Recently, CMOS Active Pixels Sensors (APS) have become strong candidates as pixel detectors to be used in high energy physics experiments. A very good resolution and an excellent detection efficiency could be obtained with these detectors. In this paper we have experimentally studied, by means of charged particle beams at a grazing angle, for four different CMOS APS the diffusion properties of electron/hole pairs formed by ionizing particles interacting within the sensitive silicon layer beneath the sensor surface. By averaging many events originating at the same distance from the surface of the device, we extracted with great accuracy the charge collection efficiency (CCE) profiles of the four sensors under test. Basic transport parameters (minority carrier diffusion length, minority carrier lifetime, width of the region at maximum CCE) have been extracted using a mathematical procedure based on the extended Ramo's theorem.

https://iopscience.iop.org/article/10.1088/1748-0221/7/09/P09011/pdf

Measurement of charge collection efficiency profiles of CMOS active pixel sensors

Geant4 is an object-oriented toolkit for the simulation of the interaction of particles and radiation with matter. It provides a snapshot of the state of a simulated particle in time, as it travels through a specified geometry. One important area of application is the modelling of radiation detector systems. Here, we extend the abilities of such modelling to include charge transport and sharing in pixelated CMOS Active Pixel Sensors (APSs); though similar effects occur in other pixel detectors. The CMOS APSs discussed were developed in the framework of the PRaVDA consortium to assist the design of custom sensors to be used in an energy-range detector for proton Computed Tomography (pCT). The development of ad-hoc classes, providing a charge transport model for a CMOS APS and its integration into the standard Geant4 toolkit, is described. The proposed charge transport model includes, charge generation, diffusion, collection, and sharing across adjacent pixels, as well as the full electronic chain for a CMOS APS. The proposed model is validated against experimental data acquired with protons in an energy range relevant for pCT.

https://iopscience.iop.org/article/10.1088/1748-0221/12/03/P03028/pdf

Geant4-based simulations of charge collection in CMOS Active Pixel Sensors

The measurement of high flux charged particle beams, specifically at medical accelerators and with small fields, poses several challenges. In this work we propose a single particle counting method based on CMOS imagers optimized for visible light collection, exploiting their very high spatial segmentation (> 3 106 pixels/cm2) and almost full efficiency detection capability. An algorithm to measure the charged particle flux with a precision of ~ 1% for fluxes up to 40 MHz/cm2 has been developed, using a non-linear calibration algorithm, and several CMOS imagers with different characteristics have been compared to find their limits on flux measurement.

https://iopscience.iop.org/article/10.1088/1748-0221/11/03/P03014/pdf

Use of CMOS imagers to measure high fluxes of charged particles

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.

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

In this work we present the results of full Geant4 and FLUKA simulations and comparison with dosimetry data of an electron LINAC of St. Maria Hospital located in Terni, Italy. The facility is being used primarily for radiotherapy and the goal of the present study is the detailed investigation of electron beam parameters to evaluate the possibility to use the e-LINAC (during time slots when it is not used for radiotherapy) to test the performance of detector systems, in particular those designed to operate in space. The critical beam parameters are electron energy, profile and flux available at the surface of device to be tested. The present work aims to extract these parameters from dosimetry calibration data available at the e-LINAC. The electron energy ranges from 4 MeV to 20 MeV. The dose measurements have been performed by using an Advanced Markus Chamber which has a small sensitive volume.

Full Geant4 and FLUKA simulations of an e-LINAC for its use in particle detectors performance tests

Silicon devices (both pixels and microstrips) have been widely used in the past years in High Energy Physics experiments and also in other applications involving the detection of ionizing radiation such as medical imaging and dosimetry. The simulation of the silicon devices response to ionizing radiation is an important step needed to understand the performances in terms of signal, noise, spatial and energy resolution as a function of several technology parameters like doping profile, geometrical dimensions, bias voltage. These simulations are routinely carried out using two separate approaches (and tools): radiation interaction with a geometrically segmented silicon material (GEANT4, FLUKA, PENELOPE) taking into account the physical processes and giving as output the deposited energy; transport of generated electron/hole pairs through the device with electronic signal formation (SENTAURUS-TCAD). In this work we propose a new combined approach using both methods, applied to the case of CMOS pixel sensor, to obtain a better understanding of the behavior of the devices.

https://iopscience.iop.org/article/10.1088/1748-0221/6/05/T05001/pdf

T Ozdemir

Papers

A combined approach to the simulation of ionizing radiation effects in silicon devices