Application of a plastic scintillating fiber array for low-energy x-ray imaging

Interest in digital imaging has led to the development of new detectors in the form of large-area displays. Most of the recent improvements are based on charge-coupled devices, a:Si photodiodes arrays, and so on. Some of these photodetectors must be coupled to scintillating screens to convert the ionizing radiation into light. Fiber-optic screens offer an advantage for achieving this interface because the length (thickness) of the interaction medium does not contribute too much to the degradation of the spatial resolution. We discuss the possibility of using a plastic scintillating fiber (PSF) array for x-ray detection and imaging in the 10-keV range. Modulation-transfer-function (MTF) measurements of the PSF array are compared with the optics MTF of the imaging system (without the sample); cross talk in the fiber array is negligible, even though the fiber array thickness is 20 mm. The optimal thickness of the array is estimated experimentally.     

Lung counting: comparison of detector performance with a four detector array that has either metal or carbon fibre end caps, and the effect on mda calculation

This study described the performance of an array of high-purity Germanium detectors, designed with two different end cap materials-steel and carbon fibre. The advantages and disadvantages of using this detector type in the estimation of the minimum detectable activity (MDA) for different energy peaks of isotope (152)Eu were illustrated. A Monte Carlo model was developed to study the detection efficiency for the detector array. A voxelised Lawrence Livermore torso phantom, equipped with lung, chest plates and overlay plates, was used to mimic a typical lung counting protocol with the array of detectors. The lung of the phantom simulated the volumetric source organ. A significantly low MDA was estimated for energy peaks at 40 keV and at a chest wall thickness of 6.64 cm.     

Interconnection of a two-dimensional array of vertical-cavity surface-emitting lasers to a receiver array by means of a fiber image guide

The implementation of a 10-channel parallel optical interconnect consisting of a two-dimensional array of vertical-cavity surface-emitting lasers, a 1.35-m fiber image guide, and a metal-semiconductor-metal receiver array is described. Transmission rates of 250 Mbits/s per channel are demonstrated with an optical cross talk of less than -27 dB and a loss of -3 dB. Coupling issues associated with image guides are analyzed and discussed.     

Coupling and cross-talk effects in 12-15 microm diameter single-mode fiber arrays for simultaneous transmission and photon collection from scattering media

We examine signal degradation effects in fiber arrays from fiber-to-fiber coupling and from cross talk attributable to backscatter from the sample medium originating from adjacent fibers in the array. An analysis of coupling and cross talk for single-mode fibers (SMFs) operating at 1310 nm with different core diameters, interaction lengths, core center spacing, and numerical apertures (NAs) is evaluated. The coupling was evaluated using beam propagation algorithms and cross talk was analyzed by using Monte Carlo methods. Several multimode fiber types that are currently used in fiber image guides were also evaluated for comparative purposes. The analysis shows that an optimum NA and core diameter can be found for a specific fiber center separation that maximizes the directly backscattered signal relative to the cross talk. The coupling between fibers can be kept less than -35 dB for interaction lengths less than 5 mm. The calculations were compared to an experimentally fabricated SMF array with 15 microm center spacing and showed good agreement. The experimental fiber array without a lens was also used in a coherent detection configuration to measure the position of a mirror. Accurate depth ranging up to a distance of 250 microm from the tip of the fiber was achieved, which was five times the Rayleigh range of the beam emitted from the fiber.     

Multimode all-fiber quasi-distributed refractometer sensor array and cross-talk mitigation

The analysis and design of a quasi-distributed multimode fiber refractometer array is presented. The main challenge in the design of a practical quasi-distributed sensor array proved to be in mitigation of otherwise pronounced cross-talk effects among the individual sensors in the network. The cross-talk effects originate from mode filtering properties and the strong mode excitation dependence on the multimode refractometer sensors that constitute the array. The introduction of mode conditioning based on fiber mode filters and mode mixers effectively reduced the cross talk to a negligible level while providing the desired sensor response at acceptable collateral losses to the network. A comprehensive experimental analysis was carried out to provide detailed insight into the multimode sensor array behavior and to obtain data necessary for an overall and effective network design.     

Expanded array for giant air shower observation at Akeno

As the first stage of a future huge air shower array, the Akeno array was expanded to about 20 km² by adding 19 scintillation counters of 2.25 m² area outside the present 1 km² array and installing a new data collection system. These detectors are connected successively by two optical fiber cables. The total number of electrons and the arrival direction of extensive air showers of 10¹⁰ GeV can be determined with accuracies of 25% and 3° respectively with this array. The present recording system is applicable to other experiments in which many sensors are connected in sequence.     

Demonstration of a multichannel optical interconnection by use of imaging fiber bundles butt coupled to optoelectronic circuits

Through five experiments, we demonstrate and characterize the basic functionality of imaging fiber bundles for optoelectronic chip-level interconnections. We demonstrate the transmission of spot arrays with spot sizes and a spot pitch roughly equal to 2 and 4 times the core pitch, respectively. We show that optoelectronic integrated circuits, including sources and detectors, can be butt coupled directly to fiber bundles without any additional optical elements. We demonstrate a 16-channel interconnect with -23 dB of cross talk, and we characterize the most significant optical loss mechanism. Finally, we show how imaging fiber bundles can be used to implement more complex interconnection structures by an example of a hybrid-bonded structure that implements a low-cost, high-connectivity solution for more advanced system architectures.     

SPAD array module for multi-dimensional photon timing applications

Multi-dimensional time correlated single-photon counting has reached a prominent position among analytical techniques employed in the medical and biological fields. The development of instruments able to perform temporal and spectral fluorescence analysis (sFLIM) at the same time is limited by the performance of single-photon detectors, since currently available arrays cannot simultaneously satisfy all the requirements. To face this rising quest, a fully-parallel eight-channel module, based on a monolithic single-photon avalanche diode (SPAD) array with great temporal resolution, high photon detection efficiency and low dark count rate, has been designed and fabricated. The system relies on a novel architecture of the single pixel, based on the integration of the timing pick-up circuit next to the photodetector, making the negative effects of electrical and optical crosstalk on photon timing performance negligible. To this end, the custom technological process used to fabricate the SPAD has been modified, allowing the integration of MOS transistors without impairing the structure and the performance of the detector. The single channel is complemented by an external active quenching circuit, fabricated in a standard CMOS technology, that ensures high maximum counting rate (>5?MHz) and low after-pulsing (<2%). Finally, the output timing signals are read and conditioned by proper CMOS electronics. The complete system shows a very good temporal resolution of about 45?ps (FWHM).     

Design of a high-resolution gamma-ray detector module for tomography applications

We present a modular gamma-ray detector design for gamma-ray tomography applications. As a key electronic component we use the APD array S8550 of Hamamatsu Corp. with 4×8 single APD elements each of 1.6 mm×1.6 mm size. For this APD array we tested and evaluated different configurations of 2 mm wide lutetium yttrium orthosilicate scintillation crystals. Emphasize was given to high counting efficiency and low dead time in order to secure applicability of the detector to tomography of objects with highly attenuating materials. For electronic processing, we designed a low-cost low-power charge-sensitive preamplifier circuit using commercially available operational amplifier ICs. The modular design of the detectors allows us to build up larger line or arc detectors.     

Electrical breakdown gas detector featuring carbon nanotube array electrodes

We demonstrate here detection of dichloro-difluoro-methane and oxygen in mixtures with helium using a carbon nanotube electrical breakdown sensor device. The sensor is comprised of an aligned array of multiwalled carbon nanotubes deposited on a nickel based super-alloy (Inconel 600) as the anode; the counter electrode is a planar nickel sheet. By monitoring the electrical breakdown characteristics of oxygen and dichloro-difluoro-methane in a background of helium, we find that the detection limit for dichloro-difluoro-methane is approximately 0.1% and the corresponding limit for oxygen is approximately 1%. A phenomenologigal model is proposed to describe the trends observed in detection of the two mixtures. These results indicate that carbon nanotube based electrical breakdown sensors show potential as end detectors in gas-chromatography devices.     

Cross talk and ghost talk in a microbeam free-space optical interconnect system with vertical-cavity surface-emitting lasers, microlenses, and metal-semiconductor-metal detectors

A diffraction-based beam-propagation model is used to study optical cross talk in microbeam free-space optical interconnection (FSOI) systems. The system consists of VCSEL's, microlenses, and metal-semiconductor-metal (MSM) detectors, with the detectors modeled as amplitude gratings with low contrast ratio (based on experimental results). Different possible cross-talk sources are studied. Results show that, in an optimized system, the cross talk caused by diffractive scattering is not an issue. However, in such systems the principal reflection from a MSM detector surface creates two problems: VCSEL coupling and ghost talk. The coupling of the reflected beam into the VCSEL's may cause power oscillation (and increase the bit error rate), whereas ghost talk will limit the distance-bandwidth product of the interconnect system. This optical system is also abstracted in hspice together with the laser driver and receiver circuits to analyze ghost talk in this system. Results show that at high speed (1 Gbit/s or more) these effects negatively affect system performance.     

Analysis of hexagonal array geometry for free-space optical interconnects with improved signal-to-noise ratio

The effect of transmitter and receiver array configurations on the performance of free-space optical interconnects (FSOIs) was investigated. Experimentally measured, spectrally resolved, near-field images of vertical-cavity surface-emitting laser (VCSEL) transverse modes were used as extended sources in our simulation model and combined with laser relative intensity noise and the receiver noise to determine the optimal array geometry. Our results demonstrate the importance of stray-light cross talk in both square and hexagonal configurations. By changing the array lattice geometry from square to hexagonal, we obtained an overall optical signal-to-noise ratio improvement of 3 dB. We demonstrated that the optical signal-to-noise ratio is optimal for the hexagonal channel arrangement regardless of the transverse mode structure of the VCSEL beam. We also determined the VCSEL drive current required for the best performance of the FSOI system.     

Imaging of high-Z material for nuclear contraband detection with a minimal prototype of a muon tomography station based on GEM detectors

Muon Tomography based on the measurement of multiple scattering of atmospheric cosmic ray muons in matter is a promising technique for detecting heavily shielded high-Z radioactive materials (U, Pu) in cargo or vehicles. The technique uses the deflection of cosmic ray muons in matter to perform tomographic imaging of high-Z material inside a probed volume. A Muon Tomography Station (MTS) requires position-sensitive detectors with high spatial resolution for optimal tracking of incoming and outgoing cosmic ray muons. Micro Pattern Gaseous Detector (MPGD) technologies such as Gas Electron Multiplier (GEM) detectors are excellent candidates for this application. We have built and operated a minimal MTS prototype based on 30 cm×30 cm GEM detectors for probing targets with various Z values inside the MTS volume. We report the first successful detection and imaging of medium-Z and high-Z targets of small volumes (∼0.03 L) using GEM-based Muon Tomography.     

Design of a low cross‐talk micro‐coil array for micro‐scale MRI

Design of magnetic resonance micro‐coil arrays with low cross‐talk among the coils can be the main challenge to improve the effectiveness of magnetic resonance micro‐imaging because the electrical cross‐talk which is mainly due to the inductive coupling perturbs the sensitivity profile of the array and causes image artifacts. In this work, a capacitive decoupling network with N(M ? 1) + (N ? 1)(M ? 2) capacitors is proposed to reduce the inductive coupling in an N × M array. A 3 × 3 array of optimized micro‐coils is designed using the finite element simulations and all the needed elements for the array equivalent circuit are extracted in order to evaluate the effectiveness of the proposed decoupling method by assessing the reduction of the coupled signals after employing the capacitive network on the circuit. The achieved results for the designed array show that the high cross‐talk level is reduced by the factor of 2.2–3.4 after employing the capacitive network. By employing this method of decoupling, the adjacent coils in each row and inner columns can be decoupled properly while the minimum decoupling belongs to the outer columns because of the lack of all necessary decoupling capacitances for these columns. The main advantages of the proposed decoupling method are its efficiency and design easiness which facilitates the design of dense arrays with the properly decoupled coils, especially the inner coils which are more coupled due to their neighbors. © 2013 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 23, 353–359, 2013     

A 2.5 MHz 2D array with Z-axis electrically conductive backing

The design, fabrication and initial testing of a prototype fully λ/2 sampled, 2500 element 2D phased array is presented. The array utilizes a unique Z-axis electrical conductivity backing layer to provide both acoustic attenuation and electrical interconnect for the signal channels. The electrical interconnect is designed to be in the acoustic shadow of the transducer elements so as to minimize the foot print of the array. A modular, demountable Pad Grid Array interconnect is used to connect to the backing of the array. Results are presented for measurements of the single element properties of electrical impedance, pulse echo waveform and spectrum, directivity, and cross talk     

An optimised method for material identification using a photon counting detector

X-ray photon counting detectors have become a competitive alternative to energy integrating systems in certain domains. However, processing methods currently used in radiography for investigating the composition of an object are an extension of dual energy methods and have started to show their limits with an increasing number of counting windows (bins). Thus, in a context of material recognition for homeland security, this study aims to introduce a new data processing method suitable for any type of detector, in integrating or counting mode, regardless of the number of bins. Additionally, a criterion to quantify the influence of the number of counting windows as well as the influence of their threshold position has been developed.The optimisation process is conducted in simulation by considering ideal detectors, and the results, for detectors with m=2, 3, 4 and 5 counting windows, are compared to those obtained with an analytical criterion developed in the literature and adapted to our study context. Both methods give identical results. In the final part, the performances of a spectrometric detector (energy windows width of 1 keV) and of optimised systems with m=2, 3, 4 and 5 bins are analysed for homogeneous plastic identification. The results show an increase in performance with increase in bin number until the performance level of the spectrometric detector is reached.Moreover, a discussion on the optimisation robustness as a function of material thickness to be identified is presented. The spectrometric detector, which does not require any optimisation of the bins thresholds, appears then to be a candidate of choice for material recognition when using X-ray photon counting detectors.     

High-density fiber-optic genosensor microsphere array capable of zeptomole detection limits

The detection limit of a fiber-optic microsensor array was investigated for simultaneous detection of multiple DNA sequences. A random array composed of oligonucleotide-functionalized 3.1-microm-diameter microspheres on the distal face of a 500-microm etched imaging fiber was monitored for binding to fluorescently labeled complementary DNA sequences. Inherent sensor redundancy in the microarray allows the use of multiple microspheres to increase the signal-to-noise ratio, further enhancing the detection capabilities. Specific hybridization was observed for each of three sequences in an array yielding a detection limit of 10(-21) mol (approximately 600 DNA molecules).     

A distributed array of automated Cherenkov telescopes for TeV gamma-ray astronomy

It is proposed that an array of widely spaced, independent detectors be used to improve the sensitivity for detection of TeV gamma-ray signals from discrete sources. The additional benefits of full automation of such a system are discussed.     

Field-effect-transistor self-electro-optic-effect-device (FET-SEED) electrically addressed differential modulator array

We describe a 6 × 6 array of electrically addressed field-effect-transistor self-electro-optic-effect-device differential modulators in which each element has a single-stage amplifier to permit an input voltage of less than 1 V to control the output modulators, which can operate at as high as 10 V. The variations in the switching voltages across the array are less than ±70 mV, and the individual array elements are operated at as high as 2 Gbits/s. We also measure cross talk between adjacent elements within the array, measure the dependence of the switching time on the input voltage swing, and calculate the dependence of the switching time that is due to the photocurrent of the modulators.     

One-dimensional collimation of laser array outputs

We consider a computer-generated hologram for the one-dimensional collimation in x of the output from a linear laser-diode array in y. Our concern is to produce one-dimensional pencil beams from each laser diode with small cross talk between the output from the separate laser diodes. Such outputs can be used in matrix-vector, neural net, and interconnection applications. The efficiency and the design of the computer-generated hologram are detailed, and initial optical laboratory results with an electron-beam recorded computer-generated hologram are presented.