Γ-ray Imaging Research Articles

Development of Double-Sided CdTe Strip Detectors for γ-Ray Imaging and Spectroscopy

By reading out both anode and cathode strips, double-sided CdTe strip detectors can achieve a large area and a high position resolution with few readout channels, which makes them very attractive for X-ray and γ-ray imaging and spectroscopy. We have developed double-sided CdTe strip detectors, 1.28 ×1.28 cm2 in size and 0.5 and 2.0 mm in thickness. Both electrodes are divided into 32 orthogonal strips with a pitch of 400 µm. For a detector of 0.5 mm thickness, the energy resolution was measured to be 1.5 keV (FWHM) at 60 keV. For the 2.0-mm-thick detector, an energy resolution of 8.0 keV (FWHM) at 662 keV was obtained using only the anode signal. By combining both the anode and cathode signals, we successfully improved the spectral performance and measured an energy resolution of 5.9 keV (FWHM) at 662 keV.

Multiple Modality $\gamma$-Ray Imager Using LaCl$_{3}$(Ce) Scintillators With Active Collimation Method

Multiple modality γ-ray imaging using mechanical and electronic collimation with different detection methods was proposed and tested by simulation. Although conventional mechanical collimators, which are made from high atomic number and density materials, passively filter the incident radiation, the mechanical collimator reported in this study consisted of LaCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (Ce) scintillators, which actively collimate the radiation and are used as the 1st detector of an electronic collimator to reconstruct a Compton image. A detector box made from six planar LaCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (Ce) scintillators was positioned behind the active collimator. The LaCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (Ce) scintillator arrays used as the active collimator and box detector consisted of voxels, each 2 mm × 2 mm × 5 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> in size. A combination of active collimators and box detectors can produce multiple modalities for both mechanical and electronic collimation. The information from each modality was combined effectively using the maximum likelihood expectation maximization (MLEM) method. Therefore, a reconstructed image from multiple modalities has inherently higher efficiency than each conventional modality. The reconstructed images were evaluated using the resolution-variance curve as a quantitative method. Although mechanical collimation is effective at low γ-ray energy and electronic collimation shows high performance at high γ-ray energy, multiple modalities are superior to the conventional single modalities for intermediate γ-ray energies (364 keV). The angular resolution and absolute efficiency were calculated for each modality and source energy.

Imaging detector development for nuclear astrophysics using pixelated CdTe

The concept of focusing telescopes in the energy range of lines of astrophysical interest (i.e., of energies around 1MeV) should allow to reach unprecedented sensitivities, essential to perform detailed studies of cosmic explosions and cosmic accelerators. Our research and development activities aim to study a detector suited for the focal plane of a γ-ray telescope mission. A CdTe/CdZnTe detector operating at room temperature, that combines high detection efficiency with good spatial and spectral resolution is being studied in recent years as a focal plane detector, with the interesting option of also operating as a Compton telescope monitor. We present the current status of the design and development of a γ-ray imaging spectrometer in the MeV range, for nuclear astrophysics, consisting of a stack of CdTe pixel detectors with increasing thicknesses. We have developed an initial prototype based on CdTe ohmic detector. The detector has 11×11 pixels, with a pixel pitch of 1mm and a thickness of 2mm. Each pixel is stud bonded to a fanout board and routed to an front end ASIC to measure pulse height and rise time information for each incident γ-ray photon. First measurements of a 133Ba and 241Am source are reported here.

Predicted γ-ray image of SN 1006 due to inverse Compton emission

We propose a method to synthesize the inverse Compton (IC) gamma-ray image of a supernova remnant starting from the radio (or hard X-ray) map and using results of the spatially resolved X-ray spectral analysis. The method is successfully applied to SN 1006. We found that synthesized IC gamma-ray images of SN 1006 show morphology in nice agreement with that reported by the H.E.S.S. collaboration. The good correlation found between the observed very-high energy gamma-ray and X-ray/radio appearance can be considered as an evidence that the gamma-ray emission of SN 1006 observed by H.E.S.S. is leptonic in origin, though the hadronic origin may not be excluded.

Introducing DeBRa: a detailed breast model for radiological studies

Currently, x-ray mammography is the method of choice in breast cancer screening programmes. As the mammography technology moves from 2D imaging modalities to 3D, conventional computational phantoms do not have sufficient detail to support the studies of these advanced imaging systems. Studies of these 3D imaging systems call for a realistic and sophisticated computational model of the breast. DeBRa (Detailed Breast model for Radiological studies) is the most advanced, detailed, 3D computational model of the breast developed recently for breast imaging studies. A DeBRa phantom can be constructed to model a compressed breast, as in film/screen, digital mammography and digital breast tomosynthesis studies, or a non-compressed breast as in positron emission mammography and breast CT studies. Both the cranial-caudal and mediolateral oblique views can be modelled. The anatomical details inside the phantom include the lactiferous duct system, the Cooper ligaments and the pectoral muscle. The fibroglandular tissues are also modelled realistically. In addition, abnormalities such as microcalcifications, irregular tumours and spiculated tumours are inserted into the phantom. Existing sophisticated breast models require specialized simulation codes. Unlike its predecessors, DeBRa has elemental compositions and densities incorporated into its voxels including those of the explicitly modelled anatomical structures and the noise-like fibroglandular tissues. The voxel dimensions are specified as needed by any study and the microcalcifications are embedded into the voxels so that the microcalcification sizes are not limited by the voxel dimensions. Therefore, DeBRa works with general-purpose Monte Carlo codes. Furthermore, general-purpose Monte Carlo codes allow different types of imaging modalities and detector characteristics to be simulated with ease. DeBRa is a versatile and multipurpose model specifically designed for both x-ray and γ-ray imaging studies.

Development of three-dimensional prompt γ-ray analysis system

A newly developed three-dimensional (3-D) neutron-induced γ-ray imaging technique is presented. Using this technique, we could reconstruct the images of isotopes in objects over the full thickness of the body, since neutrons have large penetrating power and neutron-induced prompt γ-ray analysis make them ideal probes to investigate elemental information of objects, non-destructively. In this study, we confirmed the principle of this technique and succeed in the 3-D visualization of the detailed structure based on elemental analysis for the first time.

Observation of Pulsed γ-Rays Above 25 GeV from the Crab Pulsar with MAGIC

One fundamental question about pulsars concerns the mechanism of their pulsed electromagnetic emission. Measuring the high-end region of a pulsar's spectrum would shed light on this question. By developing a new electronic trigger, we lowered the threshold of the Major Atmospheric gamma-ray Imaging Cherenkov (MAGIC) telescope to 25 giga-electron volts. In this configuration, we detected pulsed gamma-rays from the Crab pulsar that were greater than 25 giga-electron volts, revealing a relatively high cutoff energy in the phase-averaged spectrum. This indicates that the emission occurs far out in the magnetosphere, hence excluding the polar-cap scenario as a possible explanation of our measurement. The high cutoff energy also challenges the slot-gap scenario.

A simulation study of the DQE of a linear plastic scintillating fiber array for hard gamma-rays

The detector quantum efficiency (DQE) of a linear plastic scintillating fiber (PSF) array coupled with a charge-coupled device (CCD) for hard γ-ray imaging is studied using a Monte Carlo simulation. The focus is on the energy from a few MeV to about 12 MeV. The excellent characteristic of PSF offers a method to balance the detection efficiency and spatial resolution. Our simulation results indicate that the modulation transfer function (MTF) for different energies become almost the same below the certain frequency and the DQE should be better at lower frequency for imaging lower incident energy. These characteristics suggest that the PSF may be useful for detecting high energy γ-rays.

Radio Submillimeter and γ‐Ray Observations of the 2003 October 28 Solar Flare

Radio observations at 210 GHz taken by the Bernese Multibeam Radiometer for KOSMA (BEMRAK) are combined with hard X-ray and γ-ray observations from the SONG instrument on board CORONA-F and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) to investigate high-energy particle acceleration during the energetic solar flare of 2003 October 28. Two distinct components at submillimeter wavelengths are found. The first is a gradual, long-lasting (>30 minutes) component with large apparent source sizes (~60''). Its spectrum below ~200 GHz is consistent with synchrotron emission from flare-accelerated electrons producing hard X-ray and γ-ray bremsstrahlung assuming a magnetic field strength of ≥200 G in the radio source and a confinement time of the radio-emitting electrons in the source of less than 30 s. The other component is impulsive and starts simultaneously with high-energy (>200 MeV nucleon−1) proton acceleration and the production of pions. The derived radio source size is compact (≤10''), and the emission is cospatial with the location of precipitating flare-accelerated >30 MeV protons as seen in γ-ray imaging. The close correlation in time and space of radio emission with the production of pions suggests that synchrotron emission of positrons produced in charged-pion decay might be responsible for the observed compact radio source. However, order-of-magnitude approximations rather suggest that the derived numbers of positrons from charged-pion decay are probably too small to account for the observed radio emission. Synchrotron emission from energetic electrons therefore appears as the most likely emission mechanism for the compact radio source seen in the impulsive phase, although it does not account for its close correlation, in time and space, with pion production.

Very high energy γ-ray observations of Mrk 501 using the TACTIC imaging γ-ray telescope during 2005–06

In this paper we report on the Markarian 501 results obtained during our TeV γ-ray observations from 11 March to 12 May 2005 and 28 February to 7 May 2006 for 112.5 h with the TACTIC γ-ray telescope. During 2005 observations for 45.7 h, the source was found to be in a low state and we have placed an upper limit of 4.62 × 10−12 photons cm−2 s−1 at 3σ level on the integrated TeV γ-ray flux above 1 TeV from the source direction. However, during 2006 observations for 66.8 h, detailed data analysis revealed the presence of a TeV γ-ray signal from the source with a statistical significance of 7.5σ above Eγ⩾ 1 TeV. The time-averaged differential energy spectrum of the source in the energy range 1–11 TeV is found to match well with the power-law function of the form (dΦ/dE = f0E−Γ) with f0 = (1.66 ± 0.52) × 10−11 cm−2 s−1 TeV−1 and Γ = 2.80 ± 0.27.

A Minimal Spanning Tree algorithm for source detection in γ-ray images

We developed a source detection algorithm based on the Minimal Spanning Tree (MST), that is a graph-theoretical method useful for finding clusters in a given set of points. This algorithm is applied to γ-ray bi-dimensional images where the points correspond to the arrival direction of photons, and the possible sources are associated with the regions where they clusterize. Some filters to select these clusters and to reduce the spurious detections are introduced. An empirical study of the statistical properties of MST on random fields is carried out in order to derive some criteria to estimate the best filter values. We also introduce two parameters useful to verify the goodness of candidate sources. To show how the MST algorithm works in practice, we present an application to an EGRET observation of the Virgo field, at high Galactic latitude and with a low and rather uniform background, in which several sources are detected.

Simulation study on detection efficiency of plastic scintillating fiber under γ-ray radiation

The detection efficiency of the plastic scintillating fiber (PSF) for γ-ray radiation is presented through Geant4 simulation. The incident γ-rays with energy of 100–10,000 keV are considered. Absorption efficiency and scintillating response of PSF are also obtained, and the simulated results are consistent with the theoretic ones. From simulation, the main important factor of detection efficiency in the energy range is the absorption efficiency of the scintillating material. The cross talks in a fiber array are also discussed. These results could probably provide some ways of using PSF as a γ-ray imaging detector in certain applications.

Production of Multimeric Prostate-Specific Membrane Antigen Small-Molecule Radiotracers Using a Solid-Phase 99mTc Preloading Strategy

Small-molecule ligands specific for prostate-specific membrane antigen (PSMA) have the potential to improve prostate cancer imaging. However, highly charged ligands are difficult to label with 99mTc and to purify. In this study, we present an adamantane-trimerized small molecule that has nanomolar binding to PSMA and also has 12 negative charges. To convert this molecule into a clinically viable SPECT diagnostic, we have developed a simple, cartridge-based, solid-phase prelabeling strategy that, within 25 min, converts readily available and inexpensive 99mTc-pertechnetate into a chemically pure complex, with a reactive N-hydroxysuccinimide (NHS) ester, in neat organic solvent. This stable intermediate can label any amine-containing small molecule or peptide with 99mTc in 1 step, with high specific activity and without the need for high-performance liquid chromatography (HPLC). Solid-phase conversion of 99mTc-pertechnetate to 99mTc-MAS3-NHS (MAS3 is S-acetylmercaptoacetyltriserine) could be completed in 25 min, with >99% radiochemical purity and with no coligands present. This intermediate was then conjugated to adamantane-trimerized GPI (2[(3-amino-3-carboxypropyl)(hydroxy)(phosphinyl)-methyl]pentane-1,5-dioic acid) in 1 step with >95% yield and no need for HPLC purification. The final molecule bound specifically to living human tumor cells expressing PSMA on their surface. Quantitative comparison was made among GPI monomer, GPI trimer, and their 99mTc-derivatives. Our study describes a simple cartridge-based conversion of 99mTc-pertechnetate to a useful, preloaded NHS ester intermediate that takes only 25 min to prepare and results in >99% radiochemical purity. Using this chemistry, we produced a high-specific-activity, 99mTc-labeled, PSMA-targeted small molecule and demonstrate gamma-ray radioscintigraphic imaging of living human prostate cancer cells.

High-sensitivity Compton imaging with position-sensitive Si and Ge detectors

We report on the development of high-sensitivity and compact Compton imaging systems built of large and position-sensitive Si(Li) and HPGe detectors. The primary goal of this effort is to provide improved capabilities in the passive detection of nuclear materials for homeland security. Our detectors are implemented in double-sided strip configuration, which—along with digital signal processing—provides energies and three-dimensional position information of individual γ-ray interactions. γ-Ray tracking algorithms then determine the scattering sequence of the γ-ray, which in turn allows us—employing the Compton scattering formula—to reconstruct a cone of possible incident angles and ultimately an image. This Compton imaging concept enables large-field-of-view γ-ray imaging without the use of a heavy collimator or aperture. The intrinsically high-energy resolution of the detectors used, the excellent position resolution we have demonstrated, both combined with the high efficiency of large-volume detectors is the basis for high Compton imaging sensitivity. These capabilities are being developed to identify and localize potential threat sources and to potentially increase the sensitivity in detecting weak sources out of the midst of natural, medical, or commercial sources. γ-ray imaging provides a new degree of freedom to distinguish between spatial and temporal background fluctuations and compact threat sources.

A test device for isotopic γ-ray imaging with CdZnTe detector

A test device for isotopic γ-ray imaging, which consists of an isotope γ-ray source, a CdZnTe γ-ray spectrometer and other auxiliary equipment, is studied here. Compared with the conventional X-ray, the isotope y-ray, which is utilized in this project, has its own advantages in imaging. Furthermore, with a room-temperature high-energy-resolution CdZnTe detector and a modern imaging processing technique, this device is capable of effectively suppressing the background and gaining more information, thus it can obtain a better image than conventional X-ray devices. In the experiment of PCB imaging, all soldered points and chip components are sharply demonstrated.

Crystal Growth and Characterization of CdTe and Cd0.9Zn0.1Te for Nuclear Radiation Detectors

AbstractCdTe and Cd0.9Zn0.1Te (CZT) crystals have been studied extensively at EIC Laboratories, Inc. for various applications including x- and γ-ray imaging and high energy radiation detectors. The crystals were grown from in-house zone refined ultra pure precursor materials using a vertical Bridgman furnace. The growth process has been monitored, controlled and optimized by a computer simulation and modeling program (MASTRAPP). The grown crystals were thoroughly characterized after sequential surface passivations and post-growth annealing treatments with and without component overpressures. The infrared (IR) transmission images of the post-treated CdTe and CZT crystals showed average Te inclusion size of ∼10 μm for CdTe crystal and ∼8 μm for CZT crystal. The etch pit density was ≤ 5×104 cm−2 for CdTe and ≤ 3×104 cm−2 for CZT. Various planar and Frisch collar detectors were fabricated and evaluated. From the current-voltage measurements, the electrical resistivity was estimated to be ∼1.5×1010 Ω·cm for CdTe and 2-5×1011 Ω·cm for CZT. The Hecht analysis of electron and hole mobility-lifetime products (μτe and μτh) showed μτe=2×10−3 cm2/V (μτh=8×10−5 cm2/V) and μτ3-6×10−3 cm2/V (μτh=4-6×10−5 cm2/V) for CdTe and CZT, respectively. Final assessments of the detector performance have been carried out using 241Am (60 keV) and 137Cs (662 keV) energy sources and the results are presented in this paper.

Development of a Neutron Radiography Three-Dimensional Computed Tomography System for Void Fraction Measurement of Boiling Flow in Tight Lattice Rod Bundles

A neutron radiography three-dimensional computed tomography (NR3DCT) system was developed to visualize the void fraction distribution of boiling flow in tight lattice heated-rod bundles. This paper chiefly reports on the data processing and the error estimation method of NR3DCT. Practical γ-ray noise reduction and image correction techniques were studied to improve the reliability of the experimental data. Using the system and a directly heated 14-rod bundle test section, the behavior of boiling flow in a tight lattice rod bundle was clearly visualized. The effect of each data processing step on the result was also discussed. By this development, the three-dimensional vapor distribution of boiling flow in a heated bundle is made clear, and void fraction databases can be provided for verification of a thermal-hydraulic simulation code.

The electronics readout and the DAQ system of the DRAGO Anger Camera

The aim of the DRAGO project, supported by Italian INFN, is the development of a high-resolution, compact γ-ray imager, based on the Anger Camera principle. In this configuration, the light generated by a unique scintillator is read by an array of 77 Silicon Drift Detectors. In order to locate the position of interaction of the photon inside the scintillator, it is necessary to make an amplification and filtering of the detector signals followed by a processing of the acquired data. The electronics readout and processing system can be divided in two separate parts: the analog front end and the DAQ board. The analog front end is composed of 80 readout channels divided in 10 CMOS chips, produced in the 0.35 μm AMS technology, each one processing 8 channels. Each analog channel of the circuit includes a low-noise preamplifier, a sixth-order semigaussian shaping amplifier with four selectable peaking times from 1.8 μs up to 6 μs, a peak stretcher and a baseline holder. The energy resolution measured using a single channel of the chip with a Silicon Drift Detector Droplet (SD 3) is of 128 eV FWHM at 6 keV with the detector cooled at −20 °C. The 8 analog channels of the chip are multiplexed to a single analog output and fed to the acquisition system. For each γ event, this system performs the A/D conversion of all the signals of the array and sends them to a host PC, where the position reconstruction algorithm is executed. The DAQ board contains 10 ADCs, each one dedicated to a single ASIC of the analog section and having a resolution of 13 bit (ENOB). The burst conversion rate of the 10 ADCs together is 50 Ms/s resulting in a dead time of about 2 μs/event. The converted data are stored in a FIFO memory, for buffering, and then are transferred to the host PC via a USB 2.0 interface, which allows an event rate of more than 40k events/s for the whole Anger Camera, compatible with the application.

New trends in γ-ray imaging with CdZnTe/CdTe at CEA-Leti

Increasing interest in CdTe/CZT for γ-ray imaging has marked the last 10 years with progress in crystal growth, device technology, integrated electronics and signal processing. The purpose of this paper is to present recent advances in these four fields at CEA-LETI. A new method to grow very large dimension CZT up to 300 mm in diameter is presented. MS-contact process control allows design of very small electrodes (100 μm pitch) and results on long-term stability are presented. The development of low-noise front-end integrated electronics and new interconnects technologies have made “pixel detectors” feasible. Significant effort has been focused on reaching high-energy resolution as well as high detection efficiency. The combination of signal processing such as bi-parametric (BP) approaches and electrode design presents an interest in charge loss compensation for a large range of μτ products, tailing reduction due to geometric effect and depth of interaction information. New digital approaches may give drastic advantages to push the limit of spectrometric performance. In the field of γ-ray application, single-channel device for per-operative probes and small 2-D imager for molecular imaging are the most promising ones. By using a tailored “mixed” electrode (small anode and capacitive grid Frich effect) and a BP correction, an energy resolution of 1% at 662 keV has been demonstrated with CZT based 8×8×15 mm 3 detector. New results of 3-D CZT detectors for μPET imaging are presented with a measured coincidence time of 2.6 ns FWHM and an expected spatial resolution of 1 mm FWHM.

Silicon drift photodetectors for scintillation readout in medical imaging

In the attempt to improve system performances of position-sensitive γ-ray detectors with contemporary ease of use and compactness, the use of silicon photodetectors for scintillation readout has become of increasing interest. With respect to the long-established photomultiplier tubes (PMTs), silicon photodiodes (PDs) have the advantages of higher quantum efficiency (QE), smaller dimensions and lower biasing voltages. Avalanche photodiodes (APDs) combine the high QE and compactness of PDs with the benefit of a moderate avalanche multiplication gain, which reduced the electronics noise contribution. However, the statistical component is again affected by the statistics of the multiplication itself (noise factor), and the noise component is still appreciable mainly in the high energy range. Moreover, the sensitivity of the gain to temperature and biasing variations represents a potential practical drawback in the use of APDs arrays for the application. As an alternative to the mentioned photodetectors, silicon drift detectors (SDDs) have recently shown to achieve excellent performances in scintillation light detection. The SDD is a photodetector characterized by an area-independent, low-value output capacitance (∼100 fF), which is one of the major factors limiting the noise performances of the other silicon detectors’ technologies. SDDs used for CsI(Tl) scintillation readout have already shown to achieve state-of-the-art energy resolution in γ-ray spectroscopy, and monolithic arrays of SDDs have been recently experimented for the development of Anger cameras for high-resolution γ-ray imaging. In this work, is the performances achievable with SDDs in both energy and spatial resolution are discussed together with a review of the most important results achieved with first prototypes of SDD-based γ-ray detectors.