This article discusses semiconductor pixel detectors, their working principle, major applications, and recent developments.
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Semiconductor Pixel Detectors and their Working Principle
Semiconductor pixel detectors are an important class of radiation and particle detection devices based on semiconductors, such as germanium or silicon (Si). A semiconductor diode is depleted by applying a reverse bias voltage on its electrodes to synthesize a simple semiconductor ionizing radiation detector.
The incoming ionization particle in the depleted diode volume creates a charge, which is collected by the electric field. Subsequently, the charge is brought to contacts where the read-out electronics collect the charge for further processing.
The electronics typically perform signal amplification, discrimination, and counting/ analog-to-digital conversion (ADC). A semiconductor pixel detector contains a matrix of such simple semiconductor detectors/pixels. Every pixel of the matrix is connected to its respective electronics that perform signal processing.
A semiconductor pixel detector can be manufactured as a monolithic or hybrid electronic device. Monolithic pixel devices are composed of a single chip where the bulk is utilized as the sensor and the read-out electronics are formed on the surface. DEPFET Macropixel detector manufactured by MPI Semiconductor laboratory is an example of a monolithic device.
Hybrid pixel devices consist of two chips. The first chip/sensor chip is a semiconductor diode with one pixelated contact and one common backside, while the second chip is composed of read-out electronics for every pixel. Bump-bonding technique is used to connect both chips.
XPAD and Pilatus are the commonly used hybrid pixel detectors. Hybrid devices are more advantageous compared to monolithic devices as the read-out chip can be combined with the sensors of various materials, such as cadmium telluride (CdTe), gallium arsenide (GaAs), and Si, while only Si can be used in monolithic devices.
For instance, the Medipix2 detector, a hybrid single quantum counting pixel imager used for radiography, consists of a CdTe/GaAs/Si semiconductor detector chip bump-bonded to a read-out electronics chip.
The detector chip is equipped with one common backside electrode and a front side matrix of electrodes containing 256 × 256 square pixels with 55 µm pitch that yields 65536 pixels in 1.4 × 1.4 cm2 area.
Medipix2 can be used to detect all types of ionizing particles using proper sensors. Each pixel is connected to its own preamplifier, double discriminator, and digital counter integrated into the read-out chip. Thus, any particle creating charges within a specific time interval is accurately counted by the detector.
In the TimePix hybrid pixel detector, which is the successor of Medipix2, every pixel can be configured to function in any one of the three modes, including the Medipix mode where the incoming particles are counted by the integrated counter, the time over threshold (ToT) mode where the counter is utilized as a Wilkinson type ADC to directly measure energy in each pixel, and the TimePix mode where the counter functions as a timer and measures time after the detection of a particle.
Applications of Semiconductor Pixel Detectors
Semiconductor pixel detectors are used in several applications, such as photo cameras and high-energy physics. In life sciences, these detectors are used extensively for imaging.
For instance, pixel detectors are used in X-ray radiography, including energy-sensitive X-ray transmission radiography of biological objects, phase-sensitive imaging, high-resolution X-ray microradiography of soft tissue, and high-contrast X-ray transmission radiography. These detectors are also used in radiography using heavy charged particles, neutron transmission radiography, and X-ray fluorescence analysis (XRF) imaging.
In nuclear medicine, the use of semiconductor pixel detectors in place of scintillation cameras can improve sensitivity, spatial resolution, and energy resolution. A hybrid semiconductor pixel detector comprising 48 × 48 cadmium zinc telluride (CdZnTe) pixel arrays with 125 μm spatial resolution can be used for gamma-ray imaging.
Limitations of Semiconductor Pixel Detectors
Several factors can affect the performance of semiconductor pixel detectors. For instance, the lack of uniformity or partial uniformity in the noise, energy calibration, and efficiency of all individual pixels/detectors can impact the detector performance. Additionally, the signal in one pixel often affects the neighboring pixels due to several reasons, such as charge sharing.
Recent Developments
X-ray tubes are one of the most important ionizing radiation sources and are used in different methods and applications, such as irradiation, investigation of materials, and diagnostics in medicine.
In such applications, the exact properties of X-ray beams or sources, including homogeneity and divergence of beams, beam profiles, and X-ray spectra, must be obtained to calibrate or optimize the X-ray systems.
In a study recently published in the journal Radiation Physics and Chemistry, researchers investigated the properties of low-power X-ray tubes, including a microfocus X-ray tube with polycapillary focusing optics and a small laboratory X-ray tube with collimated X-ray beam, with a maximum voltage of 50 kV using semiconductor pixel detector.
A silicon pixel detector working in a single event detection mode was used to reconstruct the beam shape features. The detector was inserted into the X-ray beams at different distances from the X-ray sources.
Researchers successfully constructed the beam two-dimensional (2D) profiles and measured the approximate position and energy of individual photons to obtain detailed information about the beam properties.
Conclusion
To summarize, semiconductor pixel detectors are effective for different radiation imaging applications as they provide higher quality and amount of information compared to alternative devices.
Moreover, new pixel devices, such as Medipix3, have further improved the operations of existing devices, such as Medipix2. Medipix3 allows dead time-free operation and color imaging.
Additionally, a novel charge allocation and summing scheme is implemented at the pixel level to allow proper binning of incoming photon energy by overcoming the effects of charge diffusion and fluorescence.
Each pixel of the Medipix3 device is equipped with two counters, enabling continuous measurement using one counter for image accumulation and another counter for read-out or acquisition of two images simultaneously at different energy discriminations.
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References and Further Reading
Jakubek, J. (2009). Semiconductor Pixel detectors and their applications in life sciences. Journal of Instrumentation. https://www.researchgate.net/publication/258309647_Semiconductor_Pixel_detectors_and_their_applications_in_life_sciences
Prokeš, R., Trojek, T., Musílek, L. (2020). Determination of X-ray tubes radiation beam characteristics with semiconductor pixel detectors. Radiation Physics and Chemistry, 172, 108771. https://doi.org/10.1016/j.radphyschem.2020.108771
Barber, H. B., Apotovsky, B. A., Augustine, F. L., Barrett, H. H., Dereniak, E. L., Doty, F. P., Eskin, J. D., Hamilton, W. J., Marks, D. G., Matherson, K. J., Venzon, J. E., Woolfenden, J. M., Young, E. T. (1997). Semiconductor pixel detectors for gamma-ray imaging in nuclear medicine. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 395(3), 421-428. https://doi.org/10.1016/S0168-9002(97)00615-3
Medipix3 [Online] Available at https://medipix.web.cern.ch/medipix3 (Accessed on 09 January 2023)
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