Multilayer Detector Research Articles

A multilayer surface detector for ultracold neutrons

A multilayer surface detector for ultracold neutrons (UCNs) is described. The top 10B layer is exposed to vacuum and directly captures UCNs. The ZnS:Ag layer beneath the 10B layer is a few microns thick, which is sufficient to detect the charged particles from the 10B(n,α)7Li neutron-capture reaction, while thin enough that ample light due to α and 7Li escapes for detection by photomultiplier tubes. A 100-nm thick 10B layer gives high UCN detection efficiency, as determined by the mean UCN kinetic energy, detector materials, and other parameters. Low background, including negligible sensitivity to ambient neutrons, has also been verified through pulse-shape analysis and comparison with other existing 3He and 10B detectors. This type of detector has been configured in different ways for UCN flux monitoring, development of UCN guides and neutron lifetime research.

Multilayer Scanning Coil Detector for Improved Detection of Surface Pathogen Contamination

This paper presents an investigation into improved pathogen detection on food and food contact surfaces using a multilayer scanning coil detector combined with wireless magnetoelastic (ME) biosensors. In contrast to our previous studies of surface pathogen detection where a single-layer planar coil was employed, a more sensitive detector with a multi-layer coil design was investigated in this work. The two-fold roles of the coil detector are: (1) the application of an alternating magnetic field for the excitation of the ME sensor’s vibration and (2) detection of the resultant magnetic flux changes induced by the vibrating sensor. Surface pathogen detection can then be realized when the sensor comes into contact and binds specifically with the pathogen of interest, causing the sensor’s resonant frequency to decrease. For surfaces with large roughness and curvatures (e.g., leafy greens and cantaloupes), a long stand-off distance (distance between the sensor and detector) may be required. Hence, enhancement in the excitation power and sensitivity of coil detector with a multilayer design is a key to improving the system’s detection capabilities. Furthermore, smaller ME biosensors with higher mass sensitivity can also be measured with this sensitive multilayer coil detector. Several design parameters, including the number of layers, width, height, and spacing of coil turns, were studied. Coils were fabricated using standard microelectronic fabrication techniques (photolithograpy, electroplating, etching and sputtering) and tested.Both theoretical calculations and experimental data showed that the above coil geometric parameters have a significant impact on the signal amplitude and stand-off distance. Finally, with the newly fabricated multilayer coil detector, simultaneous measurement of multiple biosensors on spinach leaf surfaces was demonstrated. This multilayer scanning coil detector therefore facilitates the detection of pathogens on food and food contact surfaces.

Reactive Ion-Assisted Deposition of Cerium Oxide Hole-Blocking Contact for Leakage-Current Suppression in an Amorphous Selenium Multilayer Structure

Cerium oxide has proven as a proper hole-blocking contact in multilayer amorphous selenium X-ray detectors to reduce leakage current by limiting hole injection. To further improve its blocking properties, one of the key factors is to prepare a thin layer of cerium oxide with full oxidation. In this paper, a cerium oxide thin film was deposited by reactive oxygen ion assisted e-beam evaporation that utilized an independent oxygen ion source to create ionized oxygen, allowing for a refined oxidation control without requiring high process temperatures. Compared with films obtained without ion-beam assistance, oxygen ion assisted deposition enhanced the refractive index of oxide film from ~1.8 to 2.2 at a wavelength of 550 nm and improved the electrical resistivity from ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> Qcm. X-ray photoelectron spectroscopy and X-ray diffraction measurements further gave insights into the stoichiometry and the crystal structures. To investigate blocking effectiveness, multilayer selenium sandwich structures with and without the cerium oxide blocking contact were fabricated and the leakage current was evaluated.

TH‐EF‐BRB‐04: One‐Pixel Prototype of a Novel Polycrystalline CdTe‐Based Multilayer Detector for Radiation Therapy Imaging Applications

Purpose:Recent developments in polycrystalline CdTe‐based photovoltaics have resulted in a technology of inexpensive, large‐area structures possessing strong built‐in fields and superior radiation hardness. Stacking a large number of such structures (number of layers N&gt;&gt;1) into a detector with signal readout from individual layers produces a device with sufficient absorption under an MV beam, low signal spreading, no charge trapping, and low noise due to obviating the need for external biasing. We tested one‐pixel multilayer detector prototypes under a 6MV beam, compared their electrical parameters with those predicted by device modeling, and used the layer number‐signal data for verification of the energy spectrum unfolding algorithm.Methods:We modeled the energy deposition with depth of a CdTe‐based device under a 6MV beam with the Monte Carlo simulation package MCNP5. The results yielded charge carrier generation profiles in individual layers, which were then combined into structures of 10–12 layers. The profiles were used as input for modeling of current‐voltage (I‐V) characteristics with the device operation modeling software SCAPS‐1D. Modeling results were compared to measurements on several 1‐pixel multilayer structures under the 6MV beam of a Varian TrueBeam accelerator.Results:Measurements of I‐V characteristics for all layers of 1‐pixel multilayer structures established electrical parameters of best devices to be close to those modeled. A regularization algorithm for extracting the energy spectrum was successfully tested with these structures.Conclusion:The proposed multilayer structure adds a third dimension, depth‐signal reading, to the traditional 2‐D pixelation, allowing for the photon energy and source position to be obtained through signal processing. The latter processing relies on regularization approaches, similar to those developed for computed tomography, and thus have potential to significantly improve the quality of images under MV energy x‐ray sources of medical linear accelerators.This research is supported with NRC grant No. NRC‐HQ‐12‐G‐38‐0042

Handling missing data for the identification of charged particles in a multilayer detector: A comparison between different imputation methods

Identification of charged particles in a multilayer detector by the energy loss technique may also be achieved by the use of a neural network. The performance of the network becomes worse when a large fraction of information is missing, for instance due to detector inefficiencies. Algorithms which provide a way to impute missing information have been developed over the past years. Among the various approaches, we focused on normal mixtures’ models in comparison with standard mean imputation and multiple imputation methods. Further, to account for the intrinsic asymmetry of the energy loss data, we considered skew-normal mixture models and provided a closed form implementation in the Expectation-Maximization (EM) algorithm framework to handle missing patterns. The method has been applied to a test case where the energy losses of pions, kaons and protons in a six-layers’ Silicon detector are considered as input neurons to a neural network. Results are given in terms of reconstruction efficiency and purity of the various species in different momentum bins.

3D monolithically stacked CMOS Active Pixel Sensors for particle position and direction measurements

In this work we propose a 3D monolithically stacked, multi-layer detectors based on CMOS Active Pixel Sensors (APS) layers which allows at the same time accurate estimation of the impact point and of the incidence angle an ionizing particle. The whole system features two fully-functional CMOS APS matrix detectors, including both sensing area and control/signal elaboration circuitry, stacked in a monolithic device by means of Through Silicon Via (TSV) connections thanks to the capabilities of the CMOS vertical scale integration (3D-IC) 130 nm Chartered/Tezzaron technology. In order to evaluate the suitability of the two layer monolithic active pixel sensor system to reconstruct particle tracks, tests with proton beams have been carried out at the INFN LABEC laboratories in Florence (Italy) with 3 MeV proton beam.

Single-shot dual-energy x-ray imaging with a flat-panel sandwich detector for preclinical imaging

We describe a multi-layer (“sandwich”) configuration detector consisting of two x-ray imaging flat-panel detectors for single-shot (single-kV) dual-energy imaging. An intermediate copper filter is used to increase spectral separation between the two detectors to improve contrast at the expense of image noise. Monte Carlo and cascaded-systems analyses of the signal and noise performance are described that quantify performance characteristics. Image quality of dual-energy images obtained from a prototype sandwich-detector system is evaluated using a figure of merit (FOM), defined as the squared contrast-to-noise ratio normalized by x-ray exposure for a mouse phantom for preclinical imaging. Demonstration dual-energy bone and soft-tissues images of a postmortem mouse are obtained using the prototype system. While the FOM with the single-shot detector is lower than that achieved using a conventional dual-shot (dual-kV) method, the single-shot approach may be preferable when imaging speed or insensitivity to motion artifacts is a primary concern.

SU-8 as a Material for Microfabricated Particle Physics Detectors

Several recent detector technologies developed for particle physics applications are based on microfabricated structures. Detectors built with this approach generally exhibit the overall best performance in terms of spatial and time resolution. Many properties of the SU-8 photoepoxy make it suitable for the manufacturing of microstructured particle detectors. This article aims to review some emerging detector technologies making use of SU-8 microstructuring, namely micropattern gaseous detectors and microfluidic scintillation detectors. The general working principle and main process steps for the fabrication of each device are reported, with a focus on the advantages brought to the device functionality by the use of SU-8. A novel process based on multiple bonding steps for the fabrication of thin multilayer microfluidic scintillation detectors developed by the authors is presented. Finally, a brief overview of the applications for the discussed devices is given.

Composite boron nitride neutron detectors

Single phase polycrystalline hexagonal boron nitride (BN) or mixed with boron carbide (BxC) embedded in an insulating polymeric matrix acting as a binder and forming a composite material as well as pure submicron size polycrystalline BN has been tested as a thermal neutron converter in a multilayer thermal neutron detector design. Metal sheet electrodes were covered with 20–50μm thick layers of composite materials and assembled in a multi-layer sandwich configuration. High voltage was applied to the metal electrodes to create an interspacing electric field. The spacing volume could be filled with air, nitrogen or argon. Thermal neutrons were captured in converter layers due to the presence of the 10B isotope. The resulting nuclear reaction produced α-particles and 7Li ions which ionized the gas in the spacing volume. Electron-ion pairs were collected by the field to create an electrical signal proportional to the intensity of the neutron source. The detection efficiency of the multilayer neutron detectors is found to increase with the number of active converter layers. Pixel structures of such neutron detectors necessary for imaging applications and incorporation of internal moderator materials for field measurements of fast neutron flux intensities are discussed as well.

Development of multi-layer crystal detector and related front end electronics

A crystal (diamond) particle detector has been developed and tested, whose constitute elements are a multi-layer polycrystalline diamond and a pick-up system capable of collecting in parallel the charge produced in the layers. The charge is read with a charge-to-voltage amplifier (5–6mV/fC) realized with bipolar junction transistors in order to minimize the effect of the detector capacitance. The tests performed with cosmic rays and at the beam test facility of Frascati with 500MeV electrons in single electron mode operation have shown that a detector with 4–5 layers of 250μm thickness each and 9mm2 active area exhibits an upper limit of 150ps time resolution for minimum ionizing particles at an operating voltage of about 350V.

Design of a Novel Multilayer Detector Based on Thin-Film Cadmium Telluride for High Energy Photon Imaging in Radiation Therapy Applications

Design of a Novel Multilayer Detector Based on Thin-Film Cadmium Telluride for High Energy Photon Imaging in Radiation Therapy Applications

Construction and performance of a dose-verification scintillation-fiber detector for proton therapy

A multilayer scintillation-fiber detector has been developed for precision measurement of time-dependent dose verification in proton therapy. In order to achieve the time and position sensitivity required for the precision dose measurements, a prototype detector was constructed with double-clad 1-mm-thick scintillation fibers and 128-channel silicon photodiodes. The hole charges induced in each channel of the silicon photodiodes were amplified and processed with a charge-integration mode. The detector was tested with 45-MeV proton beams provided by the MC50 cyclotron at the Korea Institute of Radiological and Medical Science (KIRAMS). The detector response for a 45-MeV proton beam was agreed fairly well with the predicted by GEANT4 simulations. Furthermore, the quantitative accuracy appearing in the spatial distribution of the detector response measured for 20s is in the order of 1%, whose accuracy is satisfactory to verify beam-induced dose in proton therapy. We anticipate that the detector composed of scintillation fibers and operating in the charge-integration mode allows us to perform quality measurement of dynamic therapeutic beams.

A multilayer scintillation detector of high-energy particles for satellite experiments

We describe the detecting system of a high-energy charged particle telescope spectrometer for a space experiment to be performed on the outer surface of the Russian section of the International Space Station and other spacecraft, including small satellites. The spectrometer’s detecting system is a multilayer scintillation detector (MSD) of polystyrene plates scanned by photomultipliers. It allows high-intensity fluxes of electrons (up to ∼105 cm−2 s−1) several milliseconds long with energies of 3 to 30 MeV to be measured with an accuracy of ∼1 μs, along with time profiles and the evolution of particle energy spectra. The MSD is characterized by an energy resolution no worse than 10%, an angular resolution of ∼10 degrees, a geometric factor of ∼40 cm2 sr, and a trigger system time resolution of ∼20 ns.

Analytical modeling of thin film neutron converters and its application to thermal neutron gas detectors

A simple model is explored mainly analytically tocalculate and understand the PHS of single and multi-layer thermalneutron detectors and to help optimize the design in differentcircumstances. Several theorems are deduced that can help guide thedesign.

The multilayer scintillation detector of high-energy charged particles for satellite experiments

We present the detection system of high-energy charged particle telescope-spectrometer for space experiment, which is scheduled to take place on the outside the Russian segment of The International Space Station (ISS) and other spacecrafts and small satellites. One of the scientific objectives of experiment is to study electron beams propagation in the magnetosphere. Such a beam can form in acceleration electrons by high-altitude electrical discharge and are injected in the magnetosphere. The detection system for this experiment is developed on the basis of the multilayer scintillation detector (MSD). The MSD is made by polystyrene plates viewed photomultipliers. It can detect 3-30 MeV intense electron beams (up to∼105 cm−2s−1) of up several milliseconds, can measure time profiles with accuracy ∼ 1 microsecond and energy spectra of particles evolution. MSD's main parameters: geometric factor ∼40 cm2·sr, trigger system time resolution ∼20 ns, energy resolution 5-10%, angular resolution ∼ 10 grad.

Investigation of Ghosting Recovery Mechanisms in Selenium X-ray Detector Structures for Mammography

The ghosting recovery mechanisms in multilayer selenium X-ray detector structures for mammography are experimentally and theoretically investigated. The experiments have been carried out under low positive applied electric field <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(\sim 1\ {\rm V}/\mu{\rm m})$</tex></formula> . A ghost removal technique is investigated by reversing the bias polarity during the natural recovery process. The theoretical model considers accumulated trapped charges and their effects (trap filling, recombination, detrapping, structural relaxation and electric field dependent electron-hole pair creation), and effects of charge injection from the metal contacts. Carrier trapping in both charged and neutral defect states has been considered in the model. It has been found that the X-ray induced deep trap centers are charged defects. A faster sensitivity recovery is found by reversing the bias during the natural recovery process. During the reverse bias, a huge number of carriers are injected from the metal contacts, and fill the existing trap centers. This results in an abrupt recovery of the relative sensitivity. However, the relative sensitivity slightly decreases with time after this abrupt recovery due to the release of the trapped electrons as well as the long recovery time of the induced trap centers. The theoretical model shows a very good agreement with the experimental results.

Multi-layer boron thin-film detectors for neutrons

Abstract Intrinsic efficiencies of multi-layer boron-10 thin-film detectors were studied theoretically and experimentally. For multi-layer schemes based on an optimized single-layer film thickness, the practical efficiency is limited to about 42% for thermal neutrons. This is about half the efficiency of a moderated 3He detectors in commercial use for portal monitoring. The efficiency limitation is due to charged particle loss in the boron layers and substrates. The same loss mechanism will prevent all substrate-based boron detectors from ever reaching the intrinsic efficiencies of high-pressure 3He tubes, independent of substrate geometry and material composition. Experimental data also indicate that the multi-layer detector configuration can have an efficiency approaching the theoretical limit. Excellent n / γ discrimination has also been achieved using an ionization chamber.

A combined γ-neutron detector for fissile material monitoring using the pulse photonuclear technology

Prompt and delayed neutrons and secondary γ rays are the informational radiation in the photonuclear technology for recording fissile materials. Due to the high cost of detectors and difficulties in their placing around a container, it is desirable that the same detectors be used to record all kinds of informational radiation. The design of a multilayer γ-neutron detector is proposed, and the effect of the layer thickness on the recording efficiency is estimated. It is shown that background from prompt neutrons in recording of γrays can be reduced by proper selection of the time interval for γ-ray measurements.

Performance simulation of an x-ray detector for spectral CT with combined Si and Cd[Zn]Te detection layers

The most obvious problem in obtaining spectral information with energy-resolving photon counting detectors in clinical computed tomography (CT) is the huge x-ray flux present in conventional CT systems. At high tube voltages (e.g. 140 kVp), despite the beam shaper, this flux can be close to 109 Mcps mm−2 in the direct beam or in regions behind the object, which are close to the direct beam. Without accepting the drawbacks of truncated reconstruction, i.e. estimating missing direct-beam projection data, a photon-counting energy-resolving detector has to be able to deal with such high count rates. Sub-structuring pixels into sub-pixels is not enough to reduce the count rate per pixel to values that today's direct converting Cd[Zn]Te material can cope with (⩽10 Mcps in an optimistic view). Below 300 µm pixel pitch, x-ray cross-talk (Compton scatter and K-escape) and the effect of charge diffusion between pixels are problematic. By organising the detector in several different layers, the count rate can be further reduced. However this alone does not limit the count rates to the required level, since the high stopping power of the material becomes a disadvantage in the layered approach: a simple absorption calculation for 300 µm pixel pitch shows that the required layer thickness of below 10 Mcps/pixel for the top layers in the direct beam is significantly below 100 µm. In a horizontal multi-layer detector, such thin layers are very difficult to manufacture due to the brittleness of Cd[Zn]Te. In a vertical configuration (also called edge-on illumination (Ludqvist et al 2001 IEEE Trans. Nucl. Sci. 48 1530–6, Roessl et al 2008 IEEE NSS-MIC-RTSD 2008, Conf. Rec. Talk NM2-3)), bonding of the readout electronics (with pixel pitches below 100 µm) is not straightforward although it has already been done successfully (Pellegrini et al 2004 IEEE NSS MIC 2004 pp 2104–9). Obviously, for the top detector layers, materials with lower stopping power would be advantageous. The possible choices are, however, quite limited, since only ‘mature’ materials, which operate at room temperature and can be manufactured reliably should reasonably be considered. Since GaAs is still known to cause reliability problems, the simplest choice is Si, however with the drawback of strong Compton scatter which can cause considerable inter-pixel cross-talk. To investigate the potential and the problems of Si in a multi-layer detector, in this paper the combination of top detector layers made of Si with lower layers made of Cd[Zn]Te is studied by using Monte Carlo simulated detector responses. It is found that the inter-pixel cross-talk due to Compton scatter is indeed very high; however, with an appropriate cross-talk correction scheme, which is also described, the negative effects of cross-talk are shown to be removed to a very large extent.

Proton range verification using a range probe: definition of concept and initial analysis

Proton therapy can be particularly sensitive to changes or errors in range. Thus, methods for the in vivo measurement of range could be of great use to improve the quality and accuracy of proton-based radiotherapy. In this paper, we introduce the concept of the ‘range probe’. This is a low-dose, high-energy proton pencil beam that would pass through a patient, and whose integral Bragg peak would be measured on the exit side using a multi-layer detector. We propose that by comparing the measured integral Bragg peak with that calculated based on the patient's planning CT, such a range probe could provide useful information about the accuracy of range calculations in vivo. To study the feasibility of this approach, a Monte Carlo-based study has been performed. Using a patient's planning CT, MC simulations (VMCpro) have been made for single pencil beams laterally traversing the head and stopping in a simulated range telescope behind the patient. Range probes have been calculated for different locations, and the residual range from the Bragg peak ‘signal’ in the range telescope has been assessed for different assumed detector thicknesses. The sensitivity of this approach to changes in CT values, calibration curve and positional shifts of the CT have been investigated. From our analysis, range resolutions of 1 mm may be possible with a detector thickness of 4 mm for homogeneous regions. Additionally, for heterogeneous regions, changes of the Bragg peak shape due to spatial shifts of the CT could be a sensitive measure for detecting patient set-up errors directly in the treatment position. The concept of the proton ‘range probe’ appears to be feasible for high-resolution range verification. We now want to test this concept experimentally using different possible range telescope detectors.