Development of time-of-flight neutron depth profiling at Penn State University

Neutron depth profiling (NDP) is a nondestructive, near-surface technique, which utilizes a thermal/cold neutron beam to determine the concentration of specific light elements versus the depth in materials. The depth distribution is obtained by measuring the energy loss spectrum of protons, alphas, or recoil atoms in the substrate materials. For conventional NDP, the depth resolution is highly dependent on the limited ability of the detectors and associated electronics. A novel technique, the time-of-flight (TOF) method that is based on a completely different energy-measurement mechanism, can greatly improve the depth resolution for the accurate measurement of the dopant depth profile in especially shallow junction devices. Such a set of TOF NDP facility is being constructed at the 1 MW Breazeale Nuclear Reactor at Penn State University, Radiation Science and Engineering Center. In the TOF-NDP, a timing start signal is obtained from electrons emitted simultaneously with a neutron-induced recoil particle leaving the surface of the sample. The same particle generates the subsequent stop signal, whereby the residual energy of the particle is much more precisely determined from the particle flight time than currently obtained by the use of surface barrier detectors. In this paper, the Penn State conventional NDP measurement results will be presented and TOF-NDP facility will be described.     

Neutron depth profiling applications at The University of Texas research reactor

The neutron depth profiling (NDP) technique has become an increasingly important method to nondestructively measure the absolute concentration versus depth of various elements in substrates. A permanent NDP facility is operational at a tangential beam port of the 1-MW TRIGA Mark II research reactor at The University of Texas at Austin (UT). This facility was developed to perform materials research, specifically measurements of interest to the microelectronics industry. Applications of the UT-NDP facility include measurements of boron-10 profiles in borophosphosilicate glass samples and helium-3 depth profiles of implanted helium-3 in metals, alloys and amorphous materials. A study is underway to determine radiation damage and microstructural changes in stainless steel samples by helium irradiation using NDP and Transmission Electron Microscopy. Another study, currently planned, is to measure surface wear by measuring the depth profiles of implanted beryllium-7 and sodium-22 in various materials.     

In Situ Quantification and Visualization of Lithium Transport with Neutrons

A real‐time quantification of Li transport using a nondestructive neutron method to measure the Li distribution upon charge and discharge in a Li‐ion cell is reported. By using in situ neutron depth profiling (NDP), we probed the onset of lithiation in a high‐capacity Sn anode and visualized the enrichment of Li atoms on the surface followed by their propagation into the bulk. The delithiation process shows the removal of Li near the surface, which leads to a decreased coulombic efficiency, likely because of trapped Li within the intermetallic material. The developed in situ NDP provides exceptional sensitivity in the temporal and spatial measurement of Li transport within the battery material. This diagnostic tool opens up possibilities to understand rates of Li transport and their distribution to guide materials development for efficient storage mechanisms. Our observations provide important mechanistic insights for the design of advanced battery materials.     

Capillary neutron optics for prompt-gamma activation analysis

A neutron lens has been constructed to focus cold neutrons from the exit of a⁵⁸Ni neutron guide, which delivers a beam to the Prompt-Gamma Activation Analysis (PGAA) station at the NIST Cold Neutron Research Facility. The lens compresses a neutron beam of cross section 50 mm× 45 mm onto a focal spot of diameter 0.53 mm (fwhm) wich an average gain of 80 in neutron current density. PGAA measurements have been performed to demonstrate the enhanced sensitivity and detection limits for various elements and the spatial resolution in one transverse dimension. For the two test particles (a gadolinium glass bead and cadmium metal of sizes less than 0.5 mm), the gain in the -count rate with the lens is a factor of 60, and the detection limit is improved by a factor of 20. The system can be used for two-dimensional mapping of samples on a sub-millimeter scale to complement other analytical techniques such as neutron depth profiling (NDP).     

中子深度剖析技术研究可充锂金属负极

可充锂金属负极严重的界面不稳定性和安全问题极大限制了其商业化应用,对于锂的沉积/溶出行为以及锂枝晶的成核生长机理的清楚认识将有利于更高效的可充锂金属负极改性研究。然而,由于锂金属的高反应活性所带来的产物复杂性及其形貌多样性给原位谱学表征带来了诸多的困难。中子深度剖析(Neutron Depth Profiling,NDP)技术由于其高穿透特性、定量非破坏性、且对锂的高灵敏性,在实时研究锂金属电池中锂的电化学行为上显示出广阔的应用前景。本文首先简要介绍了NDP技术的测试原理及提高其空间/时间分辨率的方法,同时总结分析了近年来NDP技术在液态/固态电池体系中锂金属负极研究的应用,并展望了NDP技术今后的发展前景。     

Neutron Spectroscopy: An Under‐Utilised Tool for Organic Electronics Research?

Neutron scattering is a well‐established technique that has proven to be an invaluable tool in myriad fields of chemical and physical research. Neutrons offer unique ways to study in situ or operando functional materials due to their highly penetrating nature and specific interactions with the nuclei of different isotopes. While some neutron scattering techniques, such as neutron diffraction (ND), neutron reflectometry (NR), and small‐angle neutron scattering (SANS), have already been heavily adopted by the scientific community for use in the research of organic electronics, there are a number of techniques that are far less widely used: spectroscopic neutron scattering. This article aims to highlight these “under‐utilised” techniques, to emphasise their potential use within the field of organic electronics, and to increase awareness of their utility among new research communities.     

Profiling lithium distribution in Sn anode for lithium-ion batteries with neutrons

Measuring the distribution of lithium in high capacity lithium-ion battery (LIB) electrodes is essential to understanding the coulombic losses during the lithiation/delithiation processes that occur while charging and discharging the cell. In this research, two half-cell prototypes were fabricated by electrochemically lithiating Sn foil anodes in 1M LiBF₄ in a 1:1 (wt:wt) ethylene carbonate and dimethyl carbonate solutions at a constant potential of 0.50 and 0.67 V (vs. Li/Li⁺). The neutron depth profiling (NDP) technique was employed to study the Li distributions in the anodes. Li concentration profiles were resolved for the samples lithiated under different conditions for LIB studies. In addition, this paper demonstrated an in situ NDP measurement of an electrochemical cell with a thin window design, which reveals the dynamics of lithium distribution within the Sn anode.     

Characterization of polymer materials by scattering techniques,with applications to block copolymers

The application of six techniques—static and dynamic light scattering, small-angle neutron and X-ray scattering, neutron and X-ray reflectivity—to the characterization of polymer materials is summarized. Emphasis is placed on the similarities and differences among the various techniques, and on recent advances in experimental practice. Twelve examples from the recent literature are described, most of which concern block copolymers. A brief introduction to block copolymer properties is also provided.     

The determination of longitudinal crystal moduli in polymers by spectroscopic methods

Experimental methods for determining longitudinal crystal moduli of polymers were evaluated in light of recent proceesing methods that produced macroscopic Young's moduli which exceeded ultimate values as found by the x-ray diffraction method. The spectroscopic techniques of Raman and coherent inelastic neutron scattering yielded higher longitudinal crystal moduli than x-ray diffraction, and from calculations described herein it is concluded that these spectroscopic values are better estimates of the maximum Young's moduli in fully aligned and crystalline polymeric materials.     

On the correlation of implantation defects and implanted species

Using high-sensitive deep level transient spectroscopy (DLTS), we have determined ion-related defects in monocrystalline silicon in the asimplanted state. In comparison with secondary ion mass spectrometry (SIMS) data and neutron depth profiling (NDP) results it is demonstrated that the defect profiles and the chemical distributions have nearly identical shape. From these experimental facts it can be concluded that this electrical spectroscopy can be applied for the detection of very low concentrations (down to 10¹⁰ cm^–3) of implanted ion species.     

Development and applications of time-of-flight neutron depth profiling (TOF-NDP)

Neutron depth profiling (NDP) is a surface analysis technique based on the irradiation of samples with thermal or sub-thermal neutrons, and subsequent release of charged particles. Emitted particles rapidly lose kinetic energy primarily through interactions with the electrons of the substrate material. The depth of the reaction site can be found by using stopping power correlations. In conventional NDP, particle residual energy is measured by using a silicon semiconductor detector. In time-of-flight NDP (TOF-NDP), the energy can be determined by particle flight time. Time measurement can be made more sensitively than the energy measurement. Silicon semiconductor detectors can be replaced by microchannel plates (MCP). In this study, TOF-NDP concept will be briefly explained; Penn State TOF-NDP facility will be introduced; preliminary measurements performed with an alpha-source will be presented.     

中子成像技术在元素分析中的应用

中子成像作为一种快速、直观的无损检测技术,在核工业、航空航天、新能源、地质、考古、先进制造等多个领域得到广泛应用。中子成像利用中子不带电、穿透能力强、对轻元素敏感、可区分同位素和近邻元素等特性,非常适合开展含氢元素、近邻元素和同位素等材料的无损检测。本文概述了中子成像技术的基本原理及特点,并结合中国先进研究堆(CARR)中子成像装置上的应用案例, 重点介绍了国内外中子成像技术在储氢材料、燃料电池、岩石、核燃料元件、古代文物等领域的典型应用。随着中子成像技术的不断发展和广泛应用,有望为我国更多领域研究提供更强有力的技术支撑。     

Phase-sensitive specular neutron reflectometry for imaging the nanometer scale composition depth profile of thin-film materials

Neutron reflectometry is a powerful method for probing the molecular scale structure of both hard and soft condensed matter films. Moreover, the phase-sensitive methods which have been developed make it possible for specular neutron reflectometry to be effectively employed as an imaging device of the composition depth profile of thin film materials with a spatial resolution approaching a fraction of a nanometer. The image of the cross-sectional distribution of matter in the film obtained in such a way can be shown to be, in most cases, unambiguous to a degree limited primarily by the range and statistical uncertainty of the reflectivity data available. The application of phase-sensitive neutron reflectometry (PSNR) to the study of several types of soft matter thin film systems are illustrated by a number of specific examples from recent studies. In addition, new software tools available to the researcher to apply PSNR methods and analysis are discussed.     

Determination of toxic and trace elements in algae by instrumental neutron activation analysis

Instrumental neutron activation analysis has been applied in the determination of toxic and other trace elements in a set of three algae materials provided by the International Atomic Energy Agency, with the aim of environmental preservation through enhanced applications of nuclear analytical techniques. The quality of the analysis method has been evaluated by analyzing a number of biological standard reference materials. By adding mineral nutrients, the cultivation of algae for metals is enhanced, in particular, selected toxic heavy metals such as As, Cd, Cr, Hg, Ni and Pb. It is believed that the level of elemental concentration in algae samples are dependent on environmental conditions due to its biochemical properties. Therefore, algae materials may be useful as an indicator or controller of environmental water pollution.     

Determination of elements by nuclear analytical methods

The working principle of nuclear analytical methods (NAMs) is not influenced by the chemical bond. Consequently, they are independent counterparts to the well-known chemical procedures. NAMs obey fundamental laws or can be described and understood thoroughly. This qualifies them as candidates for reference methods. Although following similar nuclear reaction schemes, they comprise bulk analyzing capability (neutron and photon activation analysis) as well as detection power in surface near regions of solids (ion beam techniques). Prominent features of NAMs are sensitivity, selectivity, multielement determination and linearity of the calibration function covering a concentration range of several orders of magnitude. Moreover, ion beam techniques allow depth profiling with nm-resolution in several cases while the ion microprobe additionally offers a lateral resolution in the micron-scale. As NAMs require expensive apparatus (nuclear reactor, accelerator in radioactive control areas) their availability is restricted to a small number of suitably equipped institutes. However, they are able to solve complex analytical tasks, take part in key comparisons and play an essential role in the certification of reference materials.     

Complex formation in poly(vinyl chloride) thermoreversible gels

The molecular structure of gels formed by poly(vinyl chloride) with different solvents is analyzed by neutron diffraction and neutron scattering techniques. Structure parameters are determined from the scattering functions and discussed in the light of the fibrillar model. Arguments are given in favour of the formation of PVC-solvent complexes.     

Selective removal of radio-sodium from neutron-activated materials by retention on hydrated antimony pentoxide

A new method is presented to remove sodium interferences from neutron-activated materials. The method is based on passage of the samples, dissolved in a concentrated acid solution on a column of hydrated antimony pentoxide (HAP). The retention is quantitative with a maximum capacity of 31 mg Na/g HAP. Only tantalum, in addition to sodium, was retained by HAP out of the 60 elements tested. The method should find applications in neutron activation analysis, particularly of biological materials. Applications are also foreseen in other analytical techniques, such as emission spectroscopy and flame spectrophotometry, where high sodium concentrations in the sample can be a source of interference.     

Radioactivity in monitoring materials processing

Monitoring of the concentration of actinides in process streams and waste materials can be effectively carried out by detecting and measuring their radioactive emissions. Such monitoring techniques lead to more efficient control of the process, and also aid in the minimisation of losses to the waste and better accounting of the nuclear materials. This paper provides an overview of some of the techniques such as on-line alpha monitoring, passive and active neutron assay and gamma counting, and also describes the monitoring systems which have been developed in our laboratory for use in a reprocessing plant.