Cryogenic micro-calorimeters for mass spectrometric identification of neutral molecules and molecular fragments

We have systematically investigated the energy resolution of a magnetic micro-calorimeter (MMC) for atomic and molecular projectiles at impact energies ranging from $E\approx13$ to 150 keV. For atoms we obtained absolute energy resolutions down to $\Delta E \approx 120$ eV and relative energy resolutions down to $\Delta E/E\approx10^{-3}$. We also studied in detail the MMC energy-response function to molecular projectiles of up to mass 56 u. We have demonstrated the capability of identifying neutral fragmentation products of these molecules by calorimetric mass spectrometry. We have modeled the MMC energy-response function for molecular projectiles and conclude that backscattering is the dominant source of the energy spread at the impact energies investigated. We have successfully demonstrated the use of a detector absorber coating to suppress such spreads. We briefly outline the use of MMC detectors in experiments on gas-phase collision reactions with neutral products. Our findings are of general interest for mass spectrometric techniques, particularly for those desiring to make neutral-particle mass measurements

Anthropogenic versus fish‐derived nutrient effects on seagrass community structure and function

Humans are altering nutrient dynamics through myriad pathways globally. Concurrent with the addition of nutrients via municipal, industrial, and agricultural sources, widespread consumer exploitation is changing consumer‐mediated nutrient dynamics drastically. Thus, altered nutrient dynamics can occur through changes in the supply of multiple nutrients, as well as through changes in the sources of these nutrients. Seagrass ecosystems are heavily impacted by human activities, with highly altered nutrient dynamics from multiple causes. We simulate scenarios of altered nutrient supply and ratios, nitrogen:phosphorus (N:P), from two nutrient sources in seagrass ecosystems: anthropogenic fertilizer and fish excretion. In doing so we tested expectations rooted in ecological theory that suggest the importance of resource dynamics for predicting primary producer dynamics. Ecosystem functions were strongly altered by artificial fertilizer (e.g., seagrass growth increased by as much as 140%), whereas plant/algae community structure was most affected by fish‐mediated nutrients or the interaction of both treatments (e.g., evenness increased by ~140% under conditions of low fish nutrients and high anthropogenic nutrients). Interactions between the nutrient sources were found for only two of six response variables, and the ratio of nutrient supply was the best predictor for only one response. These findings show that seagrass structure and function are well predicted by supply of a single nutrient (either N or P). Importantly, no single nutrient best explained the majority of responses—measures of community structure were best explained by the primary limiting nutrient to this system (P), whereas measures of growth and density of the dominant producer in the system were best explained by N. Thus, while our findings support aspects of theoretical expectations, the complexity of producer community responses belies broad generalities, underscoring the need to manage for multiple simultaneous nutrients in these imperiled coastal ecosystems.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/145341/1/ecy2388_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145341/2/ecy2388-sup-0003-AppendixS3.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145341/3/ecy2388-sup-0005-AppendixS5.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145341/4/ecy2388-sup-0006-AppendixS6.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145341/5/ecy2388-sup-0001-AppendixS1.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145341/6/ecy2388-sup-0002-AppendixS2.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145341/7/ecy2388.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145341/8/ecy2388-sup-0004-AppendixS4.pd

Anomalous electronic susceptibility in Bi2Sr2CuO6+d and comparison with other overdoped cuprates

We report magnetic susceptibility performed on overdoped Bi2Sr2CuO6+d powders as a function of oxygen doping d and temperature T. The decrease of the spin susceptibility with increasing T is confirmed. At sufficient high temperature, the spin susceptibility Chi_s presents an unusual linear temperature dependence Chi_s ~ Chi_s0 -Chi_1 T. Moreover, a linear correlation between Chi_1 and Chi_s0 for increasing hole concentration is displayed. A temperature Tchi, independent of hole doping characterizes this scaling. Comparison with other cuprates of the literature(LSCO, Tl-2201 and Bi-2212), over the same overdoped range, shows similarities with above results. These non conventional metal features will be discussed in terms of a singular narrow-band structure.Comment: 16 pages, 4 figure

Multichannel read-out for arrays of metallic magnetic calorimeters

Metallic magnetic micro-calorimeters (MMCs) operated at millikelvin temperature offer the possibility to achieve eV-scale energy resolution with high stopping power for X-rays and massive particles in an energy range up to several tens of keV. This motivates their use in a wide range of applications in fields as particle physics, atomic and molecular physics. Present detector systems consist of MMC arrays read out by 32 two-stage SQUID read-out channels. In contrast to the design of the detector array and consequently the design of the front-end SQUIDs, which need to be optimised for the physics case and the particles to be detected in a given experiment, the read-out chain can be standardised. We present our new standardised 32-channel parallel read-out for the operation of MMC arrays to be operated in a dilution refrigerator. The read-out system consists of a detector module, whose design depends on the particular application, an amplifier module, ribbon cables from room temperature to the millikelvin platform and a data acquisition system. In particular, we describe the realisation of the read-out system prepared for the ECHo-1k experiment for the operation of two 64-pixel arrays. The same read-out concept is also used for the maXs detector systems, developed for the study of the de-excitation of highly charged heavy ions by X-rays, as well as for the MOCCA system, developed for the energy and position sensitive detection of neutral molecular fragments for the study of fragmentation when molecular ions recombine with electrons. The choice of standard modular components for the operation of 32-channel MMC arrays offer the flexibility to upgrade detector modules without the need of any changes in the read-out system and the possibility to individually exchange parts in case of damages or failures

Interplay among critical temperature, hole content, and pressure in the cuprate superconductors

Within a BCS-type mean-field approach to the extended Hubbard model, a nontrivial dependence of T_c on the hole content per unit CuO_2 is recovered, in good agreement with the celebrated non-monotonic universal behaviour at normal pressure. Evaluation of T_c at higher pressures is then made possible by the introduction of an explicit dependence of the tight-binding band and of the carrier concentration on pressure P. Comparison with the known experimental data for underdoped Bi2212 allows to single out an `intrinsic' contribution to d T_c / d P from that due to the carrier concentration, and provides a remarkable estimate of the dependence of the inter-site coupling strength on the lattice scale.Comment: REVTeX 8 pages, including 5 embedded PostScript figures; other required macros included; to be published in Phys. Rev. B (vol. 54

New result for the neutron β\beta-asymmetry parameter A0A_0 from UCNA

The neutron $\beta$-decay asymmetry parameter $A_0$ defines the correlation between the spin of the neutron and the momentum of the emitted electron, which determines $\lambda=\frac{g_{A}}{g_{V}}$, the ratio of the axial-vector to vector weak coupling constants. The UCNA Experiment, located at the Ultracold Neutron facility at the Los Alamos Neutron Science Center, is the first to measure such a correlation coefficient using ultracold neutrons (UCN). Following improvements to the systematic uncertainties and increased statistics, we report the new result $A_0 = -0.12054(44)_{\mathrm{stat}}(68)_{\mathrm{syst}}$ which yields $\lambda\equiv \frac{g_{A}}{g_{V}}=-1.2783(22)$. Combination with the previous UCNA result and accounting for correlated systematic uncertainties produces $A_0=-0.12015(34)_{\mathrm{stat}}(63)_{\mathrm{syst}}$ and $\lambda\equiv \frac{g_{A}}{g_{V}}=-1.2772(20)$.Comment: 9 pages, 7 figures, updated to as-published versio

Search for neutron dark decay: n → χ + e⁺e⁻

In January, 2018, Fornal and Grinstein proposed that a previously unobserved neutron decay branch to a dark matter particle (χ) could account for the discrepancy in the neutron lifetime observed in two different types of experiments. One of the possible final states discussed includes a single χ along with an e⁺e⁻ pair. We use data from the UCNA (Ultracold Neutron Asymmetry) experiment to set limits on this decay channel. Coincident electron-like events are detected with ∼ 4π acceptance using a pair of detectors that observe a volume of stored Ultracold Neutrons (UCNs). We use the timing information of coincidence events to select candidate dark sector particle decays by applying a timing calibration and selecting events within a physically-forbidden timing region for conventional n → p + e⁻ + ν̅_e decays. The summed kinetic energy (E_(e⁺e⁻)) from such events is reconstructed and used to set limits, as a function of the χ mass, on the branching fraction for this decay channel