Yanfeng Du

A modeling method to calibrate the interaction depth in 3-D position sensitive CdZnTe gamma-ray spectrometers

The gamma ray interaction depth in 3-D position sensitive CdZnTe detectors is currently determined by the pulse height ratio of the cathode signal to the anode pixel signal (C/A ratio). In experiments with our 3-D CdZnTe detectors, the photopeak area as a function of the C/A ratio deviates from the expected exponential attenuation with depth. This indicates that the C/A ratio is not proportional to the true interaction depth. This paper proposes a method to calibrate the measured C/A ratio to the interaction depth by modeling the signals from the cathode and anode pixels. Knowing the detectors mobility-lifetime products of the electrons and holes from measurements, the expected pulse heights of the signals from the cathode and anode pixels can be calculated for different interaction depths. The relationship between the C/A ratios and the interaction depths can then be determined and used as the calibration. The calculation for our 3-D CdZnTe detectors shows that an 8% error in depth determination is incurred without the calibration.

Simulation of CT dose and contrast-to-noise as function of bowtie shape

Dose is becoming increasingly important for computed tomography clinical practice. It is of general interest to understand the impact that system design can have on dose and image quality. This study addresses the effect of bowtie shape on the dose and contrast-to-noise across the field of view. Simulation of the CT acquisition is used to calculate the energy deposition throughout a numerical phantom for a set of relevant system operating parameters and bowtie shapes. Mean absorbed dose is calculated by summing over the phantom volume and is compared with other typical dose specifications. A more aggressive attenuation profile of the bowtie which offers higher attenuation in the periphery of the field of view can offer the benefit of lower dose but at the expense of reduced contrast-to-noise at the edge of the cross-sectional image.

GE intelligent personal radiation locator system

The GE Intelligent Personal Radiation Locator (IPRL) system consists of multiple hand held radiation detectors and a base station. Each mobile unit has a CZT Compton camera radiation detector and can identify isotopes and determine the direction from which the radiation is detected. Using GPS and internal orientation sensors, the system continuously transforms all directional data into real-world coordinates. Detected radiation is wirelessly transmitted to the base station for system-wide analysis and situational awareness. Data can also be exchanged wirelessly between peers to enhance the overall detection efficiency of the system. The key design features and performance characteristics of the GE IPRL system are described.

Temporal response of CZT detectors under intense irradiation

The temporal response of CZT detectors is measured under different X-ray flux, spectra, and detector bias conditions. A comprehensive model has been developed to investigate the detector response under these conditions. The calculations have been compared with our measured results. Reasonable qualitative agreement is shown between the model and measurement results. This model provides a powerful tool to understand the detector temporal response, photocurrent dependence on the irradiation intensity, bias voltage, and defect characteristics. Understanding the detector response from a microscopic level can provide a guide to improve material properties and detector device design.

Estimate of large CZT detector absolute efficiency

This work presents a simulation study about the absolute spectroscopic performances of two large CZT coplanar detectors 1.5/spl times/1.5 cm/sup 2/ area, 1.0 cm thick. A code based on the Geant libraries and classical Monte-Carlo sampling has been developed for the simulation of the experimental scenarios. This tool adapts the Geant capabilities for simulating complex detection systems to the particular needs in spectroscopic studies. Initially, detectors with perfect charge transportation and collection have been simulated. Prior to the application to the CZT devices under research, the code is checked in the simulation of a reliable HPGe detector. The result for this detector is considered as reference for estimating the code capabilities. Some preliminary considerations are exposed in order to correctly define the true sensitive volume of the CZT detectors. Results from the simulation of this basic detector model are presented and commented. Subsequently, the effects of a possible realistic electric field profile in the coplanar detector unable to efficiently deflect the produced negative charge to the collecting anode is studied. From the analysis of the differences found between the simulated and real results, some conclusions relating the electrode design and detector quality are proposed.

Energy dispersive X-ray diffraction spectral resolution considerations for security screening applications

Energy dispersive X-ray diffraction (EDXRD) is a very effective method for explosive and narcotic threat detection in baggage screening. The XRD profiles arise from the molecular interference when X-rays are coherently scattered by a substance. The accurate identification of the target material depends on the ability to detect and resolve the peaks present in the coherent scatter profiles. A high-energy resolution High Purity Germanium (HPGe) detector is therefore generally used in such type of systems. To evaluate the suitability of cost-effective room-temperature semiconductor detectors for next-generation baggage screening systems, an assessment of the minimal requirements for the system resolution is required. In this study a hybrid Monte Carlo code has been modified to account for the molecular interference function that gives rise to the coherent scatter signature. A model for a realistic response function for Cadmium Zinc Telluride (CZT) detectors is then used to convolve the spectral output. This simulation tool is then used to assess the system design features and their influence on spectral resolution.

Dual kVp material decomposition using flat-panel detectors

In addition to a conventional Computed Tomography (CT) image, dual energy (dual kVp) imaging can be used to generate an image of the same anatomy that represents the equivalent density of a particular material, for example, calcium, iodine, water, etc. This image can be used to improve the differentiation of materials as well as improve the accuracy of absolute density measurements in a cross-sectional image. It is important to understand the certainty of the estimation of the density of the material. Both simulations and measurements are used to quantify these errors. Data are acquired using a flat-panel based volumetric CT system, by taking two scans and adjusting the maximum energy of the source spectrum (kVp). Physics based simulations are used to compare with the measurements. After validating the simulation algorithms, the accuracy of the dual kVp method is determined using the simulations in a perturbation study.

Energy resolution limiting factors of multi-pixel events in 3D position sensitive CZT gamma-ray spectrometer

The energy resolution limiting factors of a prototype 3D position-sensitive CZT gamma-ray spectrometer are investigated with both single and multi-pixel events from 662 keV gamma rays. The spectrometer was developed using a 10x10x10 mm3 CZT detector with 8×8 pixellated anode and a RENA-3 readout system from NOVA R&D. The RENA-3 system has energy and timing readout channels for the cathode and each individual pixel that enable the reconstruction of the energy depositions and interaction locations for both single and multi-interaction events in the detector by either C/A ratio or electron drift time. So 3D correction of detector response non-uniformity can be applied to all interaction events to optimize the overall energy resolution of the detector. For single interaction events in the detector, the energy resolution at 662 keV after 3D correction is ∼1% FWHM, which is mainly limited by the electronic noise of the readout system. For multi-pixel events from either multi-interaction or charge sharing in the detector, weighting potential crosstalk among multiple collecting pixels and electronic crosstalk between different ASIC channels turn out to have a big impact on energy resolution. Investigation shows that the weighting potential and electronic crosstalk could account for ≫2% FWHM energy resolution degradation at 662 keV for adjacent-pixel events. These effects are verified with 3D weighting potential modeling and electronic crosstalk measurement using test pulses. It is also demonstrated that the energy resolution of adjacent-pixel events can be improved to 1.8∼1.9% FWHM at 662 keV with proper calibration and correction.

Detective quantum efficiency of an energy resolving photon counting detector

The output response characteristics of an X-ray photon counting detector are measured experimentally and simulated using a Monte Carlo method in order to quantify the loss of statistical information due to pile-up. The analysis is applied to idealize counting detector models, but is adaptable to realistic event processing that is not amenable to analytic solution. In particular, the detective quantum efficiency (DQE) is calculated as a function of flux rate and shown to have an intermediate zero for the paralyzable case at the maximum periodic rate. The progressive degradation of the spectral response as a function of increasing flux rate is also modeled. Analogous metrics to DQE are defined in regards to the detectors ability to resolve atomic number and enhance image contrast based on atomic number differentiation. Analytic solutions are provided for the output and linearized response statistics and these interpolate well across the Monte Carlo and experimental results.

4 pi direction sensitive gamma imager with RENA-3 readout ASIC

A 4π direction-sensitive gamma imager is presented, using a 1 cm3 3D CZT detector from Yinnel Tech and the RENA-3 readout ASIC from NOVA R&D. The measured readout system electronic noise is around 4-5 keV FWHM for all anode channels. The measured timing resolution between two channels within a single ASIC is around 10 ns and the resolution is 30 ns between two separate ASIC chips. After 3D material non-uniformity and charge trapping corrections, the measured single-pixel-event energy resolution is around 1% for Cs-137 at 662 keV using a 1 cm3 CZT detector from Yinnel Tech with an 8 x 8 anode pixel array at 1.15 mm pitch. The energy resolution for two pixel events is 2.9%. A 10 uCi Cs-137 point source was moved around the detector to test the image reconstruction algorithms and demonstrate the source direction detection capability. Accurate source locations were reconstructed with around 200 two-pixel events within a total energy window ±10 keV around the 662 keV full energy peak. The angular resolution FWHM at four of the five positions tested was between 0.05-0.07 steradians.