Self-adaptive loop for external disturbance reduction in differential measurement set-up

We present a method developed to actively compensate common-mode magnetic disturbances on a multi-sensor device devoted to differential measurements. The system uses a field-programmable-gated-array card, and operates in conjunction with a high sensitivity magnetometer: compensating the common-mode of magnetic disturbances results in a relevant reduction of the difference-mode noise. The digital nature of the compensation system allows for using a numerical approach aimed at automatically adapting the feedback loop filter response. A common mode disturbance attenuation exceeding 50 dB is achieved, resulting in a final improvement of the differential noise floor by a factor of 10 over the whole spectral interval of interest.Comment: 7 pages, 8 figures, 26 ref

Restoring Narrow Linewidth to a Gradient-Broadened Magnetic Resonance by Inhomogeneous Dressing

We study the possibility of counteracting the line-broadening of atomic magnetic resonances due to inhomogeneities of the static magnetic field by means of spatially dependent magnetic dressing, driven by an alternating field that oscillates much faster than the Larmor precession frequency. We demonstrate that an intrinsic resonance linewidth of 25~Hz that has been broadened up to hundreds Hz by a magnetic field gradient, can be recovered by the application of an appropriate inhomogeneous dressing field. The findings of our experiments may have immediate and important implications, because they facilitate the use of atomic magnetometers as robust, high sensitivity detectors in ultra-low-field NMR imaging.Comment: 9 pages, 7 figures, 33 refs. This is the unedited versio

TRIGA-SPEC: A setup for mass spectrometry and laser spectroscopy at the research reactor TRIGA Mainz

The research reactor TRIGA Mainz is an ideal facility to provide neutron-rich nuclides with production rates sufficiently large for mass spectrometric and laser spectroscopic studies. Within the TRIGA-SPEC project, a Penning trap as well as a beam line for collinear laser spectroscopy are being installed. Several new developments will ensure high sensitivity of the trap setup enabling mass measurements even on a single ion. Besides neutron-rich fission products produced in the reactor, also heavy nuclides such as 235-U or 252-Cf can be investigated for the first time with an off-line ion source. The data provided by the mass measurements will be of interest for astrophysical calculations on the rapid neutron-capture process as well as for tests of mass models in the heavy-mass region. The laser spectroscopic measurements will yield model-independent information on nuclear ground-state properties such as nuclear moments and charge radii of neutron-rich nuclei of refractory elements far from stability. This publication describes the experimental setup as well as its present status.Comment: 20 pages, 17 figure

Nuclear quadrupole resonance system for landmine detection in Antioquia

Colombia ranks second in the world by number of victims from landmines; and in Colombia, Antioquia is the most affected department. As most landmines in Antioquia do not have metallic parts, metal detectors became useless, however, in most of those mines, the explosive mixture includes ammonium nitrate, and thus a system able to detect this compound could help to locate the mines. On the other side, nuclear quadrupole resonance (NQR) is a spectroscopic technique that allows the detection of some compounds very specifically. Thus, this work had the purpose of developing a system for remote sensing of ammonium nitrate in landmines by NQR. To achieve this goal, a portable NQR device was constructed as well as a probe, capable of sending radiofrequency pulses at the resonance frequency of ammonium nitrate, and capable of picking up the NQR signal after excitation. The manufactured system was tested against some environmental factors, concluding that the most affecting ones are soil conductivity and landmine shape. After achieving a functional system, a classifier based on spectral descriptors was trained, using data acquired with and without ammonium nitrate. Experimental results showed that the proposed classifier (an ensemble of 20 decision trees) had better performance, in terms of the area under the receiver operating characteristic curve, than the classical solution on the literature (which relies only on signal intensity). A final test validated the performance of the system, which detected four of five buried targets in an area of 2 x 1,6 m, having 3 false alarms.Resumen: Colombia es el segundo país con mayor número de víctimas por minas antipersona (MAP) en el mundo, siendo Antioquia es el departamento más afectado. La mayoría de las MAP en Antioquia contienen nitrato de amonio y la resonancia nuclear en cuadrupolo (NQR) es una técnica espectroscópica que permite detectar compuestos de forma muy específica. Así, este trabajo tiene el propósito desarrollar un sistema de NQR para la detección remota de nitrato de amonio en MAP. Para lograr este objetivo, se construyó un equipo portable de NQR, así como un inductor capaz de enviar pulsos a la frecuencia de resonancia del nitrato de amonio y de detectar la señal de NQR. El sistema construido fue probado en diferentes condiciones ambientales y se encontró que los factores que más lo afectan son la conductividad del suelo y la forma de la mina. Luego de lograr un sistema portable y funcional, se entrenó un clasificador basado en descriptores espectrales usando datos adquiridos con y sin nitrato de amonio. A partir de resultados experimentales se encontró que el clasificador entrenado (un ensamble de 20 árboles de decisión) tiene mejor desempeño, en términos del área bajo la curva de característica operativa del receptor, en comparación con la solución extendida en la literatura (que se basa únicamente en la intensidad de la señal). Una última prueba validó el desempeño del sistema, que fue capaz de detectar 5 muestras de nitrato de amonio ocultas en un área de 2 x 1,6 m, con 3 falsas alarmas.Doctorad

Development of solid state NMR to understand materials involved in catalytic technology used in fuel cells

The utility of the little used Field Sweep Fourier transform (FSFT) method is demonstrated for recording wideline nuclear magnetic resonance (NMR) of 195Pt resonances for various sized platinum nanoparticles, as well as platinum-tin bimetallics used in fuel cell catalysts, and various other related platinum (Pt3X; X = Al, Sc, Nb, Ti, Hf and Zr) alloys. The lineshapes observed from PtSn for both 195Pt and 119Sn suggest that it is more ordered than other closely related intermetallics, which might be expected from other measurements (e.g. XRD linewidths). From these reconstructed spectra the mean number of platinum atoms in the nanoparticle can be accurately determined along with detailed information regarding the number of atoms present effectively in each layer from the surface. This can be compared with theoretical predictions of the number of platinum atoms in these various layers for cubo-octahedral nanoparticles, thereby providing an estimate of the particle size. A comparison of the common NMR techniques used to acquire wideline spectra from spin I = 1 2 nuclei shows the advantages of the automated FSFT technique over the spin echo height/integration approach that dominates the literature. A study of small 13 atom platinum clusters, with variable particle size dispersion for which there is no experimental characterisation in the literature, provides evidence for an isotropic chemical shift of these platinum nanoparticles and provides a better basis for determining the Knight shift when compared to referencing against the primary IUPAC standard which has a different local structure. Rare earth apatite oxide ion conductors are novel candidates for electrolytes in solid oxide fuel cells. It has been shown that La8Y2Ge6O27 is an excellent oxide conductor at lower temperatures when compared to the market leader yttrium stabilised zirconia (YSZ). To understand the mechanism of its conduction 17O-labelled water was allowed to conduct through the sample and 17O solid state NMR was employed to comment on this pathway in a series of germanium and silicon substituted apatites. The linear channels running through the centre of the structure were believed to contain vacancies and as with perovskites it was commonly believed these allowed hopping of the oxygen to enable the apatite to conduct. It was shown that a limited amount of the 17O-oxygens made it to the channel and almost all of the label was located in the tetrahedra. This suggested that the mechanism of conduction was via the tetrahedral backbone. Molecular dynamics studies on these systems confirmed this SN2 mechanism of conduction as the excess oxygen hopped onto the tetrahedral site to form a five coordinate bridging oxygen which then forced a neighbouring oxygen to hop onto another tetrahedra. A comparison of analytical techniques used to characterise hydrogen bonding in benzoic acid and its corresponding group IA hemibenzoates indicates the need to draw upon multiple methods to fully understand the nature of the bond. The X-ray diffraction (XRD) data cannot confirm precisely the position of the hydrogen in the complex and hence cannot comment on the nature of the bond. Traditionally the angle at the central bonded proton and the oxygen-oxygen bond distance are used to comment on the strength of the hydrogen bonding, the results present here show the limitations of these analysis methods. Due to the oxygen-proton-oxygen bond angle variations commenting on the oxygen-oxygen length and correlating it to the hydrogen bonding is not feasible. There is heavy literature present on correlating the 1H isotropic shifts to the hydrogen bond strength, here we show a step wise change in hydrogen bonding from benzoic acid and lithium hemibenzoate down the periodic table to potassium, rubidium and cesium hemibenzoate. We show that the anisotropic tensor, 22, is pointed along the carbonyl bond and changes with the hydrogen bonding strength. However this method of characterising the bonding interaction gives a linear correlation from benzoic acid to cesium hemibenzoate. The 17O MAS of the carbonyl groups show an in ated quadrupole coupling constant when compared to the hydroxyls. There is a correlation between the anisotropic 13C 22 parameter and the quadrupole coupling (CQ), as the 22 decreases the CQ seems to give an overall increase. These oxygen results have been confirmed by multiple field double rotation results. All the crystallographic and solid state NMR data present is tied together by density functional theory calculations which show varying degrees of agreement with the achieved results