用于长城等古建筑探测的NMR探测器的磁体设计

以万里长城为代表的古建筑是世界瑰宝,更是中华民族的象征和骄傲.本文提出利用便携式核磁共振（NMR）装置来探测研究这类古建筑的建筑材料,在不对其造成损伤的基础上,发掘其隐含的科学、技术和工程相关的丰富信息.为此,作为第一步,设计了适合于探测这类古建筑的便携式单边NMR探测器组合式磁体.该探测器的磁体结构以semi-Halbach为基础,通过不同磁体模块间的组合得到对应移动探测模式、长距离探测模式和均匀磁场探测模式的磁体结构.随后根据优化结果,设计加工了磁体组件,并采用该磁体进行了流体、长城城砖和现代红砖的NMR实验,实测结果与模拟一致.该组合式磁体的优点在于通过不同磁体模块组合,实现了多种探测方式,适用于探测长城等这类古建筑物需要多种探测模式的科学研究.     

溴化镧闪烁体探测器性能及应用研究

LaBr3(Ce)探测器是一种新型闪烁体探测器,具有高光产额,高探测效率,高时间和空间分辨率,高能量分辨率,温度特性良好,抗辐射性能良好,操作简便等优点.从2001年以来,该探测器得到了迅速的研究和应用.LaBr3(Ce)探测器在核共振荧光检测、瞬发γ中子活化分析、爆炸物检测、核医学成像、环境辐射监测、空间辐射探测等方面的应用研究中取得了非常良好的效果.该探测器表现出优于以往用于这些领域的探测器的性能(例如NaI(Tl)探测器、BGO探测器、HPGe探测器等).介绍了LaBr3(Ce)探测器的性能及其应用研究进展,对代表性文献进行了简析和综述,阐明了其良好的应用前景.     

单边核磁共振传感器研究及其应用

单边核磁共振(single-sided NMR)传感器具有体积小、造价低和结构开放等特点,特别适用于大型样品的现场无损检测,具有广阔的应用前景.该文介绍了一种均匀度较高的单边核磁共振传感器,在10 mm×10 mm的水平目标区域内,静态磁场均匀度为338×10–6,在垂直方向上静态磁场具有4.6 T/m的恒定梯度.结合该磁场分布的梯度特性,提出了一种基于快速傅里叶变换和带通滤波处理的回波信号处理方法,可提取薄层样品的核磁共振信号,从而实现对样品纵深方向的高分辨率一维测量(分辨率达到50?m).最后使用该传感器对复合绝缘子高分子聚合物、液体的扩散行为,以及植物叶片的枯萎过程进行了测量.展现了该方法在高分子材料分析、液体测量以及农业应用方面的潜力.     

PEO-PPO-PEO嵌段共聚物的NMR研究进展

PEO-PPO-PEO嵌段共聚物是重要的非离子型高分子表面活性剂,在药物载体和基因治疗等领域有着广阔的应用前景. 核磁共振(NMR)作为重要的研究手段,在研究PEO-PPO-PEO嵌段共聚物胶团及液晶结构形成,揭示嵌段共聚物与各种添加剂或药物分子的相互作用机理,有着独特的优势. 本文重点介绍了¹H、¹³C和²H NMR波谱以及NMR弛豫时间和自扩散NMR等技术在研究PEO-PPO-PEO嵌段共聚物体系中的应用. 简要介绍了NMR技术在PEO-PPO-PEO 嵌段共聚物聚集、调控以及作为药物载体等方面的研究现状.     

具有变革性特征的红外光电探测器

现代红外光电探测技术有着近八十年的历史.从二战期间第一个可实用PbS红外探测器到第三代红外光电探测器概念的提出,红外光电探测技术经历了翻天覆地的变化.以碲镉汞、锑化铟、铟镓砷为代表的传统红外光电探测器已在军事、遥感、通信、生命科学和宇宙探索等领域发挥着至关重要的作用.随着人类对光电探测不断增长的需求,尤其近几年来在人工智能、大数据、智慧城市等方面对红外信息的探测和智能感知有着强烈的需求,大幅降低红外光电探测器的尺寸(size)、重量(weight)、功耗(power)和价格(price),以及提高探测器的性能(performance)迫在眉睫.因此,要满足上述需求,必须要寻找具有变革性特征的红外光电探测器件.当前红外探测器正处于新旧更迭的时代,一大批新型红外光电探测器涌出.本文系统地介绍了一些具有变革性特征的红外探测器前沿内容,主要包括:人工光子微结构调控的新型红外探测器、基于能带工程的红外探测器、新型低维材料红外探测器,以及传统红外探测器的新方向.最后,展望了红外光电探测未来发展面临的机遇和挑战.     

氨基酸合成方法研究—(XI)N-取代亚氨酸酯与芳醛缩合产物—去氢氨基酸衍生物的构型及其NMR研究

通过~1H NMR、NOE差谱和门控去偶~(13)C NMR等的研究确定了新合成的五对α-去氢氨基酸衍生物的构型,并利用部分弛豫、APT~(13)C NMR二维碳氢相关谱及~(13)C-~(19)F偶合常数等对NMR谱线进行了归属.     

高增益型气体电子倍增微网结构探测器的性能研究

随着微结构气体探测器的不断发展, 不同的探测需求相继提出.为了实现气体探测器在高增益和低打火率的条件下长时间稳定工作, 结合气体电子倍增器(GEM)与微网结构气体探测器(MicroMegas)的探测优势, 成功研制出一种基于GEM作为预放大的MicroMegas探测器, 详细介绍了探测器结构和工作原理, 并利用⁵⁵Fe放射源对探测器增益、打火率、能量分辨和工作稳定性等性能进行了实验测量. 分析结果显示GEM-MicroMegas探测器可以连续工作30 h 以上, 探测器增益可以超过10⁶, 相对于无GEM膜的MicroMegas探测器, 相同增益下打火率可以降低近100倍. 关键词： 微网结构气体探测器 能量分辨率 增益 打火率     

溴化镧闪烁体探测器性能及应用研究（英文）

LaBr3(Ce)探测器是一种新型闪烁体探测器,具有高光产额,高探测效率,高时间和空间分辨率,高能量分辨率,温度特性良好,抗辐射性能良好,操作简便等优点。从2001年以来,该探测器得到了迅速的研究和应用。LaBr3(Ce)探测器在核共振荧光检测、瞬发γ中子活化分析、爆炸物检测、核医学成像、环境辐射监测、空间辐射探测等方面的应用研究中取得了非常良好的效果。该探测器表现出优于以往用于这些领域的探测器的性能(例如NaI(Tl)探测器、BGO探测器、HPGe探测器等)。介绍了LaBr3(Ce)探测器的性能及其应用研究进展,对代表性文献进行了简析和综述,阐明了其良好的应用前景。     

Micromegas探测器计数曲线、增益以及能量分辨特性的研究

介绍了自行研制的Micromegas探测器的基本结构和工作原理. 在不同工作气体(Ar和CO₂)配比条件下,利用⁵⁵Fe放射源对该探测器的能量分辨、计数曲线和气体增益等特性进行了较为详尽的测试. 对相关结果进行了讨论. 关键词： Micromegas探测器 计数曲线 能量分辨率 增益     

新型“CaF2(Eu)+液闪”复合WIMP探测器的实验研究

介绍北京组新近提出的“CaF2 (Eu) +液闪”复合WIMP探测器的设计特点 .通过对探测器单元内部光收集效率的研究 ,初步确定了复合探测器的设计结构 .另外 ,对探测器内部放射性杂质衰变在复合探测器单元中形成的本底情况进行模拟 ,结果表明了复合探测器具有良好的排除本底的能力 .     

An analytical methodology for magnetic field control in unilateral NMR

Traditionally, unilateral NMR systems such as the NMR-MOUSE have used the fringe field between two bar magnets joined with a yoke in a 'U' geometry. This allows NMR signals to be acquired from a sensitive volume displaced from the magnets, permitting large samples to be investigated. The drawback of this approach is that the static field (B0) generated in this configuration is inhomogeneous, and has a large, nonlinear, gradient. As a consequence, the sensitive volume of the instrument is both small and ill defined. Empirical redesign of the permanent magnet array producing the B0 field has yielded instruments with magnetic field topologies acceptable for varying applications. The drawback of current approaches is the lack of formalism in the control of B0. Rather than tailoring the magnet geometry to NMR investigations, measurements must be tailored to the available magnet geometry. In this work, we present a design procedure whereby the size, shape, field strength, homogeneity, and gradients in the sensitive spot of a unilateral NMR sensor can be controlled. Our design uses high permeability pole pieces, shaped according to the contours of an analytical expression, to control B0, allowing unilateral NMR instruments to be designed to generate a controlled static field topology. We discuss the approach in the context of previously published design techniques, and explain the advantages inherent in our strategy as compared to other optimization methods. We detail the design, simulation, and construction of a unilateral magnet array using our approach. It is shown that the fabricated array exhibits a B0 topology consistent with the design. The utility of the design is demonstrated in a sample nondestructive testing application. Our design methodology is general, and defines a class of unilateral permanent magnet arrays in which the strength and shape of B0 within the sensitive volume can be controlled.     

Monitoring degradation in paper: non-invasive analysis by unilateral NMR. Part II

High quality paper samples have been oxidized with a specific oxidant to reproduce one of the possible causes of the aging of paper. All samples have been characterized by 13C CP-MAS NMR spectroscopy. The artificial aging of paper has been monitored using a standard NMR relaxometer and the results have been compared with the corresponding data obtained using an unilateral NMR relaxometer. Experimental values obtained with both techniques are in agreement, demonstrating that unilateral NMR relaxometric measurements constitute a suitable non-invasive method for assessing the degradation process of cellulose-based materials. The sensitivity of the non-invasive NMR method allows the detection of degradation even at a very early stage. Effects due to the sample volume and to the penetration depth have been investigated.     

A mobile one-sided NMR sensor with a homogeneous magnetic field: the NMR-MOLE

A new portable NMR sensor with a novel one-sided access magnet design, termed NMR-MOLE (MObile Lateral Explorer), has been characterised in terms of sensitivity and depth penetration. The magnet has been designed to be portable and create a volume with a relatively homogeneous magnetic field, 15,000 ppm over a region from 4 to 16 mm away from the probe, with maximum sensitivity at a depth of 10 mm. The proton NMR frequency is 3.3 MHz. We have demonstrated that with this approach a highly sensitive, portable, unilateral NMR sensor can be built. Such a design is especially suited for the characterisation of liquids in situations where unilateral or portable access is required.     

Time-Resolved Study of the Photo-Curing Process of Dental Resins with the NMR-MOUSE

The photo-curing reaction of dental resins has been examined with unilateral nuclear magnetic resonance (NMR-MOUSE) allowing nondestructive high-resolution measurement of depth profiles as a function of time and space. The NMR signal is sensitive to both the monomer concentration and changes in molecular mobility. Upon irradiation with blue light, it first increases due to molecular mobility enhanced by the reaction heat and then decreases exponentially with the monomer concentration as the polymer signal is lost in the dead time of the instrument upon curing. The space and time dependence of the NMR signal can be described by the photo-polymerization reaction kinetics together with a heuristic approximation of the temperature dependence.     

NMR spectroscopy applied to the Cultural Heritage: a preliminary study on ancient wood characterisation

High and low resolution solid state NMR methods have been applied to characterise a few samples of ancient wood. In an ancient larch wood sample, by applying ¹H low resolution NMR methods as a function of the temperature, the average pore size and its distribution have been determined. In addition, high resolution NMR techniques have allowed addressing of the question of the proximity of water pools to cellulose and lignin. In particular, a model can be hypothesized in which water pools are surrounded by thin layers of amorphous cellulose and/or lignin while the crystalline domains of cellulose surround the layers of amorphous cellulose. Preliminary results obtained using a fully non invasive and portable NMR unilateral relaxometer, the Eureka-Mouse10 (EM10), are reported. This instrumentation is shown to be perfectly suitable for characterizing degradation in ancient wood samples. PACS 76.60 k     

Advances of unilateral mobile NMR in nondestructive materials testing

Unilateral mobile NMR employs portable instrumentation with sensors, which are applied to the object from one side. Based on the principles of well-logging NMR, a hand-held sensor, the NMR-MOUSE (MObile Universal Surface Explorer) has been developed for nondestructive materials testing. In the following, a number of new applications of unilateral NMR in materials science are reviewed. They are the state assessment of polyethylene pipes, the characterization of wood, the in situ evaluation of stone conservation treatment, high-resolution profiling of rubber tubes and 2-D imaging for defect analysis in rubber products.     

Quantitative NMR monitoring of liquid ingress into repellent heterogeneous layered fabrics

Fabrics which are water repellent and repellent to other liquids are often constructed using multiple layers of material. Such a construction is preferable to a single layer of a liquid-repellent textile because, under the action of an applied pressure, ingress of a liquid through the first layer can be halted by the second or subsequent layers. In the quantitative investigation of this problem, current techniques provide limited information on the progress and distribution of the liquid as it ingresses into a fabric. Moreover, many techniques require that the material is delaminated prior to analysis, and cannot be conducted in real time to measure the progress of a liquid through the textile substrate. In this work we demonstrate that unilateral NMR, which allows signal to be collected from a volume of interest in a material residing above the instrument, can be a powerful tool to quantitatively monitor the ingress of a liquid through a layered sample exhibiting pronounced heterogeneities in repellency. A known volume of oil was placed on the top of a model textile sample composed of three 80 microm thick layers. Spatially resolved one dimensional vertical NMR profiles of the system were acquired as a function of the pressure vertically applied to the top of the sample. These profiles show that the absolute liquid volume present in each layer of textile can routinely be measured within 4 min with a spatial resolution of 15 microm. If each individual layer exhibits different repellency to the test liquid, the complexity of the dynamics of the ingress can be investigated in great detail. An elegant application of the unilateral instrument was obtained in which the sensitive volume matched the region of interest of the individual layers of the textile under investigation.     

Selective NMR excitation in strongly inhomogeneous magnetic fields

The NMR-MOUSE is a unilateral and mobile NMR sensor which operates with highly inhomogeneous magnetic fields. To produce a mobile NMR unit, RF excitation is sought, which can be produced with the most simple equipment, in particular nonlinear, low-power amplifiers, and to observe a free induction decay in strongly inhomogeneous fields, the excitation needs to be selective. The possibility to produce selective excitation by sequences of hard low-power radiofrequency pulses in the strongly inhomogeneous magnetic fields of the NMR-MOUSE is explored. The use of the DANTE sequence for selection of magnetization from parts of the sensitive volume was investigated for longitudinal and transverse magnetization by computer simulations and experiments. The spectra of the recorded FIDs and echo signals are in good agreement with those simulated for the excitation, which verifies the concept of the DANTE excitation. The results obtained are an important step towards a low-power operation of the NMR-MOUSE to improve its mobility.     

Probing four orders of magnitude of the diffusion time in porous silica glass with unconventional NMR techniques

The combined use of two unconventional NMR diffusometry techniques permits measurements of the self-diffusion coefficient of fluids confined in porous media in the time range from 100 microseconds to seconds. The fringe field stimulated echo technique (FFStE) exploits the strong steady gradient in the fringe field of a superconducting magnet. Using a standard 9.4 T (400 MHz) wide-bore magnet, for example, the gradient is 22 T/m at 375 MHz proton resonance and reaches 60 T/m at 200 MHz. Extremely short diffusion times can be probed on this basis. The magnetization grid rotating frame imaging technique (MAGROFI) is based on gradients of the radio frequency (RF) field. The RF gradients not necessarily need be constant since the data are acquired with spatial resolution along the RF gradient direction. MAGROFI is also well suited for unilateral NMR applications where all fields are intrinsically inhomogeneous. The RF gradients reached depend largely on the RF coil diameter and geometry. Using a conic shape, a value of at least 0.3 T/m can be reached which is suitable for long-time diffusion measurements. Both techniques do not require any special hardware and can be implemented on standard high RF power NMR spectrometers. As an application, the influence of the tortuosity increasing with the diffusion time is examined in a saturated porous silica glass.     

Investigations of silicone breast implants with the NMR-MOUSE

Silicone breast implants are used for breast augmentation and breast reconstruction. The issues of concern associated with such implants are: (a) the quality control of each implant before implantation, and (b) the detection of implant bleeding after implantation. We have studied the use of the Nuclear Magnetic Resonance-MObile Universal Surface Explorer (NMR-MOUSE) for the nondestructive testing of (a) the quality of implant shells, and (b) changes in implant gel due to leakage of body fluid into the implant. Depth profiles measured nondestructively through implant shells at different positions of each implant by the Profile NMR-MOUSE assured good reproducibility of the quality and thickness of different shell layers. The leakage of implants upon rupture was mimicked by observing changes in the transverse NMR relaxation time of the implant gel upon ingress of physiological saline solution and safflower oil through the rupture. Results demonstrate that nondestructive testing with unilateral NMR is a potential method for use in the quality control of implants and for the screening of implants for rupture after implantation.