Study of interpad-gap of HPK 3.1 production LGADs with Transient Current Technique

The Phase-2 upgrade of the Large Hadron Collider (LHC) to High-Luminosity LHC (HL-LHC) allows an increase in the operational luminosity value by a factor of 5-7 that will result in delivering 3000 fb(-1) or more integrated luminosity. Due to high luminosity, the number of interactions per bunch crossings (pileup) will increase up to a value of 140-200. To cope with high pileup rates, a precision minimum ionising particles (MIPs) timing detector (MTD) with a time resolution of similar to 30-40 ps and hermetic coverage up to a pseudo-rapidity of vertical bar eta vertical bar = 3 is proposed by the Compact Muon Solenoid (CMS) experiment. An endcap part (1.6 <vertical bar eta vertical bar <3) of the MTD, called the endcap timing layer, will be based on low-gain avalanche detector (LGAD) technology. LGADs provide a good timing resolution due to a combination of a fast signal rise time and high signal-to-noise ratio. The performance of the ETL depends on optimising the crucial features of the sensors, namely; gain, signal homogeneity, fill factor, leakage current, uniformity of multiple-pad sensors and long term stability. The paper mainly focuses on the study of the fill factor of LGADs with varying temperature and irradiation at varying proton fluences as these sensors will be operated at low temperatures and are subjected to a high radiation environment. The 3.1 production of LGADs from Hamamatsu Photonics K.K. (HPK) includes 2x2 sensors with different structures, in particular, different values of narrower inactive region widths between the pads, called the no-gain region. In this paper, the term interpad-gap is used instead of no-gain region in order to follow the conventional terminology. These sensors have been designed to study their fill factor, which is the ratio of the area within the active region (gain region) to the total sensor area. A comparative study on the dependence of breakdown voltage with the interpad-gap width for the sensors has been carried out. Using infrared light (as the electron-hole pair creation by IR laser mimics closely to the traversing of MIPs) from the Scanning-Transient Current Technique (Scanning-TCT) set-up shows that the fill factor does not vary significantly with a variation in temperature and irradiation at high proton fluences.Peer reviewe

Characterization of Heavily Irradiated Dielectrics for Pixel Sensors Coupling Insulator Applications

An increase in the radiation levels during the high-luminosity operation of the Large Hadron Collider calls for the development of silicon-based pixel detectors that are used for particle tracking and vertex reconstruction. Unlike the conventionally used conductively coupled (DC-coupled) detectors that are prone to an increment in leakage currents due to radiation, capacitively coupled (AC-coupled) detectors are anticipated to be in operation in future collider experiments suitable for tracking purposes. The implementation of AC-coupling to micro-scale pixel sensor areas enables one to provide an enhanced isolation of radiation-induced leakage currents. The motivation of this study is the development of new generation capacitively coupled (AC-coupled) pixel sensors with coupling insulators having good dielectric strength and radiation hardness simultaneously. The AC-coupling insulator thin films were aluminum oxide (Al2O3) and hafnium oxide (HfO2) grown by the atomic layer deposition (ALD) method. A comparison study was performed based on the dielectric material used in MOS, MOSFET, and AC-coupled pixel prototypes processed on high resistivity p-type Magnetic Czochralski silicon (MCz-Si) substrates. Post-irradiation studies with 10 MeV protons up to a fluence of 10(15) protons/cm(2) suggest HfO2 to be a better candidate as it provides higher sensitivity with negative charge accumulation on irradiation. Furthermore, even though the nature of the dielectric does not affect the electric field within the AC-coupled pixel sensor, samples with HfO2 are comparatively less susceptible to undergo an early breakdown due to irradiation. Edge-transient current technique (e-TCT) measurements show a prominent double-junction effect as expected in heavily irradiated p-type detectors, in accordance with the simulation studies.Peer reviewe

Multispectral photon-counting for medical imaging and beam characterization - A project review

Central focus of the MPMIB project – funded via the Academy of Finland’s RADDESS 2018–2021 programme – has been research towards a next-generation radiation detection system operating in a photon-counting (PC) multispectral mode: The extraction of energy spectrum per detector pixel data will lead to better efficacy in medical imaging with ionizing radiation. Therefore, it can be an important asset for diagnostic imaging and radiotherapy, enabling better diagnostic outcome with lower radiation dose as well as more versatile characterization of the radiation beam, leading for example to more accurate patient dosimetry. We present our approach of fabricating direct-conversion detectors based on cadmium telluride (CdTe) semiconductor material hybridized with PC mode capable application-specific integrated circuits (ASICs), and will give a review on our achievements, challenges and lessons learned. The CdTe crystals were processed at Micronova, Finland’s national research infrastructure for micro- and nanotechnology, employing techniques such as surface passivation via atomic layer deposition, and flip chip bonding of processed sensors to ASIC. Although CdTe has excellent photon radiation absorption properties, it is a brittle material that can include large concentrations of defects. We will therefore also emphasize our quality assessment of CdTe crystals and processed detectors, and present experimental data obtained with prototype detectors in X-ray and Co-60 beams at a standards laboratoryPeer reviewe

Quality Assessment of Detector Material and Prototype Detectors for Multispectral Medical Imaging, with the Focus on Cadmium Telluride and Infrared Microscopy

Photon-counting (PC) technology for medical applications has become a hot topic in recent years and is believed to revolutionise medical imaging. One material of particular interest for PC R&D efforts is cadmium telluride (CdTe), a high-Z material that outperforms the well-established semiconductor silicon (Si) in terms of absorption properties. However, it still proves to be a challenging material regarding crystal growth and processing the crystals into detectors. This is partly due to the material fragility and material quality differences, such as the amount of inherent tellurium (Te) inclusions. The research for this thesis has been part of the project “Multispectral Photon-Counting for Medical Imaging and Beam Characterization” (MPMIB). It focuses on two main objectives: the quality assessment of CdTe crystals before the complex processing procedure into radiation detectors suitable for PC, and the testing of prototype PC detectors for imaging as well as for beam profile measurements. For the first part, the Te defect density is chosen as an estimator for the crystal quality. A 3D infrared microscopy (IRM) setup facilitates a thorough scan of the crystals’ interior, thereby making Te defects visible. A neural network facilitates an automated Te defect detection in the IRM image data. Pre- and post-processing steps of the obtained data are introduced to convert the data into a format that allows further analysis and visualisation. A key output is the visualisation of the defect density within the CdTe crystals, allowing rapid comparison of crystals and providing information on areas of higher defect density. For the second part, a miniature tomographic proof of concept setup was built to test the performance of CdTe pixel detectors. It is shown that spectrum-per-pixel data can be extracted, a prerequisite to differentiate between materials. For beam profile measurements, a Si pixel detector of a similar design to the CdTe prototype is used. Initial measurement results indicate that a Si pixel detector may be of interest for beam calibration. Additionally, a clustering algorithm and further data analysis steps are introduced to gain access to the spectral data collected by the detector. The spectrum extracted after applying the clustering algorithm agrees well with a simulated spectrum for the detector, verifying the functionality of the algorithm. To conclude: With photon-counting technology on the rise, CdTe is a material of focal interest and quality assessment of the material, as presented in this thesis, is an essential ingredient towards ultimate detector performance.Lääketieteellisissä sovelluksissa käytettävän fotonilaskennan (photon counting, PC) teknologian uskotaan mullistavan lääketieteellisen kuvantamisen. Yksi PC:n tutkimus- ja kehitystyön kannalta erityisen kiinnostava materiaali on kadmiumtelluridi (CdTe), joka on korkean järjestysluvun materiaali ja jolla on paremmat absorptio-ominaisuu kuin vakiintuneesti käytetyllä piillä (Si). Se on kuitenkin edelleen haasteellinen materiaali kiteiden kasvattamisen ja ilmaisimien valmistuksen kannalta. Tämä johtuu osittain materiaalin hauraudesta ja materiaalin laatuun liittyvistä vaihteluista, kuten telluurin (Te) sulkeumien määrästä. Tämän tutkielman tutkimus on osa ”Multispectral Photon-Counting for Medical Imaging and Beam Characterization” (MPMIB)-hanketta. Siinä keskitytään kahteen päätavoitteeseen: CdTe-kiteiden laadun arviointiin ennen monimutkaista käsittelyä PC:hen soveltuviksi säteilyilmaisimiksi, ja PC-ilmaisimien prototyyppien testaus kuvantamista ja hiukkassäteen profiilin mittauksia varten. Ensimmäisessä osassa kiteen laadun arvioimiseksi tutkitaan telluurin (Te) vikatiheyttä. 3D-infrapunamikroskopia (IRM) mahdollistaa kiteen sisäosan perusteellisen skannauksen, jolloin Te-virheet tulevat näkyviin. Konvoluutioneuroverkkoa (CNN) käytetään havaitsemaan automaattisesti Te-virheet IRM-kuvatiedoista. Kun CNN:n poimimat tiedot on jälkikäsitelty, saadaan visualisoitua CdTe-kiteen sisäinen vikatiheys, joka antaa tietoa alueista, joilla vikatiheys on suuri, mikä mahdollistaa nopean vertailun kiteiden välillä. Toisessa koejärjestelyssä rakennettiin pienoistomografia-asetelma, jolla testataan CdTe-pikseli-ilmaisimien suorituskykyä. Havaitaan, että pikselikohtainen spektri on mahdollista poimia kyseisellä ilmaisimella, mikä on edellytys materiaalien erottamiselle. Lisäksi testattiin CdTe-prototyyppiä vastaavan Si-pikselidetektorin soveltuvuutta säteen profiilin mittauksiin. Esitetään klusterointialgoritmi, jonka avulla päästään käsiksi ilmaisimen keräämiin spektritietoihin. Klusterointialgoritmin soveltamisen jälkeen poimittu spektri vastaa hyvin ilmaisimen simuloitua spektriä, mikä todentaa algoritmin toimivuuden. Lopuksi: CdTe on kiinnostava materiaali koska sitä voidaan käyttää yleistyvän fotonilaskentateknologian sovelluksissa. Tässä työssä esitetty materiaalin laadunarviointi on olennainen osa ilmaisimen suorituskyvyn optimointia

Effect of Thermal Donors Induced in Bulk and Variation in P-stop Dose on the No-gain Region Width Measurements of LGADs

Peer reviewe

Pixelated silicon detector for radiation beam profile measurements

A pixelated silicon detector, developed originally for particle physics experiments, was used for a beam profile measurement of a cobalt-60 (Co-60) irradiator in a water phantom. The beam profile was compared to a profile measured with a pinpoint ionization chamber. The differences in the pixel detector and pinpoint chamber relative profiles were within approximately 2% of profile maximum, and after calculating correction factors with Monte Carlo simulations for the pixel detector, the maximum difference was decreased to approximately 1% of profile maximum. The detector's capability to measure pulse-height was used to record an electron pulse-height spectrum in water in the Co-60 beam, and the results agreed well with simulations.Peer reviewe

Multispectral photon-counting for medical imaging and beam characterization

We present the current status of our project of developing a photon counting detector for medical imaging. An example motivation lays in producing a monitoring and dosimetry device for boron neutron capture therapy, currently not commercially available. Our approach combines in-house developed detectors based on cadmium telluride or thick silicon with readout chip technology developed for particle physics experiments at CERN. Here we describe the manufacturing process of our sensors as well as the processing steps for the assembly of first prototypes. The prototypes use currently the PSI46digV2.1-r readout chip. The accompanying readout electronics chain that was used for first measurements will also be discussed. Finally we present an advanced algorithm developed by us for image reconstruction using such photon counting detectors with focus on boron neutron capture therapy. This work is conducted within a consortium of Finnish research groups from Helsinki Institute of Physics, Aalto University, Lappeenranta-Lahti University of Technology LUT and Radiation and Nuclear Safety Authority (STUK) under the RADDESS program of Academy of Finland.Peer reviewe

Characterisation of gamma-irradiated MCz-silicon detectors with a high-K negative oxide as field insulator

Peer reviewe

VBSCan Thessaloniki 2018 Workshop Summary

International audienceThis document reports the first year of activity of the VBSCan COST Action network, as summarised by the talks and discussions happened during the VBSCan Thessaloniki 2018 workshop. The VBSCan COST action is aiming at a consistent and coordinated study of vector-boson scattering from the phenomenological and experimental point of view, for the best exploitation of the data that will be delivered by existing and future particle colliders