David Krapohl

A Geant4 based framework for pixel detector simulation

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 readou ...

Probing Defects in a Small Pixellated CdTe Sensor Using an Inclined Mono Energetic X-Ray Micro Beam

High quantum efficiency is important in X-ray imaging applications. This means using high-Z sensor materials. Unfortunately many of these materials suffer from defects that cause non-ideal charge transport. In order to increase the understanding of these defects, we have mapped the 3D response of a number of defects in two 1 mm thick CdTe sensors with different pixel sizes (55 μm and 110 μm) using a monoenergetic microbeam at 79 keV. The sensors were bump bonded to Timepix read out chips. Data was collected in photon counting as well as time-over-threshold mode. The time-over-threshold mode is a very powerful tool to investigate charge transport properties and fluorescence in pixellated detectors since the signal from the charge that each photon deposits in each pixel can be analyzed. Results show distorted electrical field around the defects, indications of excess leakage current and large differences in behavior between electron collection and hole collection mode. The experiments were carried out on the Extreme Conditions Beamline I15 at Diamond Light Source.

Fabrication, characterization and simulation of channel stop for n in p-substrate silicon pixel detectors

Silicon detectors made on p-substrates are expected to have a better radiation hardness as compared to detectors made on n-substrates. However, the fixed positive oxide charges induce an inversion layer of electrons in the substrate, which connects the pixels. The common means of solving this problem is by using a p-spray, individual p-stops or a combination of the two. Here, we investigate the use of field 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 a structure using biased field plates with a width of 20 μm. By using a spice model for simulation of the preamplifier, a cross talk of about 1.6% is achieved with this detector structure. The cross talk is caused by capacitive and resistive coupling between the pixels.

Comparison of energy resolution spectra of CdTe TIMEPIX detector working in photon counting and time-over-threshold mode

Hybrid pixel detectors like the Medipix and TIMEPIX developed by the Medipix collaboration combined with high-z materials are of growing interest. In particular the TIMEPIX detector can be operated in photon counting mode and time-over-threshold mode (ToT) to obtain spectral information. Previous studies showed that 110 μm pixel sizes obtain a better energy resolution than 55 μm pixel sizes. Furthermore, threshold scans obtained a better spectral resolution than operation in ToT mode. In this work the influence of noise sources in different measurement approaches on the spectral response of a TIMEPIX chip bump-bonded to CdTe sensor are presented. Two 1mm thick CdTe sensors with pixel sizes of 55 μm and 110 μm, bump-bonded to a TIMEPIX readout chip, were evaluated at the Diamond Light Source synchrotron. A finely collimated, perpendicular pencil beam with x-ray energies of 25 keV and 79 keV was used to investigate single pixels. A small area of 10×10 pixels was investigated in ToT-mode and compared to a threshold scan of the same pixels on both detectors. The measurements are compared to an analytical SPICE/Python simulation that emulates photon counting and time-over-threshold mode.

Spectral resolution in pixel detectors with single photon processing

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.

Measurement of the sensitive profile in a solid state silicon detector, irradiated by X-rays

A newly constructed solid state silicon dose profile detector is characterized concerning its sensitive profile. The use of the MEDIPIX2 sensor system displays an excellent method to align an image of an X-ray slit to a sample under test. The scanning from front to reverse side of the detector, show a decrease in sensitivity of 20%, which indicates a minority charge carrier lifetime of 0.18 ms and a diffusion length of 460 μm. The influence of diced edges results in a volumetric efficiency of 59%, an active volume of 1.2 mm2 of total 2.1 mm2.

Readout cross-talk for alpha-particle measurements in a pixelated sensor system

Simulations in Medici are performed to quantify crosstalk and charge sharing in a hybrid pixelated silicon detector. Crosstalk and charge sharing degrades the spatial and spectral resolution of single photon processing X-ray imaging systems. For typical medical X-ray imaging applications, the process is dominated by charge sharing between the pixels in the sensor. For heavier particles each impact generates a large amount of charge and the simulation seems to over predict the charge collection efficiency. This indicates that some type of non modelled degradation of the charge transport efficiency exists, like the plasma effect where the plasma might shield the generated charges from the electric field and hence distorts the charge transport process. Based on the simulations it can be reasoned that saturation of the amplifiers in the Timepix system might generate crosstalk that increases the charge spread measured from ion impact on the sensor.

Hard x-ray imaging and particle detection with TIMEPIX3

CMOS pixel electronics open up for applications with single photon or particle processing. TIMEPIX3 is a readout chip in the MEDIPIX family with the ability to simultaneously determine energy and time of interaction in the pixel. The device is fully event driven, sending out data on each interaction at a maximum speed of about 40 Mhits/s. The concept allows for off-line processing to correct for charge sharing or to find the interaction point in multi pixel events. The timing resolution of 1.56 ns allows for three dimensional tracking of charged particles in a thick sensor due to the drift time for the charge in the sensor. The experiments in this presentation have been performed with silicon sensors bonded MEDIPIX family chips with special focus on TIMEPIX3. This presentation covers basic performance of the chip, spectral imaging with hard X-rays, detection and imaging with charged particles and neutrons. Cluster identification, centroiding and charge summing is extensively used to determine energy and position of the interaction. For neutron applications a converter layer was placed on top of the sensor.

Validation of Geant4 Pixel Detector Simulation Framework by Measurements With the Medipix Family Detectors

Monte Carlo simulations are an extensively used tool for developing and understanding radiation detector systems. In this work, we used results of several chips and readout modes of the Medipix detector family to validate a Geant4 based pixel detector framework, developed in our group, that is capable of simulating particle tracking, charge transport in the sensor material and different readout schemes. We experimentally verified the simulation with different detector geometries in terms of pixel pitch and size as well as sensor material and sensor thickness. The single pixel mode (SPM) and charge summing mode (CSM) in Medipix3 were evaluated with fluorescence and synchrotron radiation. The integration of the charge sensitive amplifier functionality in the simulation framework allowed to simulate the time-over-threshold mode of the Timepix chip. Simulation and measurement have been compared in terms of spectral resolution using threshold scans in photon counting mode (Medipix3) and time over threshold mode (Timepix). Further comparisons were done using X-ray tube spectra and beta decay to cover a broad energy range. Additionally, TCAD simulations are performed as a comparison to a well-established simulation method. The results show good agreement between simulation and measurement.

Investigation of charge collection in a CdTe-Timepix detector

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].