Trap ionization in phosphors in a strong electric field

Temperature dependences of the intensity of Gudden-Pohl flashes in zinc sulfide crystals in various electric fields were measured in order to study trap ionization by electric fields. The decrease in the flash intensity from trap levels of a given depth in a certain temperature range is due to a decrease in the probability for the liberation of an electron from a trap as the temperature decreases. The temperature dependences of the flash intensity found in various electric fields are compared with the theoretical temperature dependence of the probability for electron liberation from a trap. The agreement between experimental and theoretical curves indicates that electrons are liberated from the deeper levels through ionization by an electric field involving many phonons.Translated from Izvestiya VUZ. Fizika, No. 10, pp. 53–58, October, 1969.     

On the nature of bipolar flashes that share the same channel to ground

The paper examines the relationship between sequential events in bipolar flashes of two groups sharing the same channel to ground: one initiated by an upward positive leader, the other by a natural flash. Two quite different mechanisms are involved in the occurrence of the bipolar flashes of each group: The bipolar events that start as a natural flash are results of recoil leaders sharing parts of the initial branching structure; the bipolar events that start as upward positive leaders are the result of the interaction between these leaders' initial branching structures and the branches of the intracloud flashes they triggered.     

Atmospheric ultraviolet and red-infrared flashes from Universitetsky-Tatiana-2 satellite data

Millisecond ultraviolet (240–400 nm) and red-infrared (610–800 nm) flashes were detected in the nighttime atmosphere with the scientific payload installed onboard the Universitetsky-Tatiana-2 micro-satellite. Flashes with various numbers of photons, from 10²⁰ to 10²⁶, were detected within the atmospheric area 300 km in diameter observed by the detector. The flashes differ in duration and temporal profile: from single short flashes ～1 ms in duration to flashes with a complex profile more than 100 ms in duration. Different global geographic distributions are observed for flashes with different numbers of photons. Flashes with fewer than 10²² photons are distributed uniformly over the Earth’s map. Flashes with more than 10²² photons are concentrated near the equator and above the continents. Series of flashes were observed in one turn of the satellite when flying not only over thunderstorm regions but also over cloudless ones. The flash number distribution has been derived from the ratio of the numbers of red-infrared and ultraviolet photons. As applied to discharges in the upper atmosphere, whose glow is dominated by the emission in the first and second positive systems of molecular nitrogen bands (1PN₂ and 2PN₂), this distribution is equivalent to the flash altitude distribution in the atmosphere. The observed ratio of the numbers of photons in red-infrared and ultraviolet flashes agrees with the calculated one for electric discharges at altitudes higher than 50 km. In-orbit measurements of the charged particle flux (with a threshold energy for electrons of 1 MeV) provide no evidence for a synchronous occurrence of an ultraviolet flash and a burst in the particle flux in the orbit.     

视觉电生理闪光参数测量误差研究

闪光参数准确与否对于视觉电生理检查结果有效性具有重要意义。针对视觉电生理闪光参数特点,建立了闪光测量系统等效电路模型,基于传递函数理论分析了闪光测量系统参数对闪光强度、时程和波形测量误差的影响,以氙灯和LED为实验对象研究了系统参数在测量不同类型闪光时的误差。提出了减小测量误差以及消除测量闪光强度和LED闪光时程理论误差的方法并进行了实验验证。采用所述方法在不同系统参数下测量LED闪光强度结果的标准差为0.17%,LED闪光时程结果标准差为0.07%,可认为基本不变,实验与理论分析结果吻合。     

Characteristics of lightning flashes striking the CN Tower below its tip

It has been observed that tall structures are sometimes struck by lightning below their tops. This phenomenon necessitates the use of special procedures for the protection of tall structures from lightning hazards. This paper focuses on the analysis of lightning flashes that struck the CN Tower below its tip over an extended period of time (1991–2005). Through the use of video images recorded during the reporting period, it was found that out of 404 flashes to the tower, only 16 hit the tower below its tip (3.96%). The distance between the tower's tip and the strike location varied from 5.4 to 70 m. Statistical analysis of flash characteristics of strikes below the tower's tip generally shows that the flash duration and the number of strokes per flash are considerably lower than those for the majority of cases, when the tower is struck at its tip. Furthermore, a trend of decrease in flash duration is found as the distance between the tower's tip and the strike location increases. Therefore, based on the analysis of the available data, lightning strikes below the top of a tall structure do not seem to pose greater danger than strikes to the structure's top, assuming that the structure is also protected along its sides. It is hoped that the presented observations and statistical analyses will be of value in the development of sophisticated measures against lightning hazards at elevated objects.     

Defining the unknowns of sonoluminescence

As the intensity of a standing sound wave is increased the pulsations of a bubble of gas trapped at a velocity node attain sufficient amplitude so as to emit picosecond flashes of light with a broadband spectrum that increases into the ultraviolet. The acoustic resonator can be tuned so that the flashes of light occur with a clocklike regularity: one flash for each cycle of sound with a jitter in the time between flashes that is also measured in picoseconds. This phenomenon (sonoluminescence or “SL”) is remarkable because it is the only means of generating picosecond flashes of light that does not use a laser and the input acoustic energy density must be concentrated by twelve orders of magnitude in order to produce light. Light scattering measurements indicate that the bubble wall is collapsing at more than 4 times the ambient speed of sound in the gas just prior to the light emitting moment when the gas has been compressed to a density determined by its van der Waals hard core. Experiments indicate that the collapse is remarkably spherical, water is the best fluid for SL, some noble gas is essential for stable SL, and that the light intensity increases as the ambient temperature is lowered. In the extremely stable experimental configuration consisting of an air bubble in water, measurements indicate that the bubble chooses an ambient radius that is not explained by mass diffusion. Experiments have not yet been able to map out the complete spectrum because above 6 eV it is obscured by the cutoff imposed by water, and furthermore experiments have only determined an upper bound on the flash widths. In addition to the above puzzles, the theory for the light emitting mechanism is still open. The scenario of a supersonic bubble collapse launching an imploding shock wave which ionizes the bubble contents so as to cause it to emit Bremsstrahlung radiation is the best candidate theory but it has not been shown how to extract from it the richness of this phenomenon. Most exciting is the issue of whether SL is a classical effect or whether Planck's constant should be invoked to explain how energy which enters a medium at the macroscopic scale holds together and focuses so as to be emitted at the microscopic scale.     

强流二极管绝缘体滑闪监测及其影响

 以闪光二号加速器为研究平台, 建立了微分环阵列和Rogowski线圈同时监测二极管电流的方法,监测了二极管绝缘体表面的滑闪现象。根据电流探头测量结果的差异,分析了绝缘体滑闪对电子束流参数的影响。二极管绝缘体出现滑闪,位置附近的微分环波形严重畸变,其它位置的微分环和Rogowski线圈测量结果基本一致。采用距离滑闪位置较远的微分环结果处理二极管束流参数,相对于不出现滑闪时的结果,束流强度和总能量没有明显的变化。绝缘体滑闪沿面局部放电,能量损失较小,尚未对电子束流造成较大影响。     

二元混合溶液闪点行为的激光拉曼光谱研究

本文以乙醇-甲酰胺,乙腈-甲酰胺,乙醇-环己酮,乙腈-环己酮四种混合溶液为研究对象,采用激光拉曼光谱对四种溶液体系的闪点行为进行讨论和研究。通过测定四种溶液体系的拉曼光谱和闪点,结合溶液拉曼光谱的特征峰强度比和拉曼位移的变化,探索四种溶液闪点的变化规律。试验结果表明,随着溶液浓度的增大,溶质的拉曼特征峰强线性增加,混合溶液的闪点降低。采用三次多项式分别对四种溶液体系的闪点与拉曼特征峰、溶液浓度的关系进行拟合,拟合结果良好。混合溶液的闪点随拉曼特征峰强比、溶液浓度的变化趋势相同。该试验方法将有助于从分子结构角度分析混合溶液的闪点变化规律,同时该方法可用于混合溶液闪点的实时在线检测。     

Increasing the sensitivity of time-resolved photo-CIDNP experiments by multiple laser flashes and temporary storage in the rotating frame

Pulse sequences have been developed that add up time-resolved photo-CIDNP signals from n successive laser flashes not in the acquisition computer of the NMR spectrometer but in the experiment itself, resulting in a greatly improved signal-to-noise ratio. For this accumulation, CIDNP is first stored in the transverse plane and then on the z axis, and finally superimposed on CIDNP produced by the next flash. These storage cycles also result in a very efficient background suppression. Because only one free induction decay is acquired for n flashes, the noise is digitized only once. The signal gain is demonstrated experimentally and analyzed theoretically. Losses are mostly due to nuclear spin relaxation, and to a small extent to instrument imperfections. With 10 laser flashes, a signal increase by a factor of about 7.5 was realized. As their main advantage compared to signal averaging in the usual way, these sequences yield the same signal-to-noise ratio with fewer laser flashes; the theoretical improvement is by a factor of n.     

旋翼叶片回波建模与闪烁现象机理分析

对旋翼叶片回波建模与闪烁现象进行了综合研究.基于散射点散射系数和分布情况,构建了旋翼叶片回波的散射点模型,并分析了散射点分布对回波的影响;在此基础上研究了回波时域闪烁现象的物理散射机理,并结合时频分析和横向分辨率分析了微多普勒特征及时频域闪烁现象;对两类不同分布间隔的散射点模型进行了仿真,并与积分模型进行对比性实验,结果验证了闪烁现象物理分析的合理性.该研究成果在旋翼目标的探测识别领域具有一定的理论与应用价值.     

超高速碰撞2A12铝板产生闪光辐射的空间演化规律

为了描述超高速碰撞2A12铝板产生闪光辐射的空间演化规律,利用瞬态光纤高温计测量系统并结合二级轻气炮加载系统,开展了弹丸以30°的入射角度和不同碰撞速度条件下的超高速撞击实验。基于闪光辐射强度和辐射温度的实验数据处理得到了超高速碰撞2A12铝板在撞击点附近产生的最大闪光辐射强度和最大闪光辐射温度,基于大量实验,建立了撞击点附近最大闪光辐射的空间演化模型。并结合Origin软件对实验所得数据的拟合,得到了最大闪光辐射强度和辐射温度随探测点到着靶点间距离变化的拟合函数关系式。实验结果还表明:在相近碰撞速度、相同碰撞角度条件下,在同一椭球面上不同探测点位置处的最大闪光辐射强度和最大闪光温度差别不大,验证了撞击产生的闪光辐射以近似椭球的形状向外膨胀,随着等离子体云的向外膨胀,离碰撞点越远产生的最大闪光辐射强度和最大闪光辐射温度均越小;在相同碰撞角度、不同碰撞速度条件下,在同一椭球面上不同探测点位置处的最大闪光辐射强度和最大闪光温度均随碰撞速度的增加而增大。该研究在导弹拦截、天体物理及深空探测领域具有重要的应用价值。     

Estimation of charge neutralized by negative cloud-to-ground flashes in Catalonia thunderstorms

Charges neutralized by lightning flashes have been usually located and inferred from the quasi-static changes in the vertical component of electric field (ΔE) obtained from multiple synchronized measurement stations. In this paper, the charges neutralized by negative cloud-to-ground (?CG) lightning flashes are located and inferred using single station electric field measurements in combination with total lightning (cloud-to-ground and intra-cloud) data from lightning detection networks in the north-eastern region of Spain. The altitude of the negative charge region in 8 thunderstorms examined here was in the range of 5.8–7.2 km as inferred from temperature soundings. Charge locations are assumed to be in the region where the VHF (Very High Frequency) sources associated with the discharge are located. The quantity of charge neutralized is calculated using a point charge model and the measured vertical component of electric field (ΔE) associated with each flash. The results are compared with those previously obtained from measurements carried out in the NASA Kennedy Space Center (KSC) in Florida. The analysis of 260 ?CG flashes from eight thunderstorms reveals a median charge value of ?9.7 C with 95% of the values ranging between ?4.5 C and ?45 C.     

Photobleaching with a subnanosecond laser flash

In standard fluorescence recovery after photobleaching (FRAP) applications for measuring lateral diffusion rates and adsorption/desorption kinetics of fluorescent molecules at biological or model membranes, irreversible bleaching is induced by a bright excitation flash of at least millisecond time scale. It has been presumed that the bleaching event is of a low probability and the significant bleached population that develops during the flash results from each molecule undergoing thousands of excitation/deexcitation cycles before a bleaching event occurs. In some FRAP experiments, notably polarized FRAP (PFRAP) for measuring molecular rotational diffusion rates, it is desirable to use much shorter (subnanosecond) bleaching pulses. However, subnanosecond pulses are shorter than the fluorescence lifetime, so that any fluorophore will experience at most only one visit to the excited state during the bleaching pulse. If bleaching occurs only by the same processes as in slower FRAP experiments, one would thereby expect only minimal bleaching regardless of the bleach intensity. Moreover, the ability of fast polarized pulses to imprint an anisotropic orientational pattern in the postbleach unbleached fluorophore, an ability essential for PFRAP, is not at all guaranteed, particularly if two-photon processes are involved in high-intensity short bleach pulses. In this study, bleaching depths are measured as a function of subnanosecond pulse intensity on a small labeled protein covalently immobilized on fused silica. We show that bright subnanosecond laser flashes do indeed produce significant bleaching, that both two photon effects and reversible bleaching are involved, and that polarized bleaching does produce an anisotropic orientational pattern of unbleached fluorophore. We also postulate a theoretical molecular state model which semiquantitatively accounts for the experimentally observed dependence of reversible bleaching on bleaching pulse intensity.     

Cooling-down of thermal thick probes after flash excitation – A measure for the real energy density?

Though flash lamps are one of the most applied heat sources in the field of Thermographic Testing (TT) using active thermography, only little is known about the actually achieved energy input into test objects. In this paper, an easy to realize sensor concept is proposed and experimentally evaluated. The concept is based on the measurement of the surface temperature of a thermal thick probe after flash excitation. After considering the sensor concept with FEM simulations the experimental investigation of four materials (two polymer and two building materials) is described. It will be shown that a suited coating is essential for the realization of the sensor concept. The experimental results prove the suitability of black rigid PVC as the most promising material. Using a coated PVC sample the energy density of short laser pulses, similar to flashes of flash lamps, could be determined exactly with an estimated relative uncertainty of only a few percent.     

Single bubble sonoluminescence driven by non-simple-harmonic ultrasounds

The dependence of the single bubble sonoluminescence (SBSL) on the waveforms of the driving ultrasound has been investigated by both experiment and numerical calculation. Three types of non-simple-harmonic waves, the rectangular, triangular and as well as the sinusoidal wave with a pulse, are used to drive the SBSL in our research. The triangular wave is the most effective, while the rectangular wave is the worst and the sinusoidal wave in the middle. However, the rectangular wave drives the brightest SBSL among those waves if the sound pressure amplitude keeps constant. When we use a simple-harmonic wave with a pulse as the driving sound, stable and periodic SBSL flashes have been observed. An increase in the flash intensity can be observed as the pulse is put at a suitable phase related to the sinusoidal wave. All of the observations are investigated numerically. Well qualitative agreements between the numerical simulations and the experimental measurements have been achieved.     

超高速碰撞2A12铝板产生的热辐射演化特征实验研究

为研究超高速碰撞2A12铝板产生的热辐射演化特征,构建了超高速碰撞产生热辐射的测量系统。采用实验测量与理论计算相结合的方法,得到了相近碰撞速度(约3 km/s)、不同弹丸入射角度(弹道与靶板平面的夹角)下的闪光辐射强度、闪光辐射温度、光谱辐射能量、辐射源面积及发光效率的演化过程。结果表明:闪光辐射强度、闪光辐射温度及光谱辐射能量均呈现急剧上升后缓慢衰减的特征,并且随着弹丸入射角度的增加而减小;在闪光辐射温度达到峰值后辐射源面积继续上升;弹丸入射角度越小发光效率越高,实验中发光效率数量级为10~(-5)。     

Zum Gudden-Pohl-Effekt bei einem Zink-Kadmium-Sulfid-Phosphor

Luminescence flashes in an excited ZnCdS(Ag)-phosphor occur when electric fields are put on. At ?120° C such flashes also occur if the electric field is applied some time after excitation is removed. The intensity depends on additional fields acting while the phosphor is excited by light. The observations can be interpreted if the flashes are assumed to arise near the surface instead of the interior of the phosphor.     

激光功率密度对Al膜靶后表面快电子发射的影响

 报道了在20 TW皮秒激光器上完成的p偏振激光与等离子体相互作用过程中产生的快电子的角分布和能谱测量结果。实验得到：当激光功率密度小于10¹⁷ W/cm²时,电子发射没有明显定向性,在激光入射面内多峰发射;当激光功率密度大于10¹⁷ W/cm2,小于10¹⁸ W/cm²时,电子主要沿靶面法线方向发射;当激光功率密度达到相对论强度时,电子主要沿激光传播方向发射;激光功率密度未达到相对论强度时,靶后表面法线方向快电子能谱拟合平均温度符合共振吸收温度定标率;激光功率密度达相对论强度以上时,靶后表面法线方向快电子能谱拟合平均温度高于已有的温度定标率。     

Elektrophotolumineszenz eines Zink-Cadmium-Sulfid-Phosphors in Abhängigkeit von Anregungswellenlänge und Feldimpulsbreite

Luminescence flashes in an excited ZnCdS (Ag) phosphor occur when electric fields are put on or off. Whether these flashes are positive or negative depends on the length of the field pulse, on the exciting wavelength, and on the polarity of the irradiated electrode. There is a critical duration of the field pulses for which no field-off flash appears. At another but related critical time, the flashes with periodic field applications change sign. These times change with the exciting wavelength. The observations are interpreted as being controlled by the gradual filling of surface traps, which in its turn is influenced by the distribution of electron and hole concentrations produced by the incident radiation and the electric field. The critical time is approximately equal to the time needed for completely filling the surface traps.     

Dose estimation and shielding calculation for X-ray hazard at high intensity laser facilities

An ionizing radiation hazard produced from the interaction between high intensity lasers and solid targets has been observed. Laser-plasma interactions create "hot" electrons, which generate bremsstrahlung X-rays when they interact with ions in the target. However, up to now only limited studies have been conducted on this laser-induced radiological protection issue. In this paper, the physical process and characteristics of the interaction between high intensity lasers and solid targets are analyzed. The parameters of the radiation sources are discussed, including the energy conversion efficiency from laser to hot electrons, hot electron energy spectrum and electron temperature, and the bremsstrahlung X-ray energy spectrum produced by hot electrons. Based on this information, the X-ray dose generated with high-Z targets for laser intensities between 10¹⁴ and 10²⁰ W/cm² is estimated. The shielding effects of common shielding items such as the glass view port, aluminum chamber wall and concrete wall are also studied using the FLUKA Monte Carlo code. This study provides a reference for the dose estimation and the shielding design of high intensity laser facilities.