Measurement of Electron Mobility-Lifetime Product in 3D Position-Sensitive CdZnTe Detectors Using the VAD_UM v2.2 Digital Readout System (#1093)
The product of electron mobility and lifetime (μeτe hereafter) was calculated in seven large-volume (2×2×1.5 cm3), 3D position-sensitive CdZnTe detectors using the two-bias and depth-fitting methods. By analyzing averaged digital pre-amplifier waveforms read out using the VAD_UM v2.2 system, the waveform filter was optimized to avoid the effects of electron drift and de-trapping. The measured μeτe values calculated using different methods at different biases were consistent. The values were also significantly higher than the results from conventional waveform filtering methods. Increased μeτe values for recent Redlen detectors were observed.
Keywords: CdZnTe, Electron mobility-lifetime product, Two-bias method, Depth-fitting method, Digital readout
Polarization of CdZnTe Radiation Detectors under High Flux X-Ray Irradiation (#1761)
R. Grill1, G. Prekas2, J. Kunc1, N. Sadeghi2, J. Pipek1, V. Dedic1, E. Belas1, P. Praus1, A. Musiienko1, I. Vasylchenko1, J. Franc1
1 Charles University, Institute of Physics, Prague 2, Czech Republic
The performance of radiation detectors is often critically disturbed by a charging of detector bulk and by build-up of internal electric field (polarization), which counteracts the applied bias and results in device failure. The formation of the space charge in biased detectors originating in the blocking or injecting contacts is further boosted by intensive irradiation when the enhanced free carrier density results in an abundant charge accumulation even in traps with energies fairly deviating from the midgap region. The utilization of the radiation detectors for the high flux applications thus accentuates demands for an excellent defect structure either from the point of low trap density or the trap distribution and resistivity homogeneity. In this contribution we report on the photoconductivity and Pockels effect measurements of pixelated CdZnTe radiation detectors subject to high flux x-ray 120kVp irradiation. Special attention is devoted to the parts of detectors with improper performance characterized by anomalous behavior of photoconductivity, incomplete charge collection and enhanced polarization. The processes in the detectors are numerically simulated in one- and two-dimensional approach based on the parallel solution of the drift-diffusion and Poisson's equations. Free carriers trapping/detrapping is described by the Shockley-Read-Hall model. While the 1D approach is used for detailed characterization of the charge dynamics in laterally homogeneous sample, the 2D model is engaged at the simulation of charge steering in laterally inhomogeneous detectors characterized respectively by local inhomogeneity of electron and hole trap density or resistivity or by lateral gradient of these quantities. The 2D simulations performed on detector with stripe-type pixelated anode allows us to evaluate the influence of respective inhomogeneity to the image warping. Results are successfully compared with experimentally observed effects.
Keywords: CdZnTe, detector polarization, high flux, x-ray, numerical simulations
Space charge oscillations in CdZnTe (#1879)
V. Dedic1, M. Rejhon1, J. Franc1, R. Grill1
1 Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic
Our work focuses on detailed analysis of non-trivial temporal evolution of the electric field measured by Pockels effect in CdZnTe planar detector after biasing. The electric field varies and in a couple of seconds it reaches steady state. We have observed an underhoot of the electric field evolution close to the cathode which is a sign of a local oscillation of the space charge density. Advanced deep level analysis based on a thermal emission results in a single hole trap with energy Ev + 0.87 eV responsible for a complex double-exponential electric field behavior.
The principle of the nonmonotonous advancement of the electric field close to the cathode is explained by a process of the space charge formation, which is initially distributed homogeneously within the detector and redistributed consecutively to the region close to the anode. The space charge damping close to the cathode is induced by the lowered electric field in this region, which results in the hole density enhancement, trapping on the hole trap and the reduction of previously formed negative space charge.
Our conclusions based on a single deep level are supported by numerical simulations based on the simultaneous solution of the drift-diffusion and Poisson’s equations. Moreover, we simulated an influence of a deep level concentration Nt on the local electric field oscillations: with higher Nt the undershoots turn to the oscillations, while with lower Nt the undershoots turn to monotonous temporal evolution of the electric field.
Polarimetry has been increasingly recognized as an important observational parameter for high energy astrophysics, However, polarimetry in high-energy astrophysics has been insufficiently explored due to the complexity of the required detection, electronic and signal processing systems. Herein we study a new prototype in 2 layers Compton configuration based on two CdTe small pixel spectro-imagers operated in coincidence. The two CdTe detectors have an anode segmented in 8´8 pixels (2 mm pitch) on 2 mm thick crystals. The new detection system configuration will allow as well assessing the scattering polarimetric performance of a 3D spectro-imager that we can simulate changing the distance between the two CdTe pixel detection layers over the 100-600 keV energy range. The prototype measured performances provide useful data to optimise a spectro-imager with polarimetric capabilities for both high efficiency Laue lens telescope focal plane and all sky advanced Compton telescope design for next generation space mission.
Investigation of the temporal and dynamic counting behavior of CdZnTe sensors operating under high flux X-ray environments. (#3665)
N. Sadeghi1, R. Grill2, G. Prekas1, P. Marthandam1, J. Kumar1, A. Musiienko2, U. El-Hanany1
1 Redlen Technologies, R&D, Saanichton, British Columbia, Canada
Over the last few years we, at Redlen technologies, have demonstrated that the operation of the CdZnTe sensors under intense irradiation of X-rays can be achieved. The ability of our CdZnTe material to operate at photon fluxes, exceeding 100s of Mcps/mm2 without significant counting and spectroscopic distortion, have made it the material of choice for many photon counting based applications, including the most challenging medical photon counting spectral CT. In order to achieve high imaging quality, artifact-free while maintaining accurate energy information it is of paramount importance to have sensors that produce stable output count rate and exhibit good energy linearly over a wide time domain; from few hundreds of milliseconds to up to several hours. In this work we will mainly focus on the temporal response and the dynamic behavior of the CdZnTe sensor under varying and rapidly alternating intense X-ray environments. Having been able to record and study the E-fields’ abrupt changes, we will explain the observed space charge build up and recovery by the trapping and de-trapping model and relate the extracted time constants to trap properties; We will also review the major underlying mechanisms that result to counting and photo-current drift in the poorly performing detectors. Concluding, we will present cases of detectors with very low trap density and homogeneous resistivity that demonstrate very good stability performance of the order of 0.2% over few seconds and less than 0.4% over several hours.
Keywords: CdZnTe, Electric field, Polarization, Pockels, Stability
Twin-Shaping Filter Method Applied to CZT Detectors Grown by the Vertical Bridgman Method (#3682)
N. Auricchio1, F. Schiavone1, E. Caroli1, J. B. Stephen1, A. Basili1, A. Zappettini2
1 Istituto di Astrofisica Spaziale e Fisica Cosmica, Bologna, Italy
CdTe/CZT is an attractive and consolidated material with which to realize detectors with good efficiency and energy resolution, operating at room temperature for a large variety of applications such as astrophysics, medical imaging and security.
However, this kind of material suffers from the low mobility of the charge carriers (particularly the holes), which are trapped and so degrade the detector response in terms of charge collection efficiency, energy resolution and photopeak efficiency. The response of a planar CdTe/CZT detector, which depends on the distance between the charge formation position and the collecting electrodes, can be improved by using two kinds of techniques, based on the optimization of the electrode geometry and/or signal compensation methods.
We are studying the feasibility and the reliability of a bi-parametric method that uses a twin pulse shaping active filter to analyze each signal from the detector twice: one “slow”, which is proportional to the energy of the incident photon, and one “fast”, which depends on the position of the interaction with respect to the collecting electrode.
In this paper we describe the bi-parametric technique applied to planar CZT detectors grown by the Vertical Bridgman method at CNR/IMEM and report the experimental results in terms of energy resolution, peak-to valley ratio and photopeak efficiency, as well as the compensated spectra obtained as a function of the bias voltage, photon energy and shaping time pairs. We also report the results obtained by using a CZT drift strip detector.
Furthermore, this technique could be implemented in an array of detectors, whose front-end electronics is composed of ASICs, where the shaping time can be selected for each channel, like the RENA-3 IC (NOVA R&D).