TL99; 采用聚乙烯转换层的气体电子倍增器(Gas Electron Multiplier,GEM)探测器具有快中子探测、较高时间分辨及位置分辨等性能,能很好地满足中国散裂中子源(Chinese Spallation Neutron Source,CSNS)大气中子谱仪束流监测的需求.采用蒙特卡罗仿真软件Geant4 (GEometry ANd Tracking,version ...4)对探测器的物理过程进行模拟,研究7 MeV、10 MeV、14 MeV三种不同能量中子的探测效率与聚乙烯转换层厚度的关系;并研究不同厚度阻止层对反冲质子出射能量、角度以及探测器探测效率的影响.此外模拟计算了反冲质子在不同厚度气体中的能量沉积以及探测器的γ射线灵敏度.最后根据模拟参数制作探测器样机并进行了初步的中子束流实验.实验结果表明:探测器采用2mm聚乙烯转换层、200μm铝阻止层以及4mm工作气体时,探测器具有良好的二维成像能力,位置分辨率可达2.9 mm.
To determine the perioperative and late outcomes for carotid endarterectomy (CEA) in treatment of patients with high-grade stenotic lesions of the extracranial internal carotid artery.
Twenty ...patients underwent 21 CEAs at the Zhongshan Hospital between May 1993 and June 2000. They were 19 men and 1 woman, with a mean age of 64 +/- 9 years. Seven patients were performed for stroke, 11 for transient ischaemic attacks (TIAs) and 2 for asymptomatic disease. Duplex scan was the primary tool of evaluation prior to surgery. Perioperative digital subtraction angiography and magnetic resonance angiography were done for 19 and 18 patients, respectively. The percentage of stenosis was calculated using NASCET criteria. Of the 21 lesions operated, 19 had a stenosis of 70% or greater, 2 had ulcerative lesions with a stenosis ranging from 60% to 69%. All CEAs were performed under cervical block anaesthesia with selective intraoperative shunting and patch angioplasty. The patients were followed up regularly with duplex scan surveillance.
There was no mortality or stroke during 30 days postoperatively. A TIA occurred in one patient and cranial nerve injury in 2 patients perioperatively. All patients were followed up for a mean interval of 31 +/- 20 months (range: 1 - 63 months). The 2-year survival rate and risk of stroke were 92.3% and 0%, respectively, and the 5-year survival rate and risk of stroke were 79.1% and 12.5%, respectively. Two asymptomatic recurrent stenoses ranging from 50% to 60% were detected on follow-up duplex scan.
For the patients in this study, CEA is associated with an acceptable perioperative outcome as well as a satisfactory long-term beneficial effect in stroke prevention.
