The high-resolution time-of-flight spectrometer TOFTOF

The TOFTOF spectrometer is a multi-disc chopper time-of-flight spectrometer for cold neutrons at the research neutron source Heinz Maier-Leibnitz (FRM II). After five reactor cycles of routine operation the characteristics of the instrument are reported in this article. The spectrometer features an excellent signal to background ratio due to its remote position in the neutron guide hall, an elaborated shielding concept and an s-shaped curved primary neutron guide which acts i.a. as a neutron velocity filter. The spectrometer is fed with neutrons from the undermoderated cold neutron source of the FRM II leading to a total neutron flux of 10¹⁰n/cm²/s in the continuous white beam at the sample position distributed over a continuous and particularly broad wavelength spectrum. A high energy resolution is achieved by the use of high speed chopper discs made of carbon-fiber-reinforced plastic. In the combination of intensity, resolution and signal to background ratio the spectrometer offers new scientific prospects in the fields of inelastic and quasielastic neutron scattering.     

Large-area scintillating-fiber time-of-flight/hodoscope detectors for particle astrophysics experiments

A new generation of detectors of ultra-heavy (UH) cosmic rays is being built which require the use of large-area, light-weight detector elements. In this paper, we present the design and operation of a new type of scintillating optical fiber detector, which can serve as both a time-of-flight (TOF) detector and coded hodoscope (i.e. trajectory detector). We demonstrate that this type of detector can make TOF measurements with a time resolution of 48 ps. We also present results of position measurements from a large area TOF/hodoscope that was flown on a high-altitude balloon flight in August, 1995. Finally, we discuss areas of modification and improvements that can be made to these detectors to enable them to be more useful for making UH cosmic ray measurements.     

Analysis of time-of-flight transient photocurrent in organic semiconductors with coplanar-blocking-electrodes configuration

Time-of-flight transient photocurrent in organic semiconductors with coplanar-blocking-electrodes configuration has been numerically analyzed. Time-of-flight measurements for such a sample configuration enable us to determine the drift mobilities of organic semiconducting materials applicable to organic field effect transistors. It is shown that the charge-carrier transit time can be determined in both non-dispersive and dispersive transport under the condition that the charge-carrier transit time is much shorter than the monomolecular and bimolecular recombination lifetimes. It is also shown that the localized-state distributions can be determined from the analysis of the transient photocurrent.     

The laser wakefield acceleration experiment at Ecole Polytechnique

A laser wakefield electron acceleration experiment has been set-up at Ecole Polytechnique. An electron beam with 3 MeV total energy is injected in a plasma wave generated by laser wakefield using the new LULI CPA laser (400 fs [FWHM], I < 10¹⁷ W/cm²). The first results show an effective acceleration of the order of 1 MeV, with a maximum when the electron density is close to the optimum value for which the laser pulse length is about half the plasma wavelength.     

Calibration of a neutron time-of-flight multidetector system for an intensity interferometry experiment

We present the details of an experiment on light particle interferometry. In particular, we focus on a time-of-flight technique which uses a cyclotron RF signal as a start and a liquid scintillator time signal as a stop, to measure neutron energy in the range of E_n≈1.8–$\text{150}\text{MeV}$. This dynamic range (up to $\text{300}\text{ns}$) is much larger than the beam bunch separation $\text{(54}\text{ns}\text{)}$ of the AGOR cyclotron (KVI). However, the problem of a short burst period is overcome by using the time information obtained from a fast projectile fragment phoswich detector. The complete analysis procedure to extract the final neutron kinetic energy spectra, is discussed.     

Composition depth profile analysis of bulk heterojunction layer by time-of-flight secondary ion mass spectrometry with gradient shaving preparation

Composition depth profile analysis of bulk heterojunction (BHJ) layer was performed by time-of-flight secondary ion mass spectrometry with gradient shaving preparation. The BHJ layer comprised of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61butyric acid methyl ester (PCBM) was formed on the substrate coated with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) followed by annealing. The P3HT component increased toward the top surface in the BHJ layer. In addition, C₈H₇SO₃⁻ was detected inside the BHJ layer, suggesting penetration of PSS. P3HT was uniformly distributed in the BHJ layer without PEDOT:PSS. The P3HT-rich distribution in the top surface may be attributed to PSS penetration.     

Proposal for a one GeV plasma wakefield acceleration experiment at SLAC

A plasma-based wakefield acceleration experiment E-157 has been approved at SLAC to study acceleration of parts of an SLC bunch by up to 1 GeV/m over a length of 1 m. A single SLC bunch is used to both induce wakefields in the 1 m long plasma and to witness the resulting beam acceleration. The experiment will explore and further develop the techniques that are needed to apply high-gradient plasma wakefield acceleration to large-scale accelerators. The 1 m length of the experiment is about two orders of magnitude larger than for other high gradient plasma wakefield acceleration experiments and the 1 GeV/m accelerating gradient is roughly ten times larger than that achieved with conventional metallic structures. Using existing SLAC facilities, the experiment will study high gradient acceleration at the forefront of advanced accelerator research.     

Optimal and effective technique for particle packing

In various fields ranging from manufacturing or pharmaceutical industry to agriculture or automotive industry air filters and -dryers filled with porous desiccant media are used for industrial air preparation. It is necessary to increase the mass of particulate material filled into the dryer for improving its efficiency. Practically this means decreasing porosity of the filler media. In order to achieve this, mechanical excitation (ensured by a vibration platform) is commonly used. However, applying outer excitation decreases the efficiency of filling procedure because of the high energy consumption of the vibratory device. To avoid unnecessary energy consumption it is recommended to examine the possibility of filling more particles into the canister without outer excitation. According to this in our present paper the gravitational deposition of particulate materials was investigated from viewpoint of porosity. Main idea of this research was the usage of the so called “snowstorm” filling technique. A conical hopper and under this multiple cylindrical rods were used to reach uniform deposition and high value of particle packing density. Thus efficiency of air preparation units may be increased as it is possible to fill more material into the container without additional energy usage compared to normal gravitational deposition.     

Plasma lens experiment at the final focus test beam

The proposed plasma lens experiment at the Final focus Test Beam (FFTB) facility of the Stanford Linear Accelerator Center has been approved by the adminstration. The experiment would allow the examination of plasma focusing devices for particle beams in the parameter regime of interest to future high-energy colliders. It is expected to lead to compact plasma lens designs capable of focusing the beam to unprecedented small spot sizes.     

ICF实验中的靶识别技术

在ICF物理实验中,靶定位精度直接影响打靶的成功率。靶定位系统包括两套CCDs和一 个多维调整架,两套CCDs从不同的视角得到靶的图像。在计算机中用边缘提取的方法分析它们的特征值,综合两个CCDs的特征值计算靶的空间坐标,它与基准点的偏差就等于靶的调整量。该系统采用了前照明,实验测得它的定位精度为4靘。     

A goal-oriented field measurement filtering technique for the identification of material model parameters

The post-processing of experiments with nonuniform fields is still a challenge: the information is often much richer, but its interpretation for identification purposes is not straightforward. However, this is a very promising field of development because it would pave the way for the robust identification of multiple material parameters using only a small number of experiments. This paper presents a goal-oriented filtering technique in which data are combined into new output fields which are strongly correlated with specific quantities of interest (the material parameters to be identified). Thus, this combination, which is nonuniform in space, constitutes a filter of the experimental outputs, whose relevance is quantified by a quality function based on global variance analysis. Then, this filter is optimized using genetic algorithms.     

Optimisation of the time-of-flight geometry for fast neutrons

It is usually assumed that the conventional ring geometry is the optimum configuration for time-of-flight measurements. It has been shown in this paper that this is not always so and that a surface of revolution geometry is often superior to a ring geometry. A geometrical method has been developed to estimate the widths of lines in the time-of-flight spectrum due to scatterers of different shapes and sizes. Assuming a paraxiality spread from 2° to 8°, the shape of lines of elastically scattered neutrons from primary fast neutrons in the 14 MeV range has been calculated. Elastic scattering off hydrogenous converters has been considered for discussing the energy spread of secondary “monoenergetic” neutrons in the 8–13 MeV range.     

Dynamic magnetic shield for the CLAS12 central TOF detector photomultiplier tubes

The Central Time-of-Flight detector for the Jefferson Laboratory 12-GeV upgrade is being designed with linear-focused photomultiplier tubes that require a robust magnetic shield against the CLAS12 main 5-T solenoid fringe fields of 100 mT (1 kG). Theoretical consideration of a ferromagnetic cylinder in an axial field has demonstrated that its shielding capability decreases with increasing length. This observation has been confirmed with finite element analysis using POISSON model software. Several shields composed of coaxial ferromagnetic cylinders have been studied. All difficulties caused by saturation effects were overcome with a novel dynamical shield, which utilizes a demagnetizing solenoid between the shielding cylinders. Basic dynamical shields for ordinary linear-focused 2-in. photomultiplier tubes were designed and tested both with models and experimental prototypes at different external field and demagnetizing current values. Our shield design reduces the 1 kG external axial field by a factor of 5000.     

Identification of the light nuclei component of cosmic rays with the PAMELA experiment

The PAMELA (Payload for Antimatter Matter Exploration and Light nuclei Astrophysics) experiment is a satellite-borne apparatus that will make long duration measurements of the cosmic radiation with a particular focus on antiparticles and light nuclei. The main scientific objective of the PAMELA mission is to investigate the nature of the dark matter that pervades the universe, the apparent absence of cosmological antimatter, the origin and evolution of matter in the Galaxy. Specifically PAMELA will measure the cosmic-ray antiproton and positron spectra over the largest energy range ever achieved and will search for antinuclei with unprecedented sensitivity. Furthermore, it will measure the light nuclear component of cosmic rays from Hydrogen up to Oxygen in the interval 200 MeV/n–150 GeV/n. Accurate measurements of the elemental composition are required in order to understand the origin, propagation and lifetime of the cosmic radiation. The primary cosmic rays (e.g. C, N and O), produced at the sources, propagate through the interstellar medium giving information about the composition at the source. Secondary elements (e.g. Li, Be, and B) are tracers of amount of matter traversed by the cosmic rays. The relative abundances of the constituents of galactic cosmic rays provide information about cosmic-ray transport within the Galaxy. PAMELA consists of a magnetic spectrometer, a Time-of-Flight and trigger system, an electromagnetic calorimeter, an anticoincidence system, a shower tail catcher scintillator and a neutron detector. The combination of these devices allows antiparticles to be reliably identified from a large background of other charged particles. This paper reviews the capability of the PAMELA subdetectors to identify light nuclei. Analysis techniques to discriminate light-charged particles will be presented.     

带粘弹减摆器旋翼系统气弹稳定性试验与阻尼识别

针对带粘弹减摆器旋翼系统气弹稳定性试验中测量所得信号可能出现的大阻尼、频率成分密集及信噪比差等情况，采用数值仿真，比较基于傅立叶级数移动矩形窗法（FSMB）、Hilbert法（HT）和传统的基于FFT移动矩形窗法（FFT—MB），从这类信号中识别阻尼的优缺点。在两米量级旋翼台上进行了带粘弹减摆器铰接式动力学相似模型旋翼气弹稳定性试验，采用角位移传感器测量摆振信号，用上述三种方法对试验结果进行稳定性分析。数值仿真和试验结果表明，角位移传感器测量信号有较高的信噪比；在大阻尼、频率成分密集及信噪比差的情况下，FSMB法与HT法较传统的移动矩形窗法有较高的识别精度。     

一种辨识Hammerstein模型的新方法

为了提高非线性Hammerstein模型的辨识精度,提出一种利用混合优化算法对非线性模型进行辨识的新方法。该算法的基本思想是把非线性系统的参数辨识问题转化为参数空间上的函数优化问题,然后利用遗传算法和改进的粒子群优化算法相结合寻求并获得参数问题的最优解。最后通过仿真研究表明,该方法对于非线性辨识具有较好的有效性和鲁棒性,获得了良好的辨识效果,是一种可行的解决非线性辨识问题的方法。