Coincidence Imaging Research Articles

An "inverse" harpoon mechanism.

Electron-transfer reactions are ubiquitous in chemistry and biology. The electrons’ quantum nature allows their transfer across long distances. For example, in the well-known harpoon mechanism, electron transfer results in Coulombic attraction between initially neutral reactants, leading to a marked increase in the reaction rate. Here, we present a different mechanism in which electron transfer from a neutral reactant to a multiply charged cation results in strong repulsion that encodes the electron-transfer distance in the kinetic energy release. Three-dimensional coincidence imaging allows to identify such “inverse” harpoon products, predicted by nonadiabatic molecular dynamics simulations to occur between H2 and HCOH2+ following double ionization of isolated methanol molecules. These dynamics are experimentally initiated by single-photon double ionization with ultrafast extreme ultraviolet pulses, produced by high-order harmonic generation. A detailed comparison of measured and simulated data indicates that while the relative probability of long-range electron-transfer events is correctly predicted, theory overestimates the electron-transfer distance.

Sequential and concerted C-C and C-O bond dissociation in the Coulomb explosion of 2-propanol.

We study the competing mechanisms involved in the Coulomb explosion of 2-propanol CH3 2CHOH2+ dication, formed by an ultrafast extreme ultraviolet pulse. Over 20 product channels are identified and characterized using 3D coincidence imaging of the ionic fragments. The momentum correlations in the three-body fragmentation channels provide evidence for a dominant sequential mechanism, starting with the cleavage of a C-C bond, ejecting CH3 + and CH3CHOH+ cations, followed by a secondary fragmentation of the hydroxyethyl cation that can be delayed for up to a microsecond after ionization. The C-O bond dissociation channels are less frequent, involving proton transfer and double proton transfer, forming H2O+ and H3O+ products, respectively, and exhibiting mixed sequential and concerted character. These results can be explained by the high potential barrier for the C-O bond dissociation seen in our ab initio quantum chemical calculations. We also observe coincident COH+ + C2Hn + ions, suggesting exotic structural rearrangements, starting from the Frank-Condon geometry of the neutral 2-propanol system. Remarkably, the relative yield of the H3 + product is suppressed compared with methanol and alkene dications. Ab initio potentials and ground state molecular dynamics simulations show that a rapid and direct C-C bond cleavage dominates the Coulomb explosion process, leaving no time for H2 roaming, which is a necessary precursor to the H3 + formation.

Compton imaging for medical applications

Compton imaging exploits inelastic scattering, known as Compton scattering, using a Compton camera consisting of a scatterer detector in the front layer and an absorber detector in the back layer. This method was developed for astronomy, and in recent years, research and development for environmental and medical applications has been actively conducted. Compton imaging can discriminate gamma rays over a wide energy range from several hundred keV to several MeV. Therefore, it is expected to be applied to the simultaneous imaging of multiple nuclides in nuclear medicine and prompt gamma ray imaging for range verification in particle therapy. In addition, multiple gamma coincidence imaging is expected to be realized, which allows the source position to be determined from a single coincidence event using nuclides that emit multiple gamma rays simultaneously, such as nuclides that emit a single gamma ray simultaneously with positron decay. This review introduces various efforts toward the practical application of Compton imaging in the medical field, including in vivo studies, and discusses its prospects.

Dynamic imaging comparison of 18F-FDG tracers with Compton imaging and PET coincidence imaging

In the clinical situation of positron emission tomography (PET) scans, an activity of an approximately few hundred megabecquerels (3.7 MBq/kg) is used for injection. Monitoring and imaging of moving radioisotopes is useful in clinical applications, such as tracer injection and leakage monitoring in PET scan protocols. We have developed a Compton imaging and PET coincidence system to monitor a moving radioisotope using 8 × 8 GAGG crystal arrays coupled to SiPM arrays with dynamic time-over-threshold-based individual readout circuits, and its imaging performance is considered. The measured resolution of PET and Compton imaging is 3.2 mm and approximately 17 degrees for a 22Na point source. In the experiment, radiotracers with activities from 11.2 MBq to 93.3 MBq moving with speeds from 1 mm/s to 10 mm/s were used for mimicking the blood flow. Both reconstructed images of PET and Compton imaging successfully visualized the movement, except for Compton imaging in the 93.3 MBq case. PET shows better activity-tracking capability and radio-tracer speeds up to 100 MBq. In contrast, Compton imaging has a wider field of view (FOV) to monitor a larger area than the limited FOV in a PET system. We believe that this work can contribute to solutions of various medical problems such as blood flow measurement and extravasation detection.

A new endstation for extreme-ultraviolet spectroscopy of free clusters and nanodroplets.

In this work, we present a new endstation for the AMOLine of the ASTRID2 synchrotron at Aarhus University, which combines a cluster and nanodroplet beam source with a velocity map imaging and time-of-flight spectrometer for coincidence imaging spectroscopy. Extreme-ultraviolet spectroscopy of free nanoparticles is a powerful tool for studying the photophysics and photochemistry of resonantly excited or ionized nanometer-sized condensed-phase systems. Here, we demonstrate this capability by performing photoelectron-photoion coincidence experiments with pure and doped superfluid helium nanodroplets. Different doping options and beam sources provide a versatile platform to generate various van der Waals clusters as well as He nanodroplets. We present a detailed characterization of the new setup and show examples of its use for measuring high-resolution yield spectra of charged particles, time-of-flight ion mass spectra, anion-cation coincidence spectra, multi-coincidence electron spectra, and angular distributions. A particular focus of the research with this new endstation is on intermolecular charge and energy-transfer processes in heterogeneous nanosystems induced by valence-shell excitation and ionization.

Microvibration Modes Reconstruction Based on Micro-Doppler Coincidence Imaging

Micro-vibration, a ubiquitous nature phenomenon, can be seen as a characteristic feature on the objects, these vibrations always have tiny amplitudes which are much less than the wavelengths of the sensing systems, thus these motions information can only be reflected in the phase item of echo. Normally the conventional radar system can detect these micro vibrations through the time frequency analyzing, but these vibration characteristics can only be reflected by time-frequency spectrum, the spatial distribution of these micro vibrations can not be reconstructed precisely. Ghost imaging (GI), a novel imaging method also known as Coincidence Imaging that originated in the quantum and optical fields, can reconstruct unknown images using computational methods. To reconstruct the spatial distribution of micro vibrations, this paper proposes a new method based on a coincidence imaging system. A detailed model of target micro-vibration is created first, taking into account two categories: discrete and continuous targets. We use the first-order field correlation feature to obtain objective different micro vibration distribution based on the complex target models and time-frequency analysis in this work.

Role of PET-CT (Positron Emission Tomography-Computed Tomography) in Cancer Evaluation and Treatment

Context: Positron emission tomography is a nuclear medicine imaging that deals with physiological function using radioisotopes. With the most PET (Positron Emission Tomography) scanners in integration with the CT scanners of late, this technology has registered phenomenal growth. The small amount of radioactive material is called Radiotracers. Objective: Like 18F- Fluro-deoxy-2-glucose has widely used. In this article, the author introduced clinical applications of PET out of 25 patients who studied hypermetabolic lesions in lymph nodes. Methods: PET imaging is coincidence imaging which is different from the other imaging technique PET image formed from multiple rings of detector crystals. Each decay positron travel in tissue annihilation reaction is going on. FDG is the most commonly used radiotracer to detect and stage various types of malignancies. Result: The field of PET/CT imaging cares for many oncology patients. PET improved localization of malignant lesions. It improved staging biopsy and therapy. Conclusion: Finally, studies to data showed 4% to 10% improvement in the overall accuracy of staging/restaging in lesions. If we use Monte Carlo simulation, OLINDA/EXM software may improve further with widely used.

Microwave Coincidence Imaging Based on Attributed Scattering Model

In this letter, a novel microwave coincidence imaging (MCI) method based on the attributed scattering model (ASM) is proposed. Unlike the classical MCI model which assumes the target as a set of discrete scatterers, the ASM-based MCI equation contains three kinds of reference matrices corresponding to the point-scatterers (PSs), the line-segment-scatterers (LSSs) and the rectangular-plate-scatterers (RPSs), respectively. Hence the ASM-based MCI could resolve richer information about the object geometries. By solving the imaging equation via the alternating direction method of multipliers (ADMM) algorithm, the scattering coefficients will be obtained and the target can be reconstructed according to the presetting parameter sets. Meantime, the ASM-based MCI also earns the superresolution ability like the classical MCI, which is brought in by the temporal-spatial orthogonal radiation field. Simulations and experiment are carried out to demonstrate the performance and superresolution ability of proposed method. The ASM-based MCI makes contributions to the progress of radar forward-looking imaging theory and technology.

Reweighted-Dynamic-Grid-Based Microwave Coincidence Imaging With Grid Mismatch

Microwave coincidence imaging (MCI) is a novel staring imaging technique with high resolution in azimuth. In MCI, the continuous imaging area is discretized into fine grids and the target-scattering centers are assumed to be exactly located at the centers of prediscretized grids. Recently, parametric methods are applied to MCI as target reconstruction algorithms with resolution enhancement and quality improvement. However, in practical applications, grid mismatch will severely degrade the imaging quality of parametric methods because the target-scattering centers will not totally locate at the grid centers no matter how fine the grids are. In this article, a reweighted-dynamic-grid-based MCI (RDG-MCI) method is proposed. In RDG-MCI, grids are evolving from coarse to dense iteratively rather than being fixed, and hence, off-grid errors can be eliminated gradually. Meanwhile, the reconstructed coefficients are used as weighting factors of grids in a form of weighting matrix in the next iteration and nonkey grids will be dropped out. Hence, the dynamic grids will be focused around the positions where target scatterers are most likely to exist. Furthermore, the matrix uncertain sparse Bayesian learning (MUSBL) algorithm is adopted to eliminate the residual off-grid errors. Finally, a preferable imaging result can be obtained based on the updated nonuniform grids. Also, the theoretical expected Cramér–Rao bound (ECRB) is also derived to evaluate the performance of the proposed method. The effectiveness of the proposed method, along with the super-resolution ability of MCI, is verified by simulations and outdoor experiments.

Coherent-Detecting and Incoherent-Modulating Microwave Coincidence Imaging With Off-Grid Errors

In this letter, a novel microwave coincidence imaging (MCI) approach is proposed based on a multiple-input single-output (MISO) radar system to deal with the low signal-to-noise ratio (SNR) scenarios and off-grid problem. First, in coherent-detecting part, a linear frequency modulated (LFM) signal is transmitted, and dechirping processes are conducted to enhance the SNR of echoes. Then, in incoherent-modulating part, the post random phase-shifting modulations are conducted on the echoes, hence the temporal–spatial orthogonal reference radiation field of MCI is constructed, which provides the potential information of super-resolution. Further, to solve the off-grid problem of target’s scatterers in MCI, a new projecting-residual-based selection criterion is also proposed, combined with the preexisting signal subspace matching (SSM) method. The proposed method could largely eliminate the off-grid errors while conduct a reference matrix selection procedure, hence the reconstruction accuracy and computational complexity can be much improved and reduced, respectively. Finally, the validity of the proposed method and the super-resolution ability of MCI are verified by experiments.

Experiment and theory elucidate the pathways for H3+formation in the ultrafast double ionization in methanol

Experiment and theory elucidate the pathways for H₃⁺formation in the ultrafast double ionization in methanol

Double-Photon Emission Imaging With High-Resolution Si/CdTe Compton Cameras

Application of Compton cameras to nuclear medicine or small animal imaging is a challenging subject. A major issue is insufficient spatial resolutions of the existing Compton camera systems. We developed an imaging system consisting of two Si/CdTe Compton cameras, which allows us to investigate γ- γ coincidence imaging for a variety of radioisotopes, including <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">111</sup> In, which emits double photons of 171 and 245 keV per decay and has been used for a long time in nuclear medicine and many commercialized radiotracers. We demonstrated with it a drastic reduction of a tail of the point spread function, compared with that of the traditional method, and obtained a spatial resolution of 4.5 mm full-width at half-maximum (FWHM) at 41.35 mm.

2光子放出核種のためのコリメータ搭載型コインシデンスイメージング

2光子放出核種のためのコリメータ搭載型コインシデンスイメージング

W-band frequency-polarization-port-diverse cavity imager with bunching random beams

A frequency-polarization-port-diverse (FPPD) cavity imager with bunching random beams working in the W-band (from 76 GHz to 81 GHz) is proposed in this paper. A random-coherent-superposition design method is utilized to design the bunching random beam. Low-correlated radiation patterns can be generated when excited either at different frequencies or by different ports. Orthogonal polarized radiation patterns also vary from each other, which can enlarge measurement modes from the polarization dimension. The FPPD cavity imager is composed of a disordered cavity, a lid etched with elliptical holes and a dual-polarized horn antenna. Frequency-port-diverse electric field distributions of the disordered cavity are achieved by loading irregular structures and FPPD radiation patterns are realized by etching elliptical holes with various minor axis lengths and rotation angels. A dual-polarized horn antenna is used to generate the bunching radiation patterns, on which we superpose the random radiation patterns. Definitions of the bunching angle and the correlation coefficient are also given to evaluate the performance of the FPPD cavity imager. Finally, a coincidence imaging experiment is used to investigate the FPPD cavity imager under different conditions and the target image has been reconstructed successfully. Advantages of the bunching characteristic are also proved by comparative experiments using the bunching radiation patterns and the normal radiation patterns. High-quality reconstruction of the target image demonstrates the feasibility of the multi-dimension joint (i.e. frequency, polarization and port) design method. The proposed method is verified by simulations and measurements.

Frequency–Polarization Sensitive Metasurface Antenna for Coincidence Imaging

A frequency-polarization sensitive metasurface antenna (FPSMA) that can generate low-correlated measurement modes both at different working frequencies and under different polarization statuses is proposed in this letter. The FPSMA is composed of a linearly polarized patch antenna as the excitation module and a frequency-polarization sensitive metasurface as the modulation module. The capacity of low-correlated measurement modes is enlarged significantly due to introducing the polarization domain. The polarization domain refers to the angle between the polarization direction of the patch antenna and the metasurface. Performances of the FPSMA, including the reflection coefficient, the radiation efficiency, and the correlation coefficients, are evaluated. A coincidence imaging experiment is also carried out using the FPSMA and the target image is reconstructed under different conditions. The working frequency of the FPSMA is from 32 to 36 GHz according to our research foundations. The design is verified by both simulated and measured results.

Cesium Rydberg-state ionization study by three-dimensional ion-electron correlation: Toward a monochromatic electron source

We study the excitation and ionization of cesium Rydberg states in an electric field ($\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}2200\phantom{\rule{4pt}{0ex}}\mathrm{V}/\mathrm{cm}$) near the classical field-ionization threshold ($\ensuremath{\approx}\ensuremath{-}280\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ binding energy) by three-dimensional (3D) ion-electron coincidence spectroscopy. Cesium atoms are produced by an effusive oven and excited with lasers to Stark-shifted Rydberg states or directly ionized. Using a double time-of-flight setup we record 3D ($X, Y$, time of flight) coincidence imaging of electrons and ions. Above-threshold photoionization creates broad images with poor electron-ion spatial correlation. Fast ionizing states produce very good correlations and the images reveal the electric-field map of the ionization region. Slow ionizing states show that the relatively high atomic velocity is detrimental to the correlations. Experimental data are accurately reproduced by detailed Monte Carlo excitation and ionization simulations based on Stark maps obtained with local-frame transformation (LFT) theory. Agreement between our spectroscopic experiment and LFT theory is very good, with better that hundreds of megahertz accuracy. But, on rare particular states, several gigahertz discrepancy is found. This study can be used to select appropriate states for the creation of ion and electron beams with high brightness, good correlation, and low energy dispersion.

Frequency-Diverse Metamaterial Cavity Antenna for Microwave Coincidence Imaging

A frequency-diverse metamaterial cavity antenna (FDMCA) that can be used for the microwave coincidence imaging is proposed in this letter. The proposed FDMCA is a slot antenna array excited by a metamaterial-based cavity, which can generate diverse radiation patterns at different frequencies. The average electric-field variation (AEV) is introduced to forecast the correlation coefficients of radiation patterns generated by the antenna. The lower the correlation coefficients are, the better the quality of the reconstructed image is. To verify the validity of the AEV and the superiority of the FDMCA, a disordered-structure-based cavity antenna (DSCA) and a lumpy-floor-based cavity antenna (LFCA) are also presented for comparison. Performances of the FDMCA, including the reflection coefficient, the radiation efficiency, and the correlation coefficients, are evaluated too. Microwave coincidence imaging experiments using the DSCA, LFCA, and FDMCA are carried out under the same condition and the target image can be reconstructed with high quality. The comparison of different imaging results illustrates the feasibility and superiority of the FDMCA. All results are validated through the simulations and measurements.

Ro-vibrational Distribution of NO+ Dissociated from NO2+ Ions in the a3B2 and b3A2 States: A Slow "Impulsive" Dissociation Example Revealed from Threshold Photoelectron-Photoion Coincidence Imaging.

To clarify the contentions about dissociative photoionization mechanism of nitrogen dioxide via the a3B2 and b3A2 ionic states, a new threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging has been conducted in the 12.8-14.0 eV energy range at the Hefei Light Source. The fine vibrational-resolved threshold photoelectron spectrum agrees well with the previous measurements. The ro-vibrational distributions of NO+, as the unique fragment ion in the dissociation of NO2+ in specific vibronic levels of a3B2 and b3A2 states, are derived from the recorded TPEPICO velocity images. A "cold" vibrational (v+ = 0) and "hot" rotational population is observed at the a3B2(0,3,0) and (0,4,0) vibronic levels, while the dissociation of NO2+ in b3A2(0,0,0) leads to the NO+ fragment with both hot vibrational and rotational populations. With the aid of the quantum chemical calculations at the time-dependent B3LYP level, minimum energy paths on the potential energy surfaces of the a3B2 and b3A2 states clarify their adiabatic dissociation mechanisms near the thresholds, and this study proposes reliable explanations for the observed internal energy distributions of fragment ions. Additionally, this study provides valuable insights into the application of the classical "impulsive" model on an overall slow dissociation process.

3-D Printing Disordered-Cavity-Based Metaimager for Coincidence Imaging

A disordered-cavity-based metaimager (DCM) that can be used for coincidence imaging is proposed in this letter. The proposed DCM possesses narrow frequency gap, low correlation coefficients, and high efficiency. Due to the all-in-one design, the DCM has good robustness and is insensitive to fabrication errors, which is suitable to be fabricated using the 3-D printing technique. The DCM working from 32 to 36 GHz contains a disordered cavity, sets of high-efficiency coupling slots and a frequency-diverse metasurface. Measurement modes generated by the DCM are low correlated due to frequency-diverse field distributions of the disordered cavity and distinguished transmission features of metamaterial elements composing the metasurface. Performances of the DCM including the reflection coefficients, the efficiency, and the correlation coefficients are evaluated. Finally, a coincidence imaging experiment is also carried out using the proposed DCM and the image of the target is reconstructed under different signal-to-noise ratio (SNR) conditions. The design is validated by both the simulated and measured results.

Ion-neutral photofragment coincidence imaging of photodissociation dynamics of ionic species

The recently constructed cryogenic cylindrical ion trap velocity map imaging spectrometer (CIT-VMI) has been upgraded for coincidence imaging of both ionic and neutral photofragments from photodissociation of ionic species. The prepared ions are cooled down in a home-made cryogenic cylindrical ion trap and then extracted for photodissociation experiments. With the newly designed electric fields for extraction and acceleration, the ion beam can be accelerated to more than 4500 eV, which is necessary for velocity imaging of the neutral photofragments by using the position-sensitive imaging detector. The setup has been tested by the 355 nm photodissociation dynamics of the argon dimer cation (Ar2+). From the recorded experimental images of both neutral Ar and ionic Ar+ fragments, we interpret velocity resolutions of Δv/v≈4.6% for neutral fragments, and Δv/v≈1.5% for ionic fragments, respectively.