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A Hybrid Pixel Detector ASIC with Energy Binning for Real-Time, Spectroscopic Dose Measurements
Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media. (Electronics Design Division)
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Hybrid pixel detectors have been demonstrated to provide excellent quality detection of ionising photon radiation, particularly in X-ray imaging. Recently, there has been interest in developing a hybrid pixel detector specifically for photon dosimetry. This thesis is on the design, implementation, and preliminary characterisation of the Dosepix readout chip.

Dosepix has 256 square pixels of 220 mm side-length, constituting 12.4 mm2 of photo-sensitive area per detector. The combination of multiple pixels provides many parallel processors with limited input flux, resulting in a radiation dose monitor which can continuously record data and provide a real-time report on personal dose equivalent. Energy measurements are obtained by measuring the time over threshold of each photon and a state machine in the pixel sorts the detected photon event into appropriate energy bins. Each pixel contains 16 digital thresholds with 16 registers to store the associated energy bins. Preliminary measurements of Dosepix chips bump bonded to silicon sensors show very promising results. The pixel has a frontend noise of 120 e-. In low power mode, each chip consumes 15 mW, permitting its use in a portable, battery-powered system. Direct time over threshold output from the hybrid pixel detector assembly reveal distinctive photo-peaks correctly identifying the nature of incident photons, and verification measurements indicate that the pixel binning state machines accurately categorise charge spectra. Personal dose equivalent reconstruction using this data has a flat response for a large range of photon energies and personal dose equivalent rates.

Place, publisher, year, edition, pages
Mid Sweden University , 2012. , 238 p.
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 128
Keyword [en]
Hybrid pixel detector, low power readout chip, photon counting, time over threshold (ToT), energy binning, active personal dosimeter (APD)
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:miun:diva-16171ISBN: 978-91-87103-20-9 (print)OAI: oai:DiVA.org:miun-16171DiVA: diva2:524757
Public defence
2012-05-30, O102, Mittuniversitetet, Campus Sundsvall, Sundsvall, 13:00 (English)
Opponent
Supervisors
Available from: 2012-06-07 Created: 2012-05-03 Last updated: 2012-08-01Bibliographically approved
List of papers
1. Timepix, a 65k programmable pixel readout chip for arrival time, energy and/or photon counting measurements
Open this publication in new window or tab >>Timepix, a 65k programmable pixel readout chip for arrival time, energy and/or photon counting measurements
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2007 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 581, no 1-2, 485-494 p.Article in journal (Refereed) Published
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-9383 (URN)10.1016/j.nima.2007.08.079 (DOI)
Available from: 2009-07-13 Created: 2009-07-13 Last updated: 2017-12-13Bibliographically approved
2. Smart dosimetry by pattern recognition using a single photon counting detector system in time over threshold mode
Open this publication in new window or tab >>Smart dosimetry by pattern recognition using a single photon counting detector system in time over threshold mode
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2012 (English)In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 7, no 1, Art. no. C01027- p.Article in journal (Refereed) Published
Abstract [en]

The function of a dosimeter is to determine the absorbed dose of radiation, for those cases in which, generally, the particular type of radiation is already known. Lately, a number of applications have emerged in which all kinds of radiation are absorbed and are sorted by pattern recognition, such as the Medipix2 application in [1]. This form of smart dosimetry enables measurements where not only the total dosage is measured, but also the contributions of different types of radiation impacting upon the detector surface. Furthermore, the use of a photon counting system, where the energy deposition can be measured in each individual pixel, ensures measurements with a high degree of accuracy in relation to the pattern recognition. In this article a Timepix [2] detector system has been used in the creation of a smart dosimeter for Alpha, Beta and Gamma radiation. When a radioactive particle hits the detector surface it generates charge clusters and those impacting upon the detector surface are read out and image processing algorithms are then used to classify each charge cluster. The individual clusters are calculated and as a result, the dosage for each type of radiation is given. In some cases, several particles can impact in roughly the same place, forming overlapping clusters. In order to handle this problem, a cluster separation method has been added to the pattern recognition algorithm. When the clusters have been separated, they are classified by shape and sorted into the correct type of radiation. The algorithms and methods used in this dosimeter have been developed so as to be simple and computationally effective, in order to enable implementation on a portable device. © 2012 IOP Publishing Ltd and SISSA.

Keyword
Dosimetry, Pattern Recognitionm, Single Photon Counting Detector, Time over Threshold Mode
National Category
Radiology, Nuclear Medicine and Medical Imaging Medical Image Processing Accelerator Physics and Instrumentation
Identifiers
urn:nbn:se:miun:diva-15495 (URN)10.1088/1748-0221/7/01/C01027 (DOI)000303806200027 ()2-s2.0-84856994290 (Scopus ID)STC (Local ID)STC (Archive number)STC (OAI)
Conference
13th International Workshop on Radiation Imaging Detectors (IWORID)
Available from: 2011-12-20 Created: 2011-12-20 Last updated: 2017-12-08Bibliographically approved
3. Design considerations for Area-Constrained In-Pixel Photon counting in Medipix3
Open this publication in new window or tab >>Design considerations for Area-Constrained In-Pixel Photon counting in Medipix3
2008 (English)In: Topical workshop on Electronics for Particle Physics, 2008, 539-543 p.Conference paper, Published paper (Refereed)
Abstract [en]

Hybrid pixel detectors process impinging photons using front-end electronics electrically connected to a segmented sensor via solder bumps. This allows for complex in-pixel processing while maintaining 100% fill factor. Medipix3 is a single photon processing chip whose 55 μm x 55 μm pixels contain analog charge-processing circuits, inter-pixel routing, and digital blocks. While a standard digital design flow would use logic gates from a standard cell library, the integration of multiple functions and configurations within the compact area of the Medipix3 pixel requires a full-custom manual layout. This work describes the various area-saving design strategies which were employed to optimize the use of available space in the digital section of the Medipix3 pixel.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-15824 (URN)
Conference
Topical Workshop on Electronics for Particle Physics, Naxos, Greece, 15 - 19 Sep 2008,
Available from: 2012-02-02 Created: 2012-02-02 Last updated: 2012-06-07Bibliographically approved
4. Medipix3: A 64 k pixel detector readout chip working in single photon counting mode with improved spectrometric performance
Open this publication in new window or tab >>Medipix3: A 64 k pixel detector readout chip working in single photon counting mode with improved spectrometric performance
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2011 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 633, no SUPPL. 1, S15-S18 p.Article in journal (Refereed) Published
Abstract [en]

Medipix3 is a 256×256 channel hybrid pixel detector readout chip working in a single photon counting mode with a new inter-pixel architecture, which aims to improve the energy resolution in pixelated detectors by mitigating the effects of charge sharing between channels. Charges are summed in all 2×2 pixel clusters on the chip and a given hit is allocated locally to the pixel summing circuit with the biggest total charge on an event-by-event basis. Each pixel contains also two 12-bit binary counters with programmable depth and overflow control. The chip is configurable such that either the dimensions of each detector pixel match those of one readout pixel or detector pixels are four times greater in area than the readout pixels. In the latter case, event-by-event summing is still possible between the larger pixels. Each pixel has around 1600 transistors and the analog static power consumption is below 15 μW in the charge summing mode and 9 μW in the single pixel mode. The chip has been built in an 8-metal 0.13 μm CMOS technology. This paper describes the chip from the pixel to the periphery and first electrical results are summarized.

Keyword
Charge sharing; Charge summing; CMOS; Medipix; Photon counting; Pixel; X-rays
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-15825 (URN)10.1016/j.nima.2010.06.108 (DOI)
Available from: 2012-02-02 Created: 2012-02-02 Last updated: 2017-12-08Bibliographically approved
5. Characterization of the Medipix3 pixel readout chip
Open this publication in new window or tab >>Characterization of the Medipix3 pixel readout chip
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2011 (English)In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 6, no 1Article in journal (Refereed) Published
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-15826 (URN)10.1088/1748-0221/6/01/C01052 (DOI)
Note
The Medipix3 chip is a hybrid pixel detector readout chip working in Single Photon Counting Mode. It has been developed with a new front-end architecture aimed at eliminating the spectral distortion produced by charge diffusion in highly segmented semiconductor detectors. In the new architecture charge deposited in overlapping clusters of four pixels is summed event-by-event and the incoming quantum is assigned as a single hit to the summing circuit with the biggest charge deposit (this mode of operation is called Charge Summing Mode (CSM)). In Single Pixel Mode (SPM) the charge reconstruction and the communication between neighbouring pixels is disabled. This is the operating mode in traditional detector systems. This paper presents the results of the characterization of the chip with electrical stimuli and radioactive sources.Available from: 2012-02-02 Created: 2012-02-02 Last updated: 2017-12-08Bibliographically approved
6. Counter architectures for a single photon-counting pixel detector such as Medipix3
Open this publication in new window or tab >>Counter architectures for a single photon-counting pixel detector such as Medipix3
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2007 (English)In: AIP Conference Proceedings, American Institute of Physics (AIP), 2007, Vol. 958, 262-263 p.Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2007
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-15822 (URN)
Available from: 2012-02-02 Created: 2012-02-02 Last updated: 2013-03-25Bibliographically approved
7. A pixel detector asic for dosimetry using time-over-threshold energy measurements
Open this publication in new window or tab >>A pixel detector asic for dosimetry using time-over-threshold energy measurements
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2011 (English)In: Radiation Measurements, ISSN 1350-4487, E-ISSN 1879-0925, Vol. 46, no 12, 1619-1623 p.Article in journal (Refereed) Published
Abstract [en]

In this work we present the design of a chip which provides the readout of a highly segmented diode array, in which signals induced by individual X-ray photons are processed discretely. There are several benefits to this approach, including the ability to achieve a high signal to noise ratio due to the inherently low sensor capacitance, and the suppression of background noise (e.g. dark current) using an analogue threshold. The segmentation also ensures a linear behaviour even at very high dose rates. A time over threshold (ToT1) energy measurement technique provides an immediate digital value corresponding to the energy deposited onto the diode by each individual photon. Deadtime-free operation is achieved by reading out a subset of the detector segments at a time while the rest of the detector continues to process signals. This paper describes the application-specific integrated circuit (ASIC) chip which was designed to provide pre-processing of photo-induced signals in the detector and readout of the processed digital data.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-15827 (URN)10.1016/j.radmeas.2011.06.061 (DOI)
Available from: 2012-02-02 Created: 2012-02-02 Last updated: 2017-12-08Bibliographically approved
8. Electrical measurements of a multi-mode hybrid pixel detector ASIC for radiation detection
Open this publication in new window or tab >>Electrical measurements of a multi-mode hybrid pixel detector ASIC for radiation detection
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2012 (English)In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 7, no 1, Art. no. C01056- p.Article in journal (Refereed) Published
Abstract [en]

We present the first electrical measurements of an application-specific integrated circuit (ASIC) to be used in a hybrid pixel detector intended for dosimetry and radiation detection. The dosimeter has three programmable modes of operation: photon counting mode, energy integration mode, and dosimetry mode. The ASIC comprises a matrix of 16 by 16 (256 total) square pixels of 220 ÎŒm pitch, providing 12.4 mm 2 of segmented active area. Each pixel can be configured to operate in one of the three radiation measurement modes, with programmable-depth counters and shift registers to tailor the data word size and optimise the readout frame-rate in a given mode. The individual energies of impinging photons are determined through programmable analogue energy threshold discrimination, time over threshold measurement, or a combination thereof. Furthermore, the dosimetry mode contains 16 digital energy thresholds and automatically sorts data into 16 corresponding energy bin registers. The chip's output is therefore pre-processed charge spectra of the radiation field. This paper discusses results from measurements taken using programmable test-pulses to inject controlled stimuli into the pixel circuits.

Keyword
Data reduction methods; Front-end electronics for detector readout; Radiation calculations; Radiation monitoring
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-15829 (URN)10.1088/1748-0221/7/01/C01056 (DOI)000303806200056 ()84856860883 (Scopus ID)
Available from: 2012-02-02 Created: 2012-02-02 Last updated: 2017-12-08Bibliographically approved

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  • modern-language-association-8th-edition
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  • sv-SE
  • Other locale
More languages
Output format
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  • asciidoc
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