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The corrosion behavior of 310S austenitic stainless steel, subjected to different surface treatments machined (MA), sandblasting (SB), and polishing (PO), was exposed to a 550°C supercritical water (SCW) environment. The aged samples were analyzed using variable‐energy slow positron beam techniques. The obtained results revealed that the plastic deformation of the near‐surface region of the MA and SB samples was substantially recovered in the SCW conditions. At least two distinct oxide layers formed, and the oxidation process created a Fe/Cr depletion zone in the inner layer. Various surface treatments, however, led to different corrosion profiles. The depth profile of slow positron beam characterization suggests that significant residual stress and deformation zones on the surfaces of the sandblasted samples probably provided a diffusion path for the oxidation of the 310S. Only the SB samples exhibited a negative weight change after SCW exposure. At the same time, the SB samples showed the highest concentration of the positron traps in this region, which was explained by open‐volume defects associated with the microcracks introduced by sandblasting. [ABSTRACT FROM AUTHOR]

We report measurements of a terrestrial gamma ray flash (TGF) detected by the Fermi Gamma‐ray Burst Monitor that was produced during a negative cloud‐to‐ground (CG) lightning leader. This is the first report of a downward directed TGF occurring during a CG flash but detected by a space‐based instrument. The gamma ray photons are produced 3 ms preceding a return stroke (−146 kA) and are essentially simultaneous with an isolated low frequency radio pulse. Based on timing, the pulse is estimated to initiate at approximately 6 km altitude, and its polarity indicates downward moving negative charge, the opposite of regular satellite‐detected upward TGFs. A likely scenario is that the runaway electrons accelerate into the upper, positively charged end of the leader in a high field region, with the reverse positron beam generating upward gamma rays detectable from space. A search for similar waveform features indicates that this type of downward CG‐TGF may occur prior to 1% of high peak current CG strokes. Extrapolating gives a global rate of 5–10% of previously known TGFs and potentially a significant fraction of global TGFs. Key Points: A TGF detected by Fermi GBM occurred 3 ms before a −CG stroke during the descending negative leader, coincident with a 120 μs radio pulseThe radio pulse polarity indicates that it is likely a downward TGF produced at ~6 km altitude with a reverse positron beam detected by Fermi GBMThis type of downward CG‐TGF may occur prior to 1% of high‐intensity CG strokes, constituting 5–10% of the previously known TGF population [ABSTRACT FROM AUTHOR]

Purpose: Recently, much research on imaging the clinical proton-irradiated volume using positron emitter nuclei based on target nuclear fragment reaction has been carried out. The purpose of this study is to develop an activity pencil beam (APB) algorithm for a simulation system for proton-activated positron-emitting imaging in clinical proton therapy using spread-out Bragg peak (SOBP) beams. Methods: The target nuclei of activity distribution calculations are 12C nuclei, 16O nuclei, and 40Ca nuclei, which are the main elements in a human body. Depth activity distributions with SOBP beam irradiations were obtained from the material information of ridge filter (RF) and depth activity distributions of compounds of the three target nuclei measured by BOLPs-RGp (beam ON-LINE PET system mounted on a rotating gantry port) with mono-energetic Bragg peak (MONO) beam irradiations. The calculated data of depth activity distributions with SOBP beam irradiations were sorted in terms of kind of nucleus, energy of proton beam, SOBP width, and thickness of fine degrader (FD), which were verified. The calculated depth activity distributions with SOBP beam irradiations were compared with the measured ones. APB kernels were made from the calculated depth activity distributions with SOBP beam irradiations to construct a simulation system using the APB algorithm for SOBP beams. Results: The depth activity distributions were prepared using the material information of RF and the measured depth activity distributions with MONO beam irradiations for clinical therapy using SOBP beams. With the SOBP width widening, the distal fall-offs of depth activity distributions and the difference from the depth dose distributions were large. The shapes of the calculated depth activity distributions nearly agreed with those of the measured ones upon comparison between the two. The APB kernels of SOBP beams were prepared by making use of the data on depth activity distributions with SOBP beam irradiations that were made from the depth activity distributions with MONO beam irradiations and sorted in terms of energy, SOBP width, and thickness of FD. The data on APB kernels of SOBP beams were determined as installment data for the simulation system using the APB algorithm for SOBP beam irradiations. Conclusions: A method of obtaining the depth activity distributions and the APB algorithm for clinical use of SOBP beams have been developed. It is suggested that the simulation system for imaging the clinical irradiated volume with the APB algorithm can be used in clinical proton therapy using SOBP beams by preparing and investigating the data on APB kernels of SOBP beams. [ABSTRACT FROM AUTHOR]

Vacancy‐type defects in Mg‐implanted GaN with and without hydrogen (H) implantation are probed by using monoenergetic positron beams. Mg+ and H+ ions are implanted into GaN(0001¯) to obtain 0.1 and 0.7‐µm‐deep box profiles with Mg and H concentrations of 1 × 1019 and 2 × 1020 cm−3, respectively. For the as‐implanted samples, the major defect species is determined to be Ga‐vacancy (VGa) related defects such as VGa, divacancy (VGaVN), and their complexes with impurities. For Mg‐implanted samples, an agglomeration of vacancies starts at 800 °C annealing, leading to the formation of vacancy clusters such as (VGaVN)3. For the samples annealed above 1000 °C, the trapping rate of positrons by vacancies is increased by illumination of a He–Cd laser. This is attributed to the capture of photon‐excited electrons by the defects and their charge transition. For Mg‐ and H‐implanted samples, the hydrogenation of vacancy‐type defects starts after 800 °C annealing. Comparing with the annealing behavior of defects for the samples without H‐implantation, the clustering of vacancy‐type defects is suppressed, which can be attributed to the interaction between Mg, H, and vacancies. [ABSTRACT FROM AUTHOR]