Evaluation of monolithic crystal detector with dual-ended readout utilizing multiplexing method.

Objective.Monolithic crystal detectors are increasingly being applied in positron emission tomography (PET) devices owing to their excellent depth-of-interaction (DOI) resolution capabilities and high detection efficiency. In this study, we constructed and evaluated a dual-ended readout monolithic crystal detector based on a multiplexing method.Approach.We employed two 12 × 12 silicon photomultiplier (SiPM) arrays for readout, and the signals from the 12 × 12 array were merged into 12 X and 12 Y channels using channel multiplexing. In 2D reconstruction, three methods based on the centre of gravity (COG) were compared, and the concept of thresholds was introduced. Furthermore, a light convolutional neural network (CNN) was employed for testing. To enhance depth localization resolution, we proposed a method by utilizing the mutual information from both ends of the SiPMs. The source width and collimation effect were simulated using GEANT4, and the intrinsic spatial resolution was separated from the measured values.Main results.At an operational voltage of 29 V for the SiPM, an energy resolution of approximately 12.5 % was achieved. By subtracting a 0.8 % threshold from the total energy in every channel, a 2D spatial resolution of approximately 0.90 mm full width at half maximum (FWHM) can be obtained. Furthermore, a higher level of resolution, approximately 0.80 mm FWHM, was achieved using a CNN, with some alleviation of edge effects. With the proposed DOI method, a significant 1.36 mm FWHM average DOI resolution can be achieved. Additionally, it was found that polishing and black coating on the crystal surface yielded smaller edge effects compared to a rough surface with a black coating.Significance.The introduction of a threshold in COG method and a dual-ended readout scheme can lead to excellent spatial resolution for monolithic crystal detectors, which can help to develop PET systems with both high sensitivity and high spatial resolution.

A hybrid coded-aperture and Compton camera based on cerium-doped Gd3Al2Ga3O12 scintillators coupled with multi-pixel photon counter arrays.

In this study, we propose a hybrid coded-aperture and Compton camera based on cerium-doped Gd3Al2Ga3O12 (GAGG:Ce) scintillator arrays coupled with Multi-Pixel Photon Counter (MPPC) arrays. The sensitive detector of the gamma camera consists of a single GAGG:Ce crystal coupled with a single-chip MPPC unit module. An impedance bridge circuit and a 64-channel data acquisition system were employed to record the code-aperture events and Compton coincidence events. After the calibration of position and energy, the total energy resolution for 662 keV gamma-rays from 137Cs was 6.6%. The hybrid camera had the characteristics of mechanical collimation and electronic collimation at the same time. In the code aperture mode, the reconstructed images were obtained by direct deconvolution and maximum likelihood expectation maximization (MLEM) methods. In the Compton imaging mode, the energy-dependent method was applied to order the sequence of Compton scatter events. The simple back-projection algorithm and list-mode MLEM algorithm were adopted for image reconstruction. Practical performances demonstrated that the angular resolutions in two modes were measured as 5.2° and 11.4°, respectively. In addition, the hybrid camera had a desirable imaging capability in a wide energy range (32 keV-2.6 MeV) and a wide field of view (∼210° in the horizontal direction). As for the sensitivity, the camera had a commercially available sensitivity level of localizing a 137Cs point source, producing ∼0.026 µSv/h in 5 min. Furthermore, the function of distinction for different radiation sources was preliminarily realized.

DESIGN, CONSTRUCTION AND TEST OF GAMMA IONIZATION CHAMBERS.

Ionization chamber is considered as the golden standard for the dosemeter. This work fabricated one graphite-walled and two polymethyl methacrylate (PMMA)-walled ionization chambers, tested respectively and compared their characteristic parameters. The performance of graphite-walled chamber was similar to that of the PMMA-walled chamber. The relative energy response of ionization chambers was lower than 4% from 50 keV to 50 MeV based on the FLUKA code simulation results. The experimental data showed that the ionization chambers performed well in linearity and repeatability. Assuming the chambers sensitive volume and HV bias was constant, the measured maximum saturation absorbed dose of ionization chambers changed with the diameter variation of chambers anode based on the comparison results of two PMMA-walled ionization chambers.

High-performance coded aperture gamma camera based on monolithic GAGG:Ce crystal.

A 50 × 50 × 10 mm3 monolithic gadolinium aluminum gallium garnet (Gd3Al2Ga3O12; GAGG):Ce crystal coupled to a 8 × 8 silicon photomultiplier (SiPM) array was developed; it showed very good system uniformity and a high energy resolution of 7.4% at 662 keV. By using a convolutional neural network-based positioning algorithm and a fan-beam calibration method, the detector achieved a position resolution of ∼1.4 mm and a depth of interaction resolution of ∼2 mm. Based on this high-performance monolithic detector, we developed a coded aperture gamma camera. A 1-mCi Cs-137 source centered at a 2-m distance from the mask could be reconstructed with a signal-to-noise ratio of 6.5 in 1 s. Furthermore, the imaging ability of a low-energy Am-241 source and a low-activity Cs-137 source when the background-to-signal ratio was approximately 1:1 and a double low-activity source (Cs-137 and Na-22) was demonstrated. It is shown that the monolithic-crystal-based coded aperture gamma camera can achieve high performance and has a large potential for further improvement.

A scintillating plastic fiber array and multiplexer based 384-channel fast neutron spectrometer.

A fast neutron detection system based on a scintillating plastic fiber array and multiplexer was designed to measure the spectrum of fast neutrons ranged 10 MeV-100 MeV. With the method of nuclear recoil, the energy of incident neutron was determined by measuring the recoil proton track and deposited energy in scintillating plastic fibers. The detection system was composed of a scintillating plastic fiber array, 6 position sensitive photomultiplier tubes, and a high-density readout electronics based on the multiplexer. The scintillating plastic fiber array was made as a staggered structure with two kinds of fibers in different sizes (0.5 mm-square fiber and 3 mm-square fiber). The structure provided a wider detection energy range and better detection efficiency than arrays made with uniform plastic fibers. A dedicated digital electronics system was well designed to control the whole readout system to provide 384-channel signal processing. The detector had a 48 mm × 48 mm effective detection area and a mechanical size of 34 cm × 34 cm × 27 cm. In the simulation of the detector model performance, the system gave an energy resolution of 23%-35% for neutrons ranged 10 MeV-100 MeV. Experimental results showed that the detector had a good energy linearity and energy resolutions were, respectively, 35.82% at 14.817 MeV, 36.84% at 21.264 MeV, 35.90% at 23.069 MeV, and 32.90% at 24.220 MeV. The optimized prototype model had potential in increasing fast neutron detection performance.

Depth discrimination method based on a multirow linear array detector for push-broom Compton scatter imaging.

A depth discrimination method is devised based on a multirow linear array detector for push-broom Compton scatter imaging. Two or more rows of detector modules are placed at different positions towards a sample. An improved parallel-hole collimator is fixed in front of the modules to restrict their fields of view. The depth information could be indicated by comparing the signal differences. In addition, an available detector and several related simulations using GEANT4 are given to support the method well.

Developing VUV spectroscopy for protein folding and material luminescence on beamline 4B8 at the Beijing Synchrotron Radiation Facility.

The new 4B8 beamline provides UV-VUV light in the wavelength range from 360 to 120 nm. It uniquely enables two kinds of spectroscopy measurements: synchrotron radiation circular dichroism spectroscopy and VUV excited fluorescence spectroscopy. The former is mainly used in protein secondary structure studies, and the latter in VUV excited luminescent materials research. Remote access to fluorescence measurement has been realised and users can collect data online. Besides steady-state measurements, fluorescence lifetime measurements have been established using the time domain method, while a laser-induced temperature jump is under development for protein folding dynamics using circular dichroism as a probe.