We present recent results on the operation of gas-avalanche detectors comprising a cascade of gas electron multipliers (GEMs) and Mico-Hole & Strip Plate (MHSP) multiplier operated in reversed-bias (R-MHSP) mode. The operation mechanism of the R-MHSP is explained and its potential contribution to ion-backflow (IBF) reduction is demonstrated. IBF values of 4Ã—10-3 were obtained in cascaded R-MHSP and GEM multipliers at gains of about 104, though at the expense of reduced effective gain in the first R-MHSP multiplier in the cascade.; http://www.sciencedirect.com/science/article/B6TJM-4GFCMS7-1/1/3106a1da456923461b6eba3e10649401
The excess noise factor (ENF) of a large area avalanche photodiode was measured as a function of gain for different temperatures, in the -40 to 27°C range. Results show that ENF does not depend significantly with temperature attaining values of about 1.8 and 2.3 for gains of 50 and 300, respectively.; http://www.sciencedirect.com/science/article/B6TJM-4CMMX05-D/1/34a0ba40f32ddab67adeb9313913fec4
We report on avalanche-ion back-flow measurements in the novel Micro-Hole and Strip-Plate (MHSP) multiplier and in gaseous photomultipliers comprising Gas Electron Multipliers (GEMs) followed by an MHSP. In a 3-GEMs/MHSP photomultiplier with reflective photocathode, avalanche-ion back-flow fraction of ~7% and ~2% were recorded for respective effective gains of 107 and 106, in Ar/CH4 (95/5) at 760 Torr. This is about one order of magnitude reduction in ion back-flow compared to the best values measured in 4-GEMs photomultiplier at the same gain. We describe the mode of operation of the MHSP and explain its ion back-flow reduction features.; http://www.sciencedirect.com/science/article/B6TJM-4BMBDH7-C/1/cdedfd1e72d3adbcfc9431895ab3e612
Performance characteristics of the response of a Peltier-cooled large-area avalanche photodiode are investigated. Detector gain, energy linearity, energy resolution and minimum detectable energy are studied at different operation temperatures. Detector energy resolution and lowest detectable X-ray energy present a strong improvement as the operation temperature is reduced from 25 to 15 °C and slower improvements are achieved for temperatures below 10 °C.; http://www.sciencedirect.com/science/article/B6TJN-48W8TPB-3/1/f2e5cc2f92ebf5121b66551085272dd4
Fernandes, L. M. P.; Antognini, A.; Boucher, M.; Conde, C. A. N.; Huot, O.; Knowles, P. E.; Kottmann, F.; Ludhova, L.; Mulhauser, F.; Pohl, R.; Santos, J. M. F. dos; Schaller, L. A.; Taqqu, D.; Veloso, J. F. C. A.
Fonte: Universidade de CoimbraPublicador: Universidade de Coimbra
Tipo: Artigo de Revista CientíficaFormato: aplication/PDF
Large-area avalanche photodiodes have been investigated as 1.9-keV X-ray detectors for the muonic hydrogen Lamb-shift experiment. We report experimental tests carried out for evaluation of the avalanche photodiode capabilities for X-ray detection in the intense radiation and low counting rate environment of experiments with muonic atoms. Several muonic atoms were used and it was shown that the electronic background of muonic atom X-ray spectra can be reduced simply by timing the X-ray signal against the gate signal produced by the muon entrance. Furthermore, the background can be eliminated using coincidences between the X-ray signal and the signal resulting from the electron due to the muon decay. This coincidence discrimination results, however, in a reduction of the X-ray detection efficiency.; http://www.sciencedirect.com/science/article/B6THN-4B6608X-11/1/4bb8c2b08d267e2eba00e1ae64b143a9
A Large Area Avalanche Photodiode was studied, aiming to access its performance as light detector at low temperatures, down to -80°C. The excess noise factor, F, was measured and found to be approximately independent of the temperature. A linear dependence of F on the APD gain with a slope of 0.00239±0.00008 was observed for gains >100. The detection of low intensity light pulses, producing only a few primary electron-hole pairs in the photodiode, is reported.; http://www.sciencedirect.com/science/article/B6TJM-48343C5-6/1/cfbd9d61bf7dfaa42e9a9b41eb71e0b1
Fernandes, L. M. P.; Antognini, A.; Boucher, M.; Conde, C. A. N.; Huot, O.; Knowles, P.; Kottmann, F.; Ludhova, L.; Mulhauser, F.; Pohl, R.; Schaller, L. A.; Santos, J. M. F. dos; Taqqu, D.; Veloso, J. F. C. A.
Fonte: Universidade de CoimbraPublicador: Universidade de Coimbra
Tipo: Artigo de Revista CientíficaFormato: aplication/PDF
The behaviour of large-area avalanche photodiodes for X-rays, visible and vacuum-ultra-violet (VUV) light detection in magnetic fields up to 5 T is described. For X-rays and visible light detection, the photodiode pulse amplitude and energy resolution were unaffected from 0 to 5 T, demonstrating the insensitivity of this type of detector to strong magnetic fields. For VUV light detection, however, the photodiode relative pulse amplitude decreases with increasing magnetic field intensity reaching a reduction of about 24% at 5 T, and the energy resolution degrades noticeably with increasing magnetic field.; http://www.sciencedirect.com/science/article/B6TJM-47K21KV-1/1/9eb7bcce781352042c63344265f8da97
The characterisation of photodiodes used as photosensors requires a determination of the number of electron-hole pairs produced by scintillation light. One method involves comparing signals produced by X-ray absorptions occurring directly in the avalanche photodiode with the light signals. When the light is derived from light-emitting diodes in the 400-600 nm range, significant non-linear behaviour is reported. In the present work, we extend the study of the linear behaviour to large-area avalanche photodiodes, of Advanced Photonix, used as photosensors of the vacuum ultraviolet (VUV) scintillation light produced by argon (128 nm) and xenon (173 nm). We observed greater non-linearities in the avalanche photodiodes for the VUV scintillation light than reported previously for visible light, but considerably less than the non-linearities observed in other commercially available avalanche photodiodes.; http://www.sciencedirect.com/science/article/B6TJM-45HFC9M-2K/1/447b7168bf09233e9122112c90b89b17
Large-area 1.9-keV X-ray detectors operating in magnetic fields up to 5 T are required for the μp-Lamb shift experiment. Xenon gas proportional scintillation counters provide high detection efficiency together with good energy and timing resolutions. Three prototypes with alternative vacuum ultraviolet photosensors of the xenon scintillation light are explored and discussed: a CsI-coated microstrip plate either integrated within the xenon envelope or in a separate chamber in a P-10 atmosphere and an avalanche photodiode integrated within the xenon envelope
An experiment with the aim to determine the Lamb shift in muonic helium has been carried out by the Charge Radius Experiments with Muonic Atoms (CREMA) Collaboration at the Paul Scherrer Institure (PSI) Switzerland.
The goal of the experiment is to measure several transitions between the 2S and 2P energy levels in muonic helium ions (μ4He+ and μ3He+) with a precision of 50 ppm, and consequently to determine the α particle and helion nuclear charge radii (RMS value) with a relative accuracy of 3×10-4.
This experiment comes after the good results attained in the muonic hydrogen
Lamb shift experiment, where the muonic hydrogen Lamb shift was measured and the proton charge radius determined, which lead to the proton radius puzzle. The next proposal of the CREMA Collaboration was to extend the Lamb shift measurements to muonic helium in an attempt to help to solve the proton radius puzzle and provide additional information about the helium nucleus.
This thesis is dedicated to the X-ray detection system to be used in the experiment, including the detectors and pre-amplifier system. The detectors chosen are avalanche photodiodes. A detailed study of two different types of avalanche photodiode (APD), reach-through avalanche photodiode (RT-APD) from Hamamatsu Photonics and large area avalanche photodiode (LAAPD) from Radiation Monitoring Devices Inc. (RMD) has been carried out. The two APD types have different depletion region thicknesses...
An indirect flat panel imager (FPI) with programmable avalanche gain and field emitter array (FEA) readout is being investigated for low-dose and high resolution x-ray imaging. It is made by optically coupling a structured x-ray scintillator, e.g., thallium (Tl) doped cesium iodide (CsI), to an amorphous selenium (a-Se) avalanche photoconductor called high-gain avalanche rushing amorphous photoconductor (HARP). The charge image created by the scintillator∕HARP (SHARP) combination is read out by the electron beams emitted from the FEA. The proposed detector is called scintillator avalanche photoconductor with high resolution emitter readout (SAPHIRE). The programmable avalanche gain of HARP can improve the low dose performance of indirect FPI while the FEA can be made with pixel sizes down to 50 μm. Because of the avalanche gain, a high resolution type of CsI (Tl), which has not been widely used in indirect FPI due to its lower light output, can be used to improve the high spatial frequency performance. The purpose of the present article is to investigate the factors affecting the spatial resolution of SAPHIRE. Since the resolution performance of the SHARP combination has been well studied, the focus of the present work is on the inherent resolution of the FEA readout method. The lateral spread of the electron beam emitted from a 50 μm×50 μm pixel FEA was investigated with two different electron-optical designs: mesh-electrode-only and electrostatic focusing. Our results showed that electrostatic focusing can limit the lateral spread of electron beams to within the pixel size of down to 50 μm. Since electrostatic focusing is essentially independent of signal intensity...
The authors are investigating the concept of a direct-conversion flat-panel imager with avalanche gain for low-dose x-ray imaging. It consists of an amorphous selenium (a-Se) photoconductor partitioned into a thick drift region for x-ray-to-charge conversion and a relatively thin region called high-gain avalanche rushing photoconductor (HARP) in which the charge undergoes avalanche multiplication. An active matrix of thin film transistors is used to read out the electronic image. The authors call the proposed imager HARP active matrix flat panel imager (HARP-AMFPI). The key advantages of HARP-AMFPI are its high spatial resolution, owing to the direct-conversion a-Se layer, and its programmable avalanche gain, which can be enabled during low dose fluoroscopy to overcome electronic noise and disabled during high dose radiography to prevent saturation of the detector elements. This article investigates key design considerations for HARP-AMFPI. The effects of electronic noise on the imaging performance of HARP-AMFPI were modeled theoretically and system parameters were optimized for radiography and fluoroscopy. The following imager properties were determined as a function of avalanche gain: (1) the spatial frequency dependent detective quantum efficiency; (2) fill factor; (3) dynamic range and linearity; and (4) gain nonuniformities resulting from electric field strength nonuniformities. The authors results showed that avalanche gains of 5 and 20 enable x-ray quantum noise limited performance throughout the entire exposure range in radiography and fluoroscopy...
A multitude of systems ranging from the Barkhausen effect in ferromagnetic materials to plastic deformation and earthquakes respond to slow external driving by exhibiting intermittent, scale-free avalanche dynamics or crackling noise. The avalanches are power-law distributed in size, and have a typical average shape: these are the two most important signatures of avalanching systems. Here we show how the average avalanche shape evolves with the universality class of the avalanche dynamics by employing a combination of scaling theory, extensive numerical simulations and data from crack propagation experiments. It follows a simple scaling form parameterized by two numbers, the scaling exponent relating the average avalanche size to its duration and a parameter characterizing the temporal asymmetry of the avalanches. The latter reflects a broken time-reversal symmetry in the avalanche dynamics, emerging from the local nature of the interaction kernel mediating the avalanche dynamics.
In the Obernberg valley, the Eastern Alps, landforms recently interpreted as moraines are re-interpreted as rock avalanche deposits. The catastrophic slope failure involved an initial rock volume of about 45 million m³, with a runout of 7.2 km over a total vertical distance of 1330 m (fahrböschung 10°). 36Cl surface-exposure dating of boulders of the avalanche mass indicates an event age of 8.6 ± 0.6 ka. A 14C age of 7785 ± 190 cal yr BP of a palaeosoil within an alluvial fan downlapping the rock avalanche is consistent with the event age. The distal 2 km of the rock-avalanche deposit is characterized by a highly regular array of transverse ridges that were previously interpreted as terminal moraines of Late-Glacial. ‘Jigsaw-puzzle structure’ of gravel to boulder-size clasts in the ridges and a matrix of cataclastic gouge indicate a rock avalanche origin. For a wide altitude range the avalanche deposit is preserved, and the event age of mass-wasting precludes both runout over glacial ice and subsequent glacial overprint. The regularly arrayed transverse ridges thus were formed during freezing of the rock avalanche deposits.
We studied a large debris-avalanche deposit of Pleistocene age in the Tenteniguada Basin, Gran Canaria Island, Spain. This deposit, which is well preserved because it is mostly covered by basanite lava flows, has distinctive matrix and block facies, hummocky topography, and internal structures typical of debris avalanches. However, neither syneruptive lavas nor some characteristic features of volcanic debris-avalanche deposits, such as a stratovolcano edifice or a horseshoe-shaped crater, are present. The occurrence of internal features characteristic of volcanic avalanche deposits could be attributed to the volcanic materials involved in the movement, rather than to the triggering of the avalanche during a volcanic eruption. The conditioning factors are shown to be associated with specific structural and hydrological conditions, such as the presence of old volcanic domes, strength reduction of the rocks, effective stress decrease, active gully erosion, and water table rise during Pleistocene humid episodes. We finally suggest that the possible triggering factor of the avalanche was a neighbouring volcanic or tectonic earthquake.; JRC.H.7-Land management and natural hazards
For a long time, it has been known that the power spectrum of Barkhausen
noise had a power-law decay at high frequencies. Up to now, the theoretical
predictions for this decay have been incorrect, or have only applied to a small
set of models. In this paper, we describe a careful derivation of the power
spectrum exponent in avalanche models, and in particular, in variations of the
zero-temperature random-field Ising model. We find that the naive exponent,
(3-tau)/(sigma nu z), which has been derived in several other papers, is in
general incorrect for small tau, when large avalanches are common. (tau is the
exponent describing the distribution of avalanche sizes, and (sigma nu z) is
the exponent describing the relationship between avalanche size and avalanche
duration.) We find that for a large class of avalanche models, including
several models of Barkhausen noise, the correct exponent for tau<2 is 1/(sigma
nu z). We explicitly derive the mean-field exponent of 2. In the process, we
calculate the average avalanche shape for avalanches of fixed duration and
scaling forms for a number of physical properties.; Comment: 16 pages, 12 figures
A novel concept of optical signal recording in two-phase avalanche detectors,
with Geiger-mode Avalanche Photodiodes (G-APD) is described.
Avalanche-scintillation photons were measured in a thick Gas Electron
Multiplier (THGEM) in view of potential applications in rare-event experiments.
The effective detection of avalanche scintillations in THGEM holes has been
demonstrated in two-phase Ar with a bare G-APD without wavelength shifter, i.e.
insensitive to VUV emission of Ar. At gas-avalanche gain of 400 and under \pm
70^\circ viewing-angle, the G-APD yielded 640 photoelectrons (pe) per 60 keV
X-ray converted in liquid Ar; this corresponds to 0.7 pe per initial (prior to
multiplication) electron. The avalanche-scintillation light yield measured by
the G-APD was about 0.7 pe per avalanche electron, extrapolated to 4pi
acceptance. The avalanche scintillations observed occurred presumably in the
near infrared (NIR) where G-APDs may have high sensitivity. The measured
scintillation yield is similar to that observed by others in the VUV. Other
related topics discussed in this work are the G-APD's single-pixel and
quenching resistor characteristics at cryogenic temperatures.; Comment: 21 pages, 18 figures. Submitted to JINST
Avalanche behavior of gravitationally-forced granular layers on a rough
inclined plane are investigated experimentally for different materials and for
a variety of grain shapes ranging from spherical beads to highly anisotropic
particles with dendritic shape. We measure the front velocity, area and the
height of many avalanches and correlate the motion with the area and height. We
also measure the avalanche profiles for several example cases. As the shape
irregularity of the grains is increased, there is a dramatic qualitative change
in avalanche properties. For rough non-spherical grains, avalanches are faster,
bigger and overturning in the sense that individual particles have down-slope
speeds $u_p$ that exceed the front speed $u_f$ as compared with avalanches of
spherical glass beads that are quantitatively slower, smaller and where
particles always travel slower than the front speed. There is a linear increase
of three quantities i) dimensionless avalanche height ii) ratio of particle to
front speed and iii) the growth rate of avalanche speed with increasing
avalanche size with increasing $\tan\theta_r$ where $\theta_r$ is the bulk
angle of repose, or with increasing $\beta_P$, the slope of the depth averaged
flow rule, where both $\theta_r$ and $\beta_P$ reflect the grain shape
irregularity. These relations provide a tool for predicting important dynamical
properties of avalanches as a function of grain shape irregularity. A
relatively simple depth-averaged theoretical description captures some
important elements of the avalanche motion...
Geiger-mode avalanche photodiode (GM APD) detectors are capable of counting single photons, measuring arrival times with high resolution, and generating zero read noise (when operated with a CMOS digital readout circuit) due to their unique internal gain characteristics. These capabilities make them exceptionally suited to tasks that require precise arrival time measurements or characterization of faint signals (low photon flux). Laser ranging systems use their arrival time measurement capabilities to build three-dimensional images, while adaptive optics applications have recently begun to capitalize on their low noise and high-speed operation for correcting wavefront imperfections due to atmospheric interference. There is now growing interest in using GM APDs for imaging applications where accurate measurements of faint signals are necessary, such as in astronomy. MIT Lincoln Laboratory and the RIT Center for Detectors have developed silicon GM APDs with unique architecture, utilizing scupper regions to minimize detector noise.
This thesis investigates the performance of these detectors in terms of dark count rate (DCR). There are a number of mechanisms that produce dark counts, the most prominent being thermal excitation of carriers. Thermal carrier generation rates are generally only dependent on the temperature of the diode and may be constant under certain controlled conditions. Afterpulsing results from the release of carriers trapped in intermediate energy states (states with energy in the band gap of the material). Unlike thermal carrier generation...
En este artículo se describe un mega deslizamiento, probablemente de tipo debris avalanche, localizado en el flanco norte del volcán Cacahuatique en el departamento de Morazán, república de El Salvador. Se estima que unos 6000 MMC (millones de metros cúbicos) de materiales volcánicos se deslizaron desde las laderas del flanco norte del volcán Cacahuatique hacia la margen izquierda del río Torola. El deslizamiento principal tiene 6 km de ancho por 5 km de longitud y unos 200 m de espesor original, deslizados probablemente hacia finales del Plioceno o inicios del Pleistoceno. La mayor parte de los materiales deslizados han sido erosionados por los ríos Los Reyes, Gualpuca, Grande y Torola, sin embargo persisten algunos relictos de la masa deslizada original como el Cerro San Lucas (morfología de hummucks). Se presentan y describen los datos geomorfológicos y geológicos que sustentan la tesis de la ocurrencia de este mega deslizamiento.