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BackgroundThe GEM (gas electron multiplier) detector with polyethylene conversion layer has the performance of fast neutron detection, high timporal resolution and spatial resolution. It is an ideal choice for fast neutron beam monitor for China spallation neutron source (CSNS).PurposeThis study aims to optimize the design of the GEM detector with polyethylene to meet its application requirements of the atmospheric neutron spectrometer of CSNS.MethodsFirst of all, the physical process of the detector was simulated by Monte Carlo simulation software Geant4 (GEometry ANd Tracking, version 4). The relationship between neutron detection efficiency of three different energies (7 MeV, 10 MeV and 14 MeV) and the thickness of polyethylene conversion layer was studied by simulation, and the influences of different thickness of barrier layer on energy and angle of recoil proton along with detection efficiency were investigated. Then, the energy deposition of recoil protons in gas with different thickness was calculated, and the γ-ray sensitivity of the detector was studied by simulation. Finally, the prototype detector was fabricated using simulated parameters, and a preliminary neutron beam experiment was carried out.ResultsThe results show that the detection has good two-dimensional imaging ability when the detector adopts 2 mm thick polyethylene, 200 µm thick aluminium barrier layer and 4 mm thick working gas. The spatial resolution of this detector can reach 2.9 mm.ConclusionsThis GEM based neutron detector meets the requirements of CSNS beam monitoring, which provides a reference for the optimal design of fast neutron beam monitor.

Background In this study, we investigate the effect of simultaneous fusion of preexisting adjacent spinal canal stenosis on the outcome of patients with lumbar spondylolisthesis. Methods Patients with lumbar spondylolisthesis who underwent transforaminal lumbar interbody fusion (TLIF) from August 2013 to March 2016 were included. The inclusion criteria were the following: single-level spondylolisthesis (L4 or L5) and mild to moderate spinal stenosis at cranial adjacent segment of the spondylolisthesis segment before operation. According to whether the adjacent stenotic segment was included in the scope of surgery, the patients were divided into two groups: group A, only the spondylolisthesis was treated with single-level TLIF, and the adjacent stenosis was not treated with any surgery; In group B, TLIF were performed in the spondylolisthesis segment and the adjacent stenotic level; no spinal stenosis was found in other levels. The patients were followed up for more than 5 years. The general information before operation, visual analog scale (VAS) scores of low back pain and leg pain, and Oswestry disability index (ODI) scores before operation and at the last follow-up were recorded. Results A total of 23 patients were included in group A, and 24 patients were included in group B. There was no significant difference between the two groups in gender distribution, age, course of disease, level of slippage, length of stay, degree of spondylolisthesis, stenotic grade of adjacent segment, and intervertebral disk degeneration grade (p > 0.05). The blood loss during surgery in group B was significantly higher than that in group A (p < 0.05). The operation time of group B was longer than that of group A, but the difference was not statistically significant (p = 0.245). There was no significant difference in preoperative VAS and ODI scores between the two groups. At the last follow-up, the VAS scores of low back pain in the two groups were almost the same. However, the VAS scores of leg pain and ODI scores in group B were slightly higher than those in group A at the last follow-up, but the difference was not statistically significant (p > 0.05). If relatives and friends have the similar disease, all the patients of group A and 87.5% of patients in group B would recommend that type of surgery. The satisfaction of group A (100%) was higher than that of group B (79.17%), but the difference was not statistically significant (p = 0.068). Conclusions For single-level lumbar spondylolisthesis with mild to moderate spinal stenosis in adjacent segment before operation, decompression and fusion on the level of spondylolisthesis only is a safe, less invasive, and economical surgical option, with good long-term clinical efficacy and high satisfaction rates. [ABSTRACT FROM AUTHOR]

The article studies a class of state-space models where the state equation is a local-to-unity process. The parameter of interest is the persistence parameter of the latent process. The large sample theory for the least squares (LS) estimator and an instrumental variable (IV) estimator of the persistent parameter in the autoregressive (AR) representation of the model is developed under two sets of conditions. In the first set of conditions, the measurement error is independent and identically distributed, and the error term in the state equation is stationary and fractionally integrated with memory parameter d ∈ (− 0.5 , 0.5) . For both estimators, the convergence rate and the asymptotic distribution crucially depend on d. The LS estimator has a severe downward bias, which is aggravated even more by the measurement error when d ≤ 0 . The IV estimator eliminates the effects of the measurement error and reduces the bias. In the second set of conditions, the measurement error is independent but not necessarily identically distributed, and the error term in the state equation is strongly mixing. In this case, the IV estimator still leads to a smaller bias than the LS estimator. Special cases of our models and results in relation to those in the literature are discussed. [ABSTRACT FROM AUTHOR]

Multi-level cell storage technology based on resistive random access memory (RRAM) with multi-level state characteristics is more attractive in achieving low-cost ultra-high-density nonvolatile memory. Although a large number of literatures have reported the multi-level state characteristics of RRAM, so far there is no unified model that can well explain the intrinsic reasons for the existence of intermediate resistance state (IRS) and the switching mechanism between different resistance states. Multi-level state characteristics are observed by I–V characteristic measurements on RRAM with TiN/HfOx/barrier layer/TiN stacks fabricated using a commercialized 28 nm CMOS process. Compared to other published resistive switching models, the proposed model based on trap-assisted tunneling is more consistent with the measured. The model can reproduce the multi-level state characteristics based on the mechanism that interaction between defects and oxygen vacancies at the interface of HfOx and TiN electrode, resulting in the formation of multiple weak conductive filaments. Furthermore, the wide spread of high resistance state (HRS) and the switching between HRS and IRS are determined by the distance of tunneling gap. As HfOx-based RRAM will soon be commercialized, it is becoming very urgent to clarify the switching mechanisms of multi-level state characteristics and propose a universal model. Consequently, this work satisfied the current demand and significantly advanced the understanding and development of commercialized, cost-effective, high-density multi-bit HfOx-based RRAM technology. [ABSTRACT FROM AUTHOR]