Development of Quality Assurance Methods for Particle Detectors

The purpose of this thesis is to develop, establish and apply novel quality assurance (QA) methods for nuclear and high-energy physics particle detectors. The detectors should be maintenance-free since devices can only be replaced during long technical shut-downs. Furthermore, the detector modules must endure handling during installation and withstand heat generation and cooling during operations. Longevity in a severe radiation environment must also be assured. Visual inspection and electrical characterisation of particle detectors are presented in this work. The detector studies included in this thesis, while based on different technologies, were united by the demand for reliable and enduring particle detectors. Four major achievements were accomplished during the the Gas Electron Multiplier (GEM) foil studies: a software analysis capable of precise foil inspection was developed, a rigorous calibration procedure for the Optical Scanning System was established, a detailed 3D GEM foil hole geometry study was performed for the first time and an impact of the hole geometry on the detector gain was confirmed. Promising results were also achieved during the solid-state detectors studies. A new technique for assuring the height uniformity of the chip interconnections in the pixel detector modules was proposed and implemented. Two semiconductor detectors (Si and GaAs) were designed, microfabricated and tested. The consistency of the QA results demonstrated the detectors reliability and preparedness to serve the needs of future particle and nuclear physics experiments. During the performed studies, strict calibration techniques and measurement uncertainties were applied to guarantee the trustworthy accuracy of the used measurement tools. Thus, all quality assurance techniques presented in this thesis were held in clean conditions at monitored temperature and humidity. The combined results of this thesis demonstrate the importance of adequate quality assurance for guaranteed accurate data collection and long operating life of the detector.Popular abstract is not obligatory for non Finnish PhD students in HI

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

Cadmium Telluride X-ray pad detectors with different passivation dielectrics

The suitability of two low-temperature dielectric passivation layer processes for the fabrication of Cadmium Telluride (CdTe) X-ray detectors has been investigated. The CdTe crystals with a size of (10 10 1) mm were coated with sputtered aluminum nitride (AlN) or with aluminum oxide (AlO) grown by the atomic layer deposition (ALD) method. The metallization contacts of the detectors were made by titanium tungsten (TiW) and gold (Au) metal sputtering depositions. The pad detector structures were patterned with proximity-contactless photolithography techniques followed by lift-off patterning of the electrodes. The detector properties were characterized at room temperature by Transient Current Technique (TCT) measurements. The obtained results were compared and verified by numerical TCAD simulations of the detector response. Our results indicate that higher signal charge was collected from samples with AlO. Furthermore, no significant laser light induced signal decay by CdTe material polarization was observed within order of 30 min of continuous illumination.Peer reviewe

Low dose rate 60^{60}Co facility in Zagreb

The Co-60 irradiation facility at Ruđer Bošković Institute in Zagreb allows for gamma irradiation in a large range of doses from a maximum of 30kGy/h down to 30Gy/h. Recently, a setup for the radiation tolerance study of readout electronics has been built inside the irradiation chamber. This setup allows for performing tests in conditions similar to ones that will be experienced at HL-LHC experiments. The irradiation dose rate is close to the dose rate at 3cm from the p-p interaction point, the temperature inside cold boxes is controlled by using chilled water and Peltier elements, and the relative humidity is defined by dry air flushed through the cold boxes.The setup is used for irradiation of ATLAS and CMS pixel detector prototype readout chips, called RD53A. This chip is built in 65 nm CMOS technology that demonstrates a radiation damage dependence on the irradiation dose rate. In this paper the setup itself and a few preliminary results of the radiation damage received at the low dose rate will be presented