uncovering Hidden Supernovae and Dusty Stellar Outbursts in Nearby Galaxies

We present a systematic study of the most luminous (M IR [Vega magnitudes] brighter than −14) infrared (IR) transients discovered by the SPitzer InfraRed Intensive Transients Survey (SPIRITS) between 2014 and 2018 in nearby galaxies (D 12) show multiple, luminous IR outbursts over several years and have directly detected, massive progenitors in archival imaging. With analyses of extensive, multiwavelength follow-up, we suggest the following possible classifications: five obscured core-collapse supernovae (CCSNe), two erupting massive stars, one luminous red nova, and one intermediate-luminosity red transient. We define a control sample of all optically discovered transients recovered in SPIRITS galaxies and satisfying the same selection criteria. The control sample consists of eight CCSNe and one Type Iax SN. We find that 7 of the 13 CCSNe in the SPIRITS sample have lower bounds on their extinction of 2 < A V < 8. We estimate a nominal fraction of CCSNe in nearby galaxies that are missed by optical surveys as high as ${38.5}_{-21.9}^{+26.0} \%$ (90% confidence). This study suggests that a significant fraction of CCSNe may be heavily obscured by dust and therefore undercounted in the census of nearby CCSNe from optical searches

Three-dimensional spatiotemporal tracking of fluorine-18 radiolabeled yeast cells via positron emission particle tracking

<div><p>A method for Positron Emission Particle Tracking (PEPT) based on optical feature point identification techniques is demonstrated for use in low activity tracking experiments. A population of yeast cells of approximately 125,000 members is activated to roughly 55 Bq/cell by ¹⁸F uptake. An <i>in vitro</i> particle tracking experiment is performed with nearly 20 of these cells after decay to 32 Bq/cell. These cells are successfully identified and tracked simultaneously in this experiment. This work extends the applicability of PEPT as a cell tracking method by allowing a number of cells to be tracked together, and demonstrating tracking for very low activity tracers.</p></div

Trajectories of yeast cells measured via PEPT in second scan.

<p>Different colors indicate different particles. Colors are coordinated by particle across panels. Trajectories that are suspected to be affected by occlusion are indicated by large square icons.</p

Average background line density grid at 5 axial locations in Inveon scanner.

<p>Images show number of CL crossings per minute, with line crossings counted on a 2 mm × 2 mm × 2 mm grid. Images are smoothed via a boxcar kernel of side width 3 voxels before averaging.</p

Trajectories of yeast cells measured via PEPT in first scan.

<p>Different colors indicate different particles. Colors are coordinated by particle across panels.</p