Fraction Of Absorbers Research Articles

Prevalence of neutral gas in centres of merging galaxies

We present Giant Metrewave Radio Telescope and Very Large Array observations of HI 21-cm absorption in ten z<=0.2 galaxy mergers that host strong radio sources. Seven of these mergers show strong absorption [with N(HI) ~10^{21-22} cm^{-2}, for spin temperature of 100 K and unit covering factor], leading to a total detection rate of 83 +/- 17% in low-z radio-loud galaxy mergers. This is ~3-4 times higher than that found for intrinsic HI 21-cm absorption in low-z radio sources in general. The fraction of intrinsic absorbers that are associated with mergers increases with increasing N(HI) threshold, i.e. ~40% and 100% of the absorbers with N(HI) >10^{21} cm^{-2} and >10^{22} cm^{-2} arise from mergers, respectively. The distribution of N(HI) among the mergers is significantly different from that found in isolated systems, with mergers giving rise to six times stronger absorption on average. The fraction of redshifted absorption components (with respect to the systemic velocity of the radio source obtained from optical emission lines) among mergers is found to be higher by two to three times compared to that found for non-interacting systems. Follow-up spatially-resolved multi-wavelength spectroscopy is essential to understand the exact connection between the presence of circumnuclear neutral gas and the AGN activity in these mergers.

Bilirubin photoisomers in rhesus monkey serum

As rhesus monkeys exhibit physiological jaundice during the neonatal period, we used rhesus monkey serum to examine changes in bilirubin photoisomers. Bilirubin-rhesus monkey serum solution was irradiated with blue light-emitting diode, and changes in the absorbance and bilirubin fraction were compared with those in bilirubin- human serum albumin (HSA) and bilirubin-rat albumin solutions. The λmax decreased with light irradiation. The mean production rate of cyclobilirubin IXα was 1.98, 199 and 0.76 × 10−2/min in rhesus monkey serum, HSA and rat albumin, respectively. There was no significant difference between rhesus monkey serum and HSA. The (ZE)-bilirubin IXα/(ZZ)-bilirubin IXα ratio was 0.33, 0.45, and 0.10, respectively, differing significantly among the groups. The (EZ)-bilirubin IXα/(ZZ)-bilirubin IXα ratio was 0.020, 0.010, and 0.062, respectively, with no significant difference between rhesus monkey serum and HSA. The production rate of (EZ)-cyclobilirubin XIIIα(= (ZE)-cyclobilirubin XIIIα) was 0.73, 1.60, and 0.51 × 10−2/min, respectively, with differing significantly among the groups. The (EZ)-bilirubin IIIα/(ZZ)-bilirubin IIIα ratio was significantly different among the groups at 0.20, 0.38, and 0.15, respectively.This is the first report demonstrating the photoisomerization of bilirubin in rhesus monkey serum and the animal with the same cyclobilirubin production rate as HSA.Rhesus monkeys may be used as an animal model for neonatal hyperbilirubinemia in humans to evaluate the efficacy of phototherapy.

The Dispersion of Fast Radio Bursts from a Structured Intergalactic Medium at Redshifts z &lt; 1.5

Abstract We analyze the sources of free electrons that produce the large dispersion measures, (in units of cm−3 pc), observed toward fast radio bursts (FRBs). Individual galaxies typically produce from ionized gas in their disk, disk-halo interface, and circumgalactic medium. Toward an FRB source at redshift z, a homogeneous intergalactic medium (IGM) containing a fraction of cosmological baryons will produce , where . A structured IGM of photoionized Lyα absorbers in the cosmic web produces similar dispersion, modeled from the observed distribution, , of H i (Lyα-forest) absorbers in column density and redshift with ionization corrections and scaling relations from cosmological simulations. An analytic formula for DM(z) applied to observed FRB dispersions suggests that for an IGM containing a significant baryon fraction, . Future surveys of the statistical distribution, DM( , of FRBs identified with specific galaxies and redshifts can be used to calibrate the IGM baryon fraction and distribution of Lyα absorbers. Fluctuations in DM at the level ±10 cm−3 pc will arise from filaments and voids in the cosmic web.

AN HST/COS SURVEY OF THE LOW-REDSHIFT INTERGALACTIC MEDIUM. I. SURVEY, METHODOLOGY, AND OVERALL RESULTS*

ABSTRACT We use high-quality, medium-resolution Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) observations of 82 UV-bright active galactic nuclei (AGNs) at redshifts z AGN &lt; 0.85 to construct the largest survey of the low-redshift intergalactic medium (IGM) to date: 5138 individual extragalactic absorption lines in H i and 25 different metal-ion species grouped into 2611 distinct redshift systems at z abs &lt; 0.75 covering total redshift pathlengths Δz H i = 21.7 and Δz O vi = 14.5. Our semi-automated line-finding and measurement technique renders the catalog as objectively defined as possible. The cumulative column density distribution of H i systems can be parametrized d &#x0E23A; ( &gt; N ) / dz ?> = C 14 ( N / 10 14 cm &minus; 2 ) &minus; ( &beta; &minus; 1 ) ?> , with C 14 = 25 ± 1 and β = 1.65 ± 0.02. This distribution is seen to evolve both in amplitude, C 14 &prop; ( 1 &plus; z ) 2.3 &PlusMinus; 0.1 ?> , and slope β(z) = 1.75–0.31 z for z ≤ 0.47. We observe metal lines in 418 systems, and find that the fraction of IGM absorbers detected in metals is strongly dependent on N H I ?> . The distribution of O vi absorbers appears to evolve in the same sense as the Lyα forest. We calculate contributions to Ω b from different components of the low-z IGM and determine the Lyα decrement as a function of redshift. IGM absorbers are analyzed via a two-point correlation function in velocity space. We find substantial clustering of H i absorbers on scales of Δv = 50–300 km s−1 with no significant clustering at Δv ≳ 1000 km s−1. Splitting the sample into strong and weak absorbers, we see that most of the clustering occurs in strong, N H i ≳ 1013.5 cm−2, metal-bearing IGM systems. The full catalog of absorption lines and fully reduced spectra is available via the Mikulski Archive for Space Telescopes (MAST) as a high-level science product at http://archive.stsci.edu/prepds/igm/.

Using in vitro bioaccessibility to refine estimates of human exposure to PAHs via incidental soil ingestion

PAH bioaccessibility in contaminated soil was determined using the organic physiologically based extraction test with the inclusion of a sorption sink. Silicone cord was incorporated into the assay in order to overcome the limited capacity of the in vitro medium to accommodate desorbable PAHs. Initially, silicone cord sorption efficacy was determined by assessing sorption kinetics using PAH-spiked sand (phenanthrene, pyrene and benzo[a]pyrene; 10–1000mgkg−1). Irrespective of PAH and concentration, >95% of the initial PAH mass partitioned into the silicone cord within 12h although rates were lower at higher concentration and with increasing hydrophobicity. When PAH bioaccessibility was assessed in contaminated soil (n=18), contamination source (i.e. pyrogenic versus petrogenic) influenced PAH bioaccessibility. Individual PAH bioaccessibility ranged up to 81.7±2.7% although mean values ranged from 2.1 (acenaphthalene) to 20.8% (benzo[k]fluoranthene) with upper 95% confidence intervals of the means of 4.5 and 28.3% respectively. Although a PAH in vivo–in vitro correlation is yet to be established, bioaccessibility approaches incorporating sorption sinks represent a robust approach for estimating PAH bioavailability as the desorbable fraction may be a conservative measure of the absorbable fraction.

Reionisation and High-Redshift Galaxies: The View from Quasar Absorption Lines

AbstractDetermining when and how the first galaxies reionised the intergalactic medium promises to shed light on both the nature of the first objects and the cosmic history of baryons. Towards this goal, quasar absorption lines play a unique role by probing the properties of diffuse gas on galactic and intergalactic scales. In this review, we examine the multiple ways in which absorption lines trace the connection between galaxies and the intergalactic medium near the reionisation epoch. We first describe how the Ly α forest is used to determine the intensity of the ionising ultraviolet background and the global ionising emissivity budget. Critically, these measurements reflect the escaping ionising radiation from all galaxies, including those too faint to detect directly. We then discuss insights from metal absorption lines into reionisation-era galaxies and their surroundings. Current observations suggest a buildup of metals in the circumgalactic environments of galaxies overz~ 6 to 5, although changes in ionisation will also affect the evolution of metal line properties. A substantial fraction of metal absorbers at these redshifts may trace relatively low-mass galaxies. Finally, we review constraints from the Ly α forest and quasar near zones on the timing of reionisation. Along with other probes of the high-redshift Universe, absorption line data are consistent with a relatively late end to reionisation (5.5 ≲z≲ 7); however, the constraints are still fairly week. Significant progress is expected to come through improved analysis techniques, increases in the number of known high-redshift quasars from optical and infrared sky surveys, large gains in sensitivity from next-generation observing facilities, and synergies with other probes of the reionisation era.

Lead in Game Meat: A Study of Bioaccessibility of Lead Metal Fragments

The presence of metal lead fragments in the tissue around the wound channel can be observed in animals killed with lead-based bullets. These parts are normally discarded as food but a limited number of fragments may be consumed accidentally. A number of studies are performed to estimate mean lead metal contamination in game meat harvested with rifle and is assumed that all lead metal fragments to its total weight that may accidentally be left in the edible muscle tissue can be taken up by the body severely increasing B-Pb level. When blood lead level, B-Pb, is an important biomarker for dietary exposure from lead several studies have been conducted comparing blood lead levels in hunters with similar population groups who do not eat wild game. Even taking into account environmental and social conditions and lead related occupations or hobbies, few conclusions can be drawn concerning risk assessment. This because lead pathways in general as well as the actual bioaccessibility of any metal lead accidently ingested, and the actual absorbable (bioavailable) fraction of this, are not compared or taken into account. The aim of this study was, by mean of in-vitro gastrointestinal simulation (IVG), to experimentally examine what percentage of any metal lead fragments present in tissue from a severely contaminated wound channel is converted to water soluble bioaccessible lead salts in the human gastrointestinal tract. Results obtained show that less than 2% of present lead metal fragments in raw meat from wound channel are converted to bioaccessible form in the human GI tract. Using this knowledge of the percentage of bioaccessible lead released, the impact on human blood lead levels from known or estimated contamination of lead metal in food can be easily calculated with use of Carlisle and Wade (1992) empirical equation for dietary exposure adopted by CONTAM panel 2010 using Slope Factor Models and absorption fractions.

Monte Carlo derived absorbed fractions for a voxelized model of Oncorhynchus mykiss, a rainbow trout.

Simple, ellipsoidal geometries have long been the standard for estimating radiation dose rates in non-human biota (NHB). With the introduction of a regulatory protection standard that emphasizes protection of NHB as its own end point, there has been interest in improved models for the calculation of dose rates in NHB. Here, we describe the creation of a voxelized model for a rainbow trout (Oncorhynchus mykiss), a freshwater aquatic salmonid. Absorbed fractions (AFs) for both photon and electron sources were tabulated at electron energies of 0.1, 0.2, 0.4, 0.5, 0.7, 1.0, 1.5, 2.0, and 4.0MeV and photon energies of 0.01, 0.015, 0.02, 0.03, 0.05, 0.1, 0.2, 0.5, 1.0, 1.5, 2.0, and 4.0MeV. A representative set of the data is made available in this publication; the entire set of absorbed fractions is available as electronic supplementary materials. These results are consistent with previous voxelized models and reinforce the well-understood relationship between the AF and the target's mass and location, as well as the energy of the incident radiation.

An Integrated 1-Dimensional Model of a Photoelectrochemical Cell for Water Splitting

A one-dimensional model of a photoelectrochemical cell for solar water splitting has been developed, with applicability to both “wired” and “wireless” designs. The model of the light absorber handles electron and hole transport. The model of the electrolyte accounts for mass transport through regions of aqueous solution, including stagnant diffusion layers and bulk regions to address mixing due to bubbles, natural convection, or other sources. A polymer membrane may be present in the electrolyte. The models of the light absorber and the electrolyte are integrated through the reactions taking place at the interface between them. Charge transfer from the semiconductor to the solution is handled using a kinetic model involving reactions between the species in both the light absorber and the electrolyte. A simplified model is also presented for use when concentration gradients in the electrolyte are negligible. The simplified model captures the effect of the electrolyte in the boundary conditions for the light absorber. The model is validated against current-potential data for a hydrogen-evolving light absorber with varying degrees of simulated solar illumination. The model then shows how overall solar-to-hydrogen yield depends on the efficiency of the light absorber and the areal fraction of absorbers in a membrane separator.

ON THE ORIGIN OF HIGHLY IONIZED X-RAY ABSORBERS DETECTED IN THE GALACTIC X-RAY BINARIES

X-ray observations of the Galactic X-ray binaries (XRB) revealed numerous highly ionized metal absorption lines. However, it is unclear whether such lines are produced by the hot interstellar medium (ISM) or the circumstellar medium (CSM) intrinsic to the binaries. Here we present a Chandra X-ray absorption line study of 28 observations on 12 X-ray binaries, with a focus on the NeIX and FeXVII lines. We report the first detections of these lines in a significant amount of observations. We do not find significant dependence of the line equivalent width on the distance of the XRBs, but we do see weak dependence on the source X-ray luminosity. We also find two out of twelve selected targets show strong temporal variation of the NeIX absorbers. While the line ratio between the two ion species suggests a temperature consistent with the previous predictions of the ISM, comparing with two theoretical models of the ISM shows the observed column densities are significantly higher than predictions. On the other hand, photoionzation by the XRBs provides reasonably good fit to the data. Our findings suggest that a significant fraction of these X-ray absorbers may originate in the hot gas intrinsic to the X-ray binaries, and that the ISM makes small, if not negligible, contribution. We briefly discuss the implications to the study of the Milky Way hot gas content.

Local structure and magnetic properties of cubic iron oxide nanoparticles formed in zeolite as deduced using Mössbauer spectroscopy

Local structure and magnetic properties of cubic iron oxide nanoparticles as prepared in zeolite matrix have been studied in a detailed manner using Mössbauer spectroscopy. Superparamagnetic particles of cubic iron oxide are observed to be strongly bound in zeolite matrix. Annealing around 870 K is understood to result in detrapping of the bound nanoparticles. This leads to the onset of coarsening of the nanoparticles thus resulting in an appearence of a broad sextet characterised by a low hyperfine field. On the otherhand, the occurrence of the sextet, due to the onset of coarsening of the bare cubic iron oxide superparamagnetic nanoparticles, has been observed subsequent to annealing around 520 K. Linewidth and quadrupole splitting corresponding to the fraction of 57Fe absorbers associated with superparamagnetic particles are shown to be sensitively dependent upon the annealing temperature with respect to detrapping and aggregation of nanoparticles formed in zeolite.

Age-Dependent Small-Animal Internal Radiation Dosimetry

Rats at various ages were observed to present with different radiosensitivity and bioavailability for radiotracers commonly used in preclinical research. We evaluated the effect of age-induced changes in body weight on radiation dose calculations. A series of rat models at different age periods were constructed based on the realistic four-dimensional digital rat whole-body (ROBY) computational model. Particle transport was simulated using the MCNPX Monte Carlo code. Absorbed fractions (AFs) and specific absorbed fraction (SAFs) of monoenergetic photons/electrons and S values of eight positron-emitting radionuclides were calculated. The SAFs and S values for most source-target pairs were inversely correlated with body weight. Differences between F-18 S values for most source-target pairs were between -1.5% and -2%/10 g difference in body weight for different computational models. For specific radiotracers, the radiation dose to organs presents a negative correlation with rat body weight. The SAFs for monoenergetic photons/electrons and S values for common positron-emitting radionuclides can be exploited in the assessment of radiation dose delivered to rats at different ages and weights. The absorbed dose to organs is significantly higher in the low-weight young rat model than in the adult model, which would result in steep secondary effects and might be a noteworthy issue in laboratory animal internal dosimetry.

Content and Bioaccessibility of Aluminium in Duplicate Diets from Southern Spain

Aluminium is found naturally in foods and beverages, but levels increase notably during processing, packaging, storage, and cooking, as a consequence of its presence in food additives and the wide use of aluminium utensils and vessels. Dietary intake of Al was estimated in 2 population groups in southern Spain (families and university students) in a duplicate diet sampling study. Diets were sampled for 7 consecutive days, and Al was determined in acid-mineralized samples with electrothermal atomization-atomic absorption spectrometry (ETA-AAS). Mean values for Al intake were 2.93 and 1.01 mg/d in families and students, respectively, ranging from 0.12 to 10.00 mg/d. Assuming an average adult weight of 60 kg, the mean dietary exposures to aluminium were 0.34 and 0.12 mg/kg body weight/week in these groups, which amounted to 17% and 6% of the 2 mg/kg body weight estimated as the tolerable weekly intake by the Joint FAO/WHO Expert Committee on Food Additives. Bioaccessibility of dietary Al tested with in vitro studies ranged from 0.30 to 17.26% (absorbable fraction). The highest aluminium intakes were observed in subjects consuming diets with a low adherence to the Mediterranean diet, which were associated to high consumption of processed and canned food. On the contrary, subjects consuming diets with a high adherence to the Mediterranean diet patterns showed the lowest Al intakes. The present findings are useful for giving both a reliable estimate of total aluminium dietary intake and tolerable intake levels according to usual dietary habits.

Pharmacokinetic-pharmacodynamic modeling of unboosted Atazanavir in a cohort of stable HIV-infected patients.

Limited data on the pharmacokinetics and pharmacodynamics (PK/PD) of unboosted atazanavir (uATV) in treatment-experienced patients are available. The aim of this work was to study the PK/PD of unboosted atazanavir in a cohort of HIV-infected patients. Data were available for 58 HIV-infected patients (69 uATV-based regimens). Atazanavir concentrations were analyzed by using a population approach, and the relationship between atazanavir PK and clinical outcome was examined using logistic regression. The final PK model was a linear one-compartment model with a mixture absorption model to account for two subgroups of absorbers. The mean (interindividual variability) of population PK parameters were as follows: clearance, 13.4 liters/h (40.7%), volume of distribution, 71.1 liters (29.7%), and fraction of regular absorbers, 0.49. Seven subjects experienced virological failure after switch to uATV. All of them were identified as low absorbers in the PK modeling. The absorption rate constant (0.38 ± 0.20 versus 0.75 ± 0.28 h(-1); P = 0.002) and ATV exposure (area under the concentration-time curve from 0 to 24 h [AUC(0-24)], 10.3 ± 2.1 versus 22.4 ± 11.2 mg · h · liter(-1); P = 0.001) were significantly lower in patients with virological failure than in patients without failure. In the logistic regression analysis, both the absorption rate constant and ATV trough concentration significantly influenced the probability of virological failure. A significant relationship between ATV pharmacokinetics and virological response was observed in a cohort of HIV patients who were administered unboosted atazanavir. This study also suggests that twice-daily administration of uATV may optimize drug therapy.

Electron specific absorbed fractions for the adult male and female ICRP/ICRU reference computational phantoms

The calculation of radiation dose from internally incorporated radionuclides is based on so-called absorbed fractions (AFs) and specific absorbed fractions (SAFs). SAFs for monoenergetic electrons were calculated for 63 source regions and 67 target regions using the new male and female adult reference computational phantoms adopted by the ICRP and ICRU and the Monte Carlo radiation transport programme package EGSnrc. The SAF values for electrons are opposed to the simplifying assumptions of ICRP Publication 30. The previously applied assumption of electrons being fully absorbed in the source organ itself is not always true at electron energies above approximately 300–500 keV. High-energy electrons have the ability to leave the source organ and, consequently, the electron SAFs for neighbouring organs can reach the same magnitude as those for photons for electron energies above 1 MeV. The reciprocity principle known for photons can be extended to electron SAFs as well, thus making cross-fire electron SAFs mass-independent. To quantify the impact of the improved electron dosimetry in comparison to the dosimetry using the simple assumptions of ICRP Publication 30, absorbed doses per administered activity of three radiopharmaceuticals were evaluated with and without explicit electron transport. The organ absorbed doses per administered activity for the two evaluation methods agree within 2%–3% for most organs for radionuclides with decay spectra having electron energies below a few hundred keV and within approximately 20% if higher electron energies are involved. An important exception is the urinary bladder wall, where the dose is overestimated by 60–150% using the simplified ICRP 30 approach for the radiopharmaceuticals of this study.

Skeletal dosimetry based on µCT images of trabecular bone: update and comparisons

Two skeletal dosimetry methods using µCT images of human bone have recently been developed: the paired-image radiation transport (PIRT) model introduced by researchers at the University of Florida (UF) in the US and the systematic–periodic cluster (SPC) method developed by researchers at the Federal University of Pernambuco in Brazil. Both methods use µCT images of trabecular bone (TB) to model spongiosa regions of human bones containing marrow cavities segmented into soft tissue volumes of active marrow (AM), trabecular inactive marrow and the bone endosteum (BE), which is a 50 µm thick layer of marrow on all TB surfaces and on cortical bone surfaces next to TB as well as inside the medullary cavities. With respect to the radiation absorbed dose, the AM and the BE are sensitive soft tissues for the induction of leukaemia and bone cancer, respectively. The two methods differ mainly with respect to the number of bone sites and the size of the µCT images used in Monte Carlo calculations and they apply different methods to simulate exposure from radiation sources located outside the skeleton. The PIRT method calculates dosimetric quantities in isolated human bones while the SPC method uses human bones embedded in the body of a phantom which contains all relevant organs and soft tissues. Consequently, the SPC method calculates absorbed dose to the AM and to the BE from particles emitted by radionuclides concentrated in organs or from radiation sources located outside the human body in one calculation step. In order to allow for similar calculations of AM and BE absorbed doses using the PIRT method, the so-called dose response functions (DRFs) have been developed based on absorbed fractions (AFs) of energy for electrons isotropically emitted in skeletal tissues. The DRFs can be used to transform the photon fluence in homogeneous spongiosa regions into absorbed dose to AM and BE. This paper will compare AM and BE AFs of energy from electrons emitted in skeletal tissues calculated with the SPC and the PIRT method and AM and BE absorbed doses and AFs calculated with PIRT-based DRFs and with the SPC method. The results calculated with the two skeletal dosimetry methods agree well if one takes the differences between the two models properly into account. Additionally, the SPC method will be updated with larger µCT images of TB.

Calculations of absorbed fractions in small water spheres for low-energy monoenergetic electrons and the Auger-emitting radionuclides123Ι and 125Ι

Purpose: To calculate the absorbed fraction (AF) of low energy electrons in small tissue-equivalent spherical volumes by Monte Carlo (MC) track structure simulation and assess the influence of phase (liquid water versus density-scaled water vapor) and of the continuous-slowing-down approximation (CSDA) used in semi-analytic calculations. Methods: An event-by-event MC code simulating the transport of electrons in both the vapor and liquid phase of water using appropriate electron-water interaction cross sections was used to quantify the energy deposition of low-energy electrons in spherical volumes. Semi-analytic calculations within the CSDA using a convolution integral of the Howell range-energy expressions are also presented for comparison. Results: The AF for spherical volumes of radii from 10–1000 nm are presented for monoenergetic electrons over the energy range 100–10,000 eV and the two Auger-emitting radionuclides 125I and 123I. The MC calculated AF for the liquid phase are found to be smaller than those of the (density scaled) gas phase by up to 10–20% for the monoenergetic electrons and 10% for the two Auger-emitters. Differences between the liquid-phase MC results and the semi-analytic CSDA calculations are up to ∼ 55% for the monoenergetic electrons and up to ∼ 35% for the two Auger-emitters. Conclusions: Condensed-phase effects in the inelastic interaction of low-energy electrons with water have a noticeable but relatively small impact on the AF for the energy range and target sizes examined. Depending on the electron energies, the semi-analytic approach may lead to sizeable errors for target sizes with linear dimensions below 1 micron.

Evaluation of absorbed doses in voxel-based and simplified models for small animals

Internal dosimetry in non-human biota is desirable from the viewpoint of radiation protection of the environment. The International Commission on Radiological Protection (ICRP) proposed Reference Animals and Plants using simplified models, such as ellipsoids and spheres and calculated absorbed fractions (AFs) for whole bodies. In this study, photon and electron AFs in whole bodies of voxel-based rat and frog models have been calculated and compared with AFs in the reference models. It was found that the voxel-based and the reference frog (or rat) models can be consistent for the whole-body AFs within a discrepancy of 25%, as the source was uniformly distributed in the whole body. The specific absorbed fractions (SAFs) and S values were also evaluated in whole bodies and all organs of the voxel-based frog and rat models as the source was distributed in the whole body or skeleton. The results demonstrated that the whole-body SAFs reflect SAFs of all individual organs as the source was uniformly distributed per mass within the whole body by about 30% uncertainties with exceptions for body contour (up to -40%) for both electrons and photons due to enhanced radiation leakages, and for the skeleton for photons only (up to +185%) due to differences in the mass attenuation coefficients. For nuclides such as (90)Y and (90)Sr, which were concentrated in the skeleton, there were large differences between S values in the whole body and those in individual organs, however the whole-body S values for the reference models with the whole body as the source were remarkably similar to those for the voxel-based models with the skeleton as the source, within about 4 and 0.3%, respectively. It can be stated that whole-body SAFs or S values in simplified models without internal organs are not sufficient for accurate internal dosimetry because they do not reflect SAFs or S values of all individual organs as the source was not distributed uniformly in whole body. Thus, voxel-based models would be good candidates for dosimetry in non-human biota if further accuracy in environmental dosimetry is desired.

Alpha dosimetry for the skeletal model of the reference adult male

Introduction: Dosimetry of bone marrow is challenging due to the complex histology of the skeleton and to the fact that the segmentation is not possible on the macroscopic level. In this context, calculation of absorbed fractions (AFs) of energy from alpha particles is of particular interest. Indeed, the high ionizing power and the short range of alpha particles makes them toxic in the case of internal contaminations or beneficial for radioimmunotherapy applications. Materials and methods: A reference voxelized adult model of the human skeleton, developed at the University of Florida using 3-D microCT images of a 40 year-old male, is used for the calculation of AFs for alpha particles with initial energy ranging from 4 to 10 MeV. The MCNPX Monte Carlo code associated to OEDIPE software will be used to simulate the transport of alphas in 32 skeletal microstructures for the following source regions: trabecular bone volume, active marrow, inactive marrow and endosteum. Radiosensitive skeletal target regions are the active marrow (risk of leukemia) and endosteum (risk for bone cancer). Siteand agespecific percentage of marrow cellularity will be considered. The Monte Carlo calculations will be benchmarked with the GEANT4 Monte Carlo code, which generates secondary electrons in addition of alpha particles. Results: Significant variations in absorbed fractions between different skeletal sites were observed. The parietal bone demonstrates the greatest difference as this particular bone site is characterized by relatively small marrow cavities and thick bone trabeculae. Differences were also observed between this model and the absorbed fractions given in ICRP Publication 30. In fact, the energy deposition in the active marrow is overestimated in the ICRP 30. On the contrary, the AFs are underestimated for the endosteum. Calculation of the AFs as a function of the marrow cellularity shows that the energy deposition decrease with the cellularity and increase with the energy of alpha particles. Conclusion: Results presented here provide a firmer basis for a more realistic dosimetry in alpha emitter radionuclide therapies through the explicit consideration of absorbed fraction variations with particle energy, skeletal site, and marrow cellularity.

SU‐E‐T‐389: Energy Deposition Characteristics of Beta‐Emitter P‐32 and Photon Emitter Pd‐103

Purpose: Comparing the energy deposition characteristics of beta‐emitter P‐32 and photon emitter Pd‐103 to evaluate potential improvement in prostate brachytherapy dosimetry.Materials /Methods: Energy deposited by an activity A after total decay is calculated by 1.44T1/2AE, where E is the energy emitted by a single disintegration. The total energy deposited in the target will be 1.44T1/2AE F, where F is the absorbed fraction (AF). Absorbed fraction is defined as the fraction of emitted energy absorbed in the target. It increases monotonously with target volume. Monte Carlo calculated AF data for uniformly distributed spherical targets, 1&lt;/sup&gt; for P‐32 and clinically obtained AF, 2&lt;/sup&gt; from Pd‐103 permanent prostate brachytherapy are used in this study. Results: A unity activity (1 mCi) of P‐32 will deliver 7.38 joules, while the same activity of Pd‐103 will deliver 0.26 joules. The total energy deposited in a volume of 20 cc will be 6.90 joules for P‐32 and 0.11 joules for Pd‐103. As the volume of the target increases to 60 cc, the total energy deposited for P‐32 will be 7.06 joules, and 0.14 joules for Pd‐103 respectively. Conclusion: From the preliminary comparison, P‐32 is about 50 to 60 times more efficient in utilizing energy than Pd‐103. This high efficiency will reduce total activity needed to achieve desired dosage in the target. Also, the fact that most energy is absorbed locally in the target suggests that the dose to healthy tissue outside the target can be significantly reduced. Further study is needed in order to fully realize these potential dosimetric advantages.