Large-aperture Detector Research Articles

An Image Post-Processing Approach Based on Fully Dense U-Net for Microwave Induced Thermo-Acoustic Tomography

In microwave induced thermo-acoustic tomography (TAT), circular scanning sensor array is the general signal acquiring mode. For higher sensitivity, large aperture detectors are widely preferred in the real TAT imaging system. However, large aperture of detector usually results in the limited spatial sampling points. Meanwhile, for considering the safety and comfort, the microwave radiation power is also limited. Due to these limitations, the signal to noise ratio (SNR) of TA signals may not be ideal to reconstruct an image with satisfactory quality. In addition, the TA image often suffers the artifacts resulted by sparsely sampling data. Herein, we propose a deep learning-based scheme to process the TA images which are reconstructed with time reversal (TR) method. Since the obvious artifacts removing performance in photoacoustic tomography (PAT), fully dense U-Net (FD U-Net) is employed to be the base deep learning architecture. Due to the amount limitation of real experimental TA data, the network is mainly trained by synthetic data which are generated in TAT simulating scheme. Furthermore, in order to ensuring the trained network could correctly detect the targets in the TA image, we propose a scheme termed data augmentation FD U-Net (DAFD U-Net) in which a few experimental TA images are added to the training procedure and perform data augmentation.The performance of DAFD U-Net is verified by an experimental image. Results show that the proposed scheme could significantly improve the quality of TAT image.

Reconstruction Algorithm for Regions of Interest in γ-Photon Images

As a nondestructive testing technology, γ-photon imaging shows immense potential in the industrial field. However, the limitations of γ-photon imaging theory and detection technology result in various problems, such as low image resolution and edge blur. The technology is particularly difficult to apply to industrial detection that requires high imaging speed and high resolution. Therefore, this study proposes a reconstruction algorithm for regions of interest (ROI) in γ-photon images. The proposed algorithm is suitable for fast industrial detection and is based on the reconstruction algorithm for sinusoidal graph data, that is, the ordered subset expectation maximization (OSEM) image reconstruction algorithm. It is an improvement of the traditional point-and-line system matrix (SM) model. In the application of the proposed algorithm, the probability weight of a pixel is determined by the solid angle of the crystal bar at both ends of the line of response (LOR) to the pixel it passes through. In this work, the known contour parameters of industrial parts are used to describe the area of nuclide distribution as the ROI. Only the pixels through which the LOR passed in the ROI are counted, and the probability weights of these pixels are calculated to construct the SM. Gaussian filters are added in each iteration to suppress the clutter of scattered noise inside the image. The effectiveness of the algorithm was verified in two model experiments. A closed cavity detection experiment on industrial hydraulic parts was also conducted to compare the image reconstruction effects before and after the improvement. Results showed that the proposed algorithm can effectively improve image resolution and image edge contours. In the tee pipe model experiment and cavity detection experiment on hydraulic parts, the image reconstruction speed increased by more than 6 and 10 times, respectively. Hence, the proposed algorithm provided a feasible solution for quickly obtaining images with clear edges and high resolution under a large aperture detector ring.

Interdisciplinary science with large aperture detectors

Large aperture detector systems to measure high energy cosmic rays also offer unique opportunities in other areas of science. Disciplines include geophysics such as seismic and volcanic activity, and atmospheric science ranging from clouds to lightning to aerosols to optical transients. This paper will discuss potential opportunities based on the ongoing experience of the Pierre Auger Observatory.

Cosmic rays above [formula omitted] eV

We briefly review the status of cosmic ray studies between 10 14 eV and the highest observed energies, namely a few times 10 20 eV. Because of the rather low incident fluxes in this energy range, the studies rely solely on ground based, large aperture detectors reconstructing the cosmic ray properties through the detection of the air-showers they generate by interacting with the atmosphere. We stress the fact that many issues such as the chemical composition of the cosmic rays, their acceleration mechanisms and the structures displayed in their energy spectrum are mostly open questions which may be answered by the next generation of experiments.

Magnetic shadowing of charged particles by an extended surface

The problem of particle shadowing by the lunar surface is examined. Shadow zones are obtained analytically for small-aperture detectors on an infinite flat surface and are shown to be in good agreement with those obtained by numerical trajectory tracing. The shadow zones cover about half of all possible external field orientations for a vertical detector, which may result in serious underestimations of the frequency of observation of a particular plasma region. Shadowing may cause wide variations between observed and actual pitch angle distributions even in the case of multiple- or large-aperture detectors. Since the particle flux can be considered approximately isotropic only for magnetic fields within 35 deg of the surface normal, the electron flux to the surface may be overestimated in current balance calculations ignoring the magnetic field direction. In addition, photoelectrons may be returned to the surface by the magnetic field.

A large aperture detector for slow positive ions

A detector is described in which slow positive ions are detected with uniform efficiency over an aperture 4 cm in diameter. It uses a single stage box and grid type of electron multiplier, the secondary electrons from which are detected with a scintillation counter. The background counting rate above a pulse amplitude at which 99% of ion pulses are registered is 0.3 counts per sec. The energy dependence of counting efficiency is also investigated.