Re-examination of electronic transports through a quantum wire coupled to a quantum dot

In this paper we re-examine the problem of electronic transports through a system consisting of a quantum dot which has well-defined discrete energy levels connected to an infinite quantum wire, using the bosonization method and phase shift representation, we show that all previously known results can be obtained through our method in a very simple way. Furthermore, the evolution of the system from ultraviolet to infrared critical fixed points appears naturally our method.Comment: latex, 26 pages, to appear in Phys. Rev. B61, January 15/200

Limitations to flat-field correction methods when using an X-ray spectrum

Flat-field correction methods are implemented in order to eliminate non-uniformities in X-ray imaging sensors. If the compensation is perfect, then the remaining variations result from noise over the detector area. The efficiency of the compensation is reduced when an object is placed in the beam. A principle cause of this effect is believed to be the spectrum hardening caused by the object. In a normal application the correction factors are calculated for a certain spectrum, meaning that the average of the correction for the individual photon energies are used. If the composition of the spectrum changes the correction factor will also change. In this paper, we present a theory for the sensitivity of the gain constants on X-ray spectra. The theory is supported by experimental data obtained with X-ray spectra and monochromatic X-rays

Probing defects in a small pixellated CdTe sensor using an inclined mono energetic X-ray micro beam

High quantum efficiency is important in X-ray imaging applications. This means using high-Z sensor materials. Unfortunately many of these materials suffer from defects that cause non-ideal charge transport. In order to increase the understanding of these defects, we have mapped the 3D response of a number of defects in two 1 mm thick CdTe sensors with different pixel sizes (55 μm and 110 μm) using a monoenergetic microbeam at 79 keV. The sensors were bump bonded to Timepix read out chips. Data was collected in photon counting as well as time-over-threshold mode. The time-over-threshold mode is a very powerful tool to investigate charge transport properties and fluorescence in pixellated detectors since the signal from the charge that each photon deposits in each pixel can be analyzed. Results show distorted electrical field around the defects, indications of excess leakage current and large differences in behavior between electron collection and hole collection mode. The experiments were carried out on the Extreme Conditions Beamline I15 at Diamond Light Source

First experimental tests with a CdTe photon counting pixel detector hybridized with a Medipix2 readout chip

We present preliminary tests of hybrid pixel detectors consisting of the Medipix2 readout chip bump-bonded to a 1-mm thick CdTe pixel detector. This room temperature imaging system for single photon counting has been developed within the Medipix2 European Collaboration for various imaging applications with X-rays and gamma rays, including dental radiography, mammography, synchrotron radiation, nuclear medicine, radiation monitoring in nuclear facilities. The Medipix2+CdTe hybrid detector features 256x256 square pixels, a pitch of 55 μm, a sensitive area of 14x14 mm2. Here we analyzed the quality of the detector and bump-bonding and the response to nuclear radiation of the first CdTe hybrids. The CdTe pixel detectors, with Pt contacts, showed an ohmic response when negatively biased to less than 100 V (electrons collection mode). Tests were also performed in holes collection mode, where a non-resistive behaviour was observed above +15 V. We performed a series of imaging tests with gamma radioactive sources and with an X-ray tube. In flood illumination, we observed for all detectors the presence of numerous, stable small-scale structures in the form of small circles, with the central pixels showing a reduced counting efficiency with respect to the periphery (in electrons counting regime)

Non-adiabatic passing electron response and outward impurity convection in gyrokinetic calculations of impurity transport in ASDEX upgrade plasmas

Gyrokinetic calculations with the GS2 code of impurity transport in ASDEX Upgrade H-mode plasmas are presented. A method to separate the diagonal and off-diagonal terms in the quasi-linear flux computed by the gyrokinetic code for a trace impurity is introduced and applied. It is shown that in the experimental conditions of strong central electron cyclotron heating, unstable modes rotating in the electron diamagnetic direction are excited in the central region of the plasma. These are generated by the non-adiabatic response of passing electrons and feature extremely elongated eigenfunctions along the field line. The related fluctuations in the electrostatic potential generate an outward convection of the impurities. These theoretical findings are in agreement with the outward convection of laser ablated Si measured in these experimental conditions. In contrast, in the outer region of the plasma, as well as everywhere in the case of discharges without central electron heating, ion temperature gradient modes produce an impurity pinch directed inwards, which is also in agreement with the experimental observations

Active pixel detector for ion beam profiling

The use of ion implanters has been vital to the downward scaling of device dimensions in silicon technology. To achieve good process control, the ion beam characteristics within the implanter must be well known. Hybrid pixel technology is advanced as a possible solution to the problem of real-time ion beam profiling and dosimetry. The proof-of-principle prototype detector is described. The read-out electronics comprises a charge integrating chip designed in Europractice. The detector has been tested in an argon ion beam. The results show real- time beam profile acquisition and current measurement. The effects of secondary electron emission are qualitatively observed for different beam energies and must be addressed for accurate beam profiling

A new pixel sensor for uniformity control in ion implantation

The constantly growing research activities based on ion beams require an increasing precision in the control of the different ion beam parameters, such as beam uniformity and intensity. This paper presents a method of measuring the ion beam profile and intensity simultaneously, not easy with existing systems. The detector built comprises an array of 400 graphite pixels connected to ASIC (application specific integrated circuit) chips, that perform the integration and multiplexing of the charge deposited by the incoming ions into each pixel. The detector has been tested at the Surrey Ion Beam Centre and profiles of an argon beam at energies between 2 and 100 keV have been recorded. Total beam current has been monitored and compared to measurements taken with standard Faraday cups. Effects of secondary electron emission on the detector resolution and beam profile accuracy are discussed

320x240 GaAs pixel detectors with improved X-ray imaging quality

We report on gain and offset corrections for GaAs X-ray pixel detectors, which were hybridised to silicon CMOS readout integrated circuits (ROICs). The whole detector array contains 320 x 240 square-shaped pixels with a pitch of 38 mum. The GaAs pixel detectors are based on semi-insulating and VPE grown substrates. The ROIC operates in the charge integration mode and provides snapshot as well as real time video images. Previously we have reported that the image quality of semi- insulating GaAs pixel detectors suffer from local variations in X-ray sensitivity. We have now developed a method to compensate for the sensitivity variations by applying suitable offset and gain corrections. The improvement in image quality is demonstrated in the measured signal-to-noise ratio of flood exposure images

FIRST EXPERIMENTAL TESTS WITH A CDTE PHOTON COUNTING PIXEL DETECTOR HYBRIDIZED WITH A MEDIPIX2 READOUT CHIP

We present preliminary tests of hybrid pixel detectors consisting of the Medipix2 readout chip bump-bonded to a 1-mm-thick CdTe pixel detector. This room temperature imaging system for single photon counting has been developed within the Medipix2 European Collaboration for various imaging applications with X-rays and gamma rays, including dental radiography, mammography, synchrotron radiation, nuclear medicine, and radiation monitoring in nuclear facilities. The Medipix2 + CdTe hybrid detector features 256 256 square pixels, a pitch of 55 m, a sensitive area of 14 14 mm2. We analyzed the quality of the detector and bump-bonding and the response to nuclear radiation of the first CdTe hybrids. The CdTe pixel detectors, with Pt ohmic contacts, showed an ohmic response when negatively biased up to less than 60 V (electrons collection mode). Tests were also performed in holes collection mode, where a nonresistive behavior was observed above +15 V. We performed a series of imaging tests at lowvoltage bias with gamma radioactive sources and with an X-ray tube. Under uniform irradiation, we observed for all detectors the presence of numerous, stable structures in the form of small circles of about 200 m diameter, with the central pixels showing a reduced counting efficiency with respect to the periphery (in electrons counting regime). Also long filament structures have been observed. Further investigations will reveal whether they are due to an intrinsic detector response (e.g., due to Te inclusions) or to the bump-bonding process

A USB 3.0 readout system for Timepix3 detectors with on-board processing capabilities

Timepix3 is a high-speed hybrid pixel detector consisting of a 256 x 256 pixel matrix with a maximum data rate of up to 5.12 Gbps (80 MHit/s). The ASIC is equipped with eight data channels that are data driven and zero suppressed making it suitable for particle tracking and spectral imaging. In this paper, we present a USB 3.0-based programmable readout system with online preprocessing capabilities. USB 3.0 is present on all modern computers and can, under real-world conditions, achieve around 320MB/s, which allows up to 40 MHit/s of raw pixel data. With on-line processing, the proposed readout system is capable of achieving higher transfer rate (approaching Timepix4) since only relevant information rather than raw data will be transmitted. The system is based on an Opal Kelly development board with a Spartan 6 FPGA providing a USB 3.0 interface between FPGA and PC via an FX3 chip. It connects to a CERN T imepix 3 chipboard with standard VHDCI connector via a custom designed mezzanine card. The firmware is structured into blocks such as detector interface, USB interface and system control and an interface for data pre-processing. On the PC side, a Qt/C++ multi-platformsoftware library is implemented to control the readout system, providing access to detector functions and handling high-speed USB 3.0 streaming of data from the detector. We demonstrate equalisation, calibration and data acquisition using a Cadmium Telluride sensor and optimise imaging data using simultaneous ToT (Time-over-Threshold) and ToA (Timeof- Arrival) information. The presented readout system is capable of other on-line processing such as analysis and classification of nuclear particles with current or larger FPGAs