Optimisation of Positron-Positronium Conversion and Positronium Laser Excitation

The AEgIS collaboration (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) at CERN has as its goal to directly measure the Earth's gravitational force on antimatter for the first time. Neutral antihydrogen atoms are an integral part of performing this measurement, and will be produced by the highly efficient charge-exchange reaction between a cloud of cold antiprotons and Rydberg-state positronium. This thesis work is done at CERN as a part of the AEgIS experiment, specifically in the positron system which is an essential part of the overall experiment. The focus of the work is on improvements of the positron system, testing different positron-positronium converter materials and the initial laser excitation of positronium. The improvements of the positron system are done in two parts: implementation of an acceleration system in order to compress the positron beam and investigating photon detectors for the detection of positron annihilation radiation. The result of accelerating the positron beam is a 3-fold temporal compression of the beam, and additionally a detector with good characteristics is found for the positron spectroscopy. Four different converter targets, each consisting of monocrystalline silicon with etched nanochannels, are then investigated for the production of positronium. The targets are one sample of p-type Si with crystal orientation (100), one p-type Si with crystal orientation (111), and two n-type Si samples with crystal orientation (100) made with two different production processes. These are investigated using lifetime spectroscopy, and only thetwo p-type samples display evidence for positronium emission. The p-type (111) is slightly more efficient than the p-type (100) and has a positronium lifetime of 142$\pm$1 ns, while the p-type (100) has a lifetime of 111$\pm$1 ns. Finally, laser excitation of positronium to the $n$=3 state with a UV laser pulse is demonstrated here for the first time, using lifetime spectroscopy

Optimisation of Positron-Positronium Conversion and Positronium Laser Excitation

The AEgIS collaboration (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) at CERN has as its goal to directly measure the Earth's gravitational force on antimatter for the first time. Neutral antihydrogen atoms are an integral part of performing this measurement, and will be produced by the highly efficient charge-exchange reaction between a cloud of cold antiprotons and Rydberg-state positronium. This thesis work is done at CERN as a part of the AEgIS experiment, specifically in the positron system which is an essential part of the overall experiment. The focus of the work is on improvements of the positron system, testing different positron-positronium converter materials and the initial laser excitation of positronium. The improvements of the positron system are done in two parts: implementation of an acceleration system in order to compress the positron beam and investigating photon detectors for the detection of positron annihilation radiation. The result of accelerating the positron beam is a 3-fold temporal compression of the beam, and additionally a detector with good characteristics is found for the positron spectroscopy. Four different converter targets, each consisting of monocrystalline silicon with etched nanochannels, are then investigated for the production of positronium. The targets are one sample of p-type Si with crystal orientation (100), one p-type Si with crystal orientation (111), and two n-type Si samples with crystal orientation (100) made with two different production processes. These are investigated using lifetime spectroscopy, and only thetwo p-type samples display evidence for positronium emission. The p-type (111) is slightly more efficient than the p-type (100) and has a positronium lifetime of 142$\pm$1 ns, while the p-type (100) has a lifetime of 111$\pm$1 ns. Finally, laser excitation of positronium to the $n$=3 state with a UV laser pulse is demonstrated here for the first time, using lifetime spectroscopy

Infrared measurements of glucose in peritoneal fluid with a tuneable quantum cascade laser

Fast and accurate continuous glucose monitoring is needed in future systems for control of blood glucose levels in type 1 diabetes patients. Direct spectroscopic measurement of glucose in the peritoneal cavity is an attractive alternative to conventional electrochemical sensors placed subcutaneously. We demonstrate the feasibility of fast glucose measurements in peritoneal fluid using a fibre-coupled tuneable mid-infrared quantum cascade laser. Mid-infrared spectra (1200–925 cm−1) of peritoneal fluid samples from pigs with physiological glucose levels (32–426 mg/dL, or 1.8–23.7 mmol/L) were acquired with a tuneable quantum cascade laser employing both transmission and attenuated total reflection (ATR) spectroscopy. Using partial least-squares regression, glucose concentrations were predicted with mean absolute percentage errors (MAPEs) of 8.7% and 12.2% in the transmission and ATR configurations, respectively. These results show that highly accurate concentration predictions are possible with mid-infrared spectroscopy of peritoneal fluid, and represent a first step towards a miniaturised optical sensor for intraperitoneal continuous glucose monitoring

Mid-infrared spectroscopy with a fiber-coupled tuneable quantum cascade laser for glucose sensing

A fiber-coupled transmission spectroscopy setup using a pulsed external-cavity quantum cascade laser (EC-QCL, 1200-900 cm−1 ) has been developed and demonstrated for measurements of aqueous solutions. The system has been characterised with regard to the laser noise and optimal optical pathlength. Solutions with glucose were used to further test the setup, and glucose concentrations down to physiologically relevant levels (0-600 mg/dl) were investigated. Albumin, lactate, urea, and fructose in various concentrations were added as interfering substances as their absorption bands overlap with those of glucose, and because they may be of interest in a clinical setting. Analyte concentrations were predicted using partial least-squares (PLS) regression, and the root-mean-square error of cross-validation for glucose was 10.7 mg/dl. The advantages of using a convolutional neural network (CNN) for regression analysis in comparison to the PLS regression were also shown. The application of a CNN gave an improved prediction error (8.3 mg/dl), and was used to identify important spectral regions. These results are comparable to state-of-the-art enzymatic glucose sensors, and are encouraging for further research on optics-based glucose sensors

Infrared Spectroscopy with a Fiber-Coupled Quantum Cascade Laser for Attenuated Total Reflection Measurements Towards Biomedical Applications

The development of rapid and accurate biomedical laser spectroscopy systems in the mid-infrared has been enabled by the commercial availability of external-cavity quantum cascade lasers (EC-QCLs). EC-QCLs are a preferable alternative to benchtop instruments such as Fourier transform infrared spectrometers for sensor development as they are small and have high spectral power density. They also allow for the investigation of multiple analytes due to their broad tuneability and through the use of multivariate analysis. This article presents an in vitro investigation with two fiber-coupled measurement setups based on attenuated total reflection spectroscopy and direct transmission spectroscopy for sensing. A pulsed EC-QCL (1200–900 cm −1 ) was used for measurements of glucose and albumin in aqueous solutions, with lactate and urea as interferents. This analyte composition was chosen as an example of a complex aqueous solution with relevance for biomedical sensors. Glucose concentrations were determined in both setup types with root-mean-square error of cross-validation (RMSECV) of less than 20 mg/dL using partial least-squares (PLS) regression. These results demonstrate accurate analyte measurements, and are promising for further development of fiber-coupled, miniaturised in vivo sensors based on mid-infrared spectroscopy

Signal enhancement in microstructured silicon attenuated total reflection elements with quantum cascade laser-based spectroscopy

Sensors in mid-infrared spectroscopy based on attenuated total reflection (ATR) sensing with internal reflection elements (IREs) facilitate easier measurements of aqueous solutions or other opaque analytes. Micromachined silicon (Si) elements are an attractive alternative to conventional IREs, as they can be produced cheaply with silicon processing. Techniques for surface modifications are also easily integrated into the wafer process, and surface structures such as micropillars or nanoparticles can thereby be used for signal enhancement. Replacing the classic Fourier transform infrared (FTIR) spectrometers with tuneable quantum cascade lasers (QCLs) also opens up new avenues for sensing. In this study, the performance of basic and signal-enhanced Si IREs has been compared for measurements in a spectroscopy setup with a fibre-coupled tuneable QCL source. These IREs had V-shaped microgrooves etched on the underside for more efficient in-coupling of light, while the signal enhanced IREs also had micropillars on the top surface. The results are also contrasted with measurements done in a standard ATR-FTIR spectrometer, using an Alpha II spectrometer with a single-reflection diamond ATR crystal. Various concentrations of glucose (0-5000 mg/dl) in aqueous solutions were used to characterise the system performance. The quality of the signal enhancement was evaluated with regard to sensitivity and noise level in the acquired spectra. The microstructured Si IREs gave a signal enhancement of up to a factor of 3.8 compared to a basic Si element, with some concomitant increase in noise. The absorbance was higher for both types of Si IREs as compared to the diamond ATR crystal. The effective enhancement and the limit of quantification improved by a factor up to 3.1 in the signal-enhanced IREs compared to the basic Si element

A Review of Optical Methods for Continuous Glucose Monitoring

Frequent glucose monitoring is a fundamental part of diabetes management, and good glucose control is important for long-term health outcomes. New types of electrochemical sensors that allow for continuous glucose monitoring (CGM) have become an important tool for diabetes management, although they still have drawbacks such as short lifetime and a need for frequent calibration. Other technologies are still being researched for CGM, in an attempt to replace the electrochemical sensors. Optical methods have several advantages for CGM, including potentially long sensor lifetimes and short measurement times, and many developments have been made over the last decades. This paper will review optical measurement methods for CGM, their challenges, and the current research status. The different methods will be compared, and the future prospects for optical methods will be discussed

A miniaturized ball-lensed fiber optic NIR transmission spectroscopy-based glucose sensor

A novel ball-lensed fiber transmission sensor is presented aimed at in vivo continuous glucose monitoring of diabetics. Preliminary results yield 20 mM RMSE, limited by mechanical instability. The design enables flexibility and further miniaturization

Towards Fiber-Optic Raman Spectroscopy for Glucose Sensing

We demonstrate a multimode optical fiber sensor for spectroscopic Raman measurements of glucose concentration for the application in intraperitoneal glucose detection in diabetic patients. A regression model with a RMSEC of 2.2 mM was obtained