Effect Of Crystal Thickness Research Articles

NaI gamma camera performance for high energies: Effects of crystal thickness, photomultiplier tube geometry and light guide thickness.

Gamma camera imaging, including single photon emission computed tomography (SPECT), is crucial for research, diagnostics, and radionuclide therapy. Gamma cameras are predominantly based on arrays of photon multipliers tubes (PMTs) that read out NaI(Tl) scintillation crystals. In this way, standard gamma cameras can localize ɣ-rays with energies typically ranging from 30 to 360keV. In the last decade, there has been an increasing interest towards gamma imaging outside this conventional clinical energy range, for example, for theragnostic applications and preclinical multi-isotope positron emission tomography (PET) and PET-SPECT. However, standard gamma cameras are typically equipped with 9.5mm thick NaI(Tl) crystals which can result in limited sensitivity for these higher energies. Here we investigate to what extent thicker scintillators can improve the photopeak sensitivity for higher energy isotopes while attempting to maintain spatial resolution. Using Monte Carlo simulations, we analyzed multiple PMT-based configurations of gamma detectors with monolithic NaI (Tl) crystals of 20 and 40mm thickness. Optimized light guide thickness together with 2-inch round, 3-inch round, 60×60 mm2 square, and 76×76 mm2 square PMTs were tested. For each setup, we assessed photopeak sensitivity, energy resolution, spatial, and depth-of-interaction (DoI) resolution for conventional (140keV) and high (511keV) energy ɣ using a maximum-likelihood algorithm. These metrics were compared to those of a "standard" 9.5mm-thick crystal detector with 3-inch round PMTs. Estimated photopeak sensitivities for 511keV were 27% and 53% for 20 and 40mm thick scintillators, which is respectively, 2.2 and 4.4 times higher than for 9.5mm thickness. In most cases, energy resolution benefits from using square PMTs instead of round ones, regardless of their size. Lateral and DoI spatial resolution are best for smaller PMTs (2-inch round and 60×60 mm2 square) which outperform the more cost-effective larger PMT setups (3-inch round and 76×76 mm2 square), while PMT layout and shape have negligible (<10%) effect on resolution. Best spatial resolution was obtained with 60×60 mm2 PMTs; for 140keV, lateral resolution was 3.5mm irrespective of scintillator thickness, improving to 2.8 and 2.9mm for 511keV with 20 and 40mm thick crystals, respectively. Using the 3-inch round PMTs, lateral resolutions of 4.5 and 3.9mm for 140keV and of 3.5 and 3.7mm for 511keV were obtained with 20 and 40mm thick crystals respectively, indicating a moderate performance degradation compared to the 3.5 and 2.9mm resolution obtained by the standard detector for 140 and 511keV. Additionally, DoI resolution for 511keV was 7.0 and 5.6mm with 20 and 40mm crystals using 60×60 mm2 square PMTs, while with 3-inch round PMTs 12.1 and 5.9mm were obtained. Depending on PMT size and shape, the use of thicker scintillator crystals can substantially improve detector sensitivity at high gamma energies, while spatial resolution is slightly improved or mildly degraded compared to standard crystals.

Thickness control of dispersion in opal photonic crystals

Opals are naturally occurring photonic crystals which can be formed easily using low-cost self-assembly methods. While the optical behaviour of opals has received significant attention over the last number of decades, there is limited information on the effect of crystal thickness on the optical properties they display. Here, the relationship between volume fraction and crystal thickness is established with an evaporation-induced self-assembly (EISA) method of formation. The extent to which thickness can be used to manipulate the optical properties of the crystals is explored, focusing on the change in the photonic band gap (PBG). Microscopical structural characterization and angle-resolved transmission spectroscopy are used to examine the quality of the photonic crystals formed using different volume fractions of polystyrene spheres, with thicknesses up to 37 layers grown from volume fractions of 0.125%. This work provides a direct correlation between sphere solution volume fraction and crystal thickness, and the associated optical fingerprint of opal photonic crystals. Maximum thickness is examined, which is shown to converge to a narrow range over several evaporation rates. We identify the criteria required to achieve thickness control in relatively fast evaporation induced self-assembly while maintaining structural quality, and the change to the spectroscopic signature to the (111) stopband and higher order (220) reflections, under conditions where a less ordered photonic crystals are formed.

Molecular mechanics and failure mechanisms in B. mori Silk Fibroin-hydroxyapatite composite interfaces: Effect of crystal thickness and surface characteristics

Bombyx mori Silk Fibroin-hydroxyapatite (B. mori SF-HA) bio-nanocomposite is a prospective biomaterial for tissue engineered graft for bone repair. Here, B. mori SF is primarily a soft and tough organic phase, and HA is a hard and stiff mineral phase. In biomaterial design, an understanding about the nanoscale mechanics of SF-HA interface, such as interfacial interaction and interface debonding mechanisms between the two phases is essential for obtaining required functionality. To investigate such nanoscale behavior, molecular dynamics method is a preferred approach. Present study focuses on understanding of the interface debonding mechanisms at SF-HA interface in B. mori SF-HA bio-nanocomposite at nanometer length scale. For this purpose, nanoscale atomistic models of SF-HA interface are also developed based on the HA crystal size and HA surface type (Ca2+ dominated and OH− dominated) in contact with SF. Mechanical behavior analysis of these SF-HA interface models under pull-out type test were performed using Molecular Dynamics (MD) simulations. Surface pull-off strength values in the range of 0.4–0.8 GPa were obtained for SF-HA interface models, for different HA crystal thicknesses, wherein, the pull-off strength values are found to increase with increase in HA thicknesses. Analyses show that deformation mechanisms in SF-HA interface deformation, is a combination of shear deformation in SF phase followed by disintegration of SF phase from HA block. Furthermore, higher rupture force values were obtained for SF-HA interface with Ca2+ dominated HA surface in contact with SF phase, indicating that SF protein has a higher affinity for Ca2+ dominated surface of HA phase. Current work contributes in developing an understanding of mechanistic interactions between organic and inorganic phases in B. mori SF-HA composite nanostructure.

Thickness-Based Dispersion in Opal Photonic Crystals

Photonic crystals (PhCs) are types of colloidal crystals which demonstrate a host of exciting optical phenomena, the nature of which can lend insight into their structure and enable a range of potential applications in fields such as optics and optoelectronics,1 energy storage,2-6 and sensors for environmental monitoring, biomedicine and food preservation.7 Artificial opals8 are PhCs which can be formed easily using low-cost self-assembly methods and their structures are composed of highly ordered, close-packed, polymeric sphere arrays. While the optical behaviour of opals has received significant attention over the last number of decades, there is limited information on the effect of crystal thickness on the optical properties they display. Most studies focus their attention on the lattice parameters and refractive index contrast within the materials.9,10 Here, the relationship between volume fraction and crystal thickness using an evaporation-induced self-assembly (EISA) method of formation is established.11-13 The extent to which thickness can be used to manipulate the optical properties of the crystals is explored, focusing on the change in the photonic band gap (PBG). Specifically, the thickness-induced changes in the properties which define the PBG for a range of volume fractions. Through meticulous structural and optical characterization, primarily in the form of scanning electron microscopy (SEM) images and angle-resolved ultraviolet-visible (UV-Vis) spectroscopy, the quality of the crystals formed is examined, with thicknesses exceeding 40 layers grown from volume fractions below 0.15 %, for the first time, to our knowledge. Crystal structure is correlated to optical fingerprint, shown in the form of transmission spectra in the UV-Vis region.We expect this work to provide our colleagues in the scientific community with a direct correlation between crystal thickness and the optical fingerprint of opal PhCs. Our analysis also stands as a manual to aid any future attempts at the growth of ultra-thick colloidal crystals. Such research may act as a precursor to further developments in materials employed in the photonics, sensing, energy storage and related communities. References 1 M. I. Shalaev, W. Walasik, A. Tsukernik, Y. Xu, and N. M. Litchinitser, Nature Nanotechnology 14, 31 (2019). 2 D. McNulty, A. Lonergan, S. O'Hanlon, and C. O'Dwyer, Solid State Ionics 314, 195 (2018). 3 D. McNulty, H. Geaney, D. Buckley, and C. O'Dwyer, Nano Energy 43, 11 (2018). 4 D. McNulty, E. Carroll, and C. O'Dwyer, Advanced Energy Materials 7, 1602291 (2017). 5 E. Armstrong, D. McNulty, H. Geaney, and C. O’Dwyer, ACS Applied Materials & Interfaces 7, 27006 (2015). 6 D. McNulty, H. Geaney, Q. Ramasse, and C. O'Dwyer, Adv. Funct. Mater. 30, 2005073 (2020). 7 J. Hou, M. Li, and Y. Song, Nano Today 22, 132 (2018). 8 E. Armstrong and C. O'Dwyer, Journal of Materials Chemistry C 3, 6109 (2015). 9 D. R. Solli and J. M. Hickmann, Optical Materials 33, 523 (2011). 10 S. G. Romanov, Physics of the Solid State 59, 1356 (2017). 11 Q. Jiang, C. Li, S. Shi, D. Zhao, L. Xiong, H. Wei, and L. Yi, Journal of Non-Crystalline Solids 358, 1611 (2012). 12 J. Zhang, Y. Li, X. Zhang, and B. Yang, Advanced Materials 22, 4249 (2010). 13 A. Lonergan, C. Hu, and C. O'Dwyer, Phys. Rev. Materials 4, 065201 (2020). 14 S. O'Hanon, D. McNulty, R. Tian, J. Coleman, and C. O'Dwyer, J. Electrochem. Soc. 167, 140532 (2020). Figure 1. Optical and structural analysis for 350 nm PS opals formed by EISA. Analysis presented in the form of transmission spectra obtained for samples formed from a range of five volume fractions, labelled by their dilution factors (a,d,g), sample plan view SEM images (b,e,f), and sample cross-sectional SEM images (c,f,i). (a-c) correspond to regions of maximum opal thickness, (d-f) to regions of lesser thickness, (g-i) to regions of minimal thickness. Figure 1

An investigation on continuous depth-of-interaction detection using a monolithic scintillator with single-ended readout

PET detectors with depth-of-interaction (DOI) capability have been studied to improve imaging resolution widely over the world. Since discrete DOI and continuous DOI detection with dual-ended readout technology have their respective limitations, we in this work focus on the continuous DOI detection with single-ended readout using a monolithic LSO scintillator and a multi-pixel photodetector. Based on a non-linear least squares data fitting method and Geant4 simulation, we studied the relationship between the spatial resolution of gamma positioning and the pixel number of photodetector. The results show that for a pixel number larger than 6x6, the positioning spatial resolution does not become significantly better when increasing the pixel number moreover. Another aspect studied is the effect of crystal thickness on the spatial resolution. Increasing the thickness of crystal leads to higher detection efficiency but lower spatial resolution.

Structure projection reconstruction from through-focus series of high-resolution transmission electron microscopy images

A structure projection reconstruction method based on contrast transfer function correction of through-focus series of high-resolution transmission electron microscopy images is presented. In this method, defocus values are determined by evaluating phase similarities of the pixels on the Fourier transforms of the images after correction using trial defocus values. Two-fold astigmatism is also determined, by measuring focus variation along different directions. Each image in the series is corrected for the effects of contrast transfer function and then combined into a structure projection image. The method works for both crystalline and non-crystalline objects. Test results with experimental images are presented. Influences of experimental parameters for imaging and effects of crystal thickness on reconstruction are discussed.

Miniature Antennas and Arrays Embedded Within Magnetic Photonic Crystals

This letter presents the performance of small antennas embedded within a new class of magnetic photonic crystals (MPCs) constructed from periodic arrangements of homogenous and anisotropic material layers. Earlier studies of these crystals demonstrated that they exhibit slow modes within their propagation bands associated with minimal reflection at the air-crystal interface, drastic slow down of the group velocity and concurrent increase in the amplitude of the field within the crystal. In this letter, we take advantage of the latter to demonstrate the extraordinary high gain that small antennas exhibit when placed within the MPC. We also examine array configuration inside the MPC to enhance power reception and to further narrow the radiation pattern beam. An initial investigation on the effect of crystal thickness and material loss due to the MPC is also considered.

A further discussion on the peculiarity of maximum entropy image deconvolution in HREM

Some important features observed in the maximum entropy image deconvolution in HREM were studied and demonstrated as a supplement to previous publications in Ultramicroscopy 35 (1991) 339 and 62 (1996) 141. It has been shown that in the process of image deconvolution how the errors of assigned electron-optical parameters can be compensated and the effect of crystal thickness can be corrected. The ‘negative-potential problem’ has been solved more perfectly than before via the normalization of sum total entropy.

Thickness dependence of structure in thin films of low-temperature silver selenide

The growth and structure of thin films of low-temperature (LT) silver selenide (Ag 2Se) were studied by HREM and electron diffraction. Samples were produced by the reaction of vacuum-deposited epitaxial silver films (1–30 nm thick) on NaCl substrates with equivalent amounts of selenium vapour. The structure of thin Ag 2Se crystals is initially found to be monoclinic, pseudo-tetragonal, while it turns to the known orthorhombic LT phase at higher thicknesses (⩾ 20 nm). It is noteworthy that the difference of phases observed is only an effect of crystal thickness. Comparisons were made between such thin crystals with those grown on continuous 30 nm thick single-crystalline silver layers. A reversible phase transition between the orthorhombic LT and cubic high-temperature (HT) phases could be induced by changing the electron beam intensity during TEM investigation (75 kV). The following orientation relationships between the body-centred cubic ( C, a = 0.498 nm) HT phase, the monoclinic, pseudo-tetragonal ( M, a = b = 0.706 nm, c = 0.498 nm, β = 90°) and orthorhombic ( O, a = 0.433, b = 0.706, c = 0.776 nm) LT phases were determined: [100] C‖[001] M and [110] M, [110] C‖[111] M and [100] M; [20 1 ] M‖[100] O, [010] M‖[010] O and [101] M‖[001] O; [ 2 22] C‖[100] O, [110] C ‖[010] O and [1 1 2] C‖[001] O. No triclinic or monoclinic (different from the determined pseudo-tetragonal) phases could be observed, although numerous references (listed in this paper) about such phases exist.

Exciton–Polariton Dispersion of Thin Anthracene Crystals in the Thickness Range of 3 to 0.1 μm

AbstractThe effect of crystal thickness on the exciton‐polariton dispersion of anthracene is studied by using melt‐grown crystals with thicknesses ranging from 3 to 0.1 μm. The refractive indices in the vicinity of the first‐exciton resonance are determined for the polarization E ‖ b and E ‖ a by analyzing interference fringes observed in the reflection spectra at 10 K. In contrast to the results reported previously, it is found that the dispersion property is independent of the crystal thickness at least down to 0.1 μm. A comparison of the present result with the thickness‐dependent dispersion formula is presented.

Determination of compositional ordering at grain boundaries in boron-doped Ni 3Al

The effects of crystal thickness and defocus on the superlattice contrast from HRTEM images have been demonstrated. The results indicate that fine, FCC fringe spacings in the vicinity of these grain boundaries can be produced if the boundary is slightly inclined to the electron beam, creating the false impression that the region is compositionally disordered. For properly chosen defocus conditions and boundary orientation, contrast typical of the ordered structure extends up to the estimated position of the boundary plane. The lack of a distinct disordered region suggests that microplasticity near grain boundaries is not significantly affected by the presence of B, and that its influence must be highly localized to the boundaries.

The effect of crystal thickness on the temperature anomaly of the shear acoustic wave velocity in the vicinity of the proper ferroelectric phase transition

Abstract It is theoretically shown that the effect of the crystal specimen thickness on the value and temperature dependence of the shear acoustic wave velocity may be essential. The neglect of the crystal dimensions and boundary conditions in comparison of the theoretical and experimental velocity anomalies may cause notable errors.

Interface phonons: effects of crystal size

The author studies the effect of crystal thickness on the vibrational properties of films and bicrystal interfaces by the Green function method. An expression for the surface Green function is obtained which depends on the crystal thickness by using a short-range interaction potential between the first and second neighbour atoms. The cases of acoustical and optical-like phonons are discussed.

Experimental and calculated images of planar defects at high resolution

Abstract Calculations of the contrast in n-beam images of isolated edge-on crystallo-graphic shear planes (CSP) have been carried out. The multi-slice method in combination with periodic continuation was used, thereby overcoming the resolution limitations inherent in the column approximation. The effects of crystal thickness, crystal tilt, CSP shear vector, defoeus, spherical aberration and objective aperture have been investigated and the results confirmed by comparison with experimental focal-series images. The applicability of the projected-charge-density approximation for the intuitive interpretation of image contrast at a resolution of about 4 A has been demonstrated for both (132) and (210) CSPs in rutile. The latter provide examples of metal-rich and metal-deficient faults, respectively.

The exciton spectrum at the absorption threshold of W 1−χTa χSe 2 single crystals

The effect of crystal thickness and composition on the energies of the adsorption edge excitons in W 1−χTa χSe 2 (with small x) has been investigated. It is found that (a) for x = 0 the exciton absorption peaks move to higher energies with decreasing crystal thickness (b) in the thickness independent region the A 1 ( n = 1) exciton moves to higher energy and decreases in intensity with increasing x. This behaviour is discussed in terms of exciton confinement and free carrier screening of the electron-hole interaction.

CsI (Tl) scintillators for recording α-particles

The preparation, performance, and properties of thalliumactivated cesium iodide crystals for recording alpha particles are given. The CsI(Tl) crystals are of large diameter (from 30 to 55 mm) and have gocd resolving power for the detection, spectrometric measurement, and other work with alpha radiation. For clarification of the performance of the crystals, the spectral resolution of the alpha line of americium-241 is given. The effects of crystal thickness and diameter on luminescence yield are shown. (N.W.R.)

Effect of Crystal Thickness and Geometry on the Alpha-Particle Resolution of CsI (Tl)

The resolution of CsI(Tl) for Po210 alpha particles has been measured as a function of crystal thickness. The best resolution of a ½-in. diam cylindrical crystal was obtained for a thickness of 0.38 mm, and the effect of thickness on the resolution is discussed. Based on the proposed model, a conical crystal was designed, which yielded a line width of 1.8% for Po210 alpha particles with a selected photomultiplier tube.