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Register a new userX-ray imaging with GEMs using $100,mu$m thick foils
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A simple X-ray imaging system using off-the-shelf electronics and simple reconstruction algorithms aiming a spatial resolution of 1.7 mm ($sim 3,%$ of the detector length) is described in this work. For this, two 100 cm$^2$ Gas Electron Multiplier (GEM) foils with a thickness of 100 mu m (2-fold thicker than the standard ones) were immersed in a mixture of argon and carbon dioxide (70:30). The charge readout with 2D position determination was done with resistive charge division. Due to their higher thickness with respect to the standard GEMs, the 100 mu m thick GEM foils were found to be less prone to damage caused by the electrical discharges. X-ray images are shown and some descriptions of the physical processes involved are presented. We describe the advantages of this method that allows counting each X-ray photon or particle entering the detector, its interaction position, as well as measuring of its energy. The results of our present work show a position resolution below 2 mm, being limited by the gas mixture used, and not the detecting system, with a very good cost effectiveness. Future work is being carried out to optimize the present system for a medical application as a proton beam monitor.
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A charge-coupled device (CCD) is a standard imager in optical region in which the image quality is limited by its pixel size. CCDs also function in X-ray region but with substantial differences in performance. An optical photon generates only one electron while an X-ray photon generates many electrons at a time. We developed a method to precisely determine the X-ray point of interaction with subpixel resolution. In particular, we found that a back-illuminated CCD efficiently functions as a fine imager. We present here the validity of our method through an actual imaging experiment.
A new concept for the simultaneous detection of primary and secondary scintillation in time projection chambers is proposed. Its core element is a type of very-thick GEM structure supplied with transparent electrodes and machined from a polyethylene naphthalate plate, a natural wavelength-shifter. Such a device has good prospects for scalability and, by virtue of its genuine optical properties, it can improve on the light collection efficiency, energy threshold and resolution of conventional micropattern gas detectors. This, together with the intrinsic radiopurity of its constituting elements, offers advantages for noble gas and liquid based time projection chambers, used for dark matter searches and neutrino experiments. Production, optical and electrical characterization, and first measurements performed with the new device are reported.
The performance of hybrid GaAs pixel detectors as X-ray imaging sensors were investigated at room temperature. These hybrids consist of 300 mu-m thick GaAs pixel detectors, flip-chip bonded to a CMOS Single Photon Counting Chip (PCC). This chip consists of a matrix of 64 x 64 identical square pixels (170 mu-m x 170 mu-m) and covers a total area of 1.2 cm**2. The electronics in each cell comprises a preamplifier, a discriminator with a 3-bit threshold adjust and a 15-bit counter. The detector is realized by an array of Schottky diodes processed on semi-insulating LEC-GaAs bulk material. An IV-charcteristic and a detector bias voltage scan showed that the detector can be operated with voltages around 200 V. Images of various objects were taken by using a standard X-ray tube for dental diagnostics. The signal to noise ratio (SNR) was also determined. The applications of these imaging systems range from medical applications like digital mammography or dental X-ray diagnostics to non destructive material testing (NDT). Because of the separation of detector and readout chip, different materials can be investigated and compared.
We report on two ultrastable lasers each stabilized to independent silicon Fabry-Perot cavities operated at 124 K. The fractional frequency instability of each laser is completely determined by the fundamental thermal Brownian noise of the mirror coatings with a flicker noise floor of $4 times 10^{-17}$ for integration times between 0.8 s and a few tens of seconds. We rigorously treat the notorious divergencies encountered with the associated flicker frequency noise and derive methods to relate this noise to observable and practically relevant linewidths and coherence times. The individual laser linewidth obtained from the phase noise spectrum or the direct beat note between the two lasers can be as small as 5 mHz at 194 THz. From the measured phase evolution between the two laser fields we derive usable phase coherence times for different applications of 11 s and 60 s.
We are currently investigating the spatial resolution of highly pixelated Cadmium Zinc Telluride (CZT) detector for imaging applications. A 20 mm {times} 20 mm {times} 5 mm CZT substrate was fabricated with 600 {mu}m pitch pixels (500 {mu}m anode pixels with 100 {mu}m gap) and coplanar cathode. Charge sharing between two pixels was studied using collimated 122 keV gamma ray source. Experiments show a resolution of 125 {mu}m FWHM for double-pixel charge sharing events when the 600 {mu}m pixelated and 5 mm thick CZT detector biased at -1000 V. In addition, we analyzed the energy response of the 600 {mu}m pitch pixelated CZT detector.
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