Distinguishing s±s^{\pm} and s++s^{++} electron pairing symmetries by neutron spin resonance in superconducting NaFe0.935_{0.935}Co0.045_{0.045}As

A determination of the superconducting (SC) electron pairing symmetry forms the basis for establishing a microscopic mechansim for superconductivity. For iron pnictide superconductors, the $s^\pm$-pairing symmetry theory predicts the presence of a sharp neutron spin resonance at an energy below the sum of hole and electron SC gap energies ($E\leq 2\Delta$) below $T_c$. On the other hand, the $s^{++}$-pairing symmetry expects a broad spin excitation enhancement at an energy above $2\Delta$ below $T_c$. Although the resonance has been observed in iron pnictide superconductors at an energy below $2\Delta$ consistent with the $s^\pm$-pairing symmetry, the mode has also be interpreted as arising from the $s^{++}$-pairing symmetry with $E\ge 2\Delta$ due to its broad energy width and the large uncertainty in determining the SC gaps. Here we use inelastic neutron scattering to reveal a sharp resonance at E=7 meV in SC NaFe$_{0.935}$Co$_{0.045}$As ($T_c = 18$ K). On warming towards $T_c$, the mode energy hardly softens while its energy width increases rapidly. By comparing with calculated spin-excitations spectra within the $s^{\pm}$ and $s^{++}$-pairing symmetries, we conclude that the ground-state resonance in NaFe$_{0.935}$Co$_{0.045}$As is only consistent with the $s^{\pm}$-pairing, and is inconsistent with the $s^{++}$-pairing symmetry.Comment: 9 pages, 8 figures. submitted to PR

An Investigation of Plastic Scintillators for Radiation Sensing and Mechanical Applications

Plastic scintillators have uses in many fields including defense, high energy physics, health physics, and space applications. In recent years, work has been done to enhance a variety of plastic scintillator properties. These enhancements have largely targeted an increase of the intrinsic gamma efficiency, photopeak efficiency, mechanical robustness, the neutron pulse shape discrimination (PSD) figure of merit, and/or the timing resolution. This body of work seeks to examine these solutions both in a general radiation detection context and in the context of using the plastic scintillator as a frame component of a given detector system; in this work a hypothetical unmanned aerial vehicle (UAV) is used for guiding questions. This examination was accomplished through a series of mechanical properties measurements, a range of simulations using GEANT4 and MCNP, and radiation measurements which serve to validate the simulations as well as further characterize both existing and novel plastic scintillators in select configurations. In particular, basic science studies were conducted to understand, quantify, and/or demonstrate: 1) the mechanical properties of the scintillators and trade-offs which may exist when these are enhanced, 2) the effect on the mechanical properties when adding organometallic molecules to select plastic scintillator matrices, 3) the methods by which moduli measurements made with a dynamic mechanical analyzer may be compared to time-domain moduli measurements, 4) methods to simulate the radiation and optical response of plastic scintillators, including nanocomposites, using Geant4, 5) validation of a Geant4 workspace and aforementioned methods, 6) the simulated scale-up of EJ256 and a 24.5 wt/% ytterbium fluoride/PVT nanocomposite scintillator highlighting emergent trade-offs, 7) a method to deconvolve latent x-ray escape peaks from photopeaks in a gamma spectrum towards determining the energy resolution, 8) a python toolkit for rapidly simulating a mobile detector system, and 9) the Cramer-Rao lower bound on the timing resolution of EJ232Q in multiple sizes and configurations