C. Puigdengoles

Performance limits of a 55-/spl mu/m pixel CdTe detector

In this work, the results from simulation of a CdTe pixel detector with a pixel size of 45 mum and a pitch of 55 mum are presented. Simulation of charge sharing between neighboring pixels has been compared with experimental results obtained by the use of the Medipix2 photon counting chip by means of a unique read-out system developed for the Dear-Mama project. Electrical simulations have been performed using the commercial software package DESSIS (ISETCAD) and combined with the Monte Carlo code GEANT4 to simulate the process of interaction and subsequent charge transport of charges induced by radiation. This allowed the analysis of charge sharing between pixel elements, an important limiting factor in imaging applications. Simulation was compared with experimental results obtained by means of a 1 mm thick CdTe detector. Each of these pixel electrodes was connected to the corresponding readout pixel on the Medipix2 chip, via an indium bump-bond. The backside contact was biased at -100 V, so that the pixel electrodes were collecting electrons. The results obtained imply that using a detector 1 mm thick with a pixel size smaller than 55 mum in single photon counting mode is unrealistic, because such fine spatial resolution cannot be attained in the corresponding image due to charge sharing

Energy and coincidence time resolution measurements of CdTe detectors for PET

We report on the characterization of 2 mm thick CdTe diode detector with Schottky contacts to be employed in a novel conceptual design of PET scanner. Results at -8°C with an applied bias voltage of -1000 V/mm show a 1.2% FWHM energy resolution at 511 keV. Coincidence time resolution has been measured by triggering on the preamplifier output signal to improve the timing resolution of the detector. Results at the same bias and temperature conditions show a FWHM of 6 ns with a minimum acceptance energy of 500 keV. These results show that pixelated CdTe Schottky diode is an excellent candidate for the development of next generation nuclear medical imaging devices such as PET, Compton gamma cameras, and especially PET-MRI hybrid systems when used in a magnetic field immune configuration.

Toward VIP-PIX: A Low Noise Readout ASIC for Pixelated CdTe Gamma-Ray Detectors for Use in the Next Generation of PET Scanners

VIP-PIX will be a low noise and low power pixel readout electronics with digital output for pixelated Cadmium Telluride (CdTe) detectors. The proposed pixel will be part of a 2D pixel-array detector for various types of nuclear medicine imaging devices such as positron-emission tomography (PET) scanners, Compton gamma cameras, and positron-emission mammography (PEM) scanners. Each pixel will include a SAR ADC that provides the energy deposited with 10-bit resolution. Simultaneously, the self-triggered pixel which will be connected to a global time-to-digital converter (TDC) with 1 ns resolution will provide the events time stamp. The analog part of the readout chain and the ADC have been fabricated with TSMC 0.25 μm mixed-signal CMOS technology and characterized with an external test pulse. The power consumption of these parts is 200 μW from a 2.5 V supply. It offers 4 switchable gains from ± 10 mV/fC to ± 40 mV/fC and an input charge dynamic range of up to ± 70 fC for the minimum gain for both polarities. Based on noise measurements, the expected equivalent noise charge (ENC) is 65 e- RMS at room temperature.

Simulation of the Expected Performance of a Seamless Scanner for Brain PET Based on Highly Pixelated CdTe Detectors

The aim of this work is the evaluation of the design for a nonconventional PET scanner, the voxel imaging PET (VIP), based on pixelated room-temperature CdTe detectors yielding a true 3-D impact point with a density of 450 channels/cm3, for a total 6 336 000 channels in a seamless ring shaped volume. The system is simulated and evaluated following the prescriptions of the NEMA NU 2-2001 and the NEMA NU 4-2008 standards. Results show that the excellent energy resolution of the CdTe detectors (1.6% for 511 keV photons), together with the small voxel pitch (1 × 1 × 2 mm3), and the crack-free ring geometry, give the design the potential to overcome the current limitations of PET scanners and to approach the intrinsic image resolution limits set by physics. The VIP is expected to reach a competitive sensitivity and a superior signal purity with respect to values commonly quoted for state-of-the-art scintillating crystal PETs. The system can provide 14 cps/kBq with a scatter fraction of 3.95% and 21 cps/kBq with a scatter fraction of 0.73% according to NEMA NU 2-2001 and NEMA NU 4-2008, respectively. The calculated NEC curve has a peak value of 122 kcps at 5.3 kBq/mL for NEMA NU 2-2001 and 908 kcps at 1.6 MBq/mL for NEMA NU 4-2008. The proposed scanner can achieve an image resolution of ~ 1 mm full-width at half-maximum in all directions. The virtually noise-free data sample leads to direct positive impact on the quality of the reconstructed images. As a consequence, high-quality high-resolution images can be obtained with significantly lower number of events compared to conventional scanners. Overall, simulation results suggest the VIP scanner can be operated either at normal dose for fast scanning and high patient throughput, or at low dose to decrease the patient radioactivity exposure. The design evaluation presented in this work is driving the development and the optimization of a fully operative prototype to prove the feasibility of the VIP concept.

Characterization of CdTe detector for use in PET

CdTe diode detectors with Schottky contact have been characterized in terms of energy resolution and time of response. A resolution of 0.98% at 511keV has been achieved with a 4 mm × 4 mm × 2 mm detector at 900 V/mm and −7 °C. At the same bias and temperature conditions, two identical CdTe detectors show a coincidence time FWHM of 25 ns. Additionally, the effect of a strong magnetic field on the charge sharing has been studied for a 9-pixel array detector of 55 µm × 55 µm × 800 µm pixel size connected to MediPix2 front end electronics. No effect on the charge sharing distribution has been observed up to 4 T. These results show that CdTe Schottky diodes are excellent candidates for the development of next generation nuclear medical imaging devices such as PET, Compton gamma camera, and especially PET-MRI when used in a magnetic field immune configuration.

Modeling, simulation, and evaluation of a compton camera based on a pixelated solid-state detector

A novel Compton camera design based on pixelated solid-state detectors is proposed and evaluated via Monte Carlo simulation, using the Geant4-based Architecture for Medicine-Oriented Simulations GAMOS. For the image reconstruction, the Stochastic Origin Ensemble (SOE) method has been used. The efficiency of the reconstruction of Compton prompt events is constant up to activities of 107 Bq. The signal-to-noise ratio (SNR), i.e., the ratio between real coincidences and mis-reconstructed ones, was above 85% for photon energies ranging from 141 to 511 keV. For a 18F isotope source, a sensitivity of 12 cps/kBq has been obtained. For a 99mTc isotope source, a sensitivity of 15 cps/kBq has been obtained. Using the NEMA NU-4 2008 standard for the PSF estimation, values for the FWHM of 1.80 mm for the spatial resolution with a 18F radioactive source and 3.82 mm with a 99mTc source were obtained.

Simulation of pseudo-clinical conditions and image quality evaluation of PET scanner based on pixelated CdTe detector

A novel conceptual design of PET scanner, known as a Voxel Imaging PET (VIP) [1], based on pixelated solid state detector, has been proposed to overcome the intrinsic limitations of state-of-art PET devices based on scintillating crystals. For this study, the VIP scanner has been simulated in different pseudo-clinical conditions in order to assess its image quality performance. The evaluation of the PET image quality follows the NEMA prescriptions. Results are also presented for both analytic and iterative image reconstruction methods. The preliminary results from simulations show that the image reconstruction performance of the VIP scanner under realistic clinical conditions provides good images with low dose. The VIP scanner is able to detect down to 1 mm in diameter hot regions in small phantoms without background activity and down to 5 mm in diameter hot spheres in big phantoms in the presence of background activity. Moreover, good quality and high contrast images can be obtained with a considerably lower number of coincidences with respect to the usual crystal PETs.

LISA and LISA PathFinder, the endeavour to detect low frequency GWs

This is a review about LISA and its technology demonstrator, LISAPathFinder. We first describe the conceptual problems which need to be overcome in order to set up a working interferometric detector of low frequency Gravitational Waves (GW), then summarise the solutions to them as currently conceived by the LISA mission team. This will show that some of these solutions require new technological abilities which are still under development, and which need proper test before being fully implemented. LISAPathFinder (LPF) is the the testbed for such technologies. The final part of the paper will address the ideas and concepts behind the PathFinder as well as their impact on LISA.

Studies of irradiated AMS H35 CMOS detectors for the ATLAS tracker upgrade

Silicon detectors based on the HV-CMOS technology are being investigated as possible candidate for the outer layers of the ATLAS pixel detector for the High Luminosity LHC. In this framework the H35Demo ASIC has been produced in the 350 nm AMS technology (H35). The H35Demo chip has a large area (18.49 × 24.40 mm2) and includes four different pixel matrices and three test structures. In this paper the radiation hardness properties, in particular the evolution of the depletion region with fluence is studied using edge-TCT on test structures. Measurements on the test structures from chips with different substrate resistivity are shown for non irradiated and irradiated devices up to a cumulative fluence of 2 ⋅ 1015 1 MeV neq / cm2.

Heliospheric influences on LISA

Laser interferometer space antenna (LISA) is the first interferometer devoted to the detection of low frequency gravitational waves in space. Control of the environment and the study of the effects of energetic particles traversing the LISA apparatus are mandatory for this experiment to be fully successful. Telescopes for solar and cosmic-ray particle monitoring will be placed on board LISA and its precursor mission LISA Pathfinder (LISA-PF). In particular, proton, helium and electron detection is recommended on LISA. In this paper, we report expected incident galactic proton and helium fluxes at the time of LISA and LISA-PF missions. The number of solar events with the fluence ranging between 10 6 and 10 11 protons cm ―2 above 30 MeV occurring during the LISA missions is estimated as well. Finally, we discuss the importance of electron detection on LISA to forecast upcoming solar energetic protons.