一個作圖題的另外解法

本通報1953年1-2月號發表的“東北敬部編譯「平面幾何」中的三個作圖題”一文中第一題“求作一圓,切於已知角的一邊上的已知點,而於另一邊上截取一弦等於已知綫段”,現有文成宜和王麗庭兩位同志提出另外的解法,但兩位同志的解法賞大致相類,為省篇幅,我們經過改寫合併發表於下。 設P是已知角XOY的OX邊上的一個已知點,l為已知綫段,假定所求圓已經作出,它切OX於P點,截OY得AB弦,有AB=l(這裹AB與OY同向)。今將P點依OY的方向平移至Q點,使PQ=l,於是PQBA為平行四邊形;再以直綫OY為軸將Q點反射得Q′點,則有     

等离子体微带开关特性

 提出一种基于微放电等离子体的微带开关。它是以“时变等离子体”取代微带线射频微机电开关的“金属悬臂”,利用等离子体的导体或介质特性使电磁波沿其表面进行传输或截止,从而实现微带线上电磁波传输的动态控制。等离子体微带开关的基本结构包括用以隔断电磁波的微带间隙和产生片状等离子体的放电装置。放电产生时,电磁波因等离子体导体性通过开关,形成“开”状态;放电停止后,电磁波被微带间隙反射,形成“关”状态。利用CST软件仿真研究了等离子体开关特性,结果表明：这种开关的带宽由等离子体密度决定,隔离度由间隙决定,而工作插损与等离子体密度和电子碰撞频率有关。等离子体位形（宽度、厚度等）对于开关性能也非常重要。     

Determinations of plasma density and decay time in the hollow cathode discharge by microwave transmission

The microwave （MW） transmission method is employed to measure both the plasma density and the plasma decay time in the hollow cathode discharge （HCD） in argon at low pressure. The plasma density in DC-driven or pulsed HCD is on the order of 1012 cm-3, which can block the X-band MW effectively. In the case of pulsed HCD, the MW transmittance shows the same waveform as the pulsed current during the rising edge if the driving frequency is low, but with a longer delay during the falling edge. The MW transmittance reaches a constant low level when the driving frequency is high enough. The plasma decay time in the HCD system is measured to be about 100 μs around a pressure of 120 Pa. The ambipolar diffusion is considered to be the major mechanism in the decay process.     

Dependence of the cross sections for Ir isotopes on the values of Qgg in the heavy ion collision

¹⁹⁷Au were irradiated with 47 MeV/u ¹²C ions. Iridium was produced via the multinucleon transfer reactions in bombardments of ¹⁹⁷Au with ¹²C. and was separated radiochemically from Au and the mixture of the reaction products. The γ radioactivities of Ir isotopes were measured by using a HPGe detector. The production cross sections of Ir isotopes were determined from activities of Ir isotopes at the end of bombardment and the other relative data. It has been found that the cross sections for neutron-rich isotopes of iridium show an exponential dependence on the values of Q_gg. Our experimental results also demonstrate lack of correlation between the cross sections and Q_gg in the case of neutron-deficient isotopes of iridium. The fact can be explained from that neutron-rich isotopes of iridium were produced in the deep inelastic transfer reactions.     

Modulation of absorption manner in helicon discharges by changing profile of low axial magnetic field

The modulation of absorption manner in helicon discharge by changing the profile of low axial magnetic field is explored experimentally in this work. The experiments are carried out in Boswell-type antenna driven by 13.56-MHz power source in 0.35-Pa argon environment. The peak of the external non-uniform magnetic field(B_(ex)) along the axis is observed in a range from 0 Gs to 250 Gs(1 Gs = 10~(-4) T), where the electron density varies from 0.5×10~(16) m~(-3) to 9×10~(16) m~(-3).When Bex is located near the tube upper end sealed by a dielectric plate, or near the tube bottom end connected with a diffusion chamber, the plasmas are centralized in the tube in the former case while the strong luminance appears between the edge of the tube and the axial line in the latter case. When Bex is located in the middle of the antenna, moreover, an effective resistance(R_(eff)) peak appears apparently with increasing magnetic field. The glow moves toward first the edge of the tube and then the two antenna legs as the magnetic field increases. The discharge in this case is caused by the absorption of Trivelpiece–Gould(TG) wave. It is suggested that B_(ex) is located in the middle of the antenna to obtain a higher efficiency of power transfer.     

电爆炸法制备纳米颗粒收集方法

金属丝电爆炸法制备纳米材料因其负载可大程度的过热和爆炸产物非平衡扩散过程得到了研究人员的广泛关注,认为是制备新型功能材料的有效方法。研究了不同收集方法对电爆炸法制备钛纳米颗粒的影响,并结合电学、光学、自辐射图像和形貌分析等诊断手段分析了不同方法下产物特性的成因。结果表明,钛丝电爆炸呈现周期型放电模式,产物通道在放电结束前（约40 μs）可膨胀至约1.7 cm处,此后有尖状突刺发展（波阵面后湍流区）,其速度约为55 m/s。为研究爆炸产物不同状态下纳米颗粒形成特性,使用了3种不同的产物收集方法,分别为:①在金属丝径向1.5 cm处放置硅片收集;②在腔体出口处预置滤网收集;③在金属丝一侧电极上通过定向喷涂收集。产物形貌表征结果表明,使用不同收集方法时产物特征存在明显差别,前2种方法爆炸产物先与介质混合再沉积于硅片,得到的产物分别为分散、链状的球状纳米颗粒和密集、堆叠的纳米颗粒团簇;后一种方法电爆炸产物具有较高的密度和定向速度（对硅片）,硅片以金属丝为轴心远近呈现出粉末状和烧结块状两种不同形式。     

水热法合成硼酸钙的场发射扫描电镜分析

以工业硼酸、硼砂和氧化钙为原料,通过改变反应温度和原料配比,用水热法合成了系列硼钙化合物.根据化学分析和XRD分析数据确定了各种合成产物的化学组成、物相名称,用场发射扫描电镜(FE-SEM)对六种硼酸钙的显微形貌、颗粒大小进行了表征,发现它们的显微形貌、颗粒大小有很大的不同.CaO·3B2O3·5H2O呈条板状,厚度在100～300 nm之间;2CaO·5B2O3·5H2O呈大纳米薄片,厚度在50～100 am的居多;2CaO·3B2O3·H2O则是纳米细线,直径在100 nm左右;4CaO·5B2O3·7H2O呈小纳米薄片状,厚度在100 nm左右;2CaO·B2O3·H2O呈球粒状,大小在500 am～1μm;CaO·B2O3呈粗针状,粗、细端直径在150～300 nm之间.     

低能实验核物理及其应用

简要地介绍了北京大学技术物理系和重离子物理研究所近年来在低能实验核物理及其应用方面的部分研究工作.Some recent studies on the low energy experimental nuclear physics and its applications at the Department of Technicsal Physics and the Institute of Heavy Ion Physics of Peking University are beiefly introduced in this paper.     

50MeV/A12C轰击197Au和209Bi核的线动量转移

用裂变碎片折叠角技术测得50MeV/A¹²C轰击¹⁹⁷Au和²⁰⁹Bi靶的线性动量转移分布,得到每个¹²C核核子线动量转移的最可几值与靶核相关,其值分别为184和173MeV/c.在线动量转移分布中,0.3附近有一小的突起,似乎可以认为是相应于弹核的α集团转移.