Soft Gamma-ray Detector for the ASTRO-H Mission

ASTRO-H is the next generation JAXA X-ray satellite, intended to carry instruments with broad energy coverage and exquisite energy resolution. The Soft Gamma-ray Detector (SGD) is one of ASTRO-H instruments and will feature wide energy band (40-600 keV) at a background level 10 times better than the current instruments on orbit. SGD is complimentary to ASTRO-H's Hard X-ray Imager covering the energy range of 5-80 keV. The SGD achieves low background by combining a Compton camera scheme with a narrow field-of-view active shield where Compton kinematics is utilized to reject backgrounds. The Compton camera in the SGD is realized as a hybrid semiconductor detector system which consists of silicon and CdTe (cadmium telluride) sensors. Good energy resolution is afforded by semiconductor sensors, and it results in good background rejection capability due to better constraints on Compton kinematics. Utilization of Compton kinematics also makes the SGD sensitive to the gamma-ray polarization, opening up a new window to study properties of gamma-ray emission processes. The ASTRO-H mission is approved by ISAS/JAXA to proceed to a detailed design phase with an expected launch in 2014. In this paper, we present science drivers and concept of the SGD instrument followed by detailed description of the instrument and expected performance.Comment: 17 pages, 15 figures, Proceedings of the SPIE Astronomical Instrumentation "Space Telescopes and Instrumentation 2010: Ultraviolet to Gamma Ray

Design and Characterization of 64K Pixels Chips Working in Single Photon Processing Mode

Progress in CMOS technology and in fine pitch bump bonding has made possible the development of high granularity single photon counting detectors for X-ray imaging. This thesis studies the design and characterization of three pulse processing chips with 65536 square pixels of 55 µm x 55 µm designed in a commercial 0.25 µm 6-metal CMOS technology. The 3 chips share the same architecture and dimensions and are named Medipix2, Mpix2MXR20 and Timepix. The Medipix2 chip is a pixel detector readout chip consisting of 256 x 256 identical elements, each working in single photon counting mode for positive or negative input charge signals. The preamplifier feedback provides compensation for detector leakage current on a pixel by pixel basis. Two identical pulse height discriminators are used to define an energy window. Every event falling inside the energy window is counted with a 13 bit pseudo-random counter. The counter logic, based in a shift register, also behaves as the input/output register for the pixel. Each cell also has an 8-bit configuration register which allows masking, test-enabling and 3-bit individual threshold adjust for each discriminator. The chip can be configured in serial mode and readout either serially or in parallel. Measurements show an electronic noise ~160 e- rms with a gain of ~9 mV/ke-. The threshold spread after equalization of ~120 e- rms brings the full chip minimum detectable charge to ~1100 e-. The analog static power consumption is ~8 µW per pixel with Vdda=2.2 V. The Mpix2MXR20 is an upgraded version of the Medipix2. The main changes in the pixel consist of: an improved tolerance to radiation, improved pixel to pixel threshold uniformity, and a 14-bit counter with overflow control. The chip periphery includes new threshold DACs with smaller step size, improved linearity, and better temperature dependence. Timepix is an evolution of the Mpix2MXR20 which provides independently in each pixel information of arrival time, time-over-threshold or event counting. Timepix uses as a time reference an external clock (Ref_Clk) up to 100 MHz which is distributed all over the pixel matrix during acquisition mode. The preamplifier is improved and there is a single discriminator with 4-bit threshold adjustment in order to reduce the minimum detectable charge limit. Measurements show an electrical noise ~100 e- rms and a gain of ~16.5 mV/ke-. The threshold spread after equalization of ~35 e- rms brings the full chip minimum detectable charge either to ~650 e- with a naked chip (i.e. gas detectors) or ~750 e- when bump-bonded to a detector. The pixel static power consumption is ~13.5 µW per pixel with Vdda=2.2 V and Ref_Clk=80 MHz. This family of chips have been used for a wide variety of applications. During these studies a number of limitations have come to light. Among those are limited energy resolution and surface area. Future developments, such as Medipix3, will aim to address those limitations by carefully exploiting developments in microelectronics

Si, CdTe and CdZnTe radiation detectors for imaging applications

The structure and operation of CdTe, CdZnTe and Si pixel detectors based on crystalline semiconductors, bump bonding and CMOS technology and developed mainly at Oy Simage Ltd. And Oy Ajat Ltd., Finland for X- and gamma ray imaging are presented. This detector technology evolved from the development of Si strip detectors at the Finnish Research Institute for High Energy Physics (SEFT) which later merged with other physics research units to form the Helsinki Institute of Physics (HIP). General issues of X-ray imaging such as the benefits of the method of direct conversion of X-rays to signal charge in comparison to the indirect method and the pros and cons of photon counting vs. charge integration are discussed. A novel design of Si and CdTe pixel detectors and the analysis of their imaging performance in terms of SNR, MTF, DQE and dynamic range are presented in detail. The analysis shows that directly converting crystalline semiconductor pixel detectors operated in the charge integration mode can be used in X-ray imaging very close to the theoretical performance limits in terms of efficiency and resolution. Examples of the application of the developed imaging technology to dental intra oral and panoramic and to real time X-ray imaging are given. A CdTe photon counting gamma imager is introduced. A physical model to calculate the photo peak efficiency of photon counting CdTe pixel detectors is developed and described in detail. Simulation results indicates that the charge sharing phenomenon due to diffusion of signal charge carriers limits the pixel size of photon counting detectors to about 250 μm. Radiation hardness issues related to gamma and X-ray imaging detectors are discussed.Työssä esitellään Suomessa kehitettyjä uudentyyppisiä digitaalisia röntgen- ja gammakuvausantureita, jotka perustuvat pii- ja CdTe-puolijohdeteknologiaan. Uuden tekniikan anturit osoitetaan toimivan hyvin lähellä teoreettista optimia. Erinomainen röntgenkuvan laatu saavutetaan pienellä säteilyannoksella. Antureiden rakenne ja toiminta selostetaan seikkaperäisesti ja niiden soveltamisesta hammaskuvaukseen ja reaaliaikaiseen videoröntgenkuvantamiseen annetaan esimerkkejä. Työssä esitellään myös fotonilaskentaan perustuvan CdTe gammakuvausanturin toiminta simuloinnein ja mittauksin sekä käsitellään antureiden säteilynkestoon liittyviä kysymyksiä

Optimization of Signal-to-Noise Ratio in Semiconductor Sensors via On-Chip Signal Amplification and Interface-Induced Noise Suppression.

Radiation detectors are now used in a large variety of fields in science and technology, and the number of applications is growing continually. This thesis presents the development of a wide band-gap solid state photomultiplier (SSPM) and the performance improvement of Si radiation detector with respect to noise suppression and resolution enhancement. Recently developed advanced scintillators, which have the ability to distinguish gamma-ray interaction events from those that accompany neutron impact, require improved quantum efficiency in the blue or near UV region of the spectrum. We utilize AlGaAs photodiode elements as components in a wide band-gap SSPM as a lower-cost, lower logistical burden and higher quantum efficiency replacement for the photomultiplier tube (PMT). We demonstrate that the diodes are responsive to blue and near UV in both linear and breakdown modes with robust electrical characteristics, which includes the leakage current and the onset of breakdown against geometric alteration in the diode design. For semiconductor direct-conversion radiation detectors, we investigated the performance enhancement of the detector via the suppression of noise induced from the semiconductor interface and the resolution improvement with on-chip amplification. The properties of the phonon-based noise are studied and methods to quench the charge mobility fluctuation via surface control, evaluating acoustic reflectance at the semiconductor metal interface by calculating reflectance coefficient via the roaming phonon microgradient (RPMG) model. Si radiation detectors are fabricated and the hypotheses evaluated with different geometries and metal types. In addition to the noise suppression, we also sought to increase the device signal by integrating an amplifying junction as part of the detector topology so that the SNR could be maximized. From this research, we demonstrated the feasibility of improving the energy resolution relative to those low-noise designs that don’t possess on-chip amplification by modeling, fabricating, and characterizing proof-of-concept planar and partitioned detectors. From the fabricated detectors, a semi-empirical result shows that the energy resolution for 81 keV gamma-rays can be reduced from 2.12 % to 0.96 % (for a k = 0.2) with a gain of ~8, which shows the best SNR optimization from our modeling.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111488/1/thnkang_1.pd

Improvements to MOS CCD technology for future X-ray astronomy missions

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.This thesis is concerned with the development of MOS charge-coupled device (CCD) technology for future applications in X-ray astronomy. Of particular interest is increased detection efficiency of high energy X-ray photons and increased pixel readout speed for large area sensors. Chapter 2 reviews the generation of X-rays, methods for extra-terrestrial X-ray observations, detectors and provides an overview of X-ray astronomy missions. Chapter 3 discusses the CCD and introduces some of the recent technological developments that improve their overall performance for optical and X-ray photon detection. Chapter 4 presents the basic laboratory equipment and methods used to carry out the experimental work of this thesis. Chapter 5 presents the characterisation of new high resistivity devices that were manufactured by e2v technologies during the work of this thesis. Chapter 6 describes a method for estimating the depletion depth of a CCD by analysing the X-ray event patterns that are generated in CCD image data. Chapter 7 presents the equipment developed and experimental measurements taken to evaluate the high energy X-ray quantum efficiency of a high resistivity CCD. Finally, Chapter 8 describes the ongoing development and characterisation of low noise ASICs that are intended for use in future X-ray astronomy missions

Characterization of the Imaging Performance of the Simultaneously Counting and Integrating X-ray Detector CIX

The CIX detector is a direct converting hybrid pixel detector designed for medical X-ray imaging applications. Its defining feature is the simultaneous operation of a photon counter as well as an integrator in every pixel cell. This novel approach offers a dynamic range of more than five orders of magnitude, as well as the ability to directly obtain the average photon energy from the measured data. Several CIX 0.2 ASICs have been successfully connected to CdTe, CdZnTe and Si sensors. These detector modules were tested with respect to the imaging performance of the simultaneously counting and integrating concept under X-ray irradiation. Apart from a characterization of the intrinsic benefits of the CIX concept, the sensor performance was also investigated. Here, the two parallel signal processing concepts offer valuable insights into material related effects like polarization and temporal response. The impact of interpixel coupling effects like charge-sharing, Compton scattering and X-ray uorescence was evaluated through simulations and measurements

Low gain avalanche detectors for particle physics and synchrotron applications

Semiconductor detectors have a wide range of uses for particle physics and synchrotron applications. This thesis concentrates on the simulation, fabrication and characterisation of a new type of detector known as the low gain avalanche detectors (LGAD). The detector’s characteristics are simulated via a full process simulation to obtain the required doping profiles which demonstrate the desired operational characteristics of high breakdown voltage (500V) and a gain of 10 at 200V reverse bias for low energy X-ray detection. The low gain avalanche detectors fabricated by Micron Semiconductor Ltd are presented. The doping profiles of the multiplication junctions were measured with Secondary ion mass spectrometry (SIMS) and reproduced by simulating the full fabrication process which enabled further development of the manufacturing process. LGADs are interesting for high energy physics experiments due to their good timing performance. The need for such a detector is explained and results for 250um thick LGADs with a gain of 5 manufactured at Micron Semiconductor show comparable results to the other vendors, of 120ps. For low energy X-ray detection it is essential to operate at low noise levels. The aim of the project was to develop LGAD detectors with a highly segmented front side which would be compatible with the Timepix3 chip, which has an array of 256x256 pixels with a pixel pitch of 55um. However, when LGAD pixels are made to these size requirements their gain uniformity and fill factor are extremely degraded. Specific development for small pixel LGAD's was undertaken through simulation and possible structures have been identified to minimize this small pixel effect

Surface versus Bulk Currents and Ionic Space-Charge Effects in CsPbBr3 Single Crystals

CsPbBr3 single crystals have potential for application in ionizing-radiation detection devices due to their optimal optoelectronic properties. Yet, their mixed ionic–electronic conductivity produces instability and hysteretic artifacts hindering the long-term device operation. Herein, we report an electrical characterization of CsPbBr3 single crystals operating up to the time scale of hours. Our fast time-of-flight measurements reveal bulk mobilities of 13–26 cm2 V–1 s–1 with a negative voltage bias dependency. By means of a guard ring (GR) configuration, we separate bulk and surface mobilities showing significant qualitative and quantitative transport differences. Our experiments of current transients and impedance spectroscopy indicate the formation of several regimes of space-charge-limited current (SCLC) associated with mechanisms similar to the Poole–Frenkel ionized-trap-assisted transport. We show that the ionic-SCLC seems to be an operational mode in this lead halide perovskite, despite the fact that experiments can be designed where the contribution of mobile ions to transport is negligible.Funding for open access charge: CRUE-Universitat Jaume

XNAP: A Novel Two-Dimensional X-Ray Detector for Time Resolved Synchrotron Applications

The XNAP project develops a demonstration system for a spatially resolving detector with timing capabilities in the nanosecond range. A dense array of avalanche photodiodes is combined with multiple readout ASICs to build the detector hybrid. On an area of nearly 1 cm2, single photons can be counted within each of the 1k pixels. After 20 years of continuous improvements during operation, the ESRF Synchrotron is going to be upgraded substantially by the replacement of major parts of the source and the beamlines. For experimental techniques that will benefit from the increased brilliance, research into X-ray detectors is required. The requirements for the novel detector are composed of the distinguished properties of multiple state-of-the-art detector systems, shifted towards technical limits. The specification is transferred into the design of the sensor, ASIC, interposing structure and the readout system. A smaller prototype detector is built to resolve implementation challenges ahead of its large-scale accomplishment. Emphasis is put on the ASIC, and parallel approaches for the interconnecting technology and the readout system are carried out. The usability of the smaller prototype system is demonstrated with measurements of microfocus X-ray and Synchrotron light. Parts of the final detector are characterized at the laboratory prior to its commissioning