Characterization of charge collection in CdTe and CZT using the transient current technique

The charge collection properties in different particle sensor materials with respect to the shape of the generated signals, the electric field within the detector, the charge carrier mobility and the carrier lifetime are studied with the transient current technique (TCT). Using the well-known properties of Si as a reference, the focus is laid on Cadmium-Telluride (CdTe) and Cadmium-Zinc-Telluride (CZT), which are currently considered as promising candidates for the efficient detection of X-rays. All measurements are based on a transient-current technique (TCT) setup, which allows the recording of current pulses generated by an 241Am alpha-source. These signals will be interpreted with respect to the build-up of space-charges inside the detector material and the subsequent deformation of the electric field. Additionally the influence of different electrode materials (i.e. ohmic or Schottky contacts) on the current pulse shapes will be treated in the case of CdTe. Finally, the effects of polarization, i.e. the time-dependent degradation of the detector signals due to the accumulation of fixed charges within the sensor, are presented.Comment: 20 pages, 17 figure

Characterization and simulation of a CdTe detector for use in PET

The Voxel Imaging PET (VIP) Path nder project got the 4 year European Research Council FP7 grant in 2010 to prove the feasibility of using CdTe detectors in a novel conceptual design of PET scanner. The work presented in this thesis is a part of the VIP project and consists of, on the one hand, the characterization of a CdTe detector in terms of energy resolution and coincidence time resolution and, on the other hand, the simulation of the setup with the single detector in order to extend the results to the full PET scanner. An energy resolution of 0.98% at 511 keV with a bias voltage of 1000 V/mm has been measured at low temperature T=-8 ºC. The coincidence time distribution of two twin detectors has been found to be as low as 6 ns FWHM for events with energies above 500 keV under the same temperature and bias conditions. The measured energy and time resolution values are compatible with similar ndings available in the literature and prove the excellent potential of CdTe for PET applications. This results have been presented in form of a poster contribution at the IEEE NSS/MIC & RTSD 2011 conference in October 2011 in Valencia and at the iWoRID 2012 conference in July 2012 in Coimbra, Portugal. They have been also submitted for publication to "Journal of Instrumentation (JINST)" in September 2012

Simulation study of signal formation in position sensitive planar p-on-n silicon detectors after short range charge injection

Segmented silicon detectors (micropixel and microstrip) are the main type of detectors used in the inner trackers of Large Hadron Collider (LHC) experiments at CERN. Due to the high luminosity and eventual high fluence of energetic particles, detectors with fast response to fit the short shaping time of 20-25 ns and sufficient radiation hardness are required. Charge collection measurements carried out at the Ioffe Institute have shown a reversal of the pulse polarity in the detector response to short-range charge injection. Since the measured negative signal is about 30-60% of the peak positive signal, the effect strongly reduces the CCE even in non-irradiated detectors. For further investigation of the phenomenon the measurements have been reproduced by TCAD simulations. As for the measurements, the simulation study was applied for the p-on-n strip detectors similar in geometry to those developed for the ATLAS experiment and for the Ioffe Institute designed p-on-n strip detectors with each strip having a window in the metallization covering the p(+) implant, allowing the generation of electron-hole pairs under the strip implant. Red laser scans across the strips and the interstrip gap with varying laser diameters and Si-SiO2 interface charge densities (Q(f)) were carried out. The results verify the experimentally observed negative response along the scan in the interstrip gap. When the laser spot is positioned on the strip p(+) implant the negative response vanishes and the collected charge at the active strip increases respectively. The simulation results offer a further insight and understanding of the influence of the oxide charge density in the signal formation. The main result of the study is that a threshold value of Q(f), that enables negligible losses of collected charges, is defined. The observed effects and details of the detector response for different charge injection positions are discussed in the context of Ramo's theorem.Peer reviewe

Etude et caractérisation d'un capteur en silicium amorphe hydrogéné déposé sur circuit intégré pour la détection de particules et de rayonnements

Next generation experiments at the European laboratory of particle physics (CERN) require particle detector alternatives to actual silicon detectors. This thesis presents a novel detector technology, which is based on the deposition of a hydrogenated amorphous silicon sensor on top of an integrated circuit. Performance and limitations of this technology have been assessed for the first time in this thesis in the context of particle detectors. Specific integrated circuits have been designed and the detector segmentation, the interface sensor â chip and the sensor leakage current have been studied in details. The signal induced by the track of an ionizing particle in the sensor has been characterized and results on the signal speed, amplitude and on the sensor resistance to radiation are presented. The results are promising regarding the use of this novel technology for radiation detection, though limitations have been shown for particle physics application

High-Flux Experiments and Simulations of Pulse-Mode Position-Sensitive CdZnTe Pixelated Detectors.

In recent years, operation of CdZnTe detectors under high photon flux irradiation has been pursued by many research groups. However, under high-flux scenarios, these detectors are limited by poor hole transport properties and other factors, causing the build-up of positive space charge as a function of irradiation flux and time, the so-called polarization effect. In this work, high-flux experiments and simulations were conducted to study this problem. In simulations, charge drift calculations were carried out by numerically solving the charge continuity equations coupled with Poisson's equation, assuming multiple electron and hole defect levels. A simplifed 3D axisymmetric model was implemented in order to reduce computational time and enable simulation of more advanced physical processes. Electron and hole transport properties used in simulations were measured by irradiating the detectors with Gamma-rays, alpha particles and K alpha X-rays. Experiments were conducted using a JL Shepherd & Associates 3 Ci (09-20-2001) Cs-137 Gamma-ray irradiator and a custom-built experimental apparatus. A series of experiments were conducted at different applied cathode bias voltages. More importantly, in this work we have developed a complete 3D framework that can be extended to other semiconductor detector materials to study and predict their performance under high-flux scenarios.Ph.D.Nuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91504/1/miesher_1.pd

Quality Assurance of Diamond Radiation Detectors

Semiconductor radiation detectors are devices used to detect electromagnetic and particle radiation. The signal formation is based on the transportation of charges between the valence band and conduction band. The interaction between the detector material and the radiation generates free electrons and holes that move in opposite directions in the electric field applied between the electrodes. The movement of charges induces a current in the external electrical circuit, which can be used for particle identification, measurement of energy or momentum, timing, or tracking. There are several different detector materials and designs and, new options are continuously developed. Diamond is a detector material that has received a great amount of interest in many fields. This is due to its many unique properties. Many of them arise from the diamond crystal structure and the strength of the bond between the carbon atoms. The tight and rigid structure makes diamond a strong and durable material, which allows operation of diamond detectors in harsh radiation environments. This, combined with the fast signal formation and short response time makes diamond detector an excellent choice for high energy physics applications. The diamond structure leads also to a wide band gap. Thanks to the wide band bap, diamond detectors have low leakage current and they can be operated even in high temperatures without protection from surrounding light. Especially electrical properties of semiconductors strongly depend on the concentration of impurities and crystal defects. Determination of electrical properties can therefore be used to study the crystal quality of the material. The electrical properties of the material determine the safe operational region of the device and knowledge of the leakage current and the charge carrier transportation mechanism are required for optimized operation of detectors. Characterization of electrical properties is therefore an important part of semiconductor device fabrication. Electrical characterization should be done at different stages of the fabrication in order to detect problems at an early stage and to get an idea of what could have caused them. This work describes the quality assurance process of single crystal CVD (chemical vapour deposition) diamond detectors for the PPS-detectors for the CMS-experiment. The quality assurance process includes visual inspection of the diamond surfaces and dimensions by optical and cross polarized light microscopy, and electrical characterization by measurement of leakage current and CCE (charge collection efficiency). The CCE measurement setup was improved with a stage controller, which allows automatic measurement of CCE in several positions on the diamond detector. The operation of the new setup and the reproducibility of the results were studied by repeated measurements of a reference diamond. The setup could successfully be used to measure CCE over the whole diamond surface. However, the measurement uncertainty is quite large. Further work is needed to reduce the measurement uncertainty and to determine the correlation between observed defects and the measured electrical properties

In-depth characterisation of diamond detectors for the Belle II experiment

This thesis aims at deepening and improving the characterisation procedure of diamond detectors used as radiation monitor and beam abort for the Belle II experiment at the SuperKEKB electron-positron collider. Belle II is one of the leading particle physics experiments at the intensity frontier. In order to accumulate 50 times more particle collisions than its predecessors, the Belle and BaBar experiments, the SuperKEKB collider is designed to reach unprecedented instantaneous luminosities. As a side effects the detector must operate with high particle rates both from collisions and from beam loss backgrounds. The Belle II radiation monitor system consists of 28 single-crystal artificial-diamond detectors mounted in the interaction region of SuperKEKB. Owing to its excellent radiation hardness, diamond has been widely used for solid-state particle detectors and dosimeters in harsh radiation environments. I assembled and fully characterised ten new diamond detectors. I installed eight of them on a new beam pipe, to allow the new upgraded vertex detector installation. The response of diamond sensors might differ from sample to sample, due to crystal imperfections from the CVD-growing process or to the details of the electrode contact formation. In order to assess the main properties of each diamond sensor a suitable characterisation procedure using , and X radiation sources has been developed. After asses the crystal quality and check the response of each sensor, I determined the current-to-dose rate calibration factor in steady conditions. I devised a current-to-dose-rate calibration method that employs a silicon diode as a reference, to greatly reduce uncertainties associated with the source activity and with the setup simulation. The method has been validated by measuring the calibration factors with X and β radiation, spanning a dose-rate range from tens of nrad/s to few rad/s. However, intense radiation bursts may lead to non-linear effects in the collection of ionisation charges and deeper investigations on diamonds' transient response to intense radiation pulses are needed. For this purpose, I studied the transient response of our diamond detectors to collimated, sub-picosecond, ∼ 1 GeV electron beam bunches, with a bunch charge of tens of pC, provided by the FERMI electron linac in Trieste. In these experimental conditions the ionisation generates large charge carrier density in the diamond bulk. This high charge causes a transient modification of diamond electrical properties, which affects the output signal shape. The observed signal evolution in the time domain shows fair agreement with a two-step numerical simulation of the diamond time response to these intense high-energy pulses. The results that I obtained represent important steps forward in the characterisation techniques for diamond sensors to be used for the radiation-monitor upgrade.This thesis aims at deepening and improving the characterisation procedure of diamond detectors used as radiation monitor and beam abort for the Belle II experiment at the SuperKEKB electron-positron collider. Belle II is one of the leading particle physics experiments at the intensity frontier. In order to accumulate 50 times more particle collisions than its predecessors, the Belle and BaBar experiments, the SuperKEKB collider is designed to reach unprecedented instantaneous luminosities. As a side effects the detector must operate with high particle rates both from collisions and from beam loss backgrounds. The Belle II radiation monitor system consists of 28 single-crystal artificial-diamond detectors mounted in the interaction region of SuperKEKB. Owing to its excellent radiation hardness, diamond has been widely used for solid-state particle detectors and dosimeters in harsh radiation environments. I assembled and fully characterised ten new diamond detectors. I installed eight of them on a new beam pipe, to allow the new upgraded vertex detector installation. The response of diamond sensors might differ from sample to sample, due to crystal imperfections from the CVD-growing process or to the details of the electrode contact formation. In order to assess the main properties of each diamond sensor a suitable characterisation procedure using , and X radiation sources has been developed. After asses the crystal quality and check the response of each sensor, I determined the current-to-dose rate calibration factor in steady conditions. I devised a current-to-dose-rate calibration method that employs a silicon diode as a reference, to greatly reduce uncertainties associated with the source activity and with the setup simulation. The method has been validated by measuring the calibration factors with X and β radiation, spanning a dose-rate range from tens of nrad/s to few rad/s. However, intense radiation bursts may lead to non-linear effects in the collection of ionisation charges and deeper investigations on diamonds' transient response to intense radiation pulses are needed. For this purpose, I studied the transient response of our diamond detectors to collimated, sub-picosecond, ∼ 1 GeV electron beam bunches, with a bunch charge of tens of pC, provided by the FERMI electron linac in Trieste. In these experimental conditions the ionisation generates large charge carrier density in the diamond bulk. This high charge causes a transient modification of diamond electrical properties, which affects the output signal shape. The observed signal evolution in the time domain shows fair agreement with a two-step numerical simulation of the diamond time response to these intense high-energy pulses. The results that I obtained represent important steps forward in the characterisation techniques for diamond sensors to be used for the radiation-monitor upgrade

Thallium Bromide as an Alternative Material for Room-Temperature Gamma-Ray Spectroscopy and Imaging.

Thallium bromide is an attractive material for room-temperature gamma-ray spectroscopy and imaging because of its high atomic number (Tl: 81, Br: 35), high density (7.56 g/cm^3), and a wide bandgap (2.68 eV). In this work, 5 mm thick TlBr detectors achieved 0.94% FWHM at 662 keV for all single-pixel events and 0.72% FWHM at 662 keV from the best pixel and depth using three-dimensional position sensing technology. However, these results were limited to stable operation at -20 C. After days to months of room-temperature operation, ionic conduction caused these devices to fail. Depth-dependent signal analysis was used to isolate room-temperature degradation effects to within 0.5 mm of the anode surface. This was verified by refabricating the detectors after complete failure at room temperature; after refabrication, similar performance and functionality was recovered. As part of this work, the improvement in electron drift velocity and energy resolution during conditioning at -20 C was quantified. A new method was developed to measure the impurity concentration without changing the gamma ray measurement setup. The new method was used to show that detector conditioning was likely the result of charged impurities drifting out of the active volume. This space charge reduction then caused a more stable and uniform electric field. Additionally, new algorithms were developed to remove hole contributions in high-hole-mobility detectors to improve depth reconstruction. These algorithms improved the depth reconstruction (accuracy) without degrading the depth uncertainty (precision). Finally, spectroscopic and imaging performance of new 11~x~11 pixelated-anode TlBr detectors was characterized. The larger detectors were used to show that energy resolution can be improved by identifying photopeak events from their Tl characteristic x-rays.PhDNuclear Engineering and Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/133299/1/koehlerw_1.pd

Development and Characterization of Lithium Indium Diselenide for Neutron Detection and Imaging Applications

Lithium indium diselenide [LISe] is under development as a single crystal semiconductor detector for neutron detection applications. Enriched in lithium-6, a neutron sensitive isotope, this wide-band gap semiconductor possesses the inherent neutron-gamma discrimination afforded by the thermal neutron capture reaction energy while providing distinct efficiency advantages over lithiated conversion layer detectors. The overarching theme of this work is to characterize the fundamental properties of this material to optimize its performance in neutron detection applications. The work presented here includes the identification of a suitable metallurgical contact for advanced detector fabrication, fundamental electronic property characterization, and proof-of-principle fast neutron imaging performance. Candidate contact materials were deposited through radio frequency magnetron sputtering. The primary metrics used to identify a robust contact were adhesion to the LISe surface and current voltage characteristics. Among the numerous contacts investigated, indium demonstrated the best adhesion properties. Its viability was demonstrated through the fabrication of a pixelated thermal neutron imaging detector (LTNI). Charge generation, transport, and trapping properties were investigated with emphasis on the stability of these properties post-operation in high thermal neutron flux fields. Neutron and alpha spectroscopy, photoinduced current transient spectroscopy, Raman spectroscopy, trap-filled limited voltage, and photoconductivity measurements were used to probe the charge transport and trapping mechanisms. Moderate transport properties were identified with respect to comparable technologies. Defect studies demonstrated that the type and density of defects strongly influenced performance of the detector. Encouraged by the performance of LTNI, an imaging detector was fabricated by coupling a LISe crystal to a 256 x 256 channel Timepix Application Specific Integrated Circuit to maximize spatial resolution. The fast neutron spatial resolution for 9MeV [electron-Volts] neutrons was investigated via a knife edge experiment. The measured efficiency was in agreement with the Evaluated Nuclear Data File cross-section database. The ultimate spatial resolution of the system was determined as 1.55 millimeters via the 10-90% decrease in contrast of the one-dimensional edge spread function. In conclusion, this material has been shown to exhibit suitable properties warranting further development for high efficiency slow neutron applications guided by the results of this work

Signal modeling of high-purity Ge detectors with a small read-out electrode and application to neutrinoless double beta decay search in Ge-76

The GERDA experiment searches for the neutrinoless double beta decay of Ge-76 using high-purity germanium detectors enriched in Ge-76. The analysis of the signal time structure provides a powerful tool to identify neutrinoless double beta decay events and to discriminate them from gamma-ray induced backgrounds. Enhanced pulse shape discrimination capabilities of "Broad Energy Germanium" detectors with a small read-out electrode have been recently reported. This paper describes the full simulation of the response of such a detector, including the Monte Carlo modeling of radiation interaction and subsequent signal shape calculation. A pulse shape discrimination method based on the ratio between the maximum current signal amplitude and the event energy applied to the simulated data shows quantitative agreement with the experimental data acquired with calibration sources. The simulation has been used to study the survival probabilities of the decays which occur inside the detector volume and are difficult to assess experimentally. Such internal decay events are produced by the cosmogenic radio-isotopes Ge-68 and Co-60 and the neutrinoless double beta decay of Ge-76. Fixing the experimental acceptance of the double escape peak of the 2.614 MeV photon to 90%, the estimated survival probabilities at Qbb = 2.039 MeV are (86+-3)% for Ge-76 neutrinoless double beta decays, (4.5+-0.3)% for the Ge-68 daughter Ga-68, and (0.9+0.4-0.2)% for Co-60 decays.Comment: 27 pages, 17 figures. v2: fixed typos and references. Submitted to JINS