Μm Pixel Pitch Research Articles

Silicon pixel-detector R&D for CLIC

The physics aims at the future CLIC high-energy linear e+e- collider set very high precision requirements on the performance of the vertex and tracking detectors. Moreover, these detectors have to be well adapted to the experimental conditions, such as the time structure of the collisions and the presence of beam-induced backgrounds. The principal challenges are: a point resolution of a few μm, ultra-low mass (∼ 0.2%X0 per layer for the vertex region and ∼ 1%X0 per layer for the outer tracker), very low power dissipation (compatible with air-flow cooling in the inner vertex region) and pulsed power operation, complemented with ∼ 10 ns time stamping capabilities. A highly granular all-silicon vertex and tracking detector system is under development, following an integrated approach addressing simultaneously the physics requirements and engineering constraints. For the vertex-detector region, hybrid pixel detectors with small pitch (25 μm) and analog readout are explored. For the outer tracking region, both hybrid concepts and fully integrated CMOS sensors are under consideration. The feasibility of ultra-thin sensor layers is validated with Timepix3 readout ASICs bump bonded to active edge planar sensors with 50 μm to 150 μm thickness. Prototypes of CLICpix readout ASICs implemented in 6525 nm CMOS technology with 25 μm pixel pitch have been produced. Hybridisation concepts have been developed for interconnecting these chips either through capacitive coupling to active HV-CMOS sensors or through bump-bonding to planar sensors. Recent R&D achievements include results from beam tests with all types of hybrid assemblies. Simulations based on Geant4 and TCAD are used to validate the experimental results and to assess and optimise the performance of various detector designs.

Two Micron Pixel Pitch Active Matrix Spatial Light Modulator Driven by Spin Transfer Switching

We have developed an active matrix-addressed magneto-optical spatial light modulator driven by spin-transfer switching (spin-SLM) which has a 100 × 100 array pixel layout with a 2 µm pixel pitch. It has pixel-selection-transistors and logic circuits which convert serial data into parallel data to reduce input terminals. We have confirmed successful magnetization switching of each pixel by injecting a pulse current generated from the logic circuit, and its optical display capability by showing digital characters.

Resolution and quality enhancement in terahertz in-line holography by sub-pixel sampling with double-distance reconstruction.

We demonstrate the enhancement of resolution and image quality in terahertz (THz) lens-free in-line digital holography by sub-pixel sampling with double-distance reconstruction. Multiple sub-pixel shifted low-resolution (LR) holograms recorded by a pyroelectric array detector (100 μm × 100 μm pixel pitch, 124 × 124 pixels) are aligned precisely to synthesize a high-resolution (HR) hologram. By this method, the lateral resolution is no more limited by the pixel pitch, and lateral resolution of 150 μm is obtained, which corresponds to 1.26λ with respect to the illuminating wavelength of 118.8 μm (2.52 THz). Compared with other published works, to date, this is the highest resolution in THz digital holography when considering the illuminating wavelength. In addition, to suppress the twin-image and zero-order artifacts, the complex amplitude distributions of both object and illuminaing background wave fields are reconstructed simultaneously. This is achieved by iterative phase retrieval between the double HR holograms and background images at two recording planes, which does not require any constraints on object plane or a priori knowledge of the sample.

Speckle reduction in laser picoprojector by combining optical phase matrix with twin green lasers and oscillating MEMS mirror for coherence suppression

The combined speckle reduction method is proposed and designed for laser picoprojectors taking small size, high optical power, and good image preservation into account. An optical phase matrix, twin green lasers, and an oscillating MEMS scanning mirror are used to suppress the speckle by reducing spatial and temporal coherences. The optical phase matrix is designed using a 5 µm pixel pitch based on Matlab simulation results and fabricated by imprinting. The twin green lasers with a modulation configuration and an oscillating MEMS scanning mirror are also applied taking the resolution of the display image and the scanning frequency of the MEMS scanning mirror into account. From the experimental results, it is confirmed that the speckle contrast is reduced by 47.47% by the optical phase matrix, 27.93% by the twin green lasers, and 18.89% by the oscillating MEMS scanning mirror. Finally, the combined speckle contrast results in a speckle reduction efficiency of 53.80% with a relatively small optical power loss of 18.21%.

The MAPS-based vertex detector for the STAR experiment: Lessons learned and performance

The PiXeL detector (PXL) of the STAR experiment at RHIC is the first application of the state-of-the-art thin Monolithic Active Pixel Sensors (MAPS) technology in a collider environment. The PXL, together with the Intermediate Silicon Tracker (IST) and the Silicon Strip Detector (SSD), form the Heavy Flavor Tracker (HFT), which has been designed to improve the vertex resolution and extend the STAR measurement capabilities in the heavy flavor domain, providing a clean probe for studying the Quark–Gluon Plasma.The two PXL layers are placed at a radius of 2.8 and 8cm from the beam line, respectively, and is based on ultra-thin high resolution MAPS sensors. The sensor features 20.7μm pixel pitch, 185.6μs readout time and 170mW/cm2 power dissipation. The detector is air-cooled, allowing a global material budget of 0.4% radiation length on the innermost layer. A novel mechanical approach to detector insertion allows for fast installation and integration of the pixel sub detector. The HFT took data in Au+Au collisions at 200GeV during the 2014 RHIC run. Modified during the RHIC shutdown to improve its reliability, material budget, and tracking capabilities, the HFT took data in p+p and p+Au collisions at √sNN=200GeV in the 2015 RHIC run.In this paper we present detector specifications, experience from the construction and operations, and lessons learned. We also show preliminary results from 2014 Au+Au data analyses, demonstrating the capabilities of charm reconstruction with the HFT.

Angular sensitivity of modeled scientific silicon charge-coupled devices to initial electron direction

Previously, scientific silicon charge-coupled devices (CCDs) with 10.5-μm pixel pitch and a thick (650μm), fully depleted bulk have been used to measure gamma-ray-induced fast electrons and demonstrate electron track Compton imaging. A model of the response of this CCD was also developed and benchmarked to experiment using Monte Carlo electron tracks.We now examine the trade-off in pixel pitch and electronic noise. We extend our CCD response model to different pixel pitch and readout noise per pixel, including pixel pitch of 2.5μm, 5μm, 10.5μm, 20μm, and 40μm, and readout noise from 0eV/pixel to 2keV/pixel for 10.5μm pixel pitch. The CCD images generated by this model using simulated electron tracks are processed by our trajectory reconstruction algorithm. The performance of the reconstruction algorithm defines the expected angular sensitivity as a function of electron energy, CCD pixel pitch, and readout noise per pixel.Results show that our existing pixel pitch of 10.5μm is near optimal for our approach, because smaller pixels add little new information but are subject to greater statistical noise.In addition, we measured the readout noise per pixel for two different device temperatures in order to estimate the effect of temperature on the reconstruction algorithm performance, although the readout is not optimized for higher temperatures. The noise in our device at 240K increases the FWHM of angular measurement error by no more than a factor of 2, from 26° to 49° FWHM for electrons between 425keV and 480keV. Therefore, a CCD could be used for electron-track-based imaging in a Peltier-cooled device.

The STAR Heavy Flavor Tracker (HFT): focus on the MAPS based PXL detector

The heavy quark hadrons are suggested as a clean probe for studying the early dynamic evolution of the dense and hot medium created in high-energy nuclear collisions. The Heavy Flavor Tracker (HFT) of the STAR experiment, designed to improve the vertex resolution and extend the measurement capabilities in the heavy flavor domain, was installed for the 2014 heavy ion run of RHIC.It is composed of three different silicon detectors arranged in four concentric cylinders close to the STAR interaction point. The two inner-most layers are based on CMOS monolithic active pixels (MAPS), featured for the first time in a collider experiment, while the two outer layers are based on pads and strips. The two innermost HFT layers are placed at a radius of 2.7 and 8 cm from the beam line and accommodate 400 ultra-thin (50 μm) high resolution MAPS sensors arranged in 10-sensor ladders to cover a total silicon area of 0.16 m2. Each sensor includes a pixel array of 928 rows and 960 columns with a 20.7 μm pixel pitch, providing a sensitive area of ∼ 3.8 cm2. The sensor features 185.6 μs readout time and 170 mW/cm2 power dissipation. The detector is air-cooled, allowing a global material budget as low as 0.39% X/X0 on the inner layer. A novel mechanical approach to detector insertion enables effective installation and integration of the pixel layers within an 8 hour shift during the on-going STAR run.After a detailed description of the design specifications and the technology implementation, the detector status and operations during the current 200 GeV Au+Au run will be presented in this paper, with a particular focus on calibration and general system operations aimed at stabilizing the running conditions. A preliminary estimation of the detector performance meeting the design requirements will be reported.

Low‐noise readout circuit for thermo‐electrical cooler‐less uncooled microbolometer infrared imager

A low-noise analogue frontend readout circuit is presented for VOx microbolometer infrared imager without thermo-electrical cooler. A modified differential mirror structure with substrate temperature and self-heating effect compensation ability is used, targeting below 40 mK noise equivalent temperature difference (NETD) with 17 μm pixel pitch. Noise characteristics are investigated to give an optimal trade-off approach to reach lower NETD at same power consumption compared with traditional readout structures.

Topmetal-II−: a direct charge sensor for high energy physics and imaging applications

Topmetal-II−, a direct charge sensor, was manufactured in an XFAB 350 nm CMOS process. The Topmetal-II− sensor features a 72 × 72 pixel array with an 83 μm pixel pitch which collects and measures charge directly from the surrounding media. We introduce the implementation of the circuitry in the sensor including an analogue readout channel and a column based digital readout channel. The analogue readout channel allows the access to the full waveform from each pixel through a time-shared multiplexing. The digital readout channel records hits identified by an individually settable threshold in each pixel. Some simulation and preliminary test results are also discussed.

Low dose high energy x-ray in-line phase sensitive imaging prototype: Investigation of optimal geometric conditions and design parameters.

The objective of this study was to investigate the optimization of a high energy in-line phase sensitive x-ray imaging prototype under different geometric and operating conditions for mammography application. A phase retrieval algorithm based on phase attenuation duality (PAD) was applied to the phase contrast images acquired by the prototype. Imaging performance was investigated at four magnification values of 1.67, 2, 2.5 and 3 using an acrylic edge, an American College of Radiology (ACR) mammography phantom and contrast detail (CD) phantom with tube potentials of 100, 120 and 140 kVp. The ACR and CD images were acquired at the same mean glandular dose (MGD) of 1.29 mGy with a computed radiography (CR) detector of 43.75 μm pixel pitch at a fixed source to image distance (SID) of 170 cm. The x-ray tube focal spot size was kept constant as 7 μm while a 2.5 mm thick aluminum (Al) filter was used for beam hardening. The performance of phase contrast and phase retrieved images were compared with computer simulations based on the relative phase contrast factor (RPF) at high x-ray energies. The imaging results showed that the x-ray tube operated at 100 kVp under the magnification of 2.5 exhibits superior imaging performance which is in accordance to the computer simulations. As compared to the phase contrast images, the phase retrieved images of the ACR and CD phantoms demonstrated improved imaging contrast and target discrimination. We compared the CD phantom images acquired in conventional contact mode with and without the anti-scatter grid using the same prototype at 1.295 mGy and 2.59 mGy using 40 kVp, a 25 μm rhodium (Rh) filter. At the same radiation dose, the phase sensitive images provided improved detection capabilities for both the large and small discs, while compared to the double dose image acquired in conventional mode, the observer study also indicated that the phase sensitive images provided improved detection capabilities for the large discs. This study therefore validates the potential of using high energy phase contrast x-ray imaging to improve lesion detection and reduce radiation dose for clinical applications such as mammography.

50 μm pixel pitch wafer-scale CMOS active pixel sensor x-ray detector for digital breast tomosynthesis

Wafer-scale CMOS active pixel sensors (APSs) have been developed recently for x-ray imaging applications. The small pixel pitch and low noise are very promising properties for medical imaging applications such as digital breast tomosynthesis (DBT). In this work, we evaluated experimentally and through modeling the imaging properties of a 50 μm pixel pitch CMOS APS x-ray detector named DynAMITe (Dynamic Range Adjustable for Medical Imaging Technology). A modified cascaded system model was developed for CMOS APS x-ray detectors by taking into account the device nonlinear signal and noise properties. The imaging properties such as modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) were extracted from both measurements and the nonlinear cascaded system analysis. The results show that the DynAMITe x-ray detector achieves a high spatial resolution of 10 mm−1 and a DQE of around 0.5 at spatial frequencies <1 mm−1. In addition, the modeling results were used to calculate the image signal-to-noise ratio (SNRi) of microcalcifications at various mean glandular dose (MGD). For an average breast (5 cm thickness, 50% glandular fraction), 165 μm microcalcifications can be distinguished at a MGD of 27% lower than the clinical value (~1.3 mGy). To detect 100 μm microcalcifications, further optimizations of the CMOS APS x-ray detector, image aquisition geometry and image reconstruction techniques should be considered.

A 65 nm 0.5 V DPS CMOS Image Sensor With 17 pJ/Frame.Pixel and 42 dB Dynamic Range for Ultra-Low-Power SoCs

Adding vision capabilities to wireless sensors nodes (WSN) for the Internet-of-Things requires imagers working at ultra-low power (ULP) in nanometer CMOS systems-on-chip (SoCs). Such performance can be obtained with time-based digital pixel sensors (DPS) working at ultra-low voltage (ULV), at the expense of lower dynamic range, higher fixed-pattern noise (FPN) and thus poorer image quality. To address this problem, three key techniques were developed in this work for DPS pixels: wide-range adaptive body biasing, low- $R_{{\rm on}}$ gating of the 2-transistor in-pixel comparator and digital readout performing delta-reset sampling with low switching activity and robust timing closure. These concepts were demonstrated by designing and fabricating a 128 $\times$ 128 CMOS image sensor array in a 65 nm low-power CMOS logic process. Operating at 0.5 V, it features an FPN of 0.66%, a dynamic range of 42 dB and a fill factor of 57% with a 4 µm pixel pitch, while consuming only 17 pJ/(frame.pixel) and 8.8 µW at 32 fps. These performances combined with the small silicon area of 0.69 mm 2 makes the imager perfectly suitable for integration in ULP SoCs, targeting WSN applications.

A 2 kfps Sub-µW/Pix Uncooled-PbSe Digital Imager With 10 Bit DR Adjustment and FPN Correction for High-Speed and Low-Cost MWIR Applications

Mid-wavelength infrared (MWIR) thermography is an emerging technology with promising applications such as industrial monitoring, medicine and automotive, but its use in high-speed cameras is not yet widespread due to the lack of inexpensive sensor integration solutions and their common reliance on bulky cooling mechanisms. This work fills the gap by presenting a monolithic uncooled high-speed imager based on vapor-phase deposition lead selenide (VPD PbSe) photoconductors and a fully digital and configurable CMOS read-out integrated circuit (ROIC) to operate the MWIR imager. This ROIC features cancellation of PbSe dark current, compensation of its output capacitance and correction of the fixed pattern noise (FPN) caused by process non-uniformities in CMOS fabrication and detector deposition. The low-cost 80 × 80 imager has been integrated using 0.35 µm 2P4M standard CMOS technology and PbSe detector post-processing with 135 µm pixel pitch and 68% fill factor values. Experimental opto-electrical performance exhibits 10 bit real-time FPN compensation and DR calibration over the entire focal plane operating at 2 kfps, sub-0.5 LSB inter-pixel crosstalk, sub-µW pixel power consumption, and an overall figure of merit of 55 mK × ms.

High Sensitivity &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$17~\mu \text{m}$ &lt;/tex-math&gt;&lt;/inline-formula&gt; Pixel Pitch &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$640\times 512$ &lt;/tex-math&gt;&lt;/inline-formula&gt; Uncooled Infrared Focal Plane Arrays Based on Amorphous Vanadium Oxide Thin Films

This letter presents a 17 μm pixel pitch 640 × 512 uncooled infrared focal plane arrays based on the double sacrificial layer microbolometer technology incorporating amorphous vanadium oxide thin film and frequency selective structure. The amorphous vanadium oxide thin-film features low-noise and high-thermal coefficient of resistance characteristics, which help to improve the radiometric performance of the microbolometer. By incorporating a frequency selective second platform, the responsivity of the microbolometer was improved by nearly 30% over 8~14 μm spectra band. Noise equivalent temperature difference of less than 35 mK was obtained at 300 K ambient temperature with f/1.0 optics.

Microbolometer Terahertz Focal Plane Array and Camera with Improved Sensitivity in the Sub-Terahertz Region

A pixel in an uncooled microbolometer terahertz (THz) focal plane array (FPA) has a suspended structure above read-out integrated circuit (ROIC) substrate. An optical cavity structure is formed between a thin metallic layer deposited on the suspended structure and a thick metallic layer deposited on the ROIC surface. The geometrical optical cavity length for our previous pixel structure, 3–4 μm, is extended three times, so that responsivity can be increased in the sub-THz region. This modification is carried out by depositing a thick SiN layer on the thick metallic layer. The modified pixel structure is applied to 640 × 480 and 320 × 240 THz-FPAs with 23.5 μm pixel pitch. Minimum detectable powers per pixel (MDP) are evaluated for these FPAs at 4.3, 2.5, 0.6, and 0.5 THz, and the MDP values are found to be improved by a factor of ten at 0.6 and 0.5 THz. The MDP values of the THz-FPAs developed in this work are compared with those of other THz detectors, such as uncooled antenna-coupled CMOS (complimentary metal-oxide semiconductor) THz-FPAs and cooled bolometer arrays. It is found that our THz-FPAs are more sensitive in the sub-THz region than the CMOS THz-FPAs, while they are much less sensitive than the cooled bolometer arrays. These THz-FPAs are incorporated into a 640 × 480 THz camera and 320 × 240 THz camera, and imaging equipment is developed. The equipment consists of a linearly polarized sub-THz source, a collimator lens, a beam homogenizer, two wire grids, a quarter-wave plate, and two THz cameras, and sub-THz images are demonstrated. It should be mentioned for the equipment that imaging of transmission and reflection is realized by moving only the quarter-wave plate, and the reflection image is taken along a direction normal to a sample surface so that the reflection image is hardly deformed.

SU-D-213-03: Towards An Optimized 3D Scintillation Dosimetry Tool for Quality Assurance of Dynamic Radiotherapy Techniques

Purpose: The purpose of this work is to simulate a multi-focus plenoptic camera used as the measuring device in a real-time three-dimensional scintillation dosimeter. Simulating and optimizing this realistic optical system will bridge the technological gap between concept validation and a clinically viable tool that can provide highly efficient, accurate and precise measurements for dynamic radiotherapy techniques. Methods: The experimental prototype, previously developed for proof of concept purposes, uses an off-the-shelf multi-focus plenoptic camera. With an array of interleaved microlenses of different focal lengths, this camera records spatial and angular information of light emitted by a plastic scintillator volume. The three distinct microlens focal lengths were determined experimentally for use as baseline parameters by measuring image-to-object magnification for different distances in object space. A simulated plenoptic system was implemented using the non-sequential ray tracing software Zemax: this tool allows complete simulation of multiple optical paths by modeling interactions at interfaces such as scatter, diffraction, reflection and refraction. The active sensor was modeled based on the camera manufacturer specifications by a 2048×2048, 5 µm-pixel pitch sensor. Planar light sources, simulating the plastic scintillator volume, were employed for ray tracing simulations. Results: The microlens focal lengths were determined to be 384, 327more » and 290 µm. A realistic multi-focus plenoptic system, with independently defined and optimizable specifications, was fully simulated. A f/2.9 and 54 mm-focal length Double Gauss objective was modeled as the system’s main lens. A three-focal length hexagonal microlens array of 250-µm thickness was designed, acting as an image-relay system between the main lens and sensor. Conclusion: Simulation of a fully modeled multi-focus plenoptic camera enables the decoupled optimization of the main lens and microlens specifications. This work leads the way to improving the 3D dosimeter’s achievable resolution, efficiency and build for providing a quality assurance tool fully meeting clinical needs. M.R. is financially supported by a Master’s Canada Graduate Scholarship from the NSERC. This research is also supported by the NSERC Industrial Research Chair in Optical Design.« less

The Dosepix detector—an energy-resolving photon-countingpixel detector for spectrometric measurements

The Dosepix detector is a hybrid photon-counting pixel detector based on ideas of the Medipix and Timepix detector family. 1 mm thick cadmium telluride and 300 μm thick silicon were used as sensor material. The pixel matrix of the Dosepix consists of 16 x 16 square pixels with 12 rows of (200 μm)2 and 4 rows of (55 μm)2 sensitive area for the silicon sensor layer and 16 rows of pixels with 220 μm pixel pitch for CdTe. Besides digital energy integration and photon-counting mode, a novel concept of energy binning is included in the pixel electronics, allowing energy-resolved measurements in 16 energy bins within one acquisition. The possibilities of this detector concept range from applications in personal dosimetry and energy-resolved imaging to quality assurance of medical X-ray sources by analysis of the emitted photon spectrum. In this contribution the Dosepix detector, its response to X-rays as well as spectrum measurements with Si and CdTe sensor layer are presented. Furthermore, a first evaluation was carried out to use the Dosepix detector as a kVp-meter, that means to determine the applied acceleration voltage from measured X-ray tubes spectra.

The PixFEL project: development of advanced X-ray pixel detectors for application at future FEL facilities

The PixFEL project aims to develop an advanced X-ray camera for imaging suited for the demanding requirements of next generation free electron laser (FEL) facilities. New technologies can be deployed to boost the performance of imaging detectors as well as future pixel devices for tracking. In the first phase of the PixFEL project, approved by the INFN, the focus will be on the development of the microelectronic building blocks, carried out with a 65 nm CMOS technology, implementing a low noise analog front-end channel with high dynamic range and compression features, a low power ADC and high density memory. At the same time PixFEL will investigate and implement some of the enabling technologies to assembly a seamless large area X-ray camera composed by a matrix of multilayer four-side buttable tiles. A pixel matrix with active edge will be developed to minimize the dead area of the sensor layer. Vertical interconnection of two CMOS tiers will be explored to build a four-side buttable readout chip with small pixel pitch and all the on-board required functionalities. The ambitious target requirements of the new pixel device are: single photon resolution, 1 to 104 photons @ 1 keV to 10 keV input dynamic range, 10-bit analog to digital conversion up to 5 MHz, 1 kevent in-pixel memory and 100 μm pixel pitch. The long term goal of PixFEL will be the development of a versatile X-ray camera to be operated either in burst mode (European XFEL), or in continuous mode to cope with the high frame rates foreseen for the upgrade phase of the LCLS-II at SLAC.

A MAPS Based Micro-Vertex Detector for the STAR Experiment

For the 2014 heavy ion run of RHIC a new micro-vertex detector called the Heavy Flavor Tracker (HFT) was installed in the STAR experiment. The HFT consists of three detector subsystems with various silicon technologies arranged in 4 approximately concentric cylinders close to the STAR interaction point designed to improve the STAR detector's vertex resolution and extend its measurement capabilities in the heavy flavor domain. The two innermost HFT layers are placed at radii of 2.8 cm and 8 cm from the beam line. These layers are constructed with 400 high resolution sensors based on CMOS Monolithic Active Pixel Sensor (MAPS) technology arranged in 10-sensor ladders mounted on 10 thin carbon fiber sectors to cover a total silicon area of 0.16 m2. Each sensor of this PiXeL (“PXL”) sub-detector combines a pixel array of 928 rows and 960 columns with a 20.7μm pixel pitch together with front-end electronics and zero-suppression circuitry in one silicon die providing a sensitive area of ∼3.8 cm2. This sensor architecture features 185.6μs readout time and 170 mW/cm2 power dissipation. This low power dissipation allows the PXL detector to be air-cooled, and with the sensors thinned down to 50μm results in a global material budget of only 0.4% radiation length per layer. A novel mechanical approach to detector insertion allows us to effectively install and integrate the PXL sub-detector within a 12 hour period during an on-going multi-month data taking period. The detector requirements, architecture and design, as well as the performance after installation, are presented in this paper.

Charge sharing effect on 600 μm pitch pixelated CZT detector for imaging applications

We are currently investigating the spatial resolution of highly pixelated Cadmium Zinc Telluride (CZT) detector for imaging application. A 20 mm×20 mm×5 mm CZT substrate was fabricated with 600 μm pitch pixels (500 μm anode pixels with 100 μm gap) and coplanar cathode. Charge sharing between two pixels was studied using collimated a 122 keV gamma ray source. Experiments show a resolution of 125 μm FWHM for double-pixel charge sharing events when the 600 μm pixelated and 5 mm thick CZT detector biased at −1000 V. In addition, we analyzed the energy response of the 600 μm pitch pixelated CZT detector.