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Monte Carlo and Charge Transport Simulation of Pixel Detector Systems
Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.ORCID iD: 0000-0002-5619-409X
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is about simulation of semiconductor X-ray and particledetectors. The simulation of a novel coating for solid state neutrondetectors is discussed as well as the implementation of a simulationframework for hybrid pixel detectors.Today’s most common thermal neutron detectors are proportionalcounters, that use 3He gas in large tubes or multi wire arrays. Globalnuclear disarmament and the increase in use for homeland securityapplications has created a shortage of the gas which poses a problemfor neutron spallation sources that require higher resolution and largersensors. In this thesis a novel material and clean room compatible pro-cess for neutron conversion are discussed. Simulations and fabricationhave been executed and analysed in measurements. It has been proventhat such a device can be fabricated and detect thermal neutrons.Spectral imaging hybrid pixel detectors like the Medipix chipare the most advanced imaging systems currently available. Thesechips are highly sophisticated with several hundreds of transistors perpixel to enable features like multiple thresholds for noise free photoncounting measurements, spectral imaging as well as time of arrivalmeasurements. To analyse and understand the behaviour of differentsensor materials bonded to the chip and to improve development offuture generations of the chip simulations are necessary. Generally, allparts of the detector system are simulated independently. However, itis favourable to have a simulation framework that is able to combineMonte Carlo particle transport, charge transport in the sensor as wellas analogue and digital response of the pixel read-out electronics. Thisthesis aims to develop such a system that has been developed withGeant4 and analytical semiconductor and electronics models. Further-more, it has been verified with data from measurements with severalMedipix and Timepix sensors as well as TCAD simulations.Results show that such a framework is feasible even for imagingsimulations. It shows great promise to be able to be extended withfuture pixel detector designs and semiconductor materials as well asneutron converters to aim for next generation imaging devices.

Place, publisher, year, edition, pages
Sundsvall: Mid Sweden University , 2015. , 95 p.
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 215
Keyword [en]
Monte Carlo Simulation, TCAD, pixel detectors, Medipix, Timepix, Finite Element Simulation
National Category
Accelerator Physics and Instrumentation
Identifiers
URN: urn:nbn:se:miun:diva-24763Local ID: STCISBN: 978-91-88025-06-7 (print)OAI: oai:DiVA.org:miun-24763DiVA: diva2:799513
Public defence
2015-04-08, M102, Mittuniversitetet, Holmgatan 10, Sundsvall, 10:00 (English)
Opponent
Supervisors
Available from: 2015-03-31 Created: 2015-03-31 Last updated: 2016-12-23Bibliographically approved
List of papers
1. Simulation of a silicon neutron detector coated with TiB 2 absorber
Open this publication in new window or tab >>Simulation of a silicon neutron detector coated with TiB 2 absorber
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2012 (English)In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 7, no 1, Art. no. C01096- p.Article in journal (Refereed) Published
Abstract [en]

Neutron radiation cannot be directly detected in semiconductor detectors and therefore needs converter layers. Planar clean-room processing can be used in the manufacturing process of semiconductor detectors with metal layers to produce a cost-effective device. We used the Geant4 Monte-Carlo toolkit to simulate the performance of a semiconductor neutron detector. A silicon photo-diode was coated with vapour deposited titanium, aluminium thin films and a titaniumdiboride (TiB 2) neutron absorber layer. The neutron capture reaction 10B(n, alpha)7Li is taken advantage of to create charged particles that can be counted. Boron-10 has a natural abundance of about SI 19.8%. The emitted alpha particles are absorbed in the underlying silicon detector. We varied the thickness of the converter layer and ran the simulation with a thermal neutron source in order to find the best efficiency of the TiB 2 converter layer and optimize the clean room process. © 2012 IOP Publishing Ltd and SISSA.

Keyword
Detector modeling and simulations I (interaction of radiation with matter, interaction of photons with matter, interaction of hadrons with matter, etc); Neutron detectors (cold, thermal, fast neutrons); Solid state detectors
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-16748 (URN)10.1088/1748-0221/7/01/C01096 (DOI)000303806200096 ()2-s2.0-84856927467 (Scopus ID)STC (Local ID)STC (Archive number)STC (OAI)
Available from: 2012-08-17 Created: 2012-08-17 Last updated: 2016-10-20Bibliographically approved
2. A thermal neutron detector based on planar silicon sensor with TiB 2 coating
Open this publication in new window or tab >>A thermal neutron detector based on planar silicon sensor with TiB 2 coating
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2012 (English)In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 7, no 1, Art. no. C01053- p.Article in journal (Refereed) Published
Abstract [en]

Neutron radiation as a non-ionizing radiation is particularly difficult to detect; therefore a conversion material is required. The conversion material converts neutrons into secondary charged particles in order for them to be detected in a silicon detector. The use of titanium diboride (TiB 2) as the conversion material deposited by an electron beam-physical vapour deposition (EB-PVD) as a part of a front-side contact of a planar silicon detector is presented. The effect of different front-side contact material compositions is discussed. The detectors behaviour was examined using alpha particles and thermal neutrons from an 241Am-Be source. Simultaneously, a Geant4 simulation was so as executed to evaluate the conversion layer functionality and to discover the conversion material thickness for the best neutron detection efficiency. © 2012 IOP Publishing Ltd and SISSA.

Keyword
Neutron detectors (cold, thermal, fast neutrons); Si microstrip and pad detectors; Solid state detectors
National Category
Other Physics Topics
Identifiers
urn:nbn:se:miun:diva-16746 (URN)10.1088/1748-0221/7/01/C01053 (DOI)000303806200053 ()2-s2.0-84856934655 (Scopus ID)STC (Local ID)STC (Archive number)STC (OAI)
Available from: 2012-08-20 Created: 2012-08-17 Last updated: 2016-10-20Bibliographically approved
3. Simulation of the Spectral Response of a Pixellated X-Ray Imaging Detector in Single Photon Processing Mode
Open this publication in new window or tab >>Simulation of the Spectral Response of a Pixellated X-Ray Imaging Detector in Single Photon Processing Mode
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2010 (English)In: 2010 Nuclear Science Symposium and Medical Imaging Conference, IEEE conference proceedings, 2010Conference paper, (Refereed)
Abstract [en]

X-ray imaging with spectral resolution, “Color X-ray imaging” is a new imaging technology that is currently attracting a lot of attention. It has however been observed that the quality of spectral response is degraded as the pixel size is reduced. This is an effect of charge sharing where the signal from a photon absorbed close to the border between two pixels is shared between pixels. This effect is caused by both diffusion during the charge transport and X-ray fluorescence in heavy detector materials. In order to understand the behavior of pixellated detectors with heavy detector materials operating in single photon processing mode, we have simulated the X-ray interaction with the sensor and the transport of the charge to the readout electrode using a Monte Carlo model for the X-ray interaction and a drift diffusion model for the charge transport. By combining these models, both signal and noise properties of the detector can be simulated.

Place, publisher, year, edition, pages
IEEE conference proceedings, 2010
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-12856 (URN)
Conference
2010 Nuclear Science Symposium and Medical Imaging Conference, Oct 30-Nov 6, Knoxville, Tennessee
Available from: 2011-01-09 Created: 2010-12-16 Last updated: 2015-03-31Bibliographically approved
4. Investigation of charge collection in a CdTe-Timepix detector
Open this publication in new window or tab >>Investigation of charge collection in a CdTe-Timepix detector
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2013 (English)In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 8, no May, Art. no. C05003- p.Article in journal (Refereed) Published
Abstract [en]

Energy calibration of CdTe detectors is usually done using known reference sources disregarding the exact amount of charge that is collected in the pixels. However, to compare detector and detector model the quantity of charge collected is needed. We characterize the charge collection in a CdTe detector comparing test pulses, measured data and an improved TCAD simulation model [1]. The 1 mm thick detector is bump-bonded to a TIMEPIX chip and operating in Time-over-Threshold (ToT) mode. The resistivity in the simulation was adjusted to match the detector properties setting a deep intrinsic donor level [2]. This way it is possible to adjust properties like trap concentration, electron/hole lifetime and mobility in the simulation characterizing the detector close to measured data cite [3].

Keyword
Charge induction; solid state detectors; x-ray detectors; models and simulations
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:miun:diva-18946 (URN)10.1088/1748-0221/8/05/C05003 (DOI)000320726000003 ()2-s2.0-84878342633 (Scopus ID)STC (Local ID)STC (Archive number)STC (OAI)
Conference
14th International Workshop on Radiation Imaging Detectors (iWoRID 2012); 20-25 June 2012; Figueira da Foz, Portugal.
Available from: 2013-05-16 Created: 2013-05-16 Last updated: 2016-10-20Bibliographically approved
5. Spectral resolution in pixel detectors with single photon processing
Open this publication in new window or tab >>Spectral resolution in pixel detectors with single photon processing
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2013 (English)In: Proceedings of SPIE - The International Society for Optical Engineering, SPIE - International Society for Optical Engineering, 2013, Art. no. 88520O- p.Conference paper (Other academic)
Abstract [en]

Pixel detectors based on photon counting or single photon processing readout are becoming popular for spectral X-ray imaging. The detector is based on deep submicron electronics with functions to determine the energy of each individual photon in every pixel. The system is virtually noiseless when it comes to the number of the detected photons. However noise and variations in system parameters affect the determination of the photon energy. Several factors affect the energy resolution in the system. In the readout electronics the most important factors are the threshold dispersion, the gain variation and the electronic noise. In the sensor contributions come from charge sharing, variations in the charge collection efficiency, leakage current and the statistical nature of the charge generation, as described by the Fano factor. The MEDIPIX technology offers a powerful tool for investigating these effects since energy spectra can be captured in each pixel. In addition the TIMEPIX chip, when operated in Time over Threshold mode, offers an opportunity to analyze individual photon interactions, thus addressing charge sharing and fluorescence. Effects of charge sharing and the properties of charge summing can be investigated using MEDIPIX3RX. Experiments are performed using both Si and CdTe detectors. In this paper we discuss the various contributions to the spectral noise and how they affect detector response. The statements are supported with experimental data from MEDIPIX-type detectors.

Place, publisher, year, edition, pages
SPIE - International Society for Optical Engineering, 2013
Series
Proceedings of SPIE, ISSN 0277-786X ; 8852
Keyword
Pixel detectors, single photon processing, X-ray
National Category
Accelerator Physics and Instrumentation
Identifiers
urn:nbn:se:miun:diva-21069 (URN)10.1117/12.2023983 (DOI)000326643800014 ()2-s2.0-84886996985 (Scopus ID)STC (Local ID)978-081949702-4 (ISBN)STC (Archive number)STC (OAI)
Conference
SPIE Conference on Hard X-Ray, Gamma-Ray and Neutron Detector Physics XV; San Diego, CA; United States; 26 August 2013 through 28 August 2013; Code 100563
Available from: 2014-01-16 Created: 2014-01-16 Last updated: 2016-11-10Bibliographically approved
6. Fabrication, Characterization and Simulation of Channel Stop for n in p-Substrate Silicon Pixel Detectors
Open this publication in new window or tab >>Fabrication, Characterization and Simulation of Channel Stop for n in p-Substrate Silicon Pixel Detectors
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2014 (English)In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 9, no 7, Art. no. C07013- p.Article in journal (Refereed) Published
Abstract [en]

Silicon detectors made on p-substrates are expected to have a better radiation hardness as compared todetectors made on n-substrates. However, the fixed positive oxide charges induce an inversion layer ofelectrons in the substrate, which connects the pixels. The common means of solving this problem isby using a p-spray, individual p-stops or a combination of the two. Here, we investigate the use offield plates to suppress the fixed positive charges and to prevent the formation of an inversion layer.The fabricated detector shows a high breakdown voltage and low interpixel leakage current for astructure using biased field plates with a width of 20 μm. By using a spice model for simulation of thepreamplifier, a cross talk of about 1.6 % is achieved with this detector structure. The cross talk iscaused by capacitive and resistive coupling between the pixels

Keyword
Electronic detector readout concepts (solid-state), Radiation-hard detectors, Solid state detectors
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-22224 (URN)10.1088/1748-0221/9/07/C07013 (DOI)000340050700013 ()2-s2.0-84905165594 (Scopus ID)STC (Local ID)STC (Archive number)STC (OAI)
Available from: 2014-06-19 Created: 2014-06-19 Last updated: 2017-03-06Bibliographically approved
7. A Geant4 based framework for pixel detector simulation
Open this publication in new window or tab >>A Geant4 based framework for pixel detector simulation
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2014 (English)In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 9, no 12, C12018Article in journal (Refereed) Published
Abstract [en]

The output from a hybrid pixel detector depends on the interaction of the radiation with the sensor material, the transport of the resulting charge in the sensor, the pulse processing in the readout circuit and processing of the resulting signal. In order to understand the full behaviour of the device and to predict the performance of future devices it is important to have a framework that can simulate the entire process in the detector system.Geant4 is a Monte Carlo based toolkit for simulation of particle interaction with matter which is developed and actively used for CERN experiments and detector development [1]. By extending the Monte Carlo code in Geant4 with a charge carrier transport model of the sensor material and basic amplifier functionality as well as read out logic, a simulation of the complete detector system is possible.The MEDIPIX is a state of the art hybrid pixel detector that allows bonding of a wide range of sensor materials [2,3]. Simulation models have been developed and tested for different chips from the MEDIPIX family. The simulation is defined using configuration files to set the geometry, sensor material properties, number of pixels, pixel pitch and chip properties. Source properties as well as filters and objects in the beam can be added for different experimental set-ups. The interaction of radiation with the sensor is taken into account in the transport of the charge carriers in the sensor material and a current induced in the pixel electrode that triggers an amplifier response. Simulation results have been verified with X-ray fluorescence and radioactive sources using MEDIPIX family chips. In this paper we present the developed simulation framework and first results.

Keyword
Performance of high energy physics detectors, Simulation methods and programs, Software architectures (event data models, frameworks and databases)
National Category
Accelerator Physics and Instrumentation
Identifiers
urn:nbn:se:miun:diva-23708 (URN)10.1088/1748-0221/9/12/C12018 (DOI)000351342900002 ()2-s2.0-84918823479 (Scopus ID)STC (Local ID)STC (Archive number)STC (OAI)
Conference
16th International Workshop on Radiation Imaging Detectors
Available from: 2014-12-12 Created: 2014-12-12 Last updated: 2017-03-06Bibliographically approved

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