P.M.M. Correia

THGEM gain calculations using Garfield++: solving discrepancies between simulation and experimental data

Discrepancies between the measured and simulated gain in Thick Micropatterned gaseous detectors (MPGD), namely THGEM, have been observed by several groups. In order to simulate the electron avalanches and the gain the community relies on the calculations performed in Garfield++, known to produce differences of 2 orders of magnitude in comparison to the experimental data for thick MPGDs. In this work, simulations performed for Ne/5%CH4, Ar/5%CH4 and Ar/30%CO2 mixtures shows that Garfield++ is able to perfectly describe the experimental data if Penning effect is included in the simulation. The comparison between the number of excitations which may lead to a Penning transfer, is shown for THGEM and GEM, explaining the less pronounced gain discrepancies observed in GEM.

Spectroscopic analysis of LYSO:Ce crystals

Rare earth orthosilicates are among the most widely used scintillator materials in the last decades. Particularly, lutetium-yttrium oxyorthosilicate (LYSO) is known to exhibit great potentialities in the field of radiation detectors for medical imaging. Consequently, an in-depth knowledge of the material properties is of utmost interest for the mentioned applications. In this work the spectroscopic properties of commercial cerium doped lutetium-yttrium oxyorthosilicate crystals (LYSO:Ce) were investigated by Raman spectroscopy, steady state photoluminescence, photoluminescence excitation and time resolved photoluminescence. Site selective excitation was used under steady state (325nm) and pulsed (266nm) conditions to separately investigate the temperature dependence of the 5d→4f Ce1 and Ce2 luminescence, allowing to establish the thermal quenching dependence of the Ce2 optical center. In the case of the Ce1 optical center, a luminescence quantum efficiency of 78% was obtained from 14K to room temperature with 266nm photon excitation.

Pressure effects on the X-ray intrinsic position resolution in noble gases and mixtures

A study of the effect of gas pressure in the position resolution of an interacting X or γ-ray photon in a gas medium is performed. The intrinsic position resolution for pure noble gases (Argon and Xenon) and their mixtures with CO2 and CH4 was calculated for several gas pressures (1–10 bar) and for photon energies between 1 and 60 keV, being possible to establish a linear relation between the intrinsic position resolution and the inverse of the gas pressure in the indicated energy range, as intuitively expected. We show how, at high pressures and low photoelectron energies, this intrinsic 1/P scaling is modified due to the diffusion of the primary ionization in the direction perpendicular to the electric field. In order to evaluate the quality of the method here described, a comparison between the available experimental data and microscopic simulations is presented in this work and discussed. In the majority of cases, a good agreement is observed. The conditions to achieve position resolutions down to 10 μm in a realistic detector are shown and discussed.

arXiv : Simulation of gain stability of THGEM gas-avalanche particle detectors

Charging-up processes affecting gain stability in Thick Gas Electron Multipliers (THGEM) were studied with a dedicated simulation toolkit. Integrated with Garfield++, it provides an effective platform for systematic phenomenological studies of charging-up processes in MPGD detectors. We describe the simulation tool and the fine-tuning of the step-size required for the algorithm convergence, in relation to physical parameters. Simulation results of gain stability over time in THGEM detectors are presented, exploring the role of electrode-thickness and applied voltage on its evolution. The results show that the total amount of irradiated charge through electrodes hole needed for reaching gain stabilization is in the range of tens to hundreds of pC, depending on the detector geometry and operational voltage. These results are in agreement with experimental observations presented previously.

The EasyPET: A novel concept for an educational cost-effective positron emission 2D scanner

The easyPET concept proposed here, protected under a patent by the University of Aveiro, aims to realize a simple and affordable small dimension Positron Emission Tomography (PET) scanner. This innovative system is based on a single pair of detectors and a rotating mechanism with two degrees of freedom reproducing the functionalities of an entire PET ring. A 2D imaging prototype has been designed, commissioned and engineered, targeted to high level education for physics, engineering and nuclear medicine students. In this paper the performance of the prototype is reported, with a focus on the imaging capability and on the measurement of the uncertainty in the reconstruction of the source position. In addition, a detailed analysis is dedicated to the slice sensitivity and in particular to the effect of the energy threshold on the coincidence event selection.

A high pressure gaseous detector as a compton camera for nuclear medical imaging

We present the assembly and first measurements on a High Pressure gas scintillation Proportional Counter envisaging a Compton Camera for nuclear medical imaging. After introducing the concept, simulation results, setup, and assembly, the detector performance will be presented. Characterization of the Gaseous Proportional Scintillator (GSPC) counter, namely the number of produced photons and energy resolution according to the gas pressure and electric fields, by using a traditional PMT coupled to it is presented.

Development of a micro PET system with improved spatial resolution through depth-of-interaction measurement

In small diameter positron emission tomography (PET) systems, the determination of the depth-of-interaction (DOI) of 511 keV gamma photons in scintillator crystals is of great importance, in order to achieve high DOI resolution with good uniformity within the entire field-of-view. In this work, we propose a new method for DOI determination, in which a single layer of LYSO crystals is read out on both ends through the use of silicon photomultipliers (SiPMs), but using wavelength-shifting fibers and a reduced number of SiPMs on one end. This design results in a simpler and less expensive readout when compared to the typical dual-ended readout method, which requires two photodetectors per crystal and corresponding readout electronics. GATE simulation of the system was carried out and experimental proof-of-concept studies were performed on a single detector cell (composed of two LYSO crystals operating in coincidence), to evaluate the amount of light detected on each side of the crystal and the achievable DOI resolution with this method, taking into account the attenuation of the light signal on the fiber side with crystal-SiPM distance. The feasibility of applying this new method in full detector rings for a small animal PET system is evaluated and discussed, considering different alternatives for position readout electronics.

Computed tomography system using a new MPGD for small animal imaging with energy resolving capability

Micropatterned Gaseous Detectors (MPGDs) have been used for several years in single photon counting and energy resolved imaging systems. This kind of detectors presents suitable characteristics for such purposes as: elimination of the electronic noise, good energy resolution and high counting rate capability, absence of dead areas and 2D intrinsic position discrimination capability. A Computed Tomography (CT) system using a Thick COBRA (THCOBRA) with energy resolution capability envisaging small animal imaging is being developed. By using this kind of single photon counting and energy resolved detector it is possible to retain the interaction position information and energy of each detected single photon, allowing the application of Energy Weighting Technique (EWT) and, therefore, improvement of image quality. Cross-sectional images of a Poly(methyl methacrylate) - PMMA - phantom and of a sea snail are presented.

Ion back flow reduction in a THGEM based detector

Cherenkov imaging counters requiring large photosensitive areas, the capability to stand high rates and to operate in magnetic field environments could benefit from the use of micropattern gas detectors based on THick Gaseous Electron Multiplier (THGEM) coupled to a solid state CsI photo-cathode. Nevertheless, the ions produced in the charge multiplication processes, which end up in the CsI photocathode Ion Back Flow (IBF) compromise the detector performance: fast ageing of the CsI photocathode, as well as electron extraction resulting in spurious signals and eventual discharges can occur. To avoid and limit these undesired events, several configurations of THGEM based detectors were considered. Some changing the whole detector geometry by changing the relative position of individual THGEMs to take advantage of the microscopic diffusion properties of electrons and ions. One other configuration changing directly the geometry of the THGEM itself to better create an ion trap, the THCOBRA, is also studied. In this work, experimental and simulation studies of these configurations is performed, particularly concerning IBF and gain. Finite element method calculations and Monte-Carlo simulations are performed for a better understanding of the results.