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Doppler broadening of positron annihilation radiation (DBAR) spectroscopy is a critical technique to provide electronic momentum information of materials. For conventional non-beam-based DBAR experiments, due to the continuous distribution of initial kinetic energy (0-545 keV) of positrons from 22Na source, two pieces of identical samples with sufficient thickness (> 1 mm for each sample with a density of 1 g/cm3) are required to stop almost all positrons in the samples. However, in many film studies, it is difficult or even impossible to meet this sample thickness requirement. In this work, judging by the output signals of a specially designed anticoincidence detector, the annihilation of positrons in a film sample and positron source are selectively collected. Furthermore, a new method is proposed to correct high-proportioned source components. Based on this, for the first time, a non-beam-based DBAR spectrometer was developed for a single piece of micron-thickness film. This approach will drastically broaden the application of DBAR spectroscopy in film characterization in future. [ABSTRACT FROM AUTHOR]

Cosmic muon imaging (muography) has gained popularity in recent years for density reconstruction in various applications. However, evaluations of image quality in reconstructed images have been lacking. This work addresses this gap by applying muon transmission imaging to a teaching building on a university campus. Multiple impacts on reconstruction image quality, including filtering methods, total counts, building thickness thresholds, and reconstruction formulas, were integrally studied. The area under the curve (AUC) of the receiver operating characteristic analysis and mean squared error (MSE) were calculated to quantitatively evaluate and optimize these parameters. Based on the simulation using the Geant4 toolkit, the recommended parameters for the building model were median filtering, a total count of 5 × 1 0 4 in the building region, a thickness threshold of 3 m, and either the constant energy loss method or cubic polynomial form fitting method. Implementing these parameters in the experimental data analysis, the reconstructed image well revealed the inner structure of the teaching building from the view angle of the detector. The optimization of the two-dimensional (2D) reconstruction method facilitates the detection of hidden cavities or regions with abnormal density in large-scale objects. [ABSTRACT FROM AUTHOR]

To precisely characterize the defects in crystalline solids including semiconductors and alloys, it is critical but challenging to improve the time resolution of positron annihilation lifetime (PAL) spectrometers. The effects of geometry and wrapping of scintillators on the time resolution of the PAL spectrometer were studied in detail. Geant4 program simulation was used to clarify the reason why cone-frustum-shaped scintillators have better time resolution than cylinder-shaped scintillators. A figure of merit was proposed to comprehensively evaluate the impacts of the transport and absorption of fluorescent photons on the time resolution of scintillators. Additionally, for the simulations of cone-frustum-shaped scintillators, polished reflective coatings can further improve the time resolution without losing count rate. Eventually, using three cone-frustum-shaped Ba F 2 scintillators and a double-stop setup, we significantly improved the time resolution of PAL spectrometer to a new record of 88.7 ps, the best time resolution up to date. This enables us to distinguish various vacancy types in crystalline solids more precisely in future. • Explained why cone-frustum-shaped scintillators have better time resolution. • Proposed a figure of merit to evaluate the time resolution of PAL spectrometer. • Achieved the best time resolution up to date for the PAL spectrometer. [ABSTRACT FROM AUTHOR]