We present a combined surface processes and tectonic model which allows us to determine the climatic and tectonic parameters that control the development of faceted spurs at normal fault scarps. ...Sensitivity tests to climatic parameter values are performed. For a given precipitation rate, when hillslope diffusion is high and channel bedrock is highly resistant to erosion, the scarp is smooth and undissected. When, instead, the bedrock is easily eroded and diffusion is limited, numerous channels develop and the scarp becomes deeply incised. Between these two end‐member states, diffusion and incision compete to produce a range of scarp morphologies, including faceted spurs. The sensitivity tests allow us to determine a dimensionless ratio of erosion, f, for which faceted spurs can develop. This study evidences a strong dependence of facet slope angle on throw rate for throw rates between 0.4 and 0.7 mm/a. Facet height is also shown to be a linear function of fault throw rate. Model performance is tested on the Wasatch Fault, Utah, using topographic, geologic, and seismologic data. A Monte Carlo inversion on the topography of a portion of the Weber segment shows that the 5 Ma long development of this scarp has been dominated by a low effective precipitation rate (∼1.1 m/a) and a moderate diffusion coefficient (0.13 m2/a). Results demonstrate the ability of our model to estimate normal fault throw rates from the height of triangular facets and to retrieve the average long‐term diffusion and incision parameters that prevailed during scarp evolution using an accurate 2‐D misfit criterion.
The response to γ-rays of long thin polished or wrapped YAP:Ce scintillator bars read out at both edges by photomultipliers has been studied by means of GEANT4 simulations with the aim to investigate ...the capability of the charge division method to reconstruct the gamma interaction point in the crystal. The GEANT4 simulations took into account the optical properties of the scintillator like absorption, reflection, and diffusion during the passage of the light through the crystal. The simulations were able to reproduce experimental measurements of light yields in polished and wrapped YAP:Ce crystals with dimensions 3.2×3.2×100
mm
3. Once validated, the simulations could show that a particular patterning of the crystal surface, which enhances the surface absorption, should lead to a significant improvement of the reconstruction precision of the charge division method, without compromising the energy resolution.
A novel concept for a positron emission tomography (PET) camera module is proposed, which provides full 3D reconstruction with high resolution over the total detector volume, free of parallax errors. ...The key components are a matrix of long scintillator crystals and hybrid photon detectors (HPDs) with matched segmentation and integrated readout electronics. The HPDs read out the two ends of the scintillator package. Both excellent spatial (x, y, z) and energy resolution are obtained. The concept allows enhancing the detection efficiency by reconstructing a significant fraction of events which underwent Compton scattering in the crystals. The proof of concept will first be demonstrated with yttrium orthoaluminate perovskite (YAP):Ce crystals, but the final design will rely on other scintillators more adequate for PET applications (e.g. LSO:Ce or LaBr3:Ce). A promising application of the proposed camera module, which is currently under development, is a high resolution 3D brain PET camera with an axial field-of-view of approximately 15 cm dedicated to brain research. The design philosophy and performance predictions based on analytical calculations and Monte Carlo simulations are presented. Image correction and reconstruction tools required to operate this transmissionless device in a research environment are also discussed. Better or similar performance parameters were obtained compared to other known designs at lower fabrication cost. The axial geometrical concept also seems to be promising for applications such as positron emission mammography.
AX-PET is a novel PET detector based on axially oriented crystals and orthogonal wavelength shifter (WLS) strips, both individually read out by silicon photo-multipliers. Its design decouples ...sensitivity and spatial resolution, by reducing the parallax error due to the layered arrangement of the crystals. Additionally the granularity of AX-PET enhances the capability to track photons within the detector yielding a large fraction of inter-crystal scatter events. These events, if properly processed, can be included in the reconstruction stage further increasing the sensitivity. Its unique features require dedicated Monte-Carlo simulations, enabling the development of the device, interpreting data and allowing the development of reconstruction codes. At the same time the non-conventional design of AX-PET poses several challenges to the simulation and modeling tasks, mostly related to the light transport and distribution within the crystals and WLS strips, as well as the electronics readout. In this work we present a hybrid simulation tool based on an analytical model and a Monte-Carlo based description of the AX-PET demonstrator. It was extensively validated against experimental data, providing excellent agreement.
The Cordilleran Ice Sheet (CIS) once covered an area comparable to that of Greenland. Previous geologic evidence and numerical models indicate that the ice sheet covered much of westernmost Canada as ...late as 12.5 thousand years ago (ka). New data indicate that substantial areas throughout westernmost Canada were ice free prior to 12.5 ka and some as early as 14.0 ka, with implications for climate dynamics and the timing of meltwater discharge to the Pacific and Arctic oceans. Early Bølling-Allerød warmth halved the mass of the CIS in as little as 500 years, causing 2.5 to 3.0 meters of sea-level rise. Dozens of cirque and valley glaciers, along with the southern margin of the CIS, advanced into recently deglaciated regions during the Bølling-Allerød and Younger Dryas.
AX-PET: A novel PET detector concept with full 3D reconstruction Braem, A.; Joram, C.; Séguinot, J. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
10/2009, Volume:
610, Issue:
1
Journal Article
Peer reviewed
We describe the concept and first experimental tests of a novel 3D axial Positron Emission Tomography (PET) geometry. It allows for a new way of measuring the interaction point in the detector with ...very high precision. It is based on a matrix of long Lutetium-Yttrium OxyorthoSilicate (LYSO) crystals oriented in the axial direction, each coupled to one Geiger Mode Avalanche Photodiode (G-APD) array. To derive the axial coordinate, Wave Length Shifter (WLS) strips are mounted orthogonally and interleaved between the crystals. The light from the WLS strips is read by custom-made G-APDs. The weighted mean of the signals in the WLS strips has proven to give very precise axial resolution. The achievable resolution along the three axes is mainly driven by the dimensions of the LYSO crystals and WLS strips. This concept is inherently free of parallax errors. Furthermore, it will allow identification of Compton interactions in the detector and for reconstruction of a fraction of them, which is expected to enhance image quality and sensitivity. We present the results of proof-of-principle tests and qualification measurements of the various components prepared to build a larger scale demonstrator consisting of two matrices of
8
×
6
LYSO crystals and 312 WLS strips.
The AX-PET demonstrator—Design, construction and characterization Beltrame, P.; Bolle, E.; Braem, A. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
10/2011, Volume:
654, Issue:
1
Journal Article
Peer reviewed
Axial PET is a novel geometrical concept for Positron Emission Tomography (PET), based on layers of long scintillating crystals axially aligned with the bore axis. The axial coordinate is obtained ...from arrays of wavelength shifting (WLS) plastic strips placed orthogonally to the crystals. This article describes the design, construction and performance evaluation of a demonstrator set-up which consists of two identical detector modules, used in coincidence. Each module comprises 48 LYSO crystals of 100
mm length and 156 WLS strips. Crystals and strips are readout by Geiger-mode Avalanche Photo Diodes (G-APDs). The signals from the two modules are processed by fully analog front-end electronics and recorded in coincidence by a VME-based data acquisition system. Measurements with point-like
22Na sources, with the modules used both individually and in coincidence mode, allowed for a complete performance evaluation up to the focal plane reconstruction of point sources. The results obtained are in good agreement with expectations and proved the set-up to be ready for the next evaluation phase with PET phantoms filled with radiotracers.
Proof of principle of G-APD based hybrid photodetectors Joram, C.; Rudge, A.; Séguinot, J.
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
09/2010, Volume:
621, Issue:
1
Journal Article
Peer reviewed
We performed a proof of principle experiment which demonstrates the suitability of pixelized Geiger mode avalanche photodiodes (G-APD) for the detection of photoelectrons at energies in the 10
keV ...range. A pumped UHV set-up with CsI photocathode, illuminated by a UV flash lamp, is used to generate photoelectrons of defined energy. The results indicate that G-APDs can be considered as anodes in hybrid photodetectors with the potential of improved performance when compared to conventional photomultiplier tubes. The concept of a G-APD based HPD has advantages but also clear drawbacks. We discuss the particular case of the X-HPD where a G-APD based anode could lead to improved detection efficiency and timing as well as to a more cost-effective production process.
The measurements presented in this paper are related to the development of a PET camera based on a 3-D axial geometry with excellent 3-D spatial, timing and energy resolution. The detector modules ...consist of matrices of long axially oriented scintillation crystal bars, which are individually coupled to photodetectors. The axial coordinate is derived from wavelength shifting (WLS) plastic strips orthogonally interleaved between the crystal bars and readout by G-APD arrays. We report on results from measurements with two LYSO crystal bars, read with PMTs, and two WLS strips readout with G-APD devices from Hamamatsu (called MPPC). The WLS strips are positioned orthogonally underneath the LYSO bars. Yields of about 80 photoelectrons from the WLS strips for an energy deposition in the LYSO crystals equivalent to the absorption of 511
keV photons are observed. The axial coordinate in the LYSO bars is reconstructed with a precision of about 1.9
mm (FWHM) using a digital reconstruction method. The resolution of an analog coordinate reconstruction method, which uses the pulse height measurement from the WLS strips is 2.8
mm (FWHM). This resolution is still compromised by the availability of only two WLS strips and will improve with a full stack of LYSO crystals interleaved with WLS strip arrays, which is presently under development for a PET demonstrator set-up.
The effective light attenuation length in thin bars of polished YAP:Ce and LYSO:Ce scintillators with lengths of the order of 10
cm has been studied for various wrappings and coatings of the crystal ...lateral surfaces. This physical parameter plays a key role in a novel 3D PET concept based on axial arrays of long scintillator bars read out at both ends by Hybrid Photodetectors (HPDs) since it influences the spatial, energy and time resolutions of such a device. In this paper we show that the effective light attenuation length of polished crystals can be reduced by wrapping their lateral surfaces with Teflon, or tuned to the desired value by depositing a coating of Cr or Au of well-defined thickness. The studies have been carried out with YAP and LYSO long scintillator bars, read out by standard photomultiplier tubes. Even if the novel PET device will use different scintillators and HPD readout, the results described here prove the feasibility of an important aspect of the concept and provide hints on the potential capabilities of the device.
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