Two-dimensional T2 distribution mapping in porous solids with phase encode MRI

Two pure phase encode MRI sequences, CPMG-prepared SPRITE and spin-echo SPI with compressed sensing, for two-dimensional (2-D) T2 distribution mapping have been presented. The sequences are 2-D extensions of their 1-D predecessors previously described and are intended for studying processes in porous solids and other samples with short relaxation times whenever 2-D T2 maps are preferable to simple 1-D profiling. The sequences were tested on model samples and natural water-saturated rocks, in a low field MRI instrument. 2-D spin-echo SPI and CPMG-SPRITE demonstrate a similar performance, enabling measurement of T2 down to 1-2 ms. Both experiments are time consuming (up to 2-2.5 h sample dependent). As such, they can be recommended mostly for measurement during steady state conditions or when studying relatively slow dynamic processes (e.g. enhanced oil recovery, cement paste hydration, curing rubber, infiltration of paramagnetic ions).     

A resolution model for mode multiplets probed with neutron resonance spin-echo spectroscopy

A resolution model, which takes into account a violation of the spin-echo conditions for inelastic scattering as it is appropriate for high resolution spin-echo measurements of mode multiplets, is experimentally tested. Phase-sensitive measurements were performed while a dispersion surface is moved through the resolution ellipsoid of a triple-axis background spectrometer. The results are found to be in agreement with the model predictions. A tunable, artificially split dispersion was realized with a double crystal setup mounted on a piezoelectric device allowing for rotation and tilt of the first crystal, while the second one was rigidly fixed. Elastic measurements with the instrument configured in Larmor diffraction geometry were used to probe the crystal orientation with high sensitivity. First inelastic measurements are in agreement with a simplified model indicating persistence of the echo modulation over the entire spin-echo time range probed by the experiment.     

Scanning laser techniques for dynamic thermo-magnetic recording onto stationary media

Scanning laser microscopes (SLMs) have been used to characterise the magnetic properties of materials for some time (J. Magn. Magn. Mater. 95(1) (1991); IEEE Trans. Magn. 31(6 Pt. 1) (1995)). An SLM has been designed to facilitate a number of operating modes: both for writing and reading magneto-optical data. The current SLM is capable of thermo-magnetically recording bits onto magneto-optical thin films. Unlike previous SLMs, the current instrument has been designed to write bits both statically and dynamically onto stationary media. It will be used to write to magneto-optic (MO) disk material thermo-magnetically prior to imaging. Images may be derived from the longitudinal and polar magneto-optic Kerr effects, which are wavelength dependent, using the appropriate laser wavelength. In this paper the two configurations for dynamic recording are described.     

Imaging with multi-aperture optical telescopes and an application

The two main types of Multi-Aperture Optical Telescopes (MAOTs) (so-called Michelson and Fizeau) and the two possible modes of optical beam combination are reviewed. Wide-field imaging with a Michelson instrument is studied and the constraints are identified. An example of application to Earth observation is given. Then, we address the optimization of the aperture configuration, a key issue in the design of a MAOT. We also stress the image restoration, a necessary component of such an instrument because of the shape of its point spread function. Finally, a MAOT seems to be a promising technical solution for high resolution Earth observation from Space on a high orbit such as a geostationary one.     

Multi‐correlation Fourier transform spectroscopy with the resolved modes of a frequency comb laser

An instrument achieving 100 KHz spectral precision using multiple correlation Fourier transform spectroscopy has been demonstrated. The instrument can measure the individual frequency comb modes of 100 MHz frequency comb lasers in air. The experiments show ～400,000 resolved modes at linewidths of 85 MHz in the region of 829 nm and ～ 182,000 resolved modes at linewidths of 28 MHz in the region of 1.5 μm, with a recording time of few minutes. The precision of the instrument, defined by the frequency positioning, attains sub‐MHz even when the scan is performed in air.     

RESEARCH ON VIBRATION MODES OF THE CHINESE CHIME STONE

Nearfield acoustical holography is a powerful tool for imaging of acoustic sources. In this article, the basic theory treating steady-state acoustic radiation with generalized holography has been discussed. By holographic imaging, the vibration characteristics of irregular objects, the Chinese ancient musical instrument, the one-tone chime stone and the two-tone chime stone provided by the Institute of Archaeology, the Museum of Hubei Province, have been researched. The results show that the ancient chime stone has two main vibration modes, which make up the base of sound produced by the instrument. As for the two-tone chime stone, it is not so long and wide as the single-tone chime stone, but thicker and smoother than the single-tone chime stone. When it is excited at different positions, although it radiates sounds of different frequencies, its vibration modes are similar. This may be related to its dimensions and shape. The research is helpful for studying the sound producing system and the radiated field characteristics of the chime stone.     

RESEARCH ON VIBRATION MODES OF THE CHINESE CHIME STONE

Nearfield acoustical holography is a powerful tool for imaging of acoustic sources. In this article, the basic theory treating steady-state acoustic radiation with generalized holography has been discussed. By holographic imaging, the vibration characteristics of irregular objects, the Chinese ancient musical instrument, the one-tone chime stone and the two-tone chime stone provided by the Institute of Archaeology, the Museum of Hubei Province, have been researched. The results show that the ancient chime stone has two main vibration modes, which make up the base of sound produced by the instrument. As for the two-tone chime stone, it is not so long and wide as the single-tone chime stone, but thicker and smoother than the single-tone chime stone. When it is excited at different positions, although it radiates sounds of different frequencies, its vibration modes are similar. This may be related to its dimensions and shape. The research is helpful for studying the sound producing system and the radiated field characteristics of the chime stone.     

Experiments with neutron polarization analysis

Neutron polarization analysis experiments of the past 25 years are reviewed. In that time the technique has progressed from a curiosity to being a useful tool to be used when needed. In early experiments, the polarization of the scattered beam was analysed in the same direction as the polarization of the incident beam but, in some later experiments, full three-dimensional polarization analysis has been employed. This article starts by writing down the interactions which the neutron has with condensed matter and deriving the cross-sections for scattering and final polarizations of the scattered beam. This is done displaying the spin state functions of the neutron explicitly. A variety of experiments is then reviewed, commencing with the elastic and inelastic scattering experiments performed by Moon, Riste and Koehler in the late 1960s. Elastic scattering experiments where it is important to separate nuclear and magnetic cross-sections such as antiferromagnetic defect scattering are reviewed together with separation out of the nuclear spin scattering for various purposes. Of particular interest are the fully three-dimensional analysis experiments which reveal more about the structure and domain populations of certain antiferromagnets. Inelastic experiments for which polarization analysis is vital are those on paramagnets at high temperatures where it is necessary to discriminate against phonon scattering. Spin glasses are treated as frozen paramagnets. Polarization analysis also has another role to play in the separation of magnetic modes in both paramagnets and ordered magnets, and several of these experiments are reviewed. Finally it is possible to tag the polarization of a neutron beam in time and space and to measure the result at another time and place and this through various techniques yields information about the change in neutron energy on scattering. The techniques of pseudo-random flipping time of flight, neutron spectral modulation and neutron spin-echo spectroscopy are briefly reviewed but the techniques of polarized-neutron-beam management are left to another review.     

POLI-HEiDi: The new polarised neutron diffractometer at the hot source (SR9) at the FRM II—Project status

The project to upgrade the existing single-crystal diffractometer Heisses Einkristal Diffractometer (HEiDi) at FRM II with a polarised neutron option, enabling the investigation of magnetic ground states in single crystals has been ongoing since autumn 2004. After a detailed investigation of the possible options it has been decided to develop and rebuild the secondary spectrometer and keep the HEiDi monocromator. The new instrument has been named POLI-HEiDi as an abbreviation of Polarisation Investigator at HEiDi. Two different zero-field polarimeters will be made available for spherical neutron polarimetry, Cryoapad and MuPAD. Both, polarisation and analysis will be performed with polarised ³He spin filters. Several new and important components of the instrument have recently been commissioned. In this report we present these components and show how they fit with the design of the whole instrument.     

Performance and testing of a four channel high-resolution heterodyne interferometer

When studying complex vibrations, simultaneous measurements at several points are indispensable if one is dealing with objects whose vibrational behavior is not guaranteed to be stable over longer periods of time, such as biological specimens, micro-mechanical elements or objects characterized by nearly resonant normal modes with different vibrational patterns. Obviously, both amplitude and phase need to be measured at each point to obtain a full characterization of the vibration. We introduce birefringent beam displacers as a highly efficient beam multiplying method to create a system of four heterodyne interferometers operating in parallel from a single laser source. The design and the performance characteristics (resolution, cross-talk) of the instrument will be discussed. The system revealed the existence of running vibrational modes on an electrically driven plate clamped along its outer edge.     

The violin: Chladni patterns,plates, shells and sounds

In this article we consider the vibrations and radiated sound of the bowed violin. The vibrations are discussed in terms of the normal modes of the instrument involving the coupled vibrations of the bowed string, the supporting bridge, the hollow shell comprising the body of the instrument and, ultimately, the acoustic modes of the performance space in which the instrument is played. We show that damping plays an important role in characterizing the normal modes in what can be distinguished as weak and strong coupling limits. The historic and modern application of Chladni pattern measurements to enhance our understanding of the acoustics and as an aid to the making of violins is highlighted, alongside the modern equivalents of experimental modal and computational finite-element analysis. The symmetry-breaking properties of the internal soundpost is shown to have a profound affect on the intensity and quality of sound radiated by the bowed instrument.     

A high resolution neutron spectrometer for quasielastic scattering on the basis of spin-echo and magnetic resonance

We propose a high resolution neutron spectrometer, which combines the spin-echo principle with the separated coil magnetic resonance technique. The introduction of magnetic resonance instead of static spin-flippers in the spin-echo spectrometer allows the replacement of its high magnetic fields by low guide fields. The new technique represents a generalisation of the conventional spin-echo spectrometer.This property also holds for the double statistical-[8] the double Fourier- [9] and the Fotof-spectrometer [10], but those have not been built to our knowledge     

Chladni's clavicylinder and some imitations

Chladni's accomplishments in the field of instrument making were until recently not nearly as well-respected as his studies on the modes of vibration of plates and rods. However, he had developed his own friction instruments based on the glass harmonica, a popular instrument of his time. The instruments, which he partially built himself, had keys, which distinguished them from the glass harmonica. Additionally, these instruments differed from traditional keyboard instruments as they enabled the crescendo and decrescendo of individual notes after the key had been struck. Although Chladni's clavicylinder fascinated audiences and prompted imitations by many instrument makers, it was largely ignored by composers and pianists and therefore never became part of standard orchestration. The Museum of Musical Instruments of the University of Leipzig features three rare examples of friction instruments which have outlasted the centuries. These instruments were built according to the Chladni principle. After a thorough analysis, including the production of individual notes, these instruments will be presented in their cultural-historical as well as their technical context, followed by a discussion of their advantages and disadvantages. These originals exhibits allow for a conclusive comprehension of Chladni's ideas and his quest for new, unusual tone colors.     

A type of spectrophotometer with both SCI and SCE measurement structures

In order to measure the color of different samples with different surface texture, multiple measurement and observation conditions are needed from the instrument. Thus, both SCI (specular component included) and SCE (specular component excluded) measurement conditions are included in most color measuring instruments today. In current design, a light trap is established in the integrating sphere, and the instrument uses mechanical control to switch between SCI and SCE measurement modes. However, establishing a light trap inside the integrating sphere will affect the SCI and SCE measurement results. This article discusses the problems in current design, proposes a dual light source structure that includes both SCI and SCE measurement conditions, and then evaluates the structure with experiments. The experiment results show that the improved structure is more suitable for color measuring instruments with both SCI and SCE modes, and it has high practical value.     

Vibration mode analysis using electronic speckle pattern interferometry

The use of electronic speckle pattern interferometry as a non-destructive testing technique has been widely reported for measuring a variety of objects. When used for vibration mode analysis, the only information presented to the operator was at the nodal area. The instrument has been developed so that, with the use of a microcomputer, the operator can now produce an isometric view of an object vibrating in a resonant mode, making the results easier to interpret. The instrument provides a real-time, non-contact alternative to other mode analysis equipment and can detect high-order modes as easily as low-order modes.     

Matrix analysis of neutron spin-echo

We describe the general action of a neutron spin-turn coil by a rotation matrix, and the nett action of a sequence of spin-turns is then evaluated as the product of the relevant matrices. In the ideal neutron spin-echo configuration, the spin-turn sequence used is shown to have a resultant action described by the unit matrix if there is only elastic scattering, so that an initially polarized beam is transmitted with unchanged polarization. This is spin-echo focussing. Measurement of changes in the final polarization then provides information on the sample dynamics.Spin-echo focussing permits high resolution to be coupled with a broad incident wavelength spread. We calculate in detail the effect of this polychromaticity on the spinturn coils involved, and present experimental confirmation. General spin-echo configurations are then considered. Finally, we take into account the effect of spin-flip scattering from spin-incoherent samples, and show that spin-flip effects may be decoupled from dynamic effects in the analysis of spin-echo intensity.The techniques evolved are relevant to both monochromatic and polychromatic spinecho spectrometry. Emphasis has been placed on practical aspects of establishing correct spin-turn conditions and analysing spectrometer response in terms of relative transmitted intensity.     

Vibroacoustic characteristics of a gothic harp

The "gothic" style of harp was popular across most of Europe from the late medieval period to the Renaissance. These harps have a one-piece, longitudinally oriented hardwood soundboard, as opposed to the transverse-oriented bonded softwood structure on a modern instrument. In addition, the one-piece back is flexible, whereas the back of a modern instrument is a rigid molded shell. To study the gothic harp, one was constructed from plans created by the Boston Museum of Fine Arts from a late German model in their collection. The vibrational behaviors of the soundboard and soundbox were measured at various stages of construction. The completed instrument was subjected to modal analysis and radiativity measurements. The sound radiation is dominated by two breathing modes at 188 and 273 Hz, each with strong motion of the back, and modes around 350 Hz. Taken together, these modes function like the A0/T1 resonance pairs seen in the soundboxes of other instruments, and a comparison is made with the guitar. Also observed is that as the frequency increases, radiation is emitted from higher up the soundboard, and from higher soundholes. This feature has been observed in other harps, and is a consequence of the harp family's unique geometry.     

Radio frequency field intensity mapping using a composite spin-echo sequence

A novel radio frequency (RF) field intensity mapping or imaging method using a composite NMR spin-echo sequence is proposed. A composite spin-echo RF pulse with 90 degrees y-180 degrees x-90 degrees y sequence makes phase change in the final image depending on the RF field intensity on the object. The resultant phase change or phase map can be used to obtain the actual RF flip-angle map for a given condition which includes the status of tuning and RF inhomogeneity, etc. Bloch equation has been solved numerically to obtain the effects of the RF field intensity as well as the main magnetic field inhomogeneity and the results are used for the mapping (imaging) of the RF field intensity. Phantom studies have been performed using a 1.5 Tesla whole body MRI system and the results are presented.     

配备于自旋回波仪的计算机系统

本文介绍配备在自旋回波仪中的计算机系统,由TP-801计算机、外围设备、接口电路和一套应用软件组成。该系统可以产生自旋回波实验所需要的多种脉冲序列,包括90°-90°、90°-180°、180°-90°、Carr-Purcell序列等。同时,计算机采集回波信号或FID,并进行数据处理,其结果可以直接由计算机或示波器、绘图仪显示,也可以存入磁带。仪器可以快速准确地测量弛豫时间T₁、T₂,也适用于核磁共振自旋回波方面的教学。在本文结束部分给出了一些样品的测量结果。本文介绍的计算机系统可以和普通的CW-NMR谱仪结合,使之增加自旋回波测量功能。