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## Pixel Detector

Session chair: He , Zhong (University of Michigan, Nuclear Engineering and Radiological Science, Ann Arbor, USA); Bolotnikov , Aleksey E. (Brookhaven National Laboratory, Instrumentation Division, Upton, USA)

Shortcut: R-02
Date: Tuesday, 19 October, 2021, 7:00 AM - 8:45 AM
Room: RTSD
Session type: RTSD Session

### Contents

Click on an contribution to preview the abstract content.

7:00 AM R-02-01

#### Simulation and measurements of the radiation dose backscatter X-ray imaging using CdZnTe Timepix in security applications(#800)

J. S. Useche Parra1, M. K. Schütz1, J. Fey1, M. Fiederle1, S. Procz1

1 University of Freiburg, Freiburg Material Research Center, Freiburg, Baden-Württemberg, Germany

Abstract

In recent years, the rise of concern about border security has motivated the use of several scanning techniques to find dangerous materials such as explosives, weapons and to look for contraband merchandise. X-rays are widely used in these security scans, and lately, X-ray backscatter imaging has been commonly adopted. The use of radiation in a security scan must always account for the health risks inflicted on a human body when intentionally or accidentally exposed. Therefore, the International Commission on Radiological Protection (ICRP) proposes the effective absorbed dose as a figure of merit to assess the possible radiation damage on the human body. In this work, the health risk of a person involved in an accidental backscatter security scan of a truck is estimated, using the simulation toolkit GATE and a general X-ray backscatter setup resembling the usage of a commercial X-ray backscatter device. The simulation uses a backscatter imaging setup, with a vertical raster scanning and a horizontal sweep, and a tungsten X-ray source operated at 120 keV. The dose absorption by the male and female ICRP-110 phantoms is investigated. Furthermore, the influence of surrounding objects and the absorbed dose by the operator are also studied. This setup is replicated experimentally, and measurements of radiation dose using a CdZnTe 2 mm thick Timepix3 sensor, as well as the MiniZ handheld backscatter as an X-ray source, are done. The characterization of the sensor as a medical dosimeter is likewise explored, obtaining preliminary results to its usage in medical standards. In this talk, the effective absorbed dose by the person inside the truck is estimated using the simulation protocol to be close to a chest X-rays. This value is compared to the experimental absorbed dose in the detector, using the mass-energy absorption coefficient of the detector to convert this value to an equivalent value in a soft-tissue sample.

7:15 AM R-02-02

#### Construction of 2-by-2 CdZnTe Array System Based on H3DD_UM v3.1 ASIC and 40×40×15 mm3 Large Detectors(#587)

Y. Zhu1, G. D. Geronimo1, Z. He1

1 University of Michigan, Nuclear Engineering and Radiological Sciences, Ann Arbor, Michigan, United States of America

Abstract

H3DD_UM ASIC is a 130-channel charge-sensitive waveform digitizer ASIC targeting at 1-keV electronic noise and up to 9-MeV dynamic range. The most recent version H3DD_UM v3.1 has achieved 1.3 keV FWHM electronic noise at the highest gain option. The development of 40×40×15 mm3 CdZnTe detectors is the latest effort to improve gamma-ray detection efficiency and coded-aperture imaging quality. Its anode is consisted of 22-by-22 pixel anodes and the cathode is separated into four quadrants. Four H3DD_UM ASICs are needed to readout the signals on one 40×40×15 mm3 detector. The paper presents the effort to build the 2-by-2 array system based on the H3DD_UM ASIC for 40×40×15 mm3 CdZnTe or alternative wide bandgap semiconductor detectors. Each system includes high voltage generation circuits, temperature regulation and environment monitoring functions.

Acknowledgment

This material is based upon work supported by the Defense Threat Reduction Agency under Contract # HDTRA1-18-C-0073. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Defense Threat Reduction Agency.

Keywords: CdZnTe, Digital ASIC, H3DD_UM
7:30 AM R-02-03

#### Simulation of polarization dynamics in semi-insulating, Cr-compensated GaAs pixelated sensors under high X-ray fluxes(#86)

P. Zambon1

1 Dectris Ltd., Baden-Daettwil, Aargau, Switzerland

Abstract

We developed a numerical solver for the drift-diffusion and Poisson equations in one-dimensional semiconductor detectors, including the possibility of arbitrary number of donors and acceptors acting as trapping centers, Schottky or Ohmic contact type, velocity saturation at high electric fields and external illumination. The temporal evolution of the system is computed by solving the full set of coupled differential equations self-consistently, at each time step. An adaptive algorithm dynamically adjusts the time step allowing to track accurately dynamics occurring at any relevant time scale within acceptable computational times. We investigated the case of semi-insulating, Cr-compensated GaAs sensor under illumination of monochromatic X-rays, showing the behavior of all the main quantities with special focus on the phenomenon of polarization, i.e. the space charge accumulation leading to the collapse of the electric field and signal loss. The model predictions are in agreement with experimental count rate efficiency data obtained with a pixelated GaAs:Cr sensor, read-out by an IBEX photon counting ASIC with pixel size 75 µm and illuminated with 55 keV X-rays over a wide range of impinging fluxes.

Keywords: GaAs:Cr, photon counting, polarization, simulation, IBEX
7:45 AM R-02-04

#### Fabrication of Small-Pixel CdZnTe Sensors and Characterization with X-rays(#508)

S. Tsigaridas3, S. Zanettini2, M. Bettelli1, N. Sarzi Amadè2, D. Calestani1, C. Ponchut3, A. Zappettini1

1 CNR, IMEM, Parma, Italy
2 Due2Lab s.r.l., Reggio Emilia, Italy
3 ESRF, Grenoble, France

Abstract

Over the past few years, sensors made from high-Z compound semiconductors have attracted quite some attention for use in applications which require the direct detection of X-rays in the energy range 30–100 keV. One of the candidate materials with promising properties is cadmium zinc telluride (CdZnTe). In this work, we demonstrate the successful fabrication of CdZnTe pixel sensors with a fine pitch of 55 μm and thickness of 1 mm and 2 mm obtained from crystals grown by Boron oxide encapsulated vertical Bridgman technique. The sensors were bonded on Timepix readout chips to evaluate their response to X-rays provided by conventional sources. Despite the issues related to single-chip fabrication procedure, reasonable uniformity was achieved along with low leakage current values at room temperature. In addition, the sensors show stable performance over time at moderate incoming fluxes, below 106 ph mm-2 s-1.

Keywords: CdZnTe, vertical Bridgman, pixel detectors, X-ray detectors, Timepix
8:00 AM R-02-05

#### Transient currents in pixelated CdZnTe sensors experiencing polarization(#1012)

R. Grill1, K. Iniewski2, G. Prekas2, J. Pipek1, M. Betusiak1, N. Sadeghi2, P. Praus1, E. Belas1

1 Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic
2 Redlen Technologies Inc., Saanichton, British Columbia, Canada

Abstract

CdZnTe (CZT) compound semiconductors are prominent materials due to their ability to directly detect and perform energy-dispersive spectroscopy of high energy radiation operating at room temperature. In spite of technological progress there remain principal issues negatively affecting the detector performance. That is especially low hole mobility-lifetime product. When the hole collection is incomplete, there appears a deficiency in the detector’s signal with the deficit depending on the location of carrier generation. Another harmful feature is the effect of polarization caused by the charging of detector’s bulk that can finally lead to the detector collapse. CZT detectors with pixelated anode conveniently resolve poor hole collection thanks to a small pixel effect that enhances the weighting potential generated by the pixelated layout. Therefore, the charge induced on the collecting pixel is mostly contributed by the drifting carriers close to the pixel, i.e. electrons. In this presentation we report on theoretical studies of the charge dynamics in pixelated CZT radiation detectors subjected to either low- or high-flux X-ray irradiation. An attention is devoted to the improper performance characterized by incomplete charge collection and enhanced polarization. Processes in the detectors are described by semi-analytical models or numerically solving coupled drift-diffusion and Poisson’s equations including free carrier (de)trapping. We avail the convenient contact geometry, in which the electric field and space charge distribution may be treated in the 1D model. The full 3D description is used through the weighting potential at the calculation of specific pixel response. We show, how the detector polarization affects the charge collection efficiency and debases X-ray spectra. Theoretical results are compared with X-ray spectra and photoconductivity measuremement.

AcknowledgmentThis work was financially supported by the Grant Agency of the Czech Republic under No. P102-19/11920S
Keywords: CdZnTe, pixelated sensor, polarization, numerical simulations
8:15 AM R-02-06

#### Initial Imaging and Spectral Performance of Finely Pixelated TlBr Detectors using a Direct Bond ASIC(#1293)

B. Allen1, H. Kim2, L. Cirignano2, S. Kanai2, J. Clayton3

1 Center for Astrophysics | Harvard-Smithsonian, Cambridge, Massachusetts, United States of America
2 Radiation Monitoring Devices, Watertown, Massachusetts, United States of America
3 Polymer Assembly Technologies, Rockford, Michigan, United States of America

Abstract

Two prototype TlBr detectors have been successfully manufactrued using the NuSTAR ASIC for readout. The NuSTAR ASIC was originally created for use with pixelated CZT detectors but has been repurposed here to enable the fabrication of test TlBr detectors with a pixel pitch of 604.8 $\mu$m.  One prototype detector is composed of a 2 × 2 array of individual 8 mm × 8 mm TlBr, 4 mm thick crystals while the other consists of a single monolithic 20 mm × 20 mm, 3 mm thick TlBr detector, both bonded directly to the NuASIC.

Here we discuss the spectral performance of these detectors after attachment to the NuASIC as well as the results of high resolution (100 $\mu$m scale) mapping conducted over the surface of these detectors and stability of the system.

AcknowledgmentThis work has been supported by NASA Contract Number 80NSSC20C0442.
Keywords: TlBr, Astronomy, Pixelated Array, Imaging, spectroscopy
8:30 AM R-02-07

#### EPIX: A High Dynamic Range Diamond Pixel Detector for Ultra-fast Electron Diffraction and Microscopy(#1258)

T. Wei1, 2, X. Wang1, 2, Z. Deng1, 2, R. Li1, 2

1 Department of Engineering Physics, Tsinghua University, Beijing, China
2 Key Laboratory of Particle & Radiation Imaging, Ministry of Education, Beijing, China

Abstract

The paper presents development of a high dynamic range diamond pixel detector for ultra-fast electron diffraction and microscopy. First the detector response has been simulated by allpix2, including energy deposition and the trajectory distribution. Compared with current silicon or CdTe detectors, diamond detectors show much less scattering for 3 MeV electrons and hence could potentially achieve better imaging resolution. Charge collection and transient signal response have also been simulated under pulsed high-flux electron radiation using our previously developed propagation module. The pulse duration of electron source is on the order of femtoseconds and each pulse contains up to 105 electrons. Primary simulation results for 200 μm diamond detectors show that as the flux increase from 102×3 MeV electrons to 103×3 MeV electrons, the charge cloud explodes by about 270%. According to the simulation results, the pixel size and the thickness of the detector were designed to be 150 μm and 200 μm respectively. A prototype readout ASIC(EPIX) was also designed. It integrates 16×16 pixels and each pixel consists of a dual range charge integrator, a correlated double sampler and a 12-bit Wilkinson type ADC. The maximum integration charge is up to 12.5 pC, corresponding to 104 injected 3 MeV electrons. The equivalent noise charge is simulated to be less than 450 electrons. The detector with 128 x 128 array and the ASIC chip are under fabrication and will be received by June. The tests of the detector and chip are well planned and the test results will be present in the paper.

AcknowledgmentThis work was supported by Tsinghua university Initiative Scientific Research Program.
Keywords: UED/UEM, ASIC, High Dynamic Range, Pixel Semiconductor Detectors

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