Pulsed UCN production using a Doppler shifter at J-PARC

We have constructed a Doppler-shifter-type pulsed ultra-cold neutron (UCN) source at the Materials and Life Science Experiment Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC). Very-cold neutrons (VCNs) with 136-$\mathrm{m/s}$ velocity in a neutron beam supplied by a pulsed neutron source are decelerated by reflection on a m=10 wide-band multilayer mirror, yielding pulsed UCN. The mirror is fixed to the tip of a 2,000-rpm rotating arm moving with 68-$\mathrm{m/s}$ velocity in the same direction as the VCN. The repetition frequency of the pulsed UCN is $8.33~\mathrm{Hz}$ and the time width of the pulse at production is $4.4~\mathrm{ms}$. In order to increase the UCN flux, a supermirror guide, wide-band monochromatic mirrors, focus guides, and a UCN extraction guide have been newly installed or improved. The $1~\mathrm{MW}$-equivalent count rate of the output neutrons with longitudinal wavelengths longer than $58~\mathrm{nm}$ is $1.6 \times 10^{2}~\mathrm{cps}$, while that of the true UCNs is $80~\mathrm{cps}$. The spatial density at production is $1.4~\mathrm{UCN/cm^{3}}$. This new UCN source enables us to research and develop apparatuses necessary for the investigation of the neutron electric dipole moment (nEDM).Comment: 32 pages, 15 fugures. A grammatical error was fixe

Pushing the limits of the NuSTAR detectors

NuSTAR (the Nuclear Spectroscopic Telescope ARray) is a NASA Small Explorer (SMEX) mission launched in June of 2012. Since its launch, NuSTAR has been the preeminent instrument for spectroscopic analysis of the hard X-ray sky over the 3-80 keV bandpass. The low energy side of the bandpass is limited by the absorption along the photon path as well as by the ability of the pixels to trigger on incident photons. The on-board calibration source does not have a low-energy line that we can use to calibrate this part of the response, so instead we use the "nearest-neighbor" readout in the NuSTAR detector architecture to calibrate the individual pixel thresholds for all 8 flight detectors on both focal plane modules (FPMs). These threshold measurements feed back into the quantum efficiency of the detectors at low (<5 keV) energies and, once well-calibrated, may allow the use of NuSTAR data below the current 3 keV limit

Effect of oxygen incorporation on normal and superconducting properties of MgB2 films

Oxygen was systematically incorporated in MBE grown MgB2 films using in-situ post-growth anneals in an oxygen environment. Connectivity analysis in combination with measurements of the critical temperature and resistivity indicate that oxygen is distributed both within and between the grains. High values of critical current densities in field (~4x10^5 A/cm^2 at 8 T and 4.2 K), extrabolated critical fields (>45 T) and slopes of critical field versus temperature (1.4 T/K) are observed. Our results suggest that low growth temperatures (300oC) and oxygen doping (>0.65%) can produce MgB2 with high Jc values in field and Hc2 for high-field magnet applications.Comment: 10 pages, 3 figure

X-ray Diagnostics of Thermal Conditions of the Hot Plasmas in the Centaurus Cluster

X-ray data of the Centaurus cluster, obtained with {\it XMM-Newton} for 45 ksec, were analyzed. Deprojected EPIC spectra from concentric thin shell regions were reproduced equally well by a single-phase plasma emission model, or by a two-phase model developed by {\it ASCA}, both incorporating cool (1.7--2.0 keV) and hot ($\sim 4$ keV) plasma temperatures. However, EPIC spectra with higher statistics, accumulated over 3-dimentional thick shell regions, were reproduced better by the two-phase model than by the singe-phase one. Therefore, hot and cool plasma phases are inferred to co-exist in the cluster core region within $\sim 70$ kpc. The iron and silicon abundances of the plasma were reconfirmed to increase significantly towards the center, while that of oxygen was consistent with being radially constant. The implied non-solar abundance ratios explains away the previously reported excess X-ray absorption from the central region. Although an additional cool ($\sim 0.7$ keV) emission was detected within $\sim 20$ kpc of the center, the RGS data gave tight upper limits on any emission with a tempeartures below $\sim 0.5$ keV. These results are compiled into a magnetosphere model, which interprets the cool phase as confined within closed magnetic loops anchored to the cD galaxy. When combined with so-called Rosner-Tucker-Vaiana mechanism which applies to solar coronae, this model can potentially explain basic properties of the cool phase, including its temperature and thermal stability.Comment: 53 pages, 11 figures, accepted for publication in Astrophysical Journa

Prospects for Improving the Intrinsic and Extrinsic Properties of Magnesium Diboride Superconducting Strands

The magnetic and transport properties of magnesium diboride films represent performance goals yet to be attained by powder-processed bulk samples and conductors. Such performance limits are still out of the reach of even the best magnesium diboride magnet wire. In discussing the present status and prospects for improving the performance of powder-based wire we focus attention on (1) the intrinsic (intragrain) superconducting properties of magnesium diboride, Hc2 and flux pinning, (2) factors that control the efficiency with which current is transported from grain-to-grain in the conductor, an extrinsic (intergrain) property. With regard to Item-(1), the role of dopants in Hc2 enhancement is discussed and examples presented. On the other hand their roles in increasing Jc, both via Hc2 enhancement as well as direct fluxoid/pining-center interaction, are discussed and a comprehensive survey of Hc2 dopants and flux-pinning additives is presented. Current transport through the powder-processed wire (an extrinsic property) is partially blocked by the inherent granularity of the material itself and the chemical or other properties of the intergrain surfaces. These and other such results indicate that in many cases less than 15% of the conductor's cross sectional area is able to carry transport current. It is pointed out that densification in association with the elimination of grain-boundary blocking phases would yield five-to ten-fold increases in Jc in relevant regimes, enabling the performance of magnesium diboride in selected applications to compete with that of Nb-Sn

Pushing the limits of the NuSTAR detectors

NuSTAR (the Nuclear Spectroscopic Telescope ARray) is a NASA Small Explorer (SMEX) mission launched in June of 2012. Since its launch, NuSTAR has been the preeminent instrument for spectroscopic analysis of the hard X-ray sky over the 3-80 keV bandpass. The low energy side of the bandpass is limited by the absorption along the photon path as well as by the ability of the pixels to trigger on incident photons. The on-board calibration source does not have a low-energy line that we can use to calibrate this part of the response, so instead we use the "nearest-neighbor" readout in the NuSTAR detector architecture to calibrate the individual pixel thresholds for all 8 flight detectors on both focal plane modules (FPMs). These threshold measurements feed back into the quantum efficiency of the detectors at low (<5 keV) energies and, once well-calibrated, may allow the use of NuSTAR data below the current 3 keV limit

Gas selection for Xe-based LCP-GEM detectors onboard the CubeSat X-ray observatory NinjaSat

We present a gas selection for Xe-based gas electron multiplier (GEM) detectors, Gas Multiplier Counters (GMCs) onboard the CubeSat X-ray observatory NinjaSat. To achieve an energy bandpass of 2-50 keV, we decided to use a Xe-based gas mixture at a pressure of 1.2 atm that is sensitive to high-energy X-rays. In addition, an effective gain of over 300 is required for a single GEM so that the 2 keV X-ray signal can be sufficiently larger than the electrical noise. At first, we measured the effective gains of GEM in nine Xe-based gas mixtures (combinations of Xe, Ar, CO2, CH4, and dimethyl ether; DME) at 1.0 atm. The highest gains were obtained with Xe/Ar/DME mixtures, while relatively lower gains were obtained with Xe/Ar/CO2, Xe/Ar/CH4, and Xe+quencher mixtures. Based on these results, we selected the Xe/Ar/DME (75%/24%/1%) mixture at 1.2 atm as the sealed gas for GMC. Then we investigated the dependence of an effective gain on the electric fields in the drift and induction gaps ranging from 100-650 V cm$^{-1}$ and 500-5000 V cm$^{-1}$, respectively, in the selected gas mixture. The effective gain weakly depended on the drift field while it was almost linearly proportional to the induction field: 2.4 times higher at 5000 V cm$^{-1}$ than at 1000 V cm$^{-1}$. With the optimal induction and drift fields, the flight model GMC achieves an effective gain of 460 with an applied GEM voltage of 590 V.Comment: 7th international conference on Micro Pattern Gaseous Detectors 2022 - MPGD2022, 3 pages, 2 figure