Calibration of Si(Li) detectors for X-ray astronomy

The characterization of a high performance, energy dispersive Si(Li) solid-state detector is presented. The response of the detector to K and L lines in the energy range 0.7–9.0 keV is obtained by using a Van de Graaff accelerator to bombard selected targets having 12 ⩽ Z ⩽ 50 with a high energy proton beam. In addition to the channel-to-energy calibration and resolution vs energy plot, an empirical relation is presented for line yield as a function of energy for the K lines. Further, spectral data are used to estimate the silicon dead layer thickness. Finally, a detailed characterization of the detector's response function, including deviations from a simple Gaussian form due to effects such as incomplete charge collection, is given. From these results, a procedure is indicated by which X-ray spectrometers used in X-ray astrophysics applications may be calibrated.     

Efficiency of Si(Li) X-ray detectors at low energies

A method for determining the efficiency of a Si(Li) X-ray detector at low energies using a fluorescence X-ray source is described. The thickness of the Be X-ray window and of the Si dead layer can also be measured. Results obtained using the method are described and the effects of the gold contact and of a possible ice layer on the surface of the detector considered.     

Developing Si(Li) nuclear radiation detectors by pulsed electric field treatment

Fabrication of Si(Li) nuclear radiation detectors using lithium ion drift under the action of a pulsed electric field is considered. Optimum treatment regime parameters are determined, including the pulse amplitude, duration, and repetition rate. Experimental data are presented, which show that the ion drift in a pulsed electric field decreases the semiconductor bulk compensation time by a factor of two to four and significantly increases the efficiency of detectors.

     

Characterising a Si(Li) detector element for the SIXA X-ray spectrometer

The detection efficiency and response function of a Si(Li) detector element for the SIXA spectrometer have been determined in the 500 eV to 5 keV energy range using synchrotron radiation emitted at a bending magnet of the electron storage ring BESSY, which is a primary radiation standard. The agreement between the measured spectrum and the model calculation is better than 2%.     

First application of calorimetric low-temperature detectors in accelerator mass spectrometry

For the first time, calorimetric low-temperature detectors were applied in accelerator mass spectrometry, a well-known method for determination of very small isotope ratios with high sensitivity. The aim of the experiment was to determine with high accuracy the isotope ratio of ²³⁶U/²³⁸U for several samples of natural uranium, ²³⁶U being known as a sensitive monitor for neutron flux. Measurements were performed at the VERA tandem accelerator at Vienna, Austria. The detectors consist of sapphire absorbers and superconducting transition edge thermometers operated at T≈ 1.5 K. The relative energy resolution obtained for 17.39 MeV ²³⁸U is ΔE/E=4–9×10⁻³, depending on the experimental conditions. This performance enabled to substantially reduce background from neighbouring isotopes and to increase the detection efficiency. Due to the high sensitivity achieved, a value of ²³⁶U/²³⁸U=6.5×10⁻¹² could be obtained, representing the smallest ²³⁶U/²³⁸U ratio measured until now.     

The effect of cathode bias (field effect) on the surface leakage current of CdZnTe detectors

Surface resistivity is an important parameter of multi-electrode CZT detectors such as coplanar-grid, strip, or pixel detectors. Low surface resistivity results in a high leakage current and affects the charge collection efficiency in the areas near contacts. Thus, it is always desirable to have the surface resistivity of the detector as high as possible. In the past the most significant efforts were concentrated to develop passivation techniques for CZT detectors. However, as we found, the field-effect caused by a bias applied on the cathode can significantly reduce the surface resistivity even though the detector surface was carefully passivated. In this paper we illustrate that the field-effect is a common feature of the CZT multi-electrode detectors, and discuss how to take advantage of this effect to improve the surface resistivity of CZT detectors.     

Planar Li growth on Li21Si5 modified Li metal for the stabilization of anode

Lithium(Li)metal is widely considered the ultimate anode for future rechargeable batteries.However,dendritic growth and related parasitic reactions during long-term cycling often lead to severe safety hazards and catastrophic failure.Herein,we fabricate a hybrid anode by coating single-phase Li21Si5 on lithium metal.The resultant electrodes show a stable cycle and depressed polarization in symmetric and half cells.A planar plating/stripping behavior is observed on the modified anode.The investigation of the interplay of Li and Li21Si5 shows relatively large adsorption energy in the Li-Si system.The deposition and stripping are surface processes,and Li21Si5 maintains its intrinsic phase structure.The deposited Li layer around Li21Si5 also has the advantage of diminished preferred orientation,which also contributes to the planar growth of Li.Both LiFePO4(LFP)and LiNi1/3Co1/3Mn1/3O2(NCM)cathodes were applied to further demonstrate the enhanced rate and cycle performance.     

Metallic magnetic calorimeters (MMC): detectors for high-resolution X-ray spectroscopy

X-ray detectors based on the concept of magnetic calorimetry are well suited for high-resolution spectroscopy. Metallic magnetic calorimeters (MMC) make use of a metallic paramagnetic temperature sensor, which is in tight thermal contact with a metallic X-ray absorber. The paramagnetic sensor is placed in a small magnetic field. Its magnetization is used to monitor the temperature, which in turn is related to the internal energy of the calorimeter. High-energy resolution can be obtained by using a low-noise, high-bandwidth DC SQUID to measure the small change in magnetization upon the absorption of an X-ray. With recent prototype detectors an energy resolution of ΔE_FWHM=3.4 eV for X-ray energies up to 6.5 keV has been achieved. We discuss general design considerations, the thermodynamic properties of such calorimeters, the energy resolution, and the various sources of noise, which are observed in MMCs.     

Use of pixelated detectors for the identification of defects and charge collection effects in mercuric iodide (HgI2) single crystal material

Physical structure of pixelated detectors provides a unique tool to evaluate the effects of different types of defects in the semiconductor material that is used to fabricate the detectors. The spectroscopic performance measured for individual pixels or groups of pixels can be used to correlate point defects or fields of inhomogeneities within the material with the charge collected from photoelectric events. A block of single crystal mercuric iodide of approximately 18×18 mm² area and between 6 and 10 mm thick is prepared. The homogeneity of this material is then investigated with light in the transparent region for HgI₂ using an optical microscope. Several types of defects can be identified in this way by the scattering of light, for example, single large inclusions or voids and areas of haziness consisting of fields of small inclusions. Standard procedures are used to fabricate from this block a pixelated detector with a 121-pixel anode structure. The performance of each pixel is measured, and differences in charge collection are correlated with the optical data. Measurement data are presented, and possible mechanisms of the interactions between the defects and the charge carriers are discussed.     

Mass Spectrometric Observations of Enhanced Rates of Association Reactions: nLiF = Li n F n (n = 2, 3) at LiF Single Crystal Surfaces

A mass spectrometric method was used to study the kinetics of lithium fluoride single-crystal sublimation. In electron impact ionization mass spectra, Li⁺, LiF⁺, Li₂F⁺, and Li₃ F ₂ ⁺ ions originating from monomer (LiF), dimer (Li₂F₂), and trimer (Li₃F₃) molecular precursors were detected in the temperature range 970–1070 K. The dimer-to-monomer and trimer-to-monomer flux ratios were found to increase progressively with increasing temperature and also in comparison with those measured under equilibrium of crystalline LiF with its saturated vapor. The temperature dependence of the ion current ratio I(Li₂F⁺)/I(Li⁺) measured over the interval 916–1087 K was shown to pass reproducibly through a minimum at about 975 K. The enhancement of the rate of association reactions at LiF crystal surfaces is discussed in light of the terrace-ledge-kink model of vaporization and surface charge concept.     

High temperature flow behaviour of SiC reinforced lithium aluminosilicate composites

The compressive flow behaviour of lithium aluminosilicate (LAS) glass, with and without SiC particulate reinforcements, was studied. The LAS glass crystallized toβ spodumene during high-temperature testing. The flow behaviour of LAS glass changed from Newtonian to non-Newtonian due to the presence of crystalline phase. Further, with the addition of 40 vol.% SiC additions, the strain rate sensitivity of flow stress decreased. While the activation energy for flow in LAS was 300 kJ/mole, it increased to 995 kJ/mole with the addition of 40 vol.% SiC reinforcements.     

The mechanical properties of lithium tetraborate (100), (011) and (112) faces

The mechanical properties of lithium tetraborate (LTB) were studied using micro- and nanoindentation on (100), (011) and (112) oriented plates. The lowest values of the hardness obtained for the LTB (100) face are caused by relatively easy cleavage parallel to the {100} planes. The hardness values decrease at the maximal load increasing for the LTB faces studied, however the indentation size effect is less pronounced for the LTB (011) face. The residual stresses of the indent area were detected using a micro-Raman spectroscopy of indents. It was found that the LTB does not exhibit irreversible phase transitions at an applied load range up to 113-130 mN.     

Study of vibrational modes in superionic lithium oxide

Lithium oxide, like other superionics, finds several technological applications. These applications range from miniature light weight high power density lithium ion batteries for heart pacemakers, mobile phones, laptops computers, etc. to high capacity energy storage devices for next generation. It is also a leading contender for future fusion reactors to convert energetic neutrons to usable heat and to breed tritium necessary to sustain D–T (deuterium–tritium) reactions. In this work, the lattice dynamics of Li₂O have been carried out to study the fast ion phase and the diffusion behavior of lithium and oxygen ions. The investigation of this simple material is a reference point for understanding more complex metal-oxides. A modified rigid ion model has been used to study the phonons in lithium oxide by considering the interatomic interactions up to third nearest neighbor. The calculated results are compared and analyzed with available experimental results.     

Neutron threshold activation detectors (TAD) for the detection of fissions

Prompt fission neutrons are one of the strongest signatures of the fission process. Depending on the fission inducing radiation, their average number ranges from 2.5 to 4 neutrons per fission. They are more energetic and abundant, by about 2 orders of magnitude, than the delayed neutrons (≈3 vs. ≈0.01) that are commonly used as indicators for the presence of fissionable materials.The detection of fission prompt neutrons, however, has to be done in the presence of extremely intense probing radiation that stimulated them. During irradiation, the fission stimulation radiation, X-rays or neutrons, overwhelms the neutron detectors and temporarily incapacitate them. Consequently, by the time the detectors recover from the source radiation, fission prompt neutrons are no longer emitted. In order to measure the prompt fission signatures under these circumstances, special measures are usually taken with the detectors such as heavy shielding with collimation, use of inefficient geometries, high pulse height bias and gamma-neutron separation via pulse-shape discrimination with an appropriate organic scintillator. These attempts to shield the detector from the flash of radiation result in a major loss of sensitivity. It can lead to a complete inability to detect the fission prompt neutrons.In order to overcome the blinding induced background from the source radiation, the detection of prompt fission neutrons needs to occur long after the fission event and after the detector has fully recovered from the source overload. A new approach to achieve this is to detect the delayed activation induced by the fission neutrons. The approach demonstrates a good sensitivity in adverse overload situations (gamma and neutron “flash”) where fission prompt neutrons could normally not be detected.The new approach achieves the required temporal separation between the detection of prompt neutrons and the detector overload by the neutron activation of the detector material. The technique, called Threshold Activation Detection (TAD), is to utilize appropriate substances that can be selectively activated by the fission neutrons and not by the source radiation and then measure the radioactively decaying activation products (typically beta and gamma rays) well after the source pulse. The activation material should possess certain properties: a suitable half-life of the order of seconds; an energy threshold below which the numerous source neutrons will not activate it (e.g., 3 MeV); easily detectable activation products (typically >1 MeV beta and gamma rays) and have a usable cross-section for the selected reaction. Ideally the substance would be a part of the scintillator.There are several good material candidates for the TAD, including fluorine, which is a major constituent of available scintillators such as BaF₂, CaF₂ and hydrogen free liquid fluorocarbon. Thus the fluorine activation products, in particular the beta particles, can be measured with a very high efficiency in the detector.The principles, applications and experimental results obtained with the fluorine based TAD are discussed.     

Terahertz emission at impurity electrical breakdown in Si(Li)

Terahertz electroluminescence caused by impurity-induced breakdown in lithium-doped silicon crystals is studied. The spectrum of the terahertz emission exhibits lines corresponding to intracenter electronic transitions between excited states of the impurity and sublevels of the ground state of the lithium donor. The spectrum also shows a background signal, which, apparently, is a manifestation of the effects due to heating at electric excitation.     

Recent research progress of alloy-containing lithium anodes in lithium-metal batteries

Lithium metal is regarded as one of the most ideal anode materials for next-generation batteries, due to its high theoretical capacity of 3860 mAh g⁻¹ and low redox potential (−3.04 V vs standard hydrogen electrode). However, practical applications of lithium anodes are impeded by the uncontrollable growth of lithium dendrite and continuous reactions between lithium and electrolyte during cycling processes. According to reports for decades, artificial solid electrolyte interface (SEI), electrolyte additives, and construction of three-dimensional (3D) structures are demonstrated essential strategies. Among numerous approaches, metals that can alloy with lithium have been employed to homogenize lithium deposition and accelerate Li ion transportation, which attract more and more attention. This review aims to summarize the lithium alloying applied in lithium anodes including the fabricating approaches of alloy-containing lithium anodes, and the action mechanism and challenges of fabricated lithium anodes. Based on summarizing the literature, shortcomings and challenges as well as the prospects are also analyzed, to impel further research of lithium anodes and lithium-based batteries.     

The Preparation of Lithium Drifted Silicon Si(Li) Detector

In this paper we describe the fabrication of lithium compensated silicon detector Si(Li) using the process of ion drift introduced by Pell [1]. We give preliminary results of electrical and nuclear characterization. This type of detector, which is under study in our laboratory, may find widespread applications in environment monitoring such as atmospheric pollution, water pollution, natural radioactivity and artificial radioactivity using microanalysis techniques (PIXE, XRF, NAA,...).