Accidental Coincidences Research Articles

A Real Time Coincidence System for High Count-Rate TOF or Non-TOF PET Cameras Using Hybrid Method Combining AND-Logic and Time-Mark Technology

A fully digital FPGA-based high count-rate coincidence system has been developed for TOF (Time of Flight) and non-TOF PET cameras. Using a hybrid of AND-logic and Time-mark technology produced both excellent timing resolution and high processing speed. In this hybrid architecture, every gamma event was synchronized by a 125 MHz system clock and generating a trigger associated with a time-mark given by an 8-bit high-resolution TDC (68.3 ps/bin). AND-logic was applied to the synchronized triggers for the real-time raw sorting of coincident events. An efficient FPGA based Time-mark fine-sort algorithm is used to select all the possible coincidence events within the preset coincidence time window. This FPGA-based coincidence system for a modular PET camera offers reprogrammable flexibility and expandability, so the coincidence system is easily employed, regardless of differences in the scale of the PET camera detector setup. A distributed processing method and pipeline technology were adopted in the design to obtain very high processing speed. In this design, both prompt and time-delayed accidental coincidences are simultaneously processed in real time. The real-time digital coincidence system supports coincidence in 2 to 12 detector module setups, capable of processing 72 million single events per second with no digital data loss and captures multiple-event coincidence for better imaging performance evaluation. The coincidence time window-size and time-offset of each coincidence event pair can be programmed independently in 68.3 ps increments (TDC LSB) during the data acquisition in different applications to optimize the signal-to-noise ratio. The complex coincidence system is integrated in one circuit board with 1.5 Gbps fiber optic interface. We demonstrated the system performance using the actual circuit and Monte Carlo simulations.

Neutron-deuteron analyzing power data at 19.0 MeV

Measurements of neutron-deuteron (n-d) analyzing power A y (θ) at E n = 19.0 MeV are reported at 16 angles from θ c.m. = 46.7 to 152.0°. The objective of the experiment is to better characterize the discrepancies between n-d data and the predictions of three-nucleon calculations for neutron energies above 16.0 MeV. The experiment used a shielded neutron source, which produced polarized neutrons via the 2 H(d,n) 3 He reaction, a deuterated liquid scintillator center detector (CD) and liquid-scintillator neutron side detectors. A coincidence between the CD and the side detectors isolated the elastic-scattering events. The CD pulse height spectrum associated with each side detector was sorted by using pulse-shape discrimination, time-of-flight techniques, and by removing accidental coincidences. A Monte Carlo computer simulation of the experiment accounted for effects due to finite geometry, multiple scattering, and CD edge effects. The resulting high-precision data (with absolute uncertainties ranging from 0.0022 to 0.0132) have a somewhat lower discrepancy with the predictions of three-body calculations, as compared to those found at lower energies.

Efficiency studies for a tracking detector based on square 1.5 m long scintillating fibers read out by SiPM

A tracking detector based on 1.5 m long scintillating fibers is being developed for the electron arm of the KAOS spectrometer at the Mainz Microtron MAMI. Measurements on light attenuation, particle detection efficiencies and accidental coincidence rates with a prototype set-up using 2 × 2 mm 2 fibers read out by silicon photomultipliers (SiPM) are presented. The highest efficiency at the lowest accidental coincidence rate was reached for high trigger thresholds at the largest SiPM bias voltages. The influence of signal attenuation and dispersion on detection efficiencies is discussed. The results are in good agreement with a Monte Carlo model that was used to predict detector characteristics for different fiber geometries.

High performance photon-pair source based on a fiber-coupled periodically poled KTiOPO_4 waveguide

We demonstrate efficient generation of photon pairs at 1316 nm in a fiber-coupled type-II phase-matched Rb-indiffused waveguide in periodically poled KTiOPO(4). The integrated waveguide source has a pair production rate of 2 x 10(7)/s/mW in a 1.08-nm bandwidth, in good agreement with a theoretical model that takes into account the transversal momentum imparted on the phase matching function by the waveguide. We achieve a Hong-Ou-Mandel quantum-interference visibility of 98.2% after subtraction of accidental coincidences, representing the highest reported value for a waveguide-based photon-pair source.

Model of hopping dc conductivity via nearest neighbor boron atoms in moderately compensated diamond crystals

Expressions for dependences of the pre-exponential factor σ 3 and the thermal activation energy ε 3 of hopping electric conductivity of holes via boron atoms on the boron atom concentration N and the compensation ratio K are obtained in the quasiclassical approximation. It is assumed that the acceptors (boron atoms) in charge states (0) and (−1) and the donors that compensate them in the charge state ( + 1 ) form a nonstoichiometric simple cubic lattice with translational period R h = [ ( 1 + K ) N ] − 1 / 3 within the crystalline matrix. A hopping event occurs only over the distance R h at a thermally activated accidental coincidence of the acceptor levels in charge states (0) and (−1). Donors block the fraction K / ( 1 − K ) of impurity lattice sites. The hole hopping conductivity is averaged over all possible orientations of the lattice with respect to the external electric field direction. It is supposed that an acceptor band is formed by Gaussian fluctuations of the potential energy of boron atoms in charge state (−1) due to Coulomb interaction only between the ions at distance R h . The shift of the acceptor band towards the top of the valence band with increasing N due to screening (in the Debye–Hückel approximation) of the impurity ions by holes hopping via acceptor states was taken into account. The calculated values of σ 3 ( N ) and ε 3 ( N ) for K ≈ 0.25 agree well with known experimental data at the insulator side of the insulator–metal phase transition. The calculation is carried out at a temperature two times lower than the transition temperature from hole transport in v -band of diamond to hopping conductance via boron atoms.

Specific Detection of CD56 (NCAM) Isoforms for the Identification of Aggressive Malignant Neoplasms with Progressive Development

Alternative splicing of transcripts from many cancer-associated genes is believed to play a major role in carcinogenesis as well as in tumor progression. Alternative splicing of one such gene, the neural cell adhesion molecule CD56 (NCAM), impacts the progression, inadequate therapeutic response, and reduced total survival of patients who suffer from numerous malignant neoplasms. Although previous investigations have determined that CD56 exists in three major isoforms (CD56(120kD), CD56(140kD), and CD56(180kD)) with individual structural and functional properties, neither the expression profiles nor the functional relevance of these isoforms in malignant tumors have been consistently investigated. Using new quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) strategies and novel CD56 isoform-specific antibodies, CD56(140kD) was shown to be exclusively expressed in a number of highly malignant CD56(+) neoplasms and was associated with the progression of CD56(+) precursor lesions of unclear malignant potential. Moreover, only CD56(140kD) induced antiapoptotic/proliferative pathways and specifically phosphorylated calcium-dependent kinases that are relevant for tumorigenesis. We conclude, therefore, that the specific detection of CD56 isoforms will help to elucidate their individual functions in the pathogenesis and progression of malignant neoplasms and may have a positive impact on the development of CD56-based immunotherapeutic strategies.

Sicko Statistics: Michael Moore and L'Ècole de Paris

Sicko Statistics: Michael Moore and L'Ècole de Paris

Stratospheric and solar cycle effects on long‐term variability of mesospheric ice clouds

Model results of mesospheric ice layers and background conditions at 69°N from 1961 to 2008 are analyzed. The model nudges to European Centre for Medium‐Range Weather Forecasts data below ∼45 km. Greenhouse gas concentrations in the mesosphere are kept constant. At polar mesospheric cloud (PMC) altitudes (83 km) temperatures decrease until the mid 1990s by ‐0.08 K/yr resulting in trends of PMC brightness, occurrence rates, and, to a lesser extent, in PMC altitudes (−0.0166 km/yr). Ice layer trends are consistent with observations by ground‐based and satellite instruments. Water vapor increases at PMC heights and decreases above due to increased freeze‐drying caused by the temperature trend. Temperature trends in the mesosphere mainly come from shrinking of the stratosphere and from dynamical effects. A solar cycle modulation of H2O is observed in the model consistent with satellite observations. The effect on ice layers is reduced because of redistribution of H2O by freeze‐drying. The accidental coincidence of low temperatures and solar cycle minimum in the mid 1990s leads to an overestimation of solar effects on ice layers. A strong correlation between temperatures and PMC altitudes is observed. Applied to historical measurements this gives negligible temperature trends at PMC altitudes (∼0.01–0.02 K/yr). Strong correlations between PMC parameters and background conditions deduced from the model confirm the standard scenario of PMC formation. The PMC sensitivity on temperatures, water vapor, and Ly‐α is investigated. PMC heights show little variation with background parameters whereas brightness and occurrence rates show large variations. None of the background parameters can be ignored regarding its influence on ice layers.

Generation of high-purity entangled photon pairs using silicon wire waveguide

We observed high-purity correlated and entangled photon pairs generated through spontaneous four-wave mixing (SFWM) in a silicon wire waveguide (SWW). Employing a nano-scale silicon waveguide with a low loss mode size converter, we obtained a high coincidence to accidental coincidence ratio (CAR) of around 200 that was larger than that of cooled dispersion shifted fiber (DSF) by a factor 3.2, and observed the two-photon interference fringe of time-bin entangled photons with > 95% visibility without subtracting the accidental coincidences.

The spatial distribution of upstream ion events from the Earth's bow shock measured by ACE, Wind, and STEREO

Using simultaneous measurements of >40 keV upstream ions observed at ACE, Wind, and STEREO‐A during the solar minimum period of 2007, day 1 through 2007, day 181, we investigate their spatial distributions by calculating their occurrence probabilities as a function of lateral and radial separation between L1 and STEREO‐A. Our main results are given as follows: (1) STEREO‐A observed upstream events even when it was separated from Earth by ∼1750 RE and ∼3800 RE in the radial and lateral directions, respectively. (2) The occurrence probability (∼20–30%) for measuring simultaneous upstream events at L1 and STEREO‐A was far greater than that expected from accidental coincidences. (3) The occurrence rate of simultaneous upstream events at L1 and STEREO‐A is significantly higher inside rarefaction regions of high‐speed solar wind flows (>500 km s−1) that follow corotating compression regions and when there exist antisunward propagating Alfvén waves. These new results confirm the global nature of the source region and place limits on the spatial size of the interplanetary structures that could either accelerate the ions in the first place or at the very least provide them with easier access by facilitating their scatter‐free transport from the Earth's foreshock into the far upstream regions traversed by STEREO‐A. We suggest that the existence of large amplitude Alfvén waves with spatial scales of the order of 0.03 AU that are embedded in and get convected past the Earth by high‐speed solar wind streams plays a critical and necessary role in the occurrence of upstream ion events near the Earth.

Generation of high-flux hyperentangled photon pairs using a microstructure-fiber Sagnac interferometer

We demonstrate generation of hyperentangled (time-bin and polarization) photon pairs via four-wave mixing in a microstructure-fiber Sagnac interferometer. The two-photon interference visibility in the time-bin (polarization) degree of freedom is $88%\ifmmode\pm\else\textpm\fi{}2%$ $(84%\ifmmode\pm\else\textpm\fi{}1%)$ without subtraction of accidental coincidences, and Bell's inequality is violated by 27 standard deviations at a $1\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}$ coincidence rate.

‘Big Think’, Disjointed Incrementalism: Chinese Economic Success and Policy Lessons for Africa, or the Case for Pan-Africanism

Chinese economic success is not the product of free market accidental coincidence. Rather, it is orchestrated by the State through a mixture of nationalism (‘big think’) and pragmatic decisions (disjointed incrementalism) in agriculture, finance and industry. Furthermore, these decisions build upon existing institutions (e.g. the Household Responsibility System, Township Village Enterprises, etc), some dating back to pre-revolutionary China (e.g. Special Economic Zones), rather than imported ones from outside China. The article explores the utility (and lack thereof) of the Chinese model in the African context, as well as the possibilities of an Africa-centred ‘big think’ (Pan-Africanism) capable of mobilizing the continent for development.

Feasibility of prompt correlated counting from photon interrogation of concealed nuclear materials

We describe new Monte Carlo codes for the simulation of coincident counts, both real and accidental, originating from photonuclear and photoatomic events in nuclear, and non-nuclear, materials. The codes perform fully analog simulation of all secondary neutrons, and photons, emitted by photonuclear, and photoatomic, events. Separate algorithms are then used to simulate the accidental coincidences. These new codes have been applied to a realistic measurement scenario. Two possible techniques for minimizing the detection of accidental coincidences are then analyzed: pulse-shape discrimination and detector lead shielding. The results showed that the use of lead-shielded liquid scintillation detectors with pulse-shape discrimination is a promising technique for the detection of actinides in shielded configurations by minimizing accidental counts. The data acquisition time for this approach would be on the order of 1 min or less for the measurement scenario considered here.

Chronic Treatment with Resveratrol Induces Redox Stress- and Ataxia Telangiectasia-mutated (ATM)-dependent Senescence in p53-positive Cancer Cells

The induction of senescence, an irreversible growth arrest, in cancer cells is regarded as a mean to halt tumor progression. The phytoalexin resveratrol (RV) is known to possess a variety of cancer-preventive, -therapeutic, and -chemosensitizing properties. We report here that chronic treatment with RV in a subapoptotic concentration induces senescence-like growth arrest in tumor cells. In contrast to the widely accepted antioxidant property of RV, we demonstrate that one causative stimulus for senescence induction by chronic RV is an increased level of reactive oxygen species (ROS). The ROS formed upon RV exposure include hydrogen peroxide and superoxide and originate largely from mitochondria. Consistently, co-incubation with the antioxidant N-acetyl cysteine interfered with RV-mediated reactivation of the senescence program. Molecular mediators on the way from increased ROS levels to the observed growth arrest include p38 MAPK, p53, and p21. Moreover, we provide evidence that RV-initiated replication stress, apparent by activation of the ataxia telangiectasia-mutated kinase pathway, is associated with increased ROS levels and senescence induction. This is the first report linking cell cycle effects with a pro-oxidant and pro-senescent effect of RV in cancer cells.

Pc1 geomagnetic pulsations as a potential hazard of the myocardial infarction

The analysis of 85,800 events (1979–1981) of Moscow ambulance calls, related to the myocardial infarction (MI), demonstrates a seasonal variation with the profound summer minima and winter maxima. Similar results were obtained by analyzing the 25-year (1970–1995) statistical monthly data on the death from infarction in Bulgaria. The estimated high correlation coefficient (0.84) between Moscow and Bulgarian data suggests a common reason. There is a great number of clinical and statistical studies confirming that the MI number rises during geomagnetic disturbances, which have a maximum of occurrence near equinox, not in winter. In order to explain this contradiction we suggest that one of the critical additional factors, which affect a human cardiovascular system, could be geomagnetic Pc1 pulsations at frequencies comparable with the human heart beat rate. The MI variations as well as the Pc1 pulsations exhibit a summer minimum. The comparative analysis of the Moscow ambulance MI data and Pc1 pulsations recorded at the geophysical observatory in Borok is presented. It is shown that in about 70% of the days when an anomalously great number of ambulance calls (AMI) has been registered Pc1 pulsations have been recorded. In the winter season the probability of the simultaneous AMI and Pc1 occurrence was 1.5 times larger than their accidental coincidence. Moreover, it was found that the effects of magnetic storms and Pc1 in AMI were much higher in winter than in summer. We suggest that the seasonal variation of the production of the pineal hormone melatonin leads to a winter instability in the human organisms and increases the sensitivity of the patient to the “negative” influence of Pc1 geomagnetic pulsations in winter.

Development of a liquid scintillator neutron multiplicity counter (LSMC)

A new neutron multiplicity counter is being developed that utilizes the fast response of liquid scintillator detectors. The ability to detect fast (vs. moderated) fission neutrons makes possible a coincidence gate of the order of tens of nanoseconds (vs. tens of microseconds). A neutron counter with such a narrow gate will be virtually insensitive to accidental coincidences, making it possible to measure items with a high single neutron background to greater accuracy in less time. This includes impure Pu items with high (α, n) rates as well as items of low-mass HEU where a strong active interrogation source is needed. Liquid scintillator detectors also allow for energy discrimination between interrogation source neutrons and fission neutrons, allowing for even greater assay sensitivity. Designing and building a liquid scintillator multiplicity counter (LSMC) requires a symbiotic effort of simulation and experiment to optimize performance and mitigate hardware costs in the final product. We present preliminary Monte-Carlo studies using the GEANT toolkit.

Generation of energy-time entangled photon pairs in 15-μm band with periodically poled lithium niobate waveguide

We report the generation of 1.5-mm-band energy-time entangled photon pairs using a periodically poled lithium niobate (PPLN) waveguide. We performed a two-photon interference experiment and obtained coincidence fringes with 77.3% visibilities without subtracting accidental coincidences.

The generation of polarization-entangled photon pairs using periodically poled lithium niobate waveguides in a fibre loop

We report the generation of polarization-entangled photon pairs at a telecom band using two cascade periodically poled lithium niobate waveguides in a fibre loop. The phase of an entangled photon state was stabilized automatically in our setup. The average visibility was about 93.5 ± 2.6% for the entangled photon pairs after subtracting accidental coincidences in our measurement.

Long-distance distribution of time-bin entangled photon pairs over 100 km using frequency up-conversion detectors

We report an experimental demonstration of the distribution of time-bin entangled photon pairs over 100 km of optical fiber. In our experiment, 1.5-mum non-degenerated time-bin entangled photon pairs were generated with a periodically poled lithium niobate (PPLN) waveguide by using the parametric down conversion process. Combining this approach with ultra-low-loss filters to eliminate the pump light and separate signal and idler photons, we obtained an efficient entangled photon pair source. To detect the photons, we used single-photon detectors based on frequency up-conversion. These detectors operated in a non-gated mode so that we could use a pulse stream of time correlated entangled photon pairs at a high repetition frequency (1 GHz). Using these elements, we distributed time-bin entangled photon pairs over 100 km of dispersion shifted fiber and performed a two-photon interference experiment. We obtained a coincidence fringe of 81.6% visibility without subtracting any background noise, such as accidental coincidence or dark count, which was good enough to violate Bell's inequality. Thus, we successfully distributed time-bin entangled photon pairs over 100 km.

Efficient source of high purity polarization-entangled photon pairs in the 1550nm telecommunication band

The authors present an efficient source of high purity polarization-entangled photon pairs in the standard 1550nm telecommunication band using a type-II degenerate parametric downconversion process. They first measured the spatial intensity distributions of the downconverted light to verify the beamlike, collinear, and noncollinear generations at the 1550nm wavelength. All four Bell states could easily be generated by selecting the intersection of two downconverted light cones in the noncollinear phase matching. They observed two-photon fringe visibilities of up to 98±3% and violations of Bell’s inequality by up to nine standard deviations when the accidental coincidences were subtracted.