Nano Hotplate Fabrication for Metal Oxide-Based Gas Sensors by Combining Electron Beam and Focused Ion Beam Lithography

Metal oxide semiconductor (MOS) gas sensors are widely used for gas detection. Typically, the hotplate element is the key component in MOS gas sensors which provide a proper and tunable operation temperature. However, the low power efficiency of the standard hotplates greatly limits the portable application of MOS gas sensors. The miniaturization of the hotplate geometry is one of the most effective methods used to reduce its power consumption. In this work, a new method is presented, combining electron beam lithography (EBL) and focused ion beam (FIB) technologies to obtain low power consumption. EBL is used to define the low-resolution section of the electrode, and FIB technology is utilized to pattern the high-resolution part. Different Au++ ion fluences in FIBs are tested in different milling strategies. The resulting devices are characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and secondary ion mass spectrometry (SIMS). Furthermore, the electrical resistance of the hotplate is measured at different voltages, and the operational temperature is calculated based on the Pt temperature coefficient of resistance value. In addition, the thermal heater and electrical stability is studied at different temperatures for 110 h. Finally, the implementation of the fabricated hotplate in ZnO gas sensors is investigated using ethanol at 250 °C.

Fabrication of a Highly NO2-Sensitive Gas Sensor Based on a Defective ZnO Nanofilm and Using Electron Beam Lithography

Hazardous substances produced by anthropic activities threaten human health and the green environment. Gas sensors, especially those based on metal oxides, are widely used to monitor toxic gases with low cost and efficient performance. In this study, electron beam lithography with two-step exposure was used to minimize the geometries of the gas sensor hotplate to a submicron size in order to reduce the power consumption, reaching 100 °C with 0.09 W. The sensing capabilities of the ZnO nanofilm against NO2 were optimized by introducing an enrichment of oxygen vacancies through N2 calcination at 650 °C. The presence of oxygen vacancies was proven using EDX and XPS. It was found that oxygen vacancies did not significantly change the crystallographic structure of ZnO, but they significantly improved the electrical conductivity and sensing behaviors of ZnO film toward 5 ppm of dry air.

Insights into the Sensing Mechanism of a Metal-Oxide Solid Solution via Operando Diffuse Reflectance Infrared Fourier Transform Spectroscopy

Recently, the influence of Nb addition in the oxide solid solution of Sn and Ti was investigated with regard to the morphological, structural and electrical properties for the production of chemoresistive gas sensors. (Sn,Ti,Nb)xO2-based sensors showed promising features for ethanol monitoring in commercial or industrial settings characterized by frequent variation in relative humidity. Indeed, the three-metal solid solution highlighted a higher response level vs. ethanol than the most widely used SnO2 and a remarkably low effect of relative humidity on the film resistance. Nevertheless, lack of knowledge still persists on the mechanisms of gas reaction occurring at the surface of these nanostructures. In this work, operando Diffuse Reflectance Infrared Fourier Transform spectroscopy was used on SnO2- and on (Sn,Ti,Nb)xO2-based sensors to combine the investigations on the transduction function, i.e., the read-out of the device activity, with the investigations on the receptor function, i.e., compositional characterization of the active sensing element in real time and under operating conditions. The sensors performance was explained by probing the interaction of H2O and ethanol molecules with the material surface sites. This information is fundamental for fine-tuning of material characteristics for any specific gas sensing applications.

New Approach for the Detection of Sub-ppm Limonene: An Investigation through Chemoresistive Metal-Oxide Semiconductors

R-(+)-limonene, one of the major constituents of citrus oils, is a monoterpene that is widely used as a fragrance additive in cosmetics, foods, and industrial solvents. Nowadays, its detection mainly relies on bulky and expensive analytical methods and only a few research works proved its revelation through affordable and portable sensors, such as electrochemical and quartz crystal microbalance sensors. In response to the demand for effective miniaturized sensing devices to be integrated into Internet of Things systems, this study represents a pioneering investigation of chemoresistive gas sensor capabilities addressed to R-(+)-limonene detection. An array of seven metal-oxide sensors was exploited to perform a complete electrical characterization of the target analyte. The experimental evidence allowed us to identify the WO₃-based sensor as the most promising candidate for R-(+)-limonene detection. The material was highly sensitive already at sub-ppm concentrations (response of 2.5 at 100 ppb), consistent with applicative parameters, and it resulted in selective vs. different gases at a lower operating temperature (200 °C) than the other sensors tested. Furthermore, it exhibited a humidity-independent behavior under real-life conditions (relative humidity > 20%). Finally, the WO₃ sensor also demonstrated a remarkable cross-selectivity, thus enabling its exploitation in cutting-edge applications.

Functionalization of Indium Oxide for Empowered Detection of CO2 over an Extra-Wide Range of Concentrations

Carbon capture, storage, and utilization have become familiar terms when discussing climate change mitigation actions. Such endeavors demand the availability of smart and inexpensive devices for CO₂ monitoring. To date, CO₂ detection relies on optical properties and there is a lack of devices based on solid-state gas sensors, which can be miniaturized and easily made compatible with Internet of Things platforms. With this purpose, we present an innovative semiconductor as a functional material for CO₂ detection. A nanostructured In₂O₃ film, functionalized by Na, proves to enhance the surface reactivity of pristine oxide and promote the chemisorption of even rather an inert molecule as CO₂. An advanced operando equipment based on surface-sensitive diffuse infrared Fourier transform is used to investigate its improved surface reactivity. The role of sodium is to increase the concentration of active sites such as oxygen vacancies and, in turn, to strengthen CO₂ adsorption and reaction at the surface. It results in a change in film conductivity, i.e., in transduction of a concentration of CO₂. The films exhibit excellent sensitivity and selectivity to CO₂ over an extra-wide range of concentrations (250-5000 ppm), which covers most indoor and outdoor applications due to the marginal influence by environmental humidity.

Elucidating the Ambient Stability and Gas Sensing Mechanism of Nickel-Decorated Phosphorene for NO2 Detection: A First-Principles Study

In the field of layered two-dimensional functional materials, black phosphorus has attracted considerable attention in many applications due to its outstanding electrical properties. It has experimentally shown superior chemical sensing performance for the room temperature detection of NO₂, highlighting high sensitivity at a ppb level. Unfortunately, pristine black phosphorus demonstrated an unstable functionality due to the fast degradation of the material when exposed to the ambient atmosphere. In the present work, a deepened investigation by density functional theory was carried out to study how nickel decoration of phosphorene can improve the stability of the material. Further, an insight into the sensing mechanism of nickel-loaded phosphorene toward NO₂ was given and compared to pristine phosphorene. This first-principles study proved that, by introducing nickel adatoms, the band gap of the material decreases and the positions of the conduction band minimum and the valence band maximum move toward each other, resulting in a drop in the conduction band minimum under the redox potential of O₂/O₂ ^-, which may result in a more stable material. Studying the adsorption of O₂ molecules on pristine phosphorene, we also proved that all oxygen molecules coming from the surrounding atmosphere react with phosphorus atoms in the layer, resulting in the oxidation of the material forming oxidized phosphorus species (PO _x ). Instead, by introducing nickel adatoms, part of the oxygen from the surrounding atmosphere reacts with nickel atoms, resulting in a decrease of the oxidation rate of the material and in subsequent long-term stability of the device. Finally, possible reaction paths for the detection of NO₂ are given by charge transfer analyses, occurring at the surface during the adsorption of oxygen molecules and the interaction with the target gas.

Design of a Metal-Oxide Solid Solution for Sub-ppm H2 Detection

Hydrogen is largely adopted in industrial processes and is one of the leading options for storing renewable energy. Due to its high explosivity, detection of H₂ has become essential for safety in industries, storage, and transportation. This work aims to design a sensing film for high-sensitivity H₂ detection. Chemoresistive gas sensors have extensively been studied for H₂ monitoring due to their good sensitivity and low cost. However, further research and development are still needed for a reliable H₂ detection at sub-ppm concentrations. Metal-oxide solid solutions represent a valuable approach for tuning the sensing properties by modifying their composition, morphology, and structure. The work started from a solid solution of Sn and Ti oxides, which is known to exhibit high sensitivity toward H₂. Such a solid solution was empowered by the addition of Nb, which─according to earlier studies on titania films─was expected to inhibit grain growth at high temperatures, to reduce the film resistance and to impact the sensor selectivity and sensitivity. Powders were synthesized through the sol-gel technique by keeping the Sn-Ti ratio constant at the optimal value for H₂ detection with different Nb concentrations (1.5-5 atom %). Such solid solutions were thermally treated at 650 and 850 °C. The sensor based on the solid solution calcined at 650 °C and with the lowest content of Nb exhibited an extremely high sensitivity toward H₂, paving the way for H₂ ppb detection. For comparison, the response to 50 ppm of H₂ was increased 6 times vs SnO₂ and twice that of (Sn,Ti)_xO₂.

Investigation on Sensing Performance of Highly Doped Sb/SnO2

Tin dioxide (SnO₂) is the most-used semiconductor for gas sensing applications. However, lack of selectivity and humidity influence limit its potential usage. Antimony (Sb) doped SnO₂ showed unique electrical and chemical properties, since the introduction of Sb ions leads to the creation of a new shallow band level and of oxygen vacancies acting as donors in SnO₂. Although low-doped SnO₂:Sb demonstrated an improvement of the sensing performance compared to pure SnO₂, there is a lack of investigation on this material. To fill this gap, we focused this work on the study of gas sensing properties of highly doped SnO₂:Sb. Morphology, crystal structure and elemental composition were characterized, highlighting that Sb doping hinders SnO₂ grain growth and decreases crystallinity slightly, while lattice parameters expand after the introduction of Sb ions into the SnO₂ crystal. XRF and EDS confirmed the high purity of the SnO₂:Sb powders, and XPS highlighted a higher Sb concentration compared to XRF and EDS results, due to a partial Sb segregation on superficial layers of Sb/SnO₂. Then, the samples were exposed to different gases, highlighting a high selectivity to NO₂ with a good sensitivity and a limited influence of humidity. Lastly, an interpretation of the sensing mechanism vs. NO₂ was proposed.

Air Stable Nickel-Decorated Black Phosphorus and Its Room-Temperature Chemiresistive Gas Sensor Capabilities

In the rapidly emerging field of layered two-dimensional functional materials, black phosphorus, the P-counterpart of graphene, is a potential candidate for various applications, e.g., nanoscale optoelectronics, rechargeable ion batteries, electrocatalysts, thermoelectrics, solar cells, and sensors. Black phosphorus has shown superior chemical sensing performance; in particular, it is selective for the detection of NO2, an environmental toxic gas, for which black phosphorus has highlighted high sensitivity at a ppb level. In this work, by applying a multiscale characterization approach, we demonstrated a stability and functionality improvement of nickel-decorated black phosphorus films for gas sensing prepared by a simple, reproducible, and affordable deposition technique. Furthermore, we studied the electrical behavior of these films once implemented as functional layers in gas sensors by exposing them to different gaseous compounds and under different relative humidity conditions. Finally, the influence on sensing performance of nickel nanoparticle dimensions and concentration correlated to the decoration technique and film thickness was investigated.

Development of a dedicated instrumentation for electrical and thermal characterization of chemiresistive gas sensors

This work presents the design and validation of a measuring instrumentation for an easy, complete, and tunable characterization of chemiresistive gas sensors based on metal-oxide semiconductors. The equipment, described in depth both as hardware and as software, was designed to monitor the electrical behavior of gas sensors in controlled thermodynamic conditions. The main goal of this setup is to synchronize the electrical characterization with different measuring conditions, i.e., operating temperature, relative humidity, and gas target concentration. This operation allows us to automate various measurement protocols, otherwise impossible to obtain manually. In particular, this instrumentation permits to correlate the response of a chemiresistive gas sensor to the applied voltage, to its working temperature, and to the gas concentration, automating the acquisition of the current-voltage characteristic and the current-temperature characteristic (Arrhenius plot) of sensing films. The experimental setup was validated by reporting the electrical characterization of a standard metal-oxide-based gas sensing material, such as SnO₂, working under different thermodynamic conditions.

First-Principles Study of Electronic Conductivity, Structural and Electronic Properties of Oxygen-Vacancy-Defected SnO₂

The use of computer simulations has become almost essential for prediction and interpretation of device's performance. In gas sensing field, the simulation of specific conditions, which determine the physical-chemical properties of widely used metal oxide semiconductors, can be used to investigate the performance of gas sensors based on these kinds of materials. The aim of this work was to evaluate the physical-chemical properties of tin dioxide employed for environmental and health gas sensing application and to investigate the influence of oxygen vacancies on its properties by means of density functional theory. Two samples, having different concentration of oxygen vacancies, were deeply studied in terms of their structural, electronic and electrical properties. It was proved the influence of oxygen vacancies on lattice parameter. By increasing oxygen vacancies concentration, the increased number of impurity states took these closer to the conduction band minimum, which can lead to an easier adsorption process of oxygen species and their availability to be exchanges with the molecules of the target gases. In this way a reduction of the operating temperature can be observed, thus reducing the power consumption of devices, while keeping the catalytic performance of the material.

Evaluation of the public health risk for autochthonous transmission of mosquito-borne viruses in southern Switzerland

Epidemics of mosquito-borne diseases such as chikungunya and dengue fever are becoming more frequent around the world. In Switzerland, autochthonous cases have not been reported so far, although the presence of the vector Aedes albopictus in urban areas of southern Switzerland increases the risk of indigenous transmissions subsequent to imported cases. In 2018, the potential risk of an outbreak of arboviral diseases was assessed in five municipalities of southern Switzerland. The population abundance of Ae. albopictus was evaluated during the mosquito active season by the mean number of Ae. albopictus bites per day per person (estimated using the human landing collection method) and the risk of outbreak in the case of the introduction of chikungunya, dengue or Zika viruses was estimated. In the five localities investigated, no epidemic risk appeared to be present for any of the arboviruses taken into consideration in the initial months (i.e. mid-May to end of July) of Ae. albopictus activity. In the case of the introduction of chikungunya (mutated or not), dengue (serotype 1) or Zika (African lineage) viruses during mid-end August, an epidemic could have occurred in all the municipalities investigated. In mid-end September, the introduction of same arboviruses could have led to an epidemic in three of the five municipalities investigated.

X-ray characterization of self-standing bent Si crystal plates for Large Hadron Collider beam extraction

Bent crystals can be used to deflect high-energy charged particles for beam extraction and/or beam collimation at accelerator facilities, thanks to the channelling phenomenon. In the present paper, two perfect silicon mono-crystals were bent using two different methods: sandblasting and the application of a carbon fibre composite. In particular, these samples were obtained for the realization of bent crystal prototypes to be used to steer the 7 TeV proton beam of the Large Hadron Collider in the context of the CRYSBEAM project. The two bending methods were selected since they allow a very homogeneous curvature of the crystals to be obtained, which is essential for high channelling efficiency. Moreover, the deformation obtained is self-standing, i.e. there is no need for any external device to keep the samples bent. Self-standing curvature can be useful because the presence of an external bender could be a severe limitation in the collider beam-pipe. The curvature of the samples was measured through high-energy X-ray diffraction at the ID11 beamline of the European Synchrotron Radiation Facility in Grenoble, France. Since the diffraction efficiencies obtained were in good agreement with theoretical expectations, it follows that the manufacturing techniques did not damage the samples, i.e. the crystallographic quality was preserved. Finally, the crystal quality of the sandblasted sample was investigated in detail at the synchrotron source at Karlsruhe Institute of Technology by X-ray white-beam topography. The measurements showed no diffusion of defects from the machined surfaces to the crystal bulk.

Nanostructured Chemoresistive Sensors for Oncological Screening and Tumor Markers Tracking: Single Sensor Approach Applications on Human Blood and Cell Samples

Preventive screening does not only allow to preemptively intervene on pathologies before they can harm the host; but also to reduce the costs of the intervention itself; boosting the efficiency of the NHS (National Health System) by saving resources for other purposes. To improve technology advancements in this field; user-friendly yet low-cost devices are required; and various applications for gas sensors have been tested and proved reliable in past studies. In this work; cell cultures and blood samples have been studied; using nanostructured chemoresistive sensors; to both verify if this technology can reliably detect tumor markers; and if correlations between responses from tumor line metabolites and the screening outcomes on human specimens could be observed. The results showed how sensors responded differently to the emanations from healthy and mutant (for cells) or tumor affected (for blood) samples, and how those results were consistent between them, since the tumoral specimens had higher responses compared to the ones of their healthy counterparts. Even though the patterns in the responses require a bigger population to be defined properly; it appeared that the different macro-groups between the same kind of samples are distinguishable from some of the sensors chosen in the study; giving promising outcomes for further research.

Bent crystals for efficient beam steering of multi TeV-particle beams

Charged particle beams can be manipulated by exploiting the channeling phenomenon in bent crystals. Two plate-like crystals, bent by mechanical holders, were manufactured and characterised for such purpose at the Sensor and Semiconductor Laboratory in Ferrara, Italy. An anticlastic curvature was obtained for these crystals, achieving a steering angle of the order of 1 mrad, which is about 20 times larger than the values currently achieved for the bent crystals used in the LHC for collimation experiments. Finally, a Geant4 simulation was performed to study the channeling efficiency for beam deflection with 400 GeV/c and 7 TeV/c proton beams. Such crystals represent technological progress in the development of bent crystals for highly energetic charged particle beams. Indeed, they are designed to impart an angular kick to a 7 TeV/c proton beam with unprecedented high efficiency. Therefore, this study demonstrates the possibility of realizing bent crystals suitable for beam extraction in high-energy hadron accelerators, such as LHC or at the future FCC. A further series of studies should be conducted to evaluate the channeling efficiency and the deflection angle of the realized crystals via a charged proton beam.

Study of inelastic nuclear interactions of 400 GeV/c protons in bent silicon crystals for beam steering purposes

Inelastic nuclear interaction probability of 400 GeV/c protons interacting with bent silicon crystals was investigated, in particular for both types of crystals installed at the CERN Large Hadron Collider for beam collimation purposes. In comparison to amorphous scattering interaction, in planar channeling this probability is ∼ 36 % for the quasi-mosaic type (planes (111)), and ∼ 27 % for the strip type (planes (110)). Moreover, the absolute inelastic nuclear interaction probability in the axial channeling orientation, along the ⟨ 110 ⟩ axis, was estimated for the first time, finding a value of 0.6 % for a crystal 2 mm long along the beam direction, with a bending angle of 55 μ rad. This value is more than two times lower with respect to the planar channeling orientation of the same crystal, and increases with the vertical angular misalignment. Finally, the correlation between the inelastic nuclear interaction probability in the planar channeling and the silicon crystal curvature is reported.

Diversity and seasonal abundances of mosquitoes at potential arboviral transmission sites in two different climate zones in Switzerland

Pathogens of medical or veterinary significance that are transmitted by mosquitoes (Diptera: Culicidae) are (re-)emerging in Europe [e.g. West Nile virus (WNV), Dirofilaria nematodes]. Little is known about the spatiotemporal abundances of mosquito species in Switzerland. Therefore, mosquito population dynamics were investigated, focusing on areas of risk for sylvatic or synanthropic transmission, such as natural sites and suburban sites on either side of the Alpine crest. Repeated collections were made using Centers for Disease Control (CDC) traps, juvenile sampling and ovitrapping. A total of 122 831 mosquito specimens of 21 taxa were identified. Levels of mosquito species richness were similar at suburban sites and in natural zones in Switzerland. Mosquito abundances and seasonality were analysed with generalized linear mixed models based on 382 CDC trap samples (29 454 females) and revealed Aedes annulipes/cantans, Aedes geniculatus, Aedes japonicus, Aedes sticticus, Aedes vexans, Coquillettidia richiardii and Culex pipiens/torrentium as the dominant species overall. Abundances of these species were season-dependent in most cases. There was an effect of site with regard to abundance (higher in natural zones), but not with respect to seasonality. Together with data on vector competence and the host preferences of different species, the present data contribute to assessments of risk for pathogen transmission. For example, both natural and suburban environments seem feasible as sites for amplification cycles of WNV and transmission to mammals.

Electromagnetic dipole moments of charged baryons with bent crystals at the LHC

We propose a unique program of measurements of electric and magnetic dipole moments of charm, beauty and strange charged baryons at the LHC, based on the phenomenon of spin precession of channeled particles in bent crystals. Studies of crystal channeling and spin precession of positively- and negatively-charged particles are presented, along with feasibility studies and expected sensitivities for the proposed experiment using a layout based on the LHCb detector.

Crystalline Microporous Organosilicates with Reversed Functionalities of Organic and Inorganic Components for Room-Temperature Gas Sensing

A deepened investigation on an innovative organic-inorganic hybrid material, referred to as ECS-14 (where ECS = Eni carbon silicates), revealed the possibility to use them as gas sensors. Indeed, among ECS phases, the crystalline state and the hexagonal microplateletlike morphology characteristic of ECS-14 seemed favorable properties to obtain continuous and uniform films. ECS-14 phase was used as functional material in screen-printable compositions and was thus deposited by drop coating for morphological, structural, thermal, and electrical characterizations. Possible operation at room temperature was investigated as technological progress, offering intrinsic safety in sensors working in harsh or industrial environments and avoiding high power consumption of most common sensors based on metal oxide semiconductors. Electrical characterization of the sensors based on ECS-14 versus concentrations of gaseous analytes gave significant results at room temperature in the presence of humidity, thereby demonstrating fundamental properties for a good quality sensor (speed, reversibility, and selectivity) that make them competitive with respect to systems currently in use. Remarkably, we observed functionality reversal of the organic and inorganic components; that is, in contrast to other hybrids, for ECS-14 the functional site has been ascribed to the inorganic phase while the organic component provided structural stability to the material. The sensing mechanism for humidity was also investigated.

Observation of Quasichanneling Oscillations

We report on the first experimental observations of quasichanneling oscillations, recently seen in simulations and described theoretically. Although above-barrier particles penetrating a single crystal are generally seen as behaving almost as in an amorphous substance, distinct oscillation peaks nevertheless appear for particles in that category. The quasichanneling oscillations were observed at SLAC National Accelerator Laboratory by aiming 20.35 GeV positrons and electrons at a thin silicon crystal bent to a radius of R=0.15  m, exploiting the quasimosaic effect. For electrons, two relatively faint quasichanneling peaks were observed, while for positrons, seven quasichanneling peaks were clearly identified.

Homogeneous self-standing curved monocrystals, obtained using sandblasting, to be used as manipulators of hard X-rays and charged particle beams

A technique to obtain self-standing curved crystals has been developed. The method is based on a sandblasting process capable of producing an amorphized layer on the substrate. It is demonstrated that the amorphized layer behaves as a thin compressive film, causing the curvature of the substrate. This procedure permits the fabrication of homogeneously curved crystals in a fast and economical way. It is shown that a sandblasted crystal can be used as an X-ray optical element for astrophysical or medical applications. A sandblasted bent crystal can also be used as an optical element for steering charged particles in accelerator beamlines. Several samples were manufactured and bent using the sandblasting method at the Sensor and Semiconductor Laboratory of Ferrara, Italy. Their curvature was verified using interferometric profilometry, showing a deformation in agreement with the Stoney formalism. The curvature of the machined samples was also tested using γ-ray diffraction at the Institut Laue–Langevin (ILL), Grenoble, France. A good agreement with the dynamical theory of diffraction was observed. In particular, the experiment showed that the crystalline quality of the bulk was preserved. Moreover, the method allowed curved samples to be obtained free of any additional material. Finally, a crystalline undulator was produced using sandblasting and tested using γ-ray diffraction at the ILL. The crystal showed a precise undulating pattern, so it will be suitable for hard X-ray production.

Origin of quasi-mosaic effect for symmetric skew planes in a silicon or germanium plate

Bent silicon and germanium crystals are used for several modern physics applications, above all for focusing of hard X-rays and for steering of charged particle beams by means of channeling and related coherent phenomena. In particular, anisotropic deformations are effectively exploited for these applications. A typical anisotropic deformation that is used is the quasi-mosaic (QM) curvature. It involves the bending of crystallographic planes that would be otherwise flat in the case of an isotropic medium. Here, the curvature the {110} planes was obtained through the quasi-mosaic effect in the symmetric configuration for the first time. This achievement is important because the {110} family of planes is highly efficient for both the applications mentioned above. Until now, the curvature of {110} planes in the QM configuration has not been used because it vanishes if the direction of the planes is aligned with the applied moment that bends the crystal plate. Indeed, to obtain the curvature of this particular family of crystallographic planes, the 〈110〉 direction has not to be aligned with respect to the imparted moment that bends the plate, i.e. the {110} planes have to be skew planes. Experimental verification of the quasi-mosaic curvature for the {110} planes was provided through hard X-ray diffraction at beamline ID15A of the European Synchrotron Radiation Facility in Grenoble, France, showing good agreement with the theoretical expectation.

Laue lens to focus an X-ray beam for radiation therapy

A Laue lens is an optical component composed of a set of crystals that produce a convergent beam exploiting X-ray diffraction in transmission geometry. Employment of a system formed by a properly designed Laue lens coupled with an X-ray unit to selectively irradiate tumours is proposed. A convergent beam leads to a depth dose profile with a pronounced peak at the focal depth, which may result in a high precision of the dose delivery. Using a custom-made Monte Carlo code and the GAMOS code, we carried out a design study to determine the geometry and the optimal features of the crystals composing the lens. As an application, a Laue lens capable of focusing a 80 keV beam 50 cm downstream of the lens has been designed. The lens is composed of an ensemble of Si crystals with curved diffracting planes. The lens produces a focal spot of 2 mm enclosing 7.64 × 106 photons for an electron charge of 1 mC impinging on the surface of the X-ray tube anode. The combination of these important figures of merit makes the proposed system suitable for irradiating both sub-cm and larger tumour masses efficiently. A dose of 2 Gy can be delivered to a small tumour in a few seconds, sparing at the same time the surrounding tissues.

Orientational Coherent Effects of High-Energy Particles in a LiNbO3 Crystal

A bent lithium niobate strip was exposed to a 400-GeV/c proton beam at the external lines of CERN Super Proton Synchrotron to probe its capabilities versus coherent interactions of the particles with the crystal such as channeling and volume reflection. Lithium niobate (LiNbO3) exhibits an interplanar electric field comparable to that of Silicon (Si) and remarkable piezoelectric properties, which could be exploited for the realization of piezo-actuated devices for the control of high-energy particle beams. In contrast to Si and germanium (Ge), LiNbO3 shows an intriguing effect; in spite of a low channeling efficiency (3%), the volume reflection maintains a high deflection efficiency (83%). Such discrepancy was ascribed to the high concentration (10(4) per cm2) of dislocations in our sample, which was obtained from a commercial wafer. Indeed, it has been theoretically shown that a channeling efficiency comparable with that of Si or Ge would be attained with a crystal at low defect concentration (less than ten per cm2). To better understand the role of dislocations on volume reflection, we have worked out computer simulation via dynecharm++ Monte Carlo code to study the effect of dislocations on volume reflection. The results of the simulations agree with experimental records, demonstrating that volume reflection is more robust than channeling in the presence of dislocations.

The `quasi-mosaic' effect in crystals and its applications in modern physics

`Quasi-mosaicity' is an effect of anisotropy in crystals that permits one to obtain a curvature of internal crystallographic planes that would be flat otherwise. The term `quasi-mosaicity' was introduced by O. Sumbaev in 1957. The concept of `quasi-mosaicity' was then retrieved about ten years ago and was applied to steering of charged-particle beams at the Super Proton Synchrotron at CERN. Beams were deviated by exploiting channeling and volume reflection phenomena in curved crystals that show the `quasi-mosaic' effect. More recently, a crystal of this kind was installed in the Large Hadron Collider at CERN for beam collimation by the UA9 collaboration. Since 2011, another important application involving the `quasi-mosaic' effect has been the focalization of hard X-rays and soft γ-rays. In particular, the possibility of obtaining both high diffraction efficiency and the focalization of a diffracted beam has been proved, which cannot be obtained using traditional diffracting crystals. A comprehensive survey of the physical properties of `quasi-mosaicity' is reported here. Finally, experimental demonstrations for adjustable values of the `quasi-mosaic' curvature are provided.

AniCryDe: calculation of elastic properties in silicon and germanium crystals

A code to calculate the anisotropic elastic properties in a silicon or germanium crystal is introduced. The program, namedAniCryDe, allows the user to select the crystallographic configuration of interest. For the selected crystallographic orientation,AniCryDecalculates several key mechanical parameters, such as Young's modulus, Poisson's ratio and the shear modulus. Furthermore, the program displays both the compliance and the stiffness tensors concerning the crystallographic orientation of interest. The code enables the user to set several parameters through a user-friendly control stage. As a result, the user obtains the complete displacement field of a deformed crystal and the curvature of any crystallographic plane. Manufacturing wafer defects such as miscut and misflat angle are also taken into account.

The experimental setup of the Interaction in Crystals for Emission of RADiation collaboration at Mainzer Mikrotron: Design, commissioning, and tests

Silicon/germanium flat/bent crystals are thin devices able to efficiently deflect charged particle GeV-energy beams up to a few hundreds of μrad; moreover, high intensity photons can be efficiently produced in the so-called Multi-Volume Reflection (MVR) and Multiple Volume Reflections in One Crystal (MVROC) conditions. In the last years, the research interest in this field has moved to the dynamic studies of light negative leptons in the low energy range: the possibility to deflect negative particles and to produce high intensity γ sources via the coherent interactions with crystals in the sub-GeV energy range has been proved by the ICE-RAD (Interaction in Crystals for Emission of RADiation) Collaboration at the MAinzer MIkrotron (MAMI, Germany). This paper describes the setup used by the ICE-RAD experiment for the crystals characterization (both in terms of deflection and radiation emission properties): a high precision goniometer is used to align the crystals with the incoming beam, while a silicon based profilometer and an inorganic scintillator reconstruct, respectively, the particle position and the photon spectra after the samples. The crystals manufacturing process and their characterization, the silicon profilometer commissioning at the CERN PS T9 beamline, and the commissioning of the whole setup installed at MAMI are presented.

Observation of deflection of a beam of multi-GeV electrons by a thin crystal

We report on an experiment performing channeling and volume reflection of a high-energy electron beam using a quasimosaic, bent silicon (111) crystal at the End Station A Test Beam at SLAC. The experiment uses beams of 3.35 and 6.3 GeV. In the channeling orientation, deflections of the beam of 400 μrad for both energies with about 22% efficiency are observed, while in the volume-reflection orientation, deflection of the beam by 120 μrad at 3.35 GeV and by 80 μrad at 6.3 GeV is observed with 86%-95% efficiency. Quantitative measurements of the channeling efficiency, surface transmission, and dechanneling length are taken. These are the first quantitative measurements of channeling and volume reflection using a primary beam of multi-GeV electrons.

Design study of a Laue lens for nuclear medicine

A Laue lens is an ensemble of crystals capable of focusing, through diffraction in transmission geometry, a fraction of the photons emitted by an X- or γ-ray source onto a small area of a detector. The present study facilitates a thorough understanding of the effect of each system parameter on the efficiency, the resolution and the field of view of the lens. In this way, the structure and the size of the crystals can be set to achieve a compact lens capable of providing a high-resolution image of the radioactivity distribution lying inside a restricted region of a patient's body. As an application, a Laue lens optimized at 140.5 keV, the γ-line emitted by99mTc, has been designed. The lens is composed of ten rings of Ge crystals with curved diffracting planes and focuses the photons onto a detector 50 cm apart from the source with 1.16 × 10−5efficiency and 0.2 mm resolution. The combination of these two important figures of merit makes the proposed device better performing than pinhole single photon emission computed tomography, which is the technique employed for top-resolution images in nuclear medicine. Finally, the imaging capability of the designed lens has been tested through simulations performed with a custom-made Monte Carlo code.

High-efficiency diffraction and focusing of X-rays through asymmetric bent crystalline planes

The grooving technique was employed for manufacturing a self-standing curved Ge crystal. The crystal focuses hard X-rays with high efficiency by diffraction in Laue geometry through asymmetric bent planes. The sample was tested at the Institut Laue–Langevin (Grenoble, France), undergoing two types of characterization. A monochromatic and low-divergence γ-ray beam was used to test the curvature of asymmetric planes, showing a diffraction performance better than for any mosaic crystal under equal conditions. Then, the focusing capability of the crystal was probed through a polychromatic and fine-focus hard X-ray beam. Asymmetric (220) planes were chosen for analysis because of the impossibility of obtaining a curvature along this family of planesviaany symmetric configuration in focusing crystals. A method for calculating the curvatures induced in any family of lattice planes is also presented.

High diffraction efficiency with hard X-rays through a thick silicon crystal bent by carbon fiber deposition

The diffraction capability of two crystalline silicon plates bent by carbon fiber deposition has been studied. The performed treatment induced a permanent curvature in the samples, resulting in an increase of the diffraction efficiency. The obtained efficiencies are constant over a wide angular range and close to the theoretical expectations, meaning that the curvatures were homogeneous. Most importantly, the bending technique allowed the manufacture of bent samples up to 5 mm thick and with a radius of curvature down to 30 m. With such a technique, the fabrication of crystals for the realization of a hard X-ray concentrator (Laue lens) for astrophysical purposes is enabled.

Steering of a sub-GeV electron beam through planar channeling enhanced by rechanneling

We report the observation of efficient steering of a 855 MeV electron beam at MAMI (MAinzer MIkrotron) facilities by means of planar channeling and volume reflection in a bent silicon crystal. A 30.5  μm thick plate of (211) oriented Si was bent to cause quasimosaic deformation of the (111) crystallographic planes, which were used for coherent interaction with the electron beam. The experimental results are analogous to those recorded some years ago at energy higher than 100 GeV, which is the only comparable study to date. Monte Carlo simulations demonstrated that rechanneling plays a considerable role in a particle's dynamics and hinders the spoiling of channeled particles. These results allow a better understanding of the dynamics of electrons subject to coherent interactions in a bent silicon crystal in the sub-GeV energy range, which is relevant for realization of innovative x-ray sources based on channeling in periodically bent crystals.

High-efficiency focusing of hard X-rays exploiting the quasi-mosaic effect in a bent germanium crystal

A germanium crystal was bent through a grid of superficial grooves, manufactured on the sample surface. The resulting diffraction planes were bent thanks to quasi-mosaicity, which is an effect of mechanical anisotropy in crystals. High integrated diffraction efficiency was achieved in symmetric Laue geometry with a monochromatic X-ray beam set at 150 and 300 keV. It is demonstrated that the sample is capable of efficiently focusing X-rays. Such crystals can be used as optical components to focalize X- and γ-rays in a high-resolution Laue lens.

Broad and intense radiation accompanying multiple volume reflection of ultrarelativistic electrons in a bent crystal

The radiation emitted by 120  GeV/c electrons traversing a single bent crystal under multiple volume reflection orientation is investigated. Multiple volume reflection in one crystal occurs as a charged particle impacts on a bent crystal at several axial channeling angles with respect to a crystal axis. The resulting energy-loss spectrum of electrons was very intense over the full energy range up to the nominal energy of the beam. As compared to the radiation emission by an individual volume reflection, the energy-loss spectrum is more intense and peaks at an energy 3 times greater. Experimental results are compared to a theoretical approach based on the direct integration of the quasiclassical Baier and Katkov formula. In this way, it is possible to determine the mean number of photons emitted by each electron and, thus, to extract the single-photon spectrum, which is broad and intense. The soft part of the radiation spectrum is due to the contribution of coherent interaction between electrons and several reflecting planes intersecting the same crystal axis, whereas the hard part is mainly connected to coherent bremsstrahlung induced by correlated scattering of electrons by atomic strings (string of strings scattering and radiation). The radiation generation by multiple volume reflection takes place over a broad angular range of the incident beam with respect to coherent bremsstrahlung and channeling radiation in straight crystals. Therefore, this type of radiation can be exploited for applications, such as beam dump and collimation devices for future linear colliders.

Ordered stacking of crystals with adjustable curvatures for hard X- and γ-ray broadband focusing

A stack of plate-like curved crystals is proposed as an optical element for X- and γ-ray focusing. Si mono-crystal plates have been bent by surface grooving and positioned one over the other to form a stack. The relative alignment of the curved diffracting planes in the stack has been tested by hard X-ray diffractometry using a polychromatic and divergent beam. The stack exhibited a single and well defined spot under X-ray diffraction, highlighting that the plates are sufficiently aligned to behave as a single crystal. The curvature of the plates in the stack can be precisely set by tightly controlling the experimental parameters of grooving. Once set, the curvature is self-standing, i.e. it is maintained without any need for an external bender. Thanks to the stacking, it would be possible to realize optical elements with arbitrarily large size. This achievement has important implications for the realization of satellite-borne experiments in astrophysics and instruments for nuclear medicine with superior resolution.

Self-standing quasi-mosaic crystals for focusing hard X-rays

A quasi mosaic bent crystal for high-resolution diffraction of X and γ rays has been realized. A net curvature was imprinted to the crystal thanks to a series of superficial grooves to keep the curvature without external devices. The crystal highlights very high diffraction efficiency due to quasi mosaic curvature. Quasi mosaic crystals of this kind are proposed for the realization of a high-resolution focusing Laue lens for hard X-rays.

Coherent effects of high-energy particles in a graded Si(1-x)Ge(x) crystal

A graded Si(1-x)Ge(x) crystal has been manufactured for operation with high-energy protons to excite coherent interactions of the particles with the crystal such as channeling and volume reflection. The crystal had the shape of a parallelepiped though its (111) atomic planes were curved at a radius of 25.6 m because of the graded Ge content. The crystal was exposed to a 400  GeV/c proton beam at the external lines of CERN Super Proton Synchrotron to probe its capability to steer high-energy particles. Measured deflection efficiency was 62.0% under planar channeling and 96.0% under volume reflection. Such values are critically compared to their counterparts for a standard bent Si crystal under peer conditions. A Monte Carlo simulation of the dynamics of channeled and volume reflected particles in a graded crystal including the effect of Ge impurities and of lattice dislocations has been carried out. We found that the effect of crystal imperfections spoiled the efficiency of channeling while it negligibly affected the performance of volume reflection. We finally propose the usage of the graded crystal as a primary scatterer to aid halo collimation for the new generation of hadronic machines. As a unique feature, a properly cut graded crystal circumvents the problem of the miscut angle, which is currently a severe limitation for implementation of crystal-assisted collimation.

Calculation of diffraction efficiency for curved crystals with arbitrary curvature radius

A model is proposed to calculate the diffraction efficiency of X-rays in Laue geometry for curved crystals with an arbitrary value of the curvature radius. The model generalizes the results based on the dynamical theory of diffraction, which are valid only for crystals with a radius of curvature lower than the critical curvature. The model is proposed for any kind of crystal, and its efficiency tends to one-half in the limit of a thick flat crystal. On the basis of this model, it was possible to reconsider the results of recently observed diffraction efficiency for curved crystals. Finally, the model sets an upper limit for diffraction efficiency of low-curvature curved crystals, this latter being useful in applications such as the construction of a hard X-ray Laue lens.

Development of a real-time PCR method for the detection of fossil 16S rDNA fragments of phototrophic sulfur bacteria in the sediments of Lake Cadagno

Lake Cadagno is a crenogenic meromictic lake situated in the southern range of the Swiss Alps characterized by a compact chemocline that has been the object of many ecological studies. The population dynamics of phototrophic sulfur bacteria in the chemocline has been monitored since 1994 with molecular methods such as 16S rRNA gene clone library analysis. To reconstruct paleo-microbial community dynamics, we developed a quantitative real-time PCR methodology for specific detection of 16S rRNA gene sequences of purple and green sulfur bacteria populations from sediment samples. We detected fossil 16S rDNA of nine populations of phototrophic sulfur bacteria down to 9-m sediment depth, corresponding to about 9500 years of the lake's biogeological history. These results provide the first evidence for the presence of 16S rDNA of anoxygenic phototrophic bacteria in Holocene sediments of an alpine meromictic lake and indicate that the water column stratification and the bacterial plume were already present in Lake Cadagno thousands of years ago. The finding of Chlorobium clathratiforme remains in all the samples analyzed shows that this population, identified in the water column only in 2001, was already a part of the lake's biota in the past.

Deflection of MeV protons by an unbent half-wavelength silicon crystal

The interaction of a 2 MeV proton beam with an ultrathin unbent Si crystal was studied through simulation and experiment. Crystal thickness along the beam was set at 92 nm, i.e., at half the oscillation wavelength of the protons in the crystal under planar channeling condition. As the nominal beam direction is inclined by less than the critical angle for planar channeling with respect to the crystal planes, under-barrier particles undergo half an oscillation and exit the crystal with the reversal of the transverse momenta; i.e., the protons are "mirrored" by the crystal planes. Over-barrier particles suffer deflection, too, to a direction opposite that of mirroring with a dynamics similar to that of volume reflection in a bent crystal. On the strength of such coherent interactions, charged particle beams can be efficiently steered through an ultrathin unbent crystal by the same physical processes as for thicker bent crystals.

Proposal for a Laue lens with quasi-mosaic crystalline tiles

Quasi-mosaicity is an effect of secondary bending within a crystal driven by crystalline anisotropy. This effect can be used to fabricate a series of curved crystals for the realization of a Laue lens. It is highlighted that crystals bent by the quasi-mosaic effect allow very high resolution focusing with respect to mosaic crystals. Under the same conditions for energy passband, crystal size and flux of incident photons, a Laue lens based on quasi-mosaic crystals would increase the signal-to-noise ratio by about an order of magnitude compared to the same lens with mosaic crystals. Moreover, no mosaic defocusing occurs for quasi-mosaic crystals.

Nanostructured (Sn,Ti, Nb)O2 Solid Solution for Hydrogen Sensing

A novel co-precipitation route for preparing pure nanograined (Ti, Sn, Nb)O2 solid solution has been accomplished. The solid solution containing the three elements has been synthesized with the molar ratios for Sn:Ti:Nb 100:42:5. Electron microscopy and X-ray diffraction have been adopted to observe the morphology, the crystalline structure and the mean grain radius. Calcining at 550, 650, 850 or 1050 °C for 2h, showed rutile-like single-phase.

The prepared powders have been deposited to produce gas sensors in form of thick films through screen-printing technology. SEM micrographs of both powders and films showed regularly-shaped particles with grain dimensions at nanometric level, the nanostructure being maintained up to 1050°C. The sensors have been tested with different reducing gases showing large responses to hydrogen and good selectivity.

High diffraction efficiency at hard X-ray energy in a silicon crystal bent by indentation

The diffraction properties of a crystalline silicon plate of which one face was mechanically indented have been studied. This treatment induced a permanent curvature in the sample, which allowed a diffraction efficiency of 88% for 150 keV photons,i.e.a reflectivity of 64% including the absorption. This efficiency is constant over 14 arcseconds and is very close to the theoretical expectation, meaning that the curvature is highly homogeneous. The technique enables the fabrication, in a very reproducible fashion, of crystals for the realization of an astronomical hard X-ray concentrator (Laue lens).

Note: Rigid holder to host and bend a crystal for multiple volume reflection of a particle beam

A holder to lodge and bend a silicon crystal to excite multivolume reflection of a high-energy particle beam has been designed and fabricated. A mechanically robust and stable structure fastens a crystal at best condition for experiments. The holder has allowed the observation of 12-time repeated volume reflection with very high efficiency. We detail the most important features behind the construction of the holder together with the characterization of the crystal being bent by the holder.

Calculation of the potential for interaction of particles with complex atomic structures

We present a method for calculation of the potential and related physical quantities experienced by a particle traversing an aligned periodic complex atomic structure. Classical physics equations and the expansion of periodic functions as a Fourier series have been used for the calculation. Based on this method, we have developed the ECHARM program, which calculates one- and two-dimensional averaged physical quantities of interest along the main axes of any orthorhombic and tetragonal structure. For the case of cubic symmetry, the calculation holds for any orientation. Complex structures such as zeolites have been worked out to show the capability of the program.

Observation of multiple volume reflection of ultrarelativistic protons by a sequence of several bent silicon crystals

The interactions of 400 GeV protons with different sequences of bent silicon crystals have been investigated at the H8 beam line of the CERN Super Proton Synchrotron. The multiple volume reflection of the proton beam has been studied in detail on a five-crystal reflector measuring an angular beam deflection theta = 52.96 +/- 0.14 microrad. The efficiency was found larger than 80% for an angular acceptance at the reflector entrance of 70 microrad, with a maximal efficiency value of epsilon = 0.90 +/- 0.01 +/- 0.03.