Adam Davis

Spatio-temporal expression patterns of anterior Hox genes during Nile tilapia (Oreochromis niloticus) embryonic development.

Hox genes encode transcription factors that function to pattern regional tissue identities along the anterior-posterior axis during animal embryonic development. Divergent nested Hox gene expression patterns within the posterior pharyngeal arches may play an important role in patterning morphological variation in the pharyngeal jaw apparatus (PJA) between evolutionarily divergent teleost fishes. Recent gene expression studies have shown the expression patterns from all Hox paralog group (PG) 2-6 genes in the posterior pharyngeal arches (PAs) for the Japanese medaka (Oryzias latipes) and from most genes of these PGs for the Nile tilapia (Oreochromis niloticus). While several orthologous Hox genes exhibit divergent spatial and temporal expression patterns between these two teleost species in the posterior PAs, several tilapia Hox gene expression patterns from PG3-6 must be documented for a full comparative study. Here we present the spatio-temporal expression patterns of hoxb3b, c3a, b4a, a5a, b5a, b5b, b6a and b6b in the neural tube and posterior PAs of the Nile tilapia. We show that several of these tilapia Hox genes exhibit divergent expression patterns in the posterior PAs from their medaka orthologs. We also compare these gene expression patterns to orthologs in other gnathostome vertebrates, including the dogfish shark.

Stomach curvature is generated by left-right asymmetric gut morphogenesis

Left-right (LR) asymmetry is a fundamental feature of internal anatomy, yet the emergence of morphological asymmetry remains one of the least understood phases of organogenesis. Asymmetric rotation of the intestine is directed by forces outside the gut, but the morphogenetic events that generate anatomical asymmetry in other regions of the digestive tract remain unknown. Here, we show in mouse and Xenopus that the mechanisms that drive the curvature of the stomach are intrinsic to the gut tube itself. The left wall of the primitive stomach expands more than the right wall, as the left epithelium becomes more polarized and undergoes radial rearrangement. These asymmetries exist across several species, and are dependent on LR patterning genes, including Foxj1, Nodal and Pitx2. Our findings have implications for how LR patterning manifests distinct types of morphological asymmetries in different contexts. Highlighted Article: Left-right patterning creates morphogenetic differences between the left and right walls of the vertebrate foregut that generate the asymmetric curvature of the stomach.

Purely Angular Effects of Photon Buildup Factors for Thin Shields of Lead, Iron, Aluminum, and Water

Abstract This study investigates purely angular effects on photon buildup factors for slabs with optical thickness up to 10 mean free paths. Photon buildup factors are determined for different slabs, upon which monoenergetic photons between 50 keV and 10 MeV are incident at angles between 0 and 1.48 radians. As the incident angle is increased, the physical slab thickness is reduced to maintain a constant slant-path optical thickness relative to incident photons. This method identifies previously unexplored angular relationships between slab thickness and incident angle. Coupled electron/photon cross sections are used to account for secondary photon effects of bremsstrahlung and electron binding energies. The discrete ordinates code PARTISN is used to determine angular photon buildup factors for ten incident energies and ten incident angles for lead, iron, aluminum, and water slabs. Portions of these results are applicable to other slab geometry buildup studies.

Slant-Path Photon Buildup Factors in Dual-Layer Radiation Shields Comprising Polyethylene and Lead

Abstract Photon buildup factors provide a convenient method for radiation protection professionals to calculate dose and exposure behind various shielding configurations. Examination of buildup factors can also provide insight into the behavior of photons in these shields. Recent work has developed dual-layer buildup factors for several shielding configurations and a limited number of energies while slant-path buildup factors have been developed for single-layer shields. This work develops slant-path buildup factors for slab-geometric, dual-layer shields comprising polyethylene and lead at 25 energies conforming to the energies used in the buildup factor standard ANSI/ANS-6.4.3-1991 (W2001), “Gamma-Ray Attenuation Coefficients and Buildup Factors for Engineering Materials,” between 10 keV and 10 MeV. Further, the increased energy resolution of the calculations performed in this work allows the energy at which the previously identified “buildup reversal” phenomenon occurs to be more precisely identified. The effect of mesh spacing and quadrature resolution on fluence through the shields is also considered.

PHOTON BUILDUP FACTORS IN LAMINATED DUAL-LAYER SHIELDS

Abstract In radiation protection, photon buildup factors provide a convenient method for calculating dose and exposure response after various shielding configurations, as well as information about the behavior of radiation in these configurations. Though many situations call for multilayer shields, few databases and derived analytical formulas for photon buildup in multilayer shields exist. This research develops buildup factors and analytical fits to these data for slab-geometric, dual-layer shields composed of various materials. The photon buildup factors were calculated for monoenergetic photon sources incident on two-layer shields of various combinations of lead, polyethylene, aluminum, and stainless steel for thicknesses varying between 2 and 20 mean free paths using the Parallel Time Independent Sn (PARTISN) discrete ordinates code. The Gauss-Lobatto S100 quadrature was used with a 244-energy-group structure and coupled photon and electron cross sections. Data from PARTISN calculations were then benchmarked for representative cases using MCNP5, and fits to a new analytical formula were developed using Mathematica 6.0.

Radiation-Shielding Properties of Foam-Surrogate Heterogeneous Slabs of Stainless Steel, Aluminum, and Polyethylene

Abstract This research investigates the effect of heterogeneity in slabs of aluminum, stainless steel, and polyethylene on photon and neutron transmission. This work considers whether novel, heterogeneous combinations of these materials provides improved photon shielding (for metal-infiltrated polyethylene) or neutron shielding (for polyethylene-infiltrated metal). Often, layers of a hydrogenous material such as polyethylene must be combined with layers of a higher-atomic-number material to provide shielding for both photons and neutrons. Several heterogeneous shield configurations are studied in which slabs of a base material are implanted with metal stud arrays ranging from 5 × 5 × 5 to 11 × 11 × 11 arrays. For metal slabs infiltrated with polyethylene studs, it is found that the performance of the heterogeneous slabs as neutron shields relative to the homogeneous material is source-energy dependent. This is a larger concern for polyethylene-infiltrated aluminum (PA) than it is for polyethylene-infiltrated stainless steel (PS) as introduction of these studs impairs PA’s performance as a photon shield (relative to solid aluminum) more than it does for PS relative to solid stainless steel. For polyethylene slabs infiltrated with aluminum or stainless steel studs, it is found that introduction of a sufficiently spaced array of metal studs with a moderate-to-high photon absorption cross section will improve the photon-shielding properties of the shield without impairing the neutron-shielding properties. Use of an insufficiently opaque material or insufficiently wide spacing of the studs will impair the photon-shielding properties, thus making it a less effective shield than homogeneous polyethylene alone. This is a larger concern for PA than it is for PS. This research demonstrates that heterogeneity is more beneficial for stainless steel shields than it is for heterogeneous aluminum shields relative to homogeneous slabs of those materials.

A New Analysis of Composite Materials for Use in Glove-Box Applications

Oil and natural gas companies use 241Am sources for well-logging applications (in the form of americium-beryllium neutron sources). Currently, the domestic supply of 241Am is depleted, and industry is now purchasing sources from Russia. The goal of the Americium Recovery Project (ARP) is to reprocess defense-waste plutonium to recover 241Am that would then be sold to oil and gas companies, providing a safe, secure domestic source for industrial applications. Because the primary radiological concern with an 241Am source is external photon exposure, the radiological workers involved in the ARP will perform operations in glove boxes featuring lead-lined gloves. Given the U.S. mandate for the reduction of lead in industrial settings and the costs associated with the disposal of leaded gloves as mixed waste, alternatives are being considered to the traditional lead-lined gloves used in glove boxes. Several composite materials were previously developed and analyzed for incident photons of energies below 400 keV using the Lambert-Beer law to calculate transmission fractions. This research extends the energy range to 10 MeV and uses a source term of interest to the ARP. Further, the Monte Carlo transport code MCNP5 is used to calculate source-normalized doses using two common response functions: H’(0.07) and H*(10). The results and calculations presented in this research are more detailed than previous calculations and present further rationale for the context-specific selection of a given material.

Calculation of Photon Exposure and Ambient Dose Slant-Path Buildup Factors for Radiological Assessment

Abstract Slant-path photon buildup factors for nine radiation shielding materials (air, aluminum, concrete, iron, lead, leaded glass, polyethylene, stainless steel, and water) are calculated with the most recent cross-section data available using Monte Carlo and discrete ordinates methods. Discrete ordinates calculations use a 244-group energy structure based on previous research at Los Alamos National Laboratory (LANL) and focus on the effects of group widths in multigroup calculations for low-energy photons. Buildup-factor calculations in discrete ordinates benefit from coupled photon/electron cross sections to account for secondary photon effects. Also, ambient dose equivalent buildup factors were analyzed at lower energies where corresponding response functions do not exist in the literature. The results of these studies are directly applicable to radiation safety at LANL, where the dose-modeling code PANDEMONIUM is used to estimate worker dose in plutonium-handling facilities. Buildup factors determined in this work will be used to enhance the code’s modeling capabilities but also should be of general interest to the radiation shielding community.