Zhong He | University of Michigan (original) (raw)
Papers by Zhong He
2009 IEEE Nuclear Science Symposium Conference Record (NSS/MIC), 2009
Computational Imaging VII, 2009
IEEE Nuclear Science Symposuim & Medical Imaging Conference, 2010
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2001
IEEE Transactions on Nuclear Science, 1995
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 1994
A technique based on ß-y coincidence has been developed to perform multiplexed gamma-ray spectrom... more A technique based on ß-y coincidence has been developed to perform multiplexed gamma-ray spectrometry of small samples using a single gamma-ray detector and a position sensitive beta detector. A system is described that uses a position sensitive photomultiplier tube coupled to a thin plastic scintillator as the beta imaging detector. Multiplexed gamma-ray spectrometry is demonstrated by results obtained with this system for a 4 X 4 array of An, Co and Ag samples. The advantages of this technique over gamma spectrometry on individual samples are the substantial reduction in total counting time and the reduction in background, which are especially significant in neutron activation analysis of particles.
AIP Conference Proceedings, 2002
This paper demonstrates the capability of compact gamma-ray imaging devices using 3dimensional po... more This paper demonstrates the capability of compact gamma-ray imaging devices using 3dimensional position sensitive CdZnTe semiconductor gamma-ray spectrometers, developed at the University of Michigan. A prototype imager was constructed and tested using two 1 cm cube 3dimensional position sensitive CdZnTe detectors. Energy resolutions of 1.5% FWHM for single pixel events at 662 keV gamma-ray energy were obtained on both detectors, and an angular resolution of about 5° FWHM was demonstrated. The capabilities of proposed devices, which can cover a wider energy range up to 2.6 MeV, are discussed.
Hard X-Ray, Gamma-Ray, and Neutron Detector Physics, 1999
A CZT Compton Camera (CCC) is being built using two three-dimensional (3-D) position-sensitive CZ... more A CZT Compton Camera (CCC) is being built using two three-dimensional (3-D) position-sensitive CZT detectors. Expected system performance was analyzed by analytical and Monte Carlo approaches. Based on the measurement of detector energy and position resolution, the expected angular resolution is &33 and &23 for a $303
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2012
A 3.5Â 3.5 Â 4.6 mm 3 thick TlBr detector with pixellated Au/Cr anodes made by Radiation Monitori... more A 3.5Â 3.5 Â 4.6 mm 3 thick TlBr detector with pixellated Au/Cr anodes made by Radiation Monitoring Devices Inc. was studied. The detector has a planar cathode and nine anode pixels surrounded by a guard ring. The pixel pitch is 1.0 mm. Digital pulse waveforms of preamplifier outputs were recorded using a multichannel GaGe PCI digitizer board. Several experiments were carried out at À 20 1C, with the detector under bias for over a month. An energy resolution of 1.7% FWHM at 662 keV was measured without any correction at À 2400 V bias. Holes generated at all depths can be collected by the cathode at À 2400 V bias which made depth correction using the cathode-to-anode ratio technique difficult since both charge carriers contribute to the signal. An energy resolution of 5.1% FWHM at 662 keV was obtained from the best pixel electrode without depth correction at þ 1000 V bias. In this positive bias case, the pixel electrode was actually collecting holes. A hole mobility-lifetime of 0.95 Â 10 À 4 cm 2 /V has been estimated from measurement data.
2011 IEEE Nuclear Science Symposium Conference Record, 2011
The complex system response of 3D position sensitive gamma-ray detectors complicates the model fo... more The complex system response of 3D position sensitive gamma-ray detectors complicates the model for the recorded measurements and makes exact expressions for detection performance intractable. This makes source detection performance difficult and expensive to com pute. Asymptotic analysis has the potential to simplify detection performance prediction with complex systems and has previously been applied to detection performance prediction with simulated gamma-ray detectors. In this work, we use asymptotic performance prediction methods to predict points on the receiver operating characteristic (ROC) curve for the illustrative task of detecting a Cs-137 source in background with an I8-detector CdZnTe array. We assume that the source position, background spectrum, and background spatial distribution are known. Our results show that the asymptotic performance prediction method accurately predicts the empirically observed performance even with real data recorded with a real system. Our results also characterize the performance of the detector array for the task of source detection. The accuracy and computational efficiency of the asymptotic detection performance prediction method make it a viable alternative to empirical performance evaluation.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2010
Thallium bromide (TlBr) is a promising semiconductor detector material due to its high atomic num... more Thallium bromide (TlBr) is a promising semiconductor detector material due to its high atomic number (Tl: 81, Br: 35), high density (7.56 g/cm 3) and wide band gap (2.68 eV). Current TlBr detectors suffer from polarization, which causes performance degradation over time when high voltage is applied. A 4.6-mm thick TlBr detector with pixellated anodes made by Radiation Monitoring Devices Inc. was used in the experiments. The detector has a planar cathode and nine anode pixels surrounded by a guard ring. The pixel pitch is 1.0-mm. Digital pulse waveforms of preamplifier outputs were recorded using a multi-channel GaGe PCI digitizer board for pulse shaping. Several experiments were carried out at À 20 1C while the detector was under bias for over a month. No polarization effect was observed and the detector's spectroscopic performance improved over time. Energy resolution of 1.5% FWHM at 662 keV has been measured without depth correction at À 2000 V cathode bias. Average electron mobility-lifetime of (5.7 7 0.8) Â 10 À 3 cm 2 /V has been measured from four anode pixels.
SPIE Proceedings, 2004
A 3D CdZnTe detector can provide 3D position information as well as energy information of each in... more A 3D CdZnTe detector can provide 3D position information as well as energy information of each individual interaction when a gamma ray is scattered or absorbed in the detector. This unique feature provides the 3D CdZnTe detector the capability to do Compton imaging with a single detector. After detector calibration, real-time data acquisition and imaging are implemented with a single detector system. Because the detector has a finite size and any point in the detector can be the first scattering position, 3D gamma-ray imaging in near field is possible. In this work we will show the result of the 4π Compton imaging with a single 15mm × 15mm × 10mm CdZnTe detector. Different algorithms for sequence and imaging reconstruction will be addressed and compared. The angular uncertainty is estimated and the most recent results from measurements are presented.
IEEE Nuclear Science Symposuim & Medical Imaging Conference, 2010
In a pixelated detector, an electron cloud can be collected by and shared between several adjacen... more In a pixelated detector, an electron cloud can be collected by and shared between several adjacent pixels. By including these charge-sharing events, the Compton-imaging efficiency can be improved for 3D position sensitive room-temperature CdZnTe gamma-ray detectors. Simulated photopeak events that trigger three separate pixels with two pixels being adjacent to each other, which are called threepixel side-neighbor photopeak events, were divided into six categories based on interaction type. Analysis of this simulation shows that the most effective strategy is to treat all side-neighbor events as charge-sharing events and to combine all side-neighbor signals into a single interaction. By including these side-neighbor events in the Compton image reconstruction, we can improve the imaging efficiency by 45% and 160% for 662 keV and 1333 keV incident photons, respectively. The simulation also shows that 76% of these combined events reconstruct to rings that pass the source direction. Measured data is presented to validate the simulation results.
IEEE Transactions on Nuclear Science, 2014
Due to favorable material properties such as high atomic number (Tl: 81, Br: 35), high density (g... more Due to favorable material properties such as high atomic number (Tl: 81, Br: 35), high density (g/cm), and a wide band gap (2.68 eV), thallium-bromide (TlBr) is currently under investigation for use as an alternative room-temperature semiconductor gamma-ray spectrometer. TlBr detectors can achieve less than 1% FWHM energy resolution at 662 keV, but these results are limited to stable operation at. After days to months of room-temperature operation, ionic conduction causes these devices to fail. This work correlates the varying leakage current with alpha-particle and gamma-ray spectroscopic performances at various operating temperatures. Depth-dependent photopeak centroids exhibit time-dependent transient behavior, which indicates trapping sites form near the anode surface during room-temperature operation. After refabrication, similar performance and functionality of failed detectors returned.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2005
The latest depth-sensing coplanar grid CdZnTe detectors have been tested. Two of these have dimen... more The latest depth-sensing coplanar grid CdZnTe detectors have been tested. Two of these have dimensions 1.5 Â 1.5 Â 1.0 cm 3 and one is a cylindrical detector with 1.5 cm diameter and 1.0 cm length, all of them using the thirdgeneration coplanar anode design. Energy resolutions of 2.0% and 2.4% FWHM at 662 keV gray energies were obtained. Detector performance has been observed experimentally as a function of depth of the gray interaction, and as a function of radial position near the anode surface. The measured results show the improvement of the thirdgeneration anode design. Material uniformity of CdZnTe crystals manufactured by eV Products have been directly observed and compared on two 1.5 Â 1.5 Â 1.0 cm 3 detectors.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 1994
A high energy gamma camera using a parallel hole tungsten collimator and a 7.62 cm square Positio... more A high energy gamma camera using a parallel hole tungsten collimator and a 7.62 cm square Position Sensitive Photomultiplier Tube (PSPMT) has been assembled and tested. The measured energy resolution is 12-20% FWHM for 137CS (662 keV) gamma rays, and 17-23% FWHM for 57 Co (122 keV). The measured spatial resolution measured is 2.8-3 .2 mm for 122 keV and 3.4-5 .3 mm for 662 keV. The variation in the energy and spatial resolution is due to the continuously varying gain across the PSPMT. Measured energy spectra and images obtained are presented.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2007
In conventional Compton camera systems, the image reconstruction is performed only in two-dimensi... more In conventional Compton camera systems, the image reconstruction is performed only in two-dimensional or three-dimensional spatial coordinates for a specific gamma-ray energy. By doing so, a priori knowledge of the incident gamma-ray energy is required, and usually an energy window is applied to select full energy deposition events. In some other applications, spectral-deconvolution algorithms were developed to estimate the incident gamma-ray spectrum by deconvolving the observed energy-loss spectrum. However, usually the spectral system response function of a non-spherical detector depends on the incident gamma-ray's direction, which cannot be modeled by those spectral-deconvolution algorithms. In this paper, we propose a new energy-imaging integrated spectral-deconvolution method, which utilizes both the Compton imaging and the spectral-deconvolution techniques. In the new method, the deconvolution takes place in a integrated spatial and energy space. This technique eliminates the requirement of knowing the gamma-ray energy in the imaging part, and removes the directional dependence in the spectral-deconvolution part. The deconvolved result provides the image at any specific energy, as well as the spectrum at any specific direction. The deconvolution method is based on the maximum likelihood expectation maximization (MLEM) algorithm, which is popular in reconstructing photon-emission images. Since the ML solution estimates the true incident gamma-ray intensity, the deconvolved energy spectrum at the source location is free of Compton continuum. To truthfully reconstruct the source distribution from the observation data, the accuracy of the system response function t ij , i.e. the probability for a photon from source pixel j to be observed as event i, is the most crucial information. Because of the large number of pixels in the energy-imaging integrated space, and the very large number of possible measurement events, it is impossible to pre-calculate the system response function t ij by simulations. In this paper, an analytical approach is introduced so that the system response function can be calculated during the reconstruction process. In order to perform Compton imaging, gamma-ray detectors are required to have position-sensing capability. The energy-imaging integrated deconvolution algorithm is applied to a three-dimensional position-sensitive CdZnTe gamma-ray imaging spectrometer, which can provide not only the energy-deposition information, but also the position information of individual gamma-ray interactions. The results demonstrate that the technique is capable of deconvolving the energy spectrum and of reconstructing the image simultaneously.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2005
This paper presents a study for exploring the limiting timing resolution that can be achieved wit... more This paper presents a study for exploring the limiting timing resolution that can be achieved with a large volume 3-D position sensitive CZT detector. The interaction timing information was obtained by fitting the measured cathode waveforms to pre-defined waveform models. We compared the results from using several different waveform models. Timing resolutions, of ~9.5 ns for 511 keV full-energy events and ~11.6 ns for all detected events with energy deposition above 250 keV, were achieved with a detailed modeling of the cathode waveform as a function of interaction location and energy deposition. This detailed modeling also allowed us to derive a theoretical lower bound for the error on estimated interaction timing. Both experimental results and theoretical predications matched well, which indicated that the best timing resolution achievable in the 1 cm 3 CZT detector tested is ~10 ns. It is also showed that the correlation between sampled amplitudes in cathode waveforms is an important limiting factor for the achievable timing resolution.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2012
A new method for measuring the electron mobility, the electron mean free drift time, and their pr... more A new method for measuring the electron mobility, the electron mean free drift time, and their product has been developed for pixelated semiconductor detectors. Using data from a standard calibration measurement, these three quantities are measured and compared against results using other methods. Since the results can be easily obtained, comparisons of many detectors have been completed and show that detector spectroscopic performance is independent of the electron trapping if the raw electron trapping is less than 6.5% from the cathode to the anode surface.
Journal of Applied Physics, 1998
This article describes novel techniques to directly measure the electron mobility and mean free d... more This article describes novel techniques to directly measure the electron mobility and mean free drift time product e e in semiconductor detectors. These methods are based on newly developed single polarity charge sensing and depth sensing techniques. Compared with conventional methods based on the Hecht relation, the new methods do not involve curve fitting, are less sensitive to the variation of pulse rise times, and allow the use of higher energy ␥ rays typical of many applications.
2009 IEEE Nuclear Science Symposium Conference Record (NSS/MIC), 2009
Computational Imaging VII, 2009
IEEE Nuclear Science Symposuim & Medical Imaging Conference, 2010
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2001
IEEE Transactions on Nuclear Science, 1995
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 1994
A technique based on ß-y coincidence has been developed to perform multiplexed gamma-ray spectrom... more A technique based on ß-y coincidence has been developed to perform multiplexed gamma-ray spectrometry of small samples using a single gamma-ray detector and a position sensitive beta detector. A system is described that uses a position sensitive photomultiplier tube coupled to a thin plastic scintillator as the beta imaging detector. Multiplexed gamma-ray spectrometry is demonstrated by results obtained with this system for a 4 X 4 array of An, Co and Ag samples. The advantages of this technique over gamma spectrometry on individual samples are the substantial reduction in total counting time and the reduction in background, which are especially significant in neutron activation analysis of particles.
AIP Conference Proceedings, 2002
This paper demonstrates the capability of compact gamma-ray imaging devices using 3dimensional po... more This paper demonstrates the capability of compact gamma-ray imaging devices using 3dimensional position sensitive CdZnTe semiconductor gamma-ray spectrometers, developed at the University of Michigan. A prototype imager was constructed and tested using two 1 cm cube 3dimensional position sensitive CdZnTe detectors. Energy resolutions of 1.5% FWHM for single pixel events at 662 keV gamma-ray energy were obtained on both detectors, and an angular resolution of about 5° FWHM was demonstrated. The capabilities of proposed devices, which can cover a wider energy range up to 2.6 MeV, are discussed.
Hard X-Ray, Gamma-Ray, and Neutron Detector Physics, 1999
A CZT Compton Camera (CCC) is being built using two three-dimensional (3-D) position-sensitive CZ... more A CZT Compton Camera (CCC) is being built using two three-dimensional (3-D) position-sensitive CZT detectors. Expected system performance was analyzed by analytical and Monte Carlo approaches. Based on the measurement of detector energy and position resolution, the expected angular resolution is &33 and &23 for a $303
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2012
A 3.5Â 3.5 Â 4.6 mm 3 thick TlBr detector with pixellated Au/Cr anodes made by Radiation Monitori... more A 3.5Â 3.5 Â 4.6 mm 3 thick TlBr detector with pixellated Au/Cr anodes made by Radiation Monitoring Devices Inc. was studied. The detector has a planar cathode and nine anode pixels surrounded by a guard ring. The pixel pitch is 1.0 mm. Digital pulse waveforms of preamplifier outputs were recorded using a multichannel GaGe PCI digitizer board. Several experiments were carried out at À 20 1C, with the detector under bias for over a month. An energy resolution of 1.7% FWHM at 662 keV was measured without any correction at À 2400 V bias. Holes generated at all depths can be collected by the cathode at À 2400 V bias which made depth correction using the cathode-to-anode ratio technique difficult since both charge carriers contribute to the signal. An energy resolution of 5.1% FWHM at 662 keV was obtained from the best pixel electrode without depth correction at þ 1000 V bias. In this positive bias case, the pixel electrode was actually collecting holes. A hole mobility-lifetime of 0.95 Â 10 À 4 cm 2 /V has been estimated from measurement data.
2011 IEEE Nuclear Science Symposium Conference Record, 2011
The complex system response of 3D position sensitive gamma-ray detectors complicates the model fo... more The complex system response of 3D position sensitive gamma-ray detectors complicates the model for the recorded measurements and makes exact expressions for detection performance intractable. This makes source detection performance difficult and expensive to com pute. Asymptotic analysis has the potential to simplify detection performance prediction with complex systems and has previously been applied to detection performance prediction with simulated gamma-ray detectors. In this work, we use asymptotic performance prediction methods to predict points on the receiver operating characteristic (ROC) curve for the illustrative task of detecting a Cs-137 source in background with an I8-detector CdZnTe array. We assume that the source position, background spectrum, and background spatial distribution are known. Our results show that the asymptotic performance prediction method accurately predicts the empirically observed performance even with real data recorded with a real system. Our results also characterize the performance of the detector array for the task of source detection. The accuracy and computational efficiency of the asymptotic detection performance prediction method make it a viable alternative to empirical performance evaluation.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2010
Thallium bromide (TlBr) is a promising semiconductor detector material due to its high atomic num... more Thallium bromide (TlBr) is a promising semiconductor detector material due to its high atomic number (Tl: 81, Br: 35), high density (7.56 g/cm 3) and wide band gap (2.68 eV). Current TlBr detectors suffer from polarization, which causes performance degradation over time when high voltage is applied. A 4.6-mm thick TlBr detector with pixellated anodes made by Radiation Monitoring Devices Inc. was used in the experiments. The detector has a planar cathode and nine anode pixels surrounded by a guard ring. The pixel pitch is 1.0-mm. Digital pulse waveforms of preamplifier outputs were recorded using a multi-channel GaGe PCI digitizer board for pulse shaping. Several experiments were carried out at À 20 1C while the detector was under bias for over a month. No polarization effect was observed and the detector's spectroscopic performance improved over time. Energy resolution of 1.5% FWHM at 662 keV has been measured without depth correction at À 2000 V cathode bias. Average electron mobility-lifetime of (5.7 7 0.8) Â 10 À 3 cm 2 /V has been measured from four anode pixels.
SPIE Proceedings, 2004
A 3D CdZnTe detector can provide 3D position information as well as energy information of each in... more A 3D CdZnTe detector can provide 3D position information as well as energy information of each individual interaction when a gamma ray is scattered or absorbed in the detector. This unique feature provides the 3D CdZnTe detector the capability to do Compton imaging with a single detector. After detector calibration, real-time data acquisition and imaging are implemented with a single detector system. Because the detector has a finite size and any point in the detector can be the first scattering position, 3D gamma-ray imaging in near field is possible. In this work we will show the result of the 4π Compton imaging with a single 15mm × 15mm × 10mm CdZnTe detector. Different algorithms for sequence and imaging reconstruction will be addressed and compared. The angular uncertainty is estimated and the most recent results from measurements are presented.
IEEE Nuclear Science Symposuim & Medical Imaging Conference, 2010
In a pixelated detector, an electron cloud can be collected by and shared between several adjacen... more In a pixelated detector, an electron cloud can be collected by and shared between several adjacent pixels. By including these charge-sharing events, the Compton-imaging efficiency can be improved for 3D position sensitive room-temperature CdZnTe gamma-ray detectors. Simulated photopeak events that trigger three separate pixels with two pixels being adjacent to each other, which are called threepixel side-neighbor photopeak events, were divided into six categories based on interaction type. Analysis of this simulation shows that the most effective strategy is to treat all side-neighbor events as charge-sharing events and to combine all side-neighbor signals into a single interaction. By including these side-neighbor events in the Compton image reconstruction, we can improve the imaging efficiency by 45% and 160% for 662 keV and 1333 keV incident photons, respectively. The simulation also shows that 76% of these combined events reconstruct to rings that pass the source direction. Measured data is presented to validate the simulation results.
IEEE Transactions on Nuclear Science, 2014
Due to favorable material properties such as high atomic number (Tl: 81, Br: 35), high density (g... more Due to favorable material properties such as high atomic number (Tl: 81, Br: 35), high density (g/cm), and a wide band gap (2.68 eV), thallium-bromide (TlBr) is currently under investigation for use as an alternative room-temperature semiconductor gamma-ray spectrometer. TlBr detectors can achieve less than 1% FWHM energy resolution at 662 keV, but these results are limited to stable operation at. After days to months of room-temperature operation, ionic conduction causes these devices to fail. This work correlates the varying leakage current with alpha-particle and gamma-ray spectroscopic performances at various operating temperatures. Depth-dependent photopeak centroids exhibit time-dependent transient behavior, which indicates trapping sites form near the anode surface during room-temperature operation. After refabrication, similar performance and functionality of failed detectors returned.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2005
The latest depth-sensing coplanar grid CdZnTe detectors have been tested. Two of these have dimen... more The latest depth-sensing coplanar grid CdZnTe detectors have been tested. Two of these have dimensions 1.5 Â 1.5 Â 1.0 cm 3 and one is a cylindrical detector with 1.5 cm diameter and 1.0 cm length, all of them using the thirdgeneration coplanar anode design. Energy resolutions of 2.0% and 2.4% FWHM at 662 keV gray energies were obtained. Detector performance has been observed experimentally as a function of depth of the gray interaction, and as a function of radial position near the anode surface. The measured results show the improvement of the thirdgeneration anode design. Material uniformity of CdZnTe crystals manufactured by eV Products have been directly observed and compared on two 1.5 Â 1.5 Â 1.0 cm 3 detectors.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 1994
A high energy gamma camera using a parallel hole tungsten collimator and a 7.62 cm square Positio... more A high energy gamma camera using a parallel hole tungsten collimator and a 7.62 cm square Position Sensitive Photomultiplier Tube (PSPMT) has been assembled and tested. The measured energy resolution is 12-20% FWHM for 137CS (662 keV) gamma rays, and 17-23% FWHM for 57 Co (122 keV). The measured spatial resolution measured is 2.8-3 .2 mm for 122 keV and 3.4-5 .3 mm for 662 keV. The variation in the energy and spatial resolution is due to the continuously varying gain across the PSPMT. Measured energy spectra and images obtained are presented.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2007
In conventional Compton camera systems, the image reconstruction is performed only in two-dimensi... more In conventional Compton camera systems, the image reconstruction is performed only in two-dimensional or three-dimensional spatial coordinates for a specific gamma-ray energy. By doing so, a priori knowledge of the incident gamma-ray energy is required, and usually an energy window is applied to select full energy deposition events. In some other applications, spectral-deconvolution algorithms were developed to estimate the incident gamma-ray spectrum by deconvolving the observed energy-loss spectrum. However, usually the spectral system response function of a non-spherical detector depends on the incident gamma-ray's direction, which cannot be modeled by those spectral-deconvolution algorithms. In this paper, we propose a new energy-imaging integrated spectral-deconvolution method, which utilizes both the Compton imaging and the spectral-deconvolution techniques. In the new method, the deconvolution takes place in a integrated spatial and energy space. This technique eliminates the requirement of knowing the gamma-ray energy in the imaging part, and removes the directional dependence in the spectral-deconvolution part. The deconvolved result provides the image at any specific energy, as well as the spectrum at any specific direction. The deconvolution method is based on the maximum likelihood expectation maximization (MLEM) algorithm, which is popular in reconstructing photon-emission images. Since the ML solution estimates the true incident gamma-ray intensity, the deconvolved energy spectrum at the source location is free of Compton continuum. To truthfully reconstruct the source distribution from the observation data, the accuracy of the system response function t ij , i.e. the probability for a photon from source pixel j to be observed as event i, is the most crucial information. Because of the large number of pixels in the energy-imaging integrated space, and the very large number of possible measurement events, it is impossible to pre-calculate the system response function t ij by simulations. In this paper, an analytical approach is introduced so that the system response function can be calculated during the reconstruction process. In order to perform Compton imaging, gamma-ray detectors are required to have position-sensing capability. The energy-imaging integrated deconvolution algorithm is applied to a three-dimensional position-sensitive CdZnTe gamma-ray imaging spectrometer, which can provide not only the energy-deposition information, but also the position information of individual gamma-ray interactions. The results demonstrate that the technique is capable of deconvolving the energy spectrum and of reconstructing the image simultaneously.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2005
This paper presents a study for exploring the limiting timing resolution that can be achieved wit... more This paper presents a study for exploring the limiting timing resolution that can be achieved with a large volume 3-D position sensitive CZT detector. The interaction timing information was obtained by fitting the measured cathode waveforms to pre-defined waveform models. We compared the results from using several different waveform models. Timing resolutions, of ~9.5 ns for 511 keV full-energy events and ~11.6 ns for all detected events with energy deposition above 250 keV, were achieved with a detailed modeling of the cathode waveform as a function of interaction location and energy deposition. This detailed modeling also allowed us to derive a theoretical lower bound for the error on estimated interaction timing. Both experimental results and theoretical predications matched well, which indicated that the best timing resolution achievable in the 1 cm 3 CZT detector tested is ~10 ns. It is also showed that the correlation between sampled amplitudes in cathode waveforms is an important limiting factor for the achievable timing resolution.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2012
A new method for measuring the electron mobility, the electron mean free drift time, and their pr... more A new method for measuring the electron mobility, the electron mean free drift time, and their product has been developed for pixelated semiconductor detectors. Using data from a standard calibration measurement, these three quantities are measured and compared against results using other methods. Since the results can be easily obtained, comparisons of many detectors have been completed and show that detector spectroscopic performance is independent of the electron trapping if the raw electron trapping is less than 6.5% from the cathode to the anode surface.
Journal of Applied Physics, 1998
This article describes novel techniques to directly measure the electron mobility and mean free d... more This article describes novel techniques to directly measure the electron mobility and mean free drift time product e e in semiconductor detectors. These methods are based on newly developed single polarity charge sensing and depth sensing techniques. Compared with conventional methods based on the Hecht relation, the new methods do not involve curve fitting, are less sensitive to the variation of pulse rise times, and allow the use of higher energy ␥ rays typical of many applications.