Avalanche Photodiode Detector Research Articles

The MAST motional Stark effect diagnostic

A motional Stark effect (MSE) diagnostic is now installed and operating routinely on the MAST spherical tokamak, with 35 radial channels, spatial resolution of ∼2.5 cm, and time resolution of ∼1 ms at angular noise levels of ∼0.5°. Conventional (albeit very narrow) interference filters isolate π or σ polarized emission. Avalanche photodiode detectors with digital phase-sensitive detection measure the harmonics of a pair of photoelastic modulators operating at 20 and 23 kHz, and thus the polarization state. The π component is observed to be significantly stronger than σ, in reasonably good agreement with atomic physics calculations, and as a result, almost all channels are now operated on π. Trials with a wide filter that admits the entire Stark pattern (relying on the net polarization of the emission) have demonstrated performance almost as good as the conventional channels. MSE-constrained equilibrium reconstructions can readily be produced between pulses.

Hacking commercial quantum cryptography systems by tailored bright illumination

The peculiar properties of quantum mechanics allow two remote parties to communicate a private, secret key, which is protected from eavesdropping by the laws of physics1,2,3,4. So-called quantum key distribution (QKD) implementations always rely on detectors to measure the relevant quantum property of single photons5. Here we demonstrate experimentally that the detectors in two commercially available QKD systems can be fully remote-controlled using specially tailored bright illumination. This makes it possible to tracelessly acquire the full secret key; we propose an eavesdropping apparatus built from off-the-shelf components. The loophole is likely to be present in most QKD systems using avalanche photodiodes to detect single photons. We believe that our findings are crucial for strengthening the security of practical QKD, by identifying and patching technological deficiencies. By using bright pulses of light to ‘blind’ the avalanche photodiode detectors used in quantum cryptography equipment, scientists in Europe have shown that it is possible to tracelessly steal the secret encryption key generated by such systems and thus compromise their security.

Four-layer depth-of-interaction PET detector for high resolution PET using a multi-pixel S8550 avalanche photodiode

Avalanche photodiodes (APDs) are being used as photodetectors in positron emission tomography (PET) because they have many advantages over photomultipliers (PMTs) typically used in PET detectors. We have developed a PET detector that consists of a multi-pixel APD and a 6×6×4 array of 1.46×1.46mm2×4.5m LYSO crystals for a small animal PET scanner. The detector can identify four-layer depth of interaction (DOI) with a position-sensitive APD coupled to the backside of a crystal array by just an optimized reflector arrangement. Since scintillation lights are shared among many pixels by the method, weaker signals in APD pixels far from the interacting crystals are affected by noise. To evaluate the performance of the four-layer DOI detector with the APD and the influence of electrical noise on our method, we constructed a prototype DOI detector and tested its performance. We found, except for crystal elements on the edge of the crystal array, all crystal elements could be identified from the 2D position histogram. An energy resolution of 16.9% was obtained for the whole crystal array of the APD detector. The results of noise dependence of detector performances indicated that the DOI detector using the APD could achieve sufficient performance even when using application-specific integrated circuits.

Bimodal MR–PET Agent for Quantitative pH Imaging

Bimodal MR–PET Agent for Quantitative pH Imaging

Investigation on new scintillators for subnanosecond time-resolved x-ray measurements

We investigated new x-ray detectors for nuclear resonant scattering measurements with high energy x rays. The organic-inorganic perovskite scintillator of phenethylamine lead halide ((C6H5C2H4NH3)2PbX4) (X:Br, I) was used. These compounds have fast light emission due to an exciton. They include heavy atoms, which make the detector to have high efficiency to high energy x rays. The merit of these scintillators is that we can make a thick crystal compared to a Si wafer which is used in an avalanche photo-diode detector. We successfully measured 67.41 keV x ray signals, the energy of 61Ni nuclear resonant scattering, with high detection efficiency of 42.5 % by the scintillator.

Investigation of avalanche silicon detectors for low energy single ion implantation applications

Avalanche silicon photodiodes have potential applications to detect low energy single ions for counting single ion impacts in shallow implant depths for the deterministic doping of nanoscale electronic devices. This paper reports the investigation of avalanche photodiode detectors in the linear operation mode for detection of 0.5–2 MeV helium ions. The measured charge gain was found to be up to 100 depending on bias voltage. The charge gain was found to saturate at a level that correlated with the ion stopping depth in silicon. The measured charge gain for energetic ions, which have a well-defined depth in a silicon substrate for the deposition of ionization energy, is compared with that of X-rays and photons, which deposit the ionization energy over a wider range of depth. This allowed the identification of a suitable structure for an avalanche photodiode optimised for the detection of sub-10 keV heavy ions with an internal charge gain above 10 achievable. This offers significant advantages over conventional PIN devices where the signals from such ions would be lost in the noise.

Monitoring Changes in Bacteriorhodopsin Conformation using Fluorescence Lifetime FRET with a Maximum Likelihood Estimator

Protein folding is a fundamental process within biological systems and changes of the protein conformation upon folding and unfolding can be studied by performing Forster resonance energy transfer (FRET) measurements. Unlike fluorescence intensity which has been traditionally used in FRET experiments, fluorescence lifetimes do not depend on fluorophore concentration, detection efficiency, illumination intensity and uniformity. Therefore, lifetime FRET overcomes the artefacts arising from intensity measurements and provides improved precision in investigating the protein unfolding procedure. Herein, we developed a novel technique for performing lifetime FRET using a maximum likelihood estimator (MLE) adapted from single molecule studies. We demonstrate the feasibility of our detection technique by monitoring the unfolding procedure of the membrane protein Bacteriorhodopsin (bR) labelled with the FRET pair Alexa Fluor 488 (donor) and Alexa Fluor 647 (acceptor). A home-built laser scanning confocal microscope and two avalanche photodiode detectors (APDs) are used for detection with high sensitivity and the fluorescence decays are collected using time correlated single photon counting (TCSPC). When determining a fluorescence lifetime with less than 2000 photons, the conventionally used least squares approach is not appropriate. Therefore, a MLE previously developed in our lab defined by multinomial statistics is used to accurately extract molecular fluorescence lifetimes from as little as 10 photons. These are then used to calculate FRET efficiencies and hence the conformational state of bR.

Förster resonance energy transfer (FRET)-based picosecond lifetime reference for instrument response evaluation

Förster resonance energy transfer (FRET) can be utilized to achieve ultrashort fluorescence responses in time-domain fluorometry. In a poly(vinyl) alcohol matrix, the presence of 60 mM Rhodamine 800 acceptor shortens the fluorescence lifetime of a pyridine 1 donor to about 20 ps. Such a fast fluorescence response is very similar to the instrument response function (IRF) obtained using scattered excitation light. A solid fluorescent sample (e.g a film) with picosecond lifetime is ideal for IRF measurements and particularly useful for time-resolved microscopy. Avalanche photodiode detectors, commonly used in this field, feature color- dependent-timing responses. We demonstrate that recording the fluorescence decay of the proposed FRET-based reference sample yields a better IRF approximation than the conventional light-scattering method and therefore avoids systematic errors in decay curve analysis.

Beam emission spectroscopy for density turbulence measurements on the MAST spherical tokamak

Beam emission spectroscopy (BES) of the energetic deuterium (D(0)) heating beams can provide a means of characterizing the density turbulence in tokamak plasmas. First such measurements have been performed on the MAST spherical tokamak using a trial BES system, which shares the collection optics of the charge-exchange recombination spectroscopy system. This system, with eight spatial channels covering the outer part of the plasma cross section, uses avalanche photodiode detectors with custom preamplifiers to provide measurements at 1 MHz bandwidth with a spatial resolution of 4 cm. Simulations of the measurement, including the beam absorption and excitation, line-of-sight integration of the emission spectrum, and the characteristics of the detection system have been benchmarked against the measured absolute intensity of the Doppler shifted Dalpha fluorescence from the 50 keV beam. This gives confidence in predictions of the performance of a two-dimensional imaging BES system planned for MAST. Correlation techniques have also provided information on the characteristics of the density turbulence at the periphery of L-mode plasmas as well as density perturbations due to coherent magnetohydrodynamic activity at the edge of H-mode plasmas. Precursor oscillations of the density in the pedestal region to edge-localized modes occurring during H-mode plasmas with a single-null diverted magnetic configuration are also observable in the raw signals from the trial BES system.

Development of a 64-channel APD detector module with individual pixel readout for submillimetre spatial resolution in PET

Visualization and quantification of biological processes by molecular imaging in small animals such as rats and especially mice require the best possible spatial resolution in positron emission tomography (PET). A new avalanche photodiode (APD) detector module, the LabPET II, was developed to achieve submillimetre spatial resolution for this purpose. The module consists of two monolithic APD arrays of 4×8 pixels, each with an active area of 1.1×1.1 mm 2 at a 1.2 mm pitch. The two APD arrays mounted in a custom ceramic PGA holder are coupled to an 8×8 tapered LYSO scintillator array designed to accommodate one-to-one coupling between individual APDs and crystal pixels. To investigate the module performance, an analog test board with four 16-channel preamplifier ASICs was designed to be interfaced with the LabPET™ digital processing electronics. At a standard APD operating bias, a mean energy resolution of 27.5±2.1% was typically obtained with a relative standard deviation of 13.8% in signal amplitude for the 64 individual pixels when irradiated with 511 keV photons. With two modules in coincidence, a mean timing resolution of 5.0±0.2 ns FWHM was measured. Finally, an intrinsic spatial resolution of 0.82 mm FWHM was measured by sweeping a 22Na point source between the two detector arrays. The LabPET II detector module demonstrates promising characteristics for dedicated small animal PET imaging at submillimetre resolution and, with some further optimization, would be suitable as the building block for a dual-modality combined PET/CT system.

Water Vapor Profiling Using a Widely Tunable, Amplified Diode-Laser-Based Differential Absorption Lidar (DIAL)

Abstract A differential absorption lidar (DIAL) instrument for automated profiling of water vapor in the lower troposphere has been designed, tested, and is in routine operation at Montana State University. The laser transmitter for the DIAL instrument uses a widely tunable external cavity diode laser (ECDL) to injection seed two cascaded semiconductor optical amplifiers (SOAs) to produce a laser transmitter that accesses the 824–841-nm spectral range. The DIAL receiver utilizes a 28-cm-diameter Schmidt–Cassegrain telescope; an avalanche photodiode (APD) detector; and a narrowband optical filter to collect, discriminate, and measure the scattered light. A technique of correcting for the wavelength-dependent incident angle upon the narrowband optical filter as a function of range has been developed to allow accurate water vapor profiles to be measured down to 225 m above the surface. Data comparisons using the DIAL instrument and collocated radiosonde measurements are presented demonstrating the capabilities of the DIAL instrument.

Derivation of the sensitivity of a Geiger mode avalanche photodiode detector from a given efficiency for quantum key distribution experiments

The detection sensitivity (DS) of a commercial single-photon receiver based on an InGaAs gate-mode avalanche photodiode is estimated. The installation of a digital-blanking system (DBS) to reduce dark current differentiates between the DS, which is the efficiency of the detector during its open-gate/active state, and the total/overall detection efficiency (DE). Using numerical simulations it is found that the average number of light-pulses blanked by DBS following a registered pulse is 0.333. The DS is estimated at 0.216, which can be used for estimating the DE for an arbitrary photon arrival rate and gating frequency of the receiver.

Multifrequency frequency-domain spectrometer for tissue analysis

In this paper we describe the modification and assessment of a standard multidistance frequency-domain near infrared spectroscopy (NIRS) instrument to perform multifrequency frequency-domain NIRS measurements. The first aim of these modifications was to develop an instrument that enables measurement of small volumes of tissue such as the cervix, which is too small to be measured using a multidistance approach. The second aim was to enhance the spectral resolution to be able to determine the absolute concentrations of oxy-, deoxy- and total hemoglobin, water, and lipids. The third aim was to determine the accuracy and error of measurement of this novel instrument in both in vitro and in vivo environments. The modifications include two frequency synthesizers with variable, freely adjustable frequency, broadband high-frequency amplifiers, the development of a novel avalanche photodiode (APD) detector and demodulation circuit, additional laser diodes with additional wavelengths, and a respective graphic user interface to analyze the measurements. To test the instrument and algorithm, phantoms with optical properties similar to those of biological tissue were measured and analyzed. The results show that the absorption coefficient can be determined with an error of <10%. The error of the scattering coefficient was <31%. Since the accuracy of the chromophore concentrations depends on the absorption coefficient and not on the scattering coefficient, the <10% error is the clinically relevant parameter. In addition, the new APD had similar accuracy as the standard photomultiplier tubes. To determine the accuracy of chromophore concentration measurements we employed liquid Intralipid(R) phantoms that contained 99% water, 1% lipid, and an increasing concentration of hemoglobin in steps of 0.010 mM. Water concentration was measured with an accuracy of 6.5% and hemoglobin concentration with an error of 0.0024 mM independent of the concentration. The measured lipid concentration was negative, which shows that the current setup is not suitable for measuring lipids. Measurements on the forearm confirmed reasonable values for water and hemoglobin concentrations, but again not for lipids. As an example of a future application, chromophore concentrations in the cervix were measured and comparable values to the forearm were found. In conclusion the modified instrument enables measurement of water concentration in addition to oxy- and deoxyhemoglobin concentrations with a single source-detector distance in small tissue samples. Future work will focus on resolving the lipid component.

High-Throughput DNA Assays Using Picoliter Reactor Volumes

The online characterization and detection of individual droplets at high speeds, low analyte concentrations, and perfect detection efficiencies is a significant challenge underpinning the application of microfluidic droplet reactors to high-throughput chemistry and biology. Herein, we describe the integration of confocal fluorescence spectroscopy as a high-efficiency detection method for droplet-based microfluidics. Issues such as surface contamination, rapid mixing, and rapid detection, as well as low detections limits have been addressed with the approach described when compared to conventional laminar flow-based fluidics. Using such a system, droplet size, droplet shape, droplet formation frequencies, and droplet compositions can be measured accurately and precisely at kilohertz frequencies. Taking advantage of this approach, we demonstrate a high-throughput biological assay based on fluorescence resonance energy transfer (FRET). By attaching a FRET donor (Alexa Fluor 488) to streptavidin and labeling a FRET acceptor (Alexa Fluor 647) on one DNA strand and biotin on the complementary strand, donor and acceptor molecules are brought in proximity due to streptavidin-biotin binding, resulting in FRET. Fluorescence bursts of the donor and acceptor from each droplet can be monitored simultaneously using separate avalanche photodiode detectors operating in single photon counting mode. Binding assays were investigated and compared between fixed streptavidin and DNA concentrations. Binding curves fit perfectly to Hill-Waud models, and the binding ratio between streptavidin and biotin was evaluated and found to be in agreement with the biotin binding sites on streptavidin. FRET efficiency for this FRET pair was also investigated from the binding results. Efficiency results show that this detection system can precisely measure FRET even at low FRET efficiencies.

Effect of electron irradiation dose on the performance of avalanche photodiode electron detectors

Avalanche photodiodes (APDs) are efficient detectors for electrons with energies below 100 keV. The damaging effects of 8 keV electron beam irradiation on the dark current and the output signal of the APD detector were investigated in this study. The APD dark current increases after electron doses exceeding 1.4×1013 cm−2. Preirradiation by high doses of 8 keV electrons further causes a deformation of the pulse height distribution of the APD output in the subsequent detection of low-flux electrons. This effect is particularly prominent when the energy of the detected electrons is lower than that of the damaging electrons. By comparing the experimental data with results of a simulation based on an electron trapping model, we conclude that the degradation of the APD performance is attributable to an enhancement of secondary-electron trapping at irradiation induced defects.

Quantitative single-molecule imaging by confocal laser scanning microscopy.

A new approach to quantitative single-molecule imaging by confocal laser scanning microscopy (CLSM) is presented. It relies on fluorescence intensity distribution to analyze the molecular occurrence statistics captured by digital imaging and enables direct determination of the number of fluorescent molecules and their diffusion rates without resorting to temporal or spatial autocorrelation analyses. Digital images of fluorescent molecules were recorded by using fast scanning and avalanche photodiode detectors. In this way the signal-to-background ratio was significantly improved, enabling direct quantitative imaging by CLSM. The potential of the proposed approach is demonstrated by using standard solutions of fluorescent dyes, fluorescently labeled DNA molecules, quantum dots, and the Enhanced Green Fluorescent Protein in solution and in live cells. The method was verified by using fluorescence correlation spectroscopy. The relevance for biological applications, in particular, for live cell imaging, is discussed.

Ultrafast ion temperature and toroidal velocity fluctuation spectroscopy diagnostic design

High sensitivity measurements of localized, long-wavelength ion temperature, and toroidal velocity fluctuations (T(i)/T(i),v(parallel)/v(parallel)) are required to address critical issues pertaining to turbulent transport. This diagnostic design exploits emission from charge exchange recombination between neutral beam atoms and the intrinsic carbon impurity. The n=8-7 transition of C VI at lambda(0)=529.05 nm will be measured. The key difference between this diagnostic design and conventional charge exchange spectrometers is the use of high-efficiency prism-coupled transmission gratings, avalanche photodiode detectors, and high-throughput collection optics. The spectrometer achieves a spectral resolution of 0.25 nm, and observes 528.0-530.0 nm with eight discrete spectral channels, with an entrance throughput of 1.6 mm(2) sr, two orders of magnitude larger than conventional charge exchange system. The diagnostic will achieve a turbulence-relevant time resolution of 1 micros. System modeling demonstrates a sensitivity of T(i)/T(i) < or = 1%.

Multipoint Thomson scattering diagnostic for the Madison Symmetric Torus reversed-field pinch

The multipoint Thomson scattering diagnostic on the Madison Symmetric Torus (MST) is now fully operational with 21 spatial points, which cover the entire minor radius. Four full electron temperature profiles can be obtained during each MST discharge, with a variable delay between each profile. This system overcomes challenges that arise from the unique machine design, location, and plasma characteristics of MST. The machine design limits the maximum porthole diameter to 11.4 cm, requiring a compact, re-entrant, seven element lens for scattered light collection. Limited space near MST necessitates a long beam path for the two Nd:YAG lasers requiring a remote beam line adjustment system to suppress drift in the beam position due to thermal expansion of the building. Due to the remote location of the laser head, substantial design effort was put into the creation of a set of safety interlocks for the laser system. The dynamic nature of MST plasmas and the wide range of operating space require a versatile scattered light detection system consisting of filter polychromators with temperature controlled avalanche photodiode detectors. We also implement an insertable integrating sphere, which travels along the laser beam path through the vacuum vessel, for the alignment of both the fiber optics and the lasers.

Enhanced red and near infrared detection in flow cytometry using avalanche photodiodes

Polychromatic flow cytometry enables detailed identification of cell phenotype using multiple fluorescent parameters. The photomultiplier tubes (PMTs) used to detect fluorescence in current instruments limit the sensitivity in the long wavelength spectral range. We demonstrate the flow cytometric applications of silicon avalanche photodiodes (APDs), which have improved red sensitivity and a working fluorescence detection range beyond 1,000 nm. A comparison of the wavelength-dependent performance of the APD and PMT was carried out using pulsed light-emitting diode sources, calibrated test beads, and biological samples. A breadboard flow cytometer test bench was constructed to compare the performance of PMTs and APD detectors. The APD used an additional amplifier stage to match the internal gain of the PMT. The resolution of the APD and PMT was compared for flow cytometry applications using a pulsed light-emitting diode source over the 500-1060 nm spectral range. These measurements showed the relative changes in the signal-to-noise performance of the APD and PMT over a broad spectral range. Both the APD and PMTs were used to measure the signal-to-noise response for a set of six peak calibration beads over the 530-800 nm wavelength range. CD4-positive cells labeled with antibody-conjugated phycoerythrin or 800 nm quantum dots were identified by simultaneous detection using the APD and the PMT. The ratios of the intensities of the CD4- and CD4+ populations were found to be similar for both detectors in the visible wavelengths, but only the APD was able to separate these populations at wavelengths above 800 nm. These measurements illustrate the differences in APD and PMT performance at different wavelengths and signal intensity levels. While the APD and PMT show similar signal-to-noise performance in the visible spectral range, the dark noise of the APD detector reduces the sensitivity at low signal levels. At wavelengths longer than 650 nm, the high quantum efficiency of the APD contributes to better signal-to-noise performance. The APD detector provides enhanced performance in the long wavelength region and may be used to extend the working range of the flow cytometer beyond 1,000 nm.

Sci-Sat AM(1): Imaging-08: Small animal APD PET detector with submillimetric resolution for molecular imaging.

Visualization and quantification of biological processes in mice, the preferred animal model in most preclinical studies, require the best possible spatial resolution in positron emission tomography (PET). A new 64-channel avalanche photodiode (APD) detector module was developed to achieve submillimeter spatial resolution for this purpose. The module consists of dual 4 × 8 APD arrays mounted in a custom ceramic holder. Individual APD pixels having an active area of 1.1 × 1.1 mm2 at a 1.2 mm pitch can be fitted to an 8 × 8 LYSO scintillator block designed to accommodate one-to-one coupling. An analog test board with four 16-channel preamplifier ASICs was designed to be interfaced with the existing LabPET digital processing electronics. At a standard APD operating bias, a mean energy resolution of 27.5 ± 0.6% was typically obtained at 511 keV with a relative standard deviation of 13.8% in signal amplitude for the 64 individual pixels. Crosstalk between pixels was found to be well below the typical lower energy threshold used for PET imaging applications. With two modules in coincidence, a global timing resolution of 5.0 ns FWHM was measured. Finally, an intrinsic spatial resolution of 0.8 mm FWHM was measured by sweeping a 22Na point source between two detector arrays. The proposed detector module demonstrates promising characteristics for dedicated mouse PET imaging at submillimiter resolution.