Russian-American tomography experiment

To intercompare various techniques used in reconstructing tomographic images, and to benchmark those results with direct observations obtained by the incoherent scatter technique, an experimental campaign and subsequent analysis program-the Russian-American Tomography experiment (RATE)-were implemented in late 1993. Russian experiment teams from the Polar Geophysical Institute in Murmansk and Moscow State University joined with American investigators from the Phillips Laboratory and the Massachusetts Institute of Technology, and an array of four receiving stations was set up in the northeastern United States and in eastern Canada to obtain data for the tomographic reconstructions. Phase-difference and total-phase tomographic reconstruction techniques have been employed and are intercompared. The spatial/altitude distribution of ionospheric electron content was observed by the MIT Millstone Hill incoherent scatter radar that scanned the ionosphere in a plane parallel to the satellite overflights. We present preliminary reconstructions of the ionospheric structure observed during a severe midlatitude ionospheric storm that took place during the campaign. The drastic large-scale changes in the ionospheric structure that accompanied the November 1993 storm were well observed by the two diagnostic techniques.©1994 John Wiley & Sons Inc     

Performance of different phoswich configurations in a balloon flight experiment

Four different phoswich units were included in a modular hard X-ray balloon experiment launched on August 10, 1982 in order to check the dependence of background level and rejection efficiency of NaI(Tl)/CsI(Na) phoswich detectors on scintillator thicknesses. The results concern the dependence of the background intensity on both the thickness of the active shield, for a fixed primary detector thickness, and on the thickness of the primary detector, given the active shield thickness. A direct comparison of phoswich detectors with passively shielded NaI(Tl) crystals is also given. As a consequence practical hints for designing new phoswich detectors are derived and the limiting sensitivity of these detectors for hard X-ray observations of celestial sources is inferred.     

High voltage generator for the power supply of photomultipliers in the time of flight system of Alpha Magnetic Spectrometer-2 experiment

In this report, the behaviour of the first prototype high voltage generator (HVG) that might be used in the time of flight (TOF) system for the AMS-2 experiment is described. The system receives a positive continuous voltage about 100–120 V as input, and it provides a programmable negative continuous voltage from −1600 to −2400 V as output, versus a total load of 50 MΩ. The most important aspect is the absence of a transformer which usually is used in the step-up DC–DC converters. In the TOF system of alpha magnetic spectrometer (AMS)-2 experiment there is a big magnetic field, higher than 2 kG, that does not allow to use a transformer, therefore this prompted us to use the Cockroft–Walton system. The power consumption is about 300 mW and the peak-to-peak high frequency ripple is lower than 0.3% of the output high voltage. We also estimated the reliability of the HVG and we obtained a failure probability lower than 0.5% after three years of continuous functioning. Besides, in this report, much importance was given to the calculation of a simple model of the system to estimate the stability margins.     

超声衍射时差法检测技术研究

描述了超声波衍射时差法的基本原理,讨论了平行扫查(B扫描)和非平行扫查(D扫描)两种扫查方式,研究了波形图和灰度图两种缺陷图谱的相位特征,推导了缺陷精确定位定量的计算公式,并通过标准试块试验对三种典型缺陷进行了图谱分析.研究结果表明,TOFD技术能识别向表面延伸的缺陷,能对缺陷精确定量和定位并且检出率高,检测速度快.该方法在国内无损检测领域将得到广泛应用.     

Measurement of the flux-flow velocity in superconducting films by a time of flight method

Flux has been pulsed in on the left side of a superconducting film with a transport current present. The current moves the flux tubes to the right, where their arrival is sensed with a movable pickup coil. The velocity has been defined by the slope of a plot of distance vs. arrival time. Velocities of 1–3 × 10⁵ cm/sec for the first onset and of 0.5–2.7 × 10⁵ cm/sec for the maximum of the flux field have been measured. The total amount of flux decreased approximately linearly with distance traveled.Supported in part by the National Science Foundation and in part by the Cryogenics Center through a grant from the DOD Themis program. Based in part on a thesis (J.K.) submitted to Stevens Institute of Technology in partial fulfillment of the requirements for the Ph.D. degree.     

Reduction of computation time for crossed-grating problems: a group-theoretic approach

A systematic approach based on group theory is established to deal with diffraction problems of crossed gratings by exploiting symmetries. With this approach, a problem in an asymmetrical incident mounting can be decomposed into a superposition of several symmetrical basis problems so that the computation efficiency is improved effectively. This methodology offers a convenient and succinct way to treat all possible symmetry cases by following only several mechanical steps instead of intricate mathematical considerations or physical intuition. It is also general, applicable to both scalar-wave and vector-wave problems and in principle can be easily adapted to any numerical method. A numerical example is presented to show its application and effectiveness.     

Upper stage flight experiment (USFE) integral structure development effort

The Air Force Research Laboratory's Space Vehicle Directorate (AFRL/VS) has established a customer focused composite tankage development program that is targeted to existing and future aerospace applications. AFRL/VS is developing a wide range of tank concepts that include linerless cryogenic tankage, self-healing cryogenic tankage, hydrogen peroxide compatible tankage, volumetrically efficient toroidal (donut shaped) geometries, and more.

This paper will summarize the Upper Stage Flight Experiment (USFE) composite integral structure development effort. The integral structure refers to the stage skirt and the propellant tankage. These two parts are bonded together to form an integral structure. The USFE tank is the world's first composite, common-bulkhead, medium-pressure vessel designed to be Class 1 Compatible with high concentrations of hydrogen peroxide. Lightweight hydrogen peroxide compatible tankage development is becoming increasingly important to the international aerospace community because it provides considerable benefits. Peroxide can be stored unpressurized and is relatively non-toxic, which makes it safer to handle and store compared to oxidizers such as hydrazine. In addition to being a viable bi-propellant oxidizer, hydrogen peroxide can also serve as a monopropellant for an attitude control system (ACS). Peroxide is not cryogenic, therefore, it does not require an on-board cryocooler, which makes it easier to meet mass budgets and to mitigate technical risk. Storability and ease of handling make high concentration hydrogen peroxide an ideal propellant for reusable launch vehicle (RLV) responsive upper stage applications.     

The ToF system for the PAMELA experiment: In flight performances

A Time of Flight (ToF) scintillator system has been developed for the PAMELA satellite-borne cosmic ray experiment. The main scientific goal of the experiment is the measurement of the antiproton and positron fluxes in the cosmic radiation over large energy ranges. The ToF system provides the fast trigger to the experiment, the rejection of albedo particles and the possibility to distinguish electrons from antiprotons up to about 1.5 GeV.In this paper we describe the performances of the ToF system measured on flight data.     

Development of an ion mobility quadrupole time of flight mass spectrometer

We describe here a new ion mobility capable mass spectrometer which comprises a drift cell for mobility separation and a quadrapole time of flight mass spectrometer for mass analysis--the MoQTOF. A commercial QToF instrument (Micromass UK Ltd., Manchester, UK) has been modified by the inclusion of an additional chamber containing a drift cell and ancillary ion optics. The drift cell is 5.1 cm long made from a copper block and is mounted from a top hat flange in a chamber situated post source optics and prior to the quadapole analyzer. Details of this instrument are provided along with information about how it can be used to acquire mobilities of ions along with their mass to charge ratios. The MoQTOF is used to examine conformations of a series of antimicrobial peptides based on a beta-defensin template. In vivo, these cationic cystine-rich amphiphilic peptides are conformationally restrained by three or more disulfide bridges, although recent findings by several groups have cast doubt on the importance of canonical disulfide pairing to antimicrobial activities. By synthesizing a panel of variants to Defb14 (the murine orthologue of HBD3), we exploit ion mobility to distinguish conformational differences which arise due to disulfide formation and to the hydrophobicity of the peptide sequence. Our gas-phase results are interpreted in terms of the antimicrobial and chemotacic properties of beta-defensins, and this mass spectrometry based approach to discern structure may have a role in future design of novel antibiotics.     

Varied threshold with laser flight time in scannerless range-gated ladar

We present a method based on maximum-likelihood estimation to calculate the threshold of the number of converted photoelectrons according to Poisson and Gaussian statistics. The calculations for detection and false-alarm probabilities are given including a proposal of error probability. An efficient approach of setting threshold according to laser flight time is discussed as the laser power decreases in propagation. Comparison of the fixed threshold and the varied threshold in Poisson and Gaussian statistics is provided. The varied threshold approach tends to keep the error probability at the lowest level. This varied threshold approach is found to have a similar effect on ranging accuracy as a threshold-fixed approach.     

飞行系统仿真精度的时序建模检测方法

飞行系统仿真实验可以用来测试飞机的性能,为使仿真测试过程正确可靠,需要对仿真的精度进行检测.采用时间序列建模分析的检测方法.对仿真系统输入与实际系统相同的控制和环境参数,比较两系统输出的飞行参数,相减得到基本平稳的误差序列,然后对该序列进行时间序列建模,并分析建模结果的稳定性,从而检测出飞行仿真系统的精度.实验表明,该方法能够有效检测仿真系统的精度.     

温度对衍射时差法超声检测结果影响

针对温度变化引起超声衍射时差技术(time of flight diffraction,TOFD)测量结果误差的现象,以含不同预置缺陷Q235A试块为测试样品,采用TOFD超声检测技术,并结合A扫结果,研究不同温度对缺陷检测结果的影响。研究发现,当试块表面温度超过40℃时,底面小缺陷的检测难度会提高,检测结果的不稳定性提升;随着试块表面温度继续升高,TOFD测量缺陷的最小分辨率会降低到3 mm,同时小缺陷波的A扫波形已经无法从背景噪声中识别。通过比较不同温度下纵波在固体介质中传播速度,发现温度升高导致纵波速度衰减,进而导致仪器分辨力的下降和实际测量误差的增大。     

Diffuse tomography: Using time‐of‐flight information in a two‐dimensional model

We show that time‐of‐flight data can be useful in a simple model of optical tomography. © 2001 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 11, 283–286, 2000     

Direct calculation of the moments of the distribution of photon time of flight in tissue with a finite-element method

Modeling of the full temporal behavior of photons propagating in diffusive materials is computationally costly. Rather than deriving intensity as a function of time to fine sampling, we may consider methods that derive a transform of this function. To derive the Fourier transform involves calculation in the (complex) frequency domain and relates to intensity-modulated experiments. We consider instead the Mellin transform and show that this relates to the moments of the original temporal distribution. A derivation of the Mellin transform given the Fourier transform that permits closed-form derivations of the temporal moments for various simple geometries is presented. For general geometries a finite-element method is presented, and it is demonstrated that the computational cost to produce the nth moment is the same as producing the first n temporal samples of the original function.     

Reduction of reequilibration time following gradient elution reversed-phase liquid chromatography

A simple and convenient method for the reduction of column reequilibration time following gradient elution reversed-phase liquid chromatography is described. This method utilizes the addition of a constant volume of 3% 1-propanol to the mobile phase throughout the solvent gradient to provide consistent solvation of the reversed-phase stationary phase. Reductions in reequilibration time of up to 78% have been observed. The effect of alkyl chain bonding density on reequilibration volume is also examined. A maximum in the mobile phase volume necessary to reequilibrate the column is found at a bonding density of about 2.9 mumol/m2. The relationship of reequilibration volume to bonding density supports the partitioning model of retention for reversed-phase liquid chromatography.     

Reduction of the average access time with thin-film head arrays

The average access time, ta, of a moving head disc file can be drastically reduced by the application of multi-track Thin-Film Head Arrays. It is shown analytically that ta= ts+ 16/15(Sm/a)^1/2, where tsis the settling time, Sm, the maximum seek length and a, the constant acceleration and deceleration. When eight arrays each containing 25 thin-film heads are used to cover a 50 mm span of disc recording surface with a track density of about 40 track/mm, the average access time on the order of 2 to 6 m sec. can be achieved depending on the settling time, with present batch fabricated vertical inductive head design and constant acceleration and deceleration of 10,000 to 20,000 cm/sec².     

Direct calculation with a finite-element method of the Laplace transform of the distribution of photon time of flight in tissue

Reconstruction methods for optical tomographic imaging require the development of models of light transport in highly scattering materials. While the simulation of the full temporal response function arising from a short source light pulse is computationally expensive, there are methods to evaluate efficiently certain transforms of the temporal profile. We previously presented methods to obtain directly the Mellin Transform, which is related to the moments of the temporal intensity distribution; We introduce a similar method to calculate directly the Laplace transform. This method provides an addtional, largely independent measurement type that can be combined with the moments to improve image quality in optical tomography, in particular with respect to the simultaneous reconstruction of absorption and scattering distribution.     

Precision of locking-in to a time scale

Summary The most precise and rapid method of locking-in local time scales consists of first synchronizing frequencies (equalizing intervals), and then locking-in the local time scale signals. The method of lockiag-in should be selected in each particular case according to the available equipment.