Unitarity constraint for threshold coherent pion photoproduction on the deuteron and chiral perturbation theory

The contribution of the two-step process gamma + d -> p + n -> pi0 + d to the imaginary part of the amplitude for coherent pion production on the deuteron is calculated exploiting unitarity constraints. The result shows that this absorptive process is not negligible and has to be considered in an extraction of the elementary neutron production amplitude from the gamma + d -> pi0 + d cross section at threshold. In addition, it is argued that a consistent calculation of gamma + d -> pi0 + d in baryon chiral perturbation theory beyond next-to-leading order requires the inclusion of this absorptive process.Comment: 11 pages revtex including 2 postscript figure

Absolute Determination of the 22Na(p,g) Reaction Rate in Novae

Gamma-ray telescopes in orbit around the Earth are searching for evidence of the elusive radionuclide 22Na produced in novae. Previously published uncertainties in the dominant destructive reaction, 22Na(p,g)23Mg, indicated new measurements in the proton energy range of 150 to 300 keV were needed to constrain predictions. We have measured the resonance strengths, energies, and branches directly and absolutely by using protons from the University of Washington accelerator with a specially designed beamline, which included beam rastering and cold vacuum protection of the 22Na implanted targets. The targets, fabricated at TRIUMF-ISAC, displayed minimal degradation over a ~ 20 C bombardment as a result of protective layers. We avoided the need to know the stopping power, and hence the target composition, by extracting resonance strengths from excitation functions integrated over proton energy. Our measurements revealed that resonance strengths for E_p = 213, 288, 454, and 610 keV are stronger by factors of 2.4 to 3.2 than previously reported. Upper limits have been placed on proposed resonances at 198-, 209-, and 232-keV. We have re-evaluated the 22Na(p,g) reaction rate, and our measurements indicate the resonance at 213 keV makes the most significant contribution to 22Na destruction in novae. Hydrodynamic simulations including our rate indicate that the expected abundance of 22Na ejecta from a classical nova is reduced by factors between 1.5 and 2, depending on the mass of the white-dwarf star hosting the nova explosion.Comment: 20 pages, 18 figures; shortened paper, accepted in Phys. Rev.

Injection of Radioactivities into the Forming Solar System

Meteorite studies have revealed the presence of short-lived radioactivities in the early solar system. The current data suggests that the origin of at least some of the radioactivities requires contribution from recent nucleosynthesis at a stellar site. This sets a strict time limit on the time available for the formation of the solar system and argues for the theory of the triggered origin of the solar system. According to this scenario, the formation of our planetary system was initiated by the impact of an interstellar shock wave on a molecular cloud core. The shock wave originated from a nearby explosive stellar event and carried with it radioactivities produced in the stellar source. In addition to triggering the collapse of the molecular cloud core, the shock wave also deposited some of the freshly synthesized radioactivities into the collapsing system. The radioactivities were then incorporated into the first solar system solids, in this manner leaving a record of the event in the meteoritic material. The viability of the scenario can be investigated through numerical simulations studying the processes involved in mixing shock wave material into the collapsing system. The high-resolution calculations presented here show that injection occurs through Rayleigh-Taylor instabilities, the injection efficiency is approximately 10%, and temporal and spatial heterogeneities in the abundances of the radioactivities existed at the time of their arrival in the forming solar system.Comment: 13 pages, including 3 figures. Better-quality figures available at http://www.public.asu.edu/~hvanhal/pubs

On the close to threshold meson production in neutron-neutron collisions

A method of measuring the close to threshold meson production in neutron-neutron collisions is described where the momenta of the colliding neutrons can be determined with the accuracy obtainable for the proton-proton reaction. The technique is based on the double quasi-free nn --> nn X^0 reaction, where deuterons are used as a source of neutronsComment: 6 pages, 2 figures, to be published in Phys. Lett.

Cross section and analyzing power of pol{p}p -> pn pi+ near threshold

The cross section and analyzing power of the pol{p}p -> pn pi+ reaction near threshold are estimated in terms of data obtained from the pol{p}p -> d pi+ and pp -> pp pi0 reactions. A simple final state interaction theory is developed which depends weakly upon the form of the pion-production operator and includes some Coulomb corrections. Within the uncertainties of the model and the input data, the approach reproduces well the measured energy dependence of the total cross section and the proton analyzing power at a fixed pion c.m. angle of 90deg, from threshold to T_p = 330 MeV. The variation of the differential cross section with pion angle is also very encouraging.Comment: 20 pages, Latex including 4 eps figure

A risk prediction model for the assessment and triage of women with hypertensive disorders of pregnancy in low-resourced settings: the miniPIERS (Pre-eclampsia Integrated Estimate of RiSk) multi-country prospective cohort study.

BACKGROUND: Pre-eclampsia/eclampsia are leading causes of maternal mortality and morbidity, particularly in low- and middle- income countries (LMICs). We developed the miniPIERS risk prediction model to provide a simple, evidence-based tool to identify pregnant women in LMICs at increased risk of death or major hypertensive-related complications. METHODS AND FINDINGS: From 1 July 2008 to 31 March 2012, in five LMICs, data were collected prospectively on 2,081 women with any hypertensive disorder of pregnancy admitted to a participating centre. Candidate predictors collected within 24 hours of admission were entered into a step-wise backward elimination logistic regression model to predict a composite adverse maternal outcome within 48 hours of admission. Model internal validation was accomplished by bootstrapping and external validation was completed using data from 1,300 women in the Pre-eclampsia Integrated Estimate of RiSk (fullPIERS) dataset. Predictive performance was assessed for calibration, discrimination, and stratification capacity. The final miniPIERS model included: parity (nulliparous versus multiparous); gestational age on admission; headache/visual disturbances; chest pain/dyspnoea; vaginal bleeding with abdominal pain; systolic blood pressure; and dipstick proteinuria. The miniPIERS model was well-calibrated and had an area under the receiver operating characteristic curve (AUC ROC) of 0.768 (95% CI 0.735-0.801) with an average optimism of 0.037. External validation AUC ROC was 0.713 (95% CI 0.658-0.768). A predicted probability ≥25% to define a positive test classified women with 85.5% accuracy. Limitations of this study include the composite outcome and the broad inclusion criteria of any hypertensive disorder of pregnancy. This broad approach was used to optimize model generalizability. CONCLUSIONS: The miniPIERS model shows reasonable ability to identify women at increased risk of adverse maternal outcomes associated with the hypertensive disorders of pregnancy. It could be used in LMICs to identify women who would benefit most from interventions such as magnesium sulphate, antihypertensives, or transportation to a higher level of care

Evolution of oxygen isotopic composition in the inner solar nebula

Changes in the chemical and isotopic composition of the solar nebula with time are reflected in the properties of different constituents that are preserved in chondritic meteorites. CR carbonaceous chondrites are among the most primitive of all chondrite types and must have preserved solar nebula records largely unchanged. We have analyzed the oxygen and magnesium isotopes in a range of the CR constituents of different formation temperatures and ages, including refractory inclusions and chondrules of various types. The results provide new constraints on the time variation of the oxygen isotopic composition of the inner (<5 AU) solar nebula - the region where refractory inclusions and chondrules most likely formed. A chronology based on the decay of short-lived 26Al (t1/2 ~ 0.73 Ma) indicates that the inner solar nebula gas was 16O-rich when refractory inclusions formed, but less than 0.8 Ma later, gas in the inner solar nebula became 16O-poor and this state persisted at least until CR chondrules formed ~1-2 Myr later. We suggest that the inner solar nebula became 16O-poor because meter-size icy bodies, which were enriched in 17,18O due to isotopic self-shielding during the ultraviolet photo dissociation of CO in the protosolar molecular cloud or protoplanetary disk, agglomerated outside the snowline, drifted rapidly towards the Sun, and evaporated at the snowline. This led to significant enrichment in 16O-depleted water, which then spread through the inner solar system. Astronomical studies of the spatial and/or temporal variations of water abundance in protoplanetary disks may clarify these processes.Comment: 27 pages, 5 figure

Spectra and waiting-time densities in firing resonant and nonresonant neurons

The response of a neural cell to an external stimulus can follow one of the two patterns: Nonresonant neurons monotonously relax to the resting state after excitation while resonant ones show subthreshold oscillations. We investigate how do these subthreshold properties of neurons affect their suprathreshold response. Vice versa we ask: Can we distinguish between both types of neuronal dynamics using suprathreshold spike trains? The dynamics of neurons is given by stochastic FitzHugh-Nagumo and Morris-Lecar models with either having a focus or a node as the stable fixpoint. We determine numerically the spectral power density as well as the interspike interval density in response to a random (noise-like) signals. We show that the information about the type of dynamics obtained from power spectra is of limited validity. In contrast, the interspike interval density gives a very sensitive instrument for the diagnostics of whether the dynamics has resonant or nonresonant properties. For the latter value we formulate a fit formula and use it to reconstruct theoretically the spectral power density, which coincides with the numerically obtained spectra. We underline that the renewal theory is applicable to analysis of suprathreshold responses even of resonant neurons.Comment: 7 pages, 8 figure

The pd <--> pi+ t reaction around the Delta resonance

The pd pi+ t process has been calculated in the energy region around the Delta-resonance with elementary production/absorption mechanisms involving one and two nucleons. The isobar degrees of freedom have been explicitly included in the two-nucleon mechanism via pi-- and rho-exchange diagrams. No free parameters have been employed in the analysis since all the parameters have been fixed in previous studies on the simpler pp pi+ d process. The treatment of the few-nucleon dynamics entailed a Faddeev-based calculation of the reaction, with continuum calculations for the initial p-d state and accurate solutions of the three-nucleon bound-state equation. The integral cross-section was found to be quite sensitive to the NN interaction employed while the angular dependence showed less sensitivity. Approximately a 4% effect was found for the one-body mechanism, for the three-nucleon dynamics in the p-d channel, and for the inclusion of a large, possibly converged, number of three-body partial states, indicating that these different aspects are of comparable importance in the calculation of the spin-averaged observables.Comment: 40 Pages, RevTex, plus 5 PostScript figure