A. J. Gonzalez

Timing Results Using an FPGA-Based TDC with Large Arrays of 144 SiPMs

Silicon photomultipliers (SiPMs) have become an alternative to traditional tubes due to several features. However, their implementation to form large arrays is still a challenge especially due to their relatively high intrinsic noise, depending on the chosen readout. In this contribution, two modules composed of 12 ×12 SiPMs with an area of roughly 50 mm×50 mm are used in coincidence. Coincidence resolving time (CRT) results with a field-programmable gate array, in combination with a time to digital converter, are shown as a function of both the sensor bias voltage and the digitizer threshold. The dependence of the CRT on the sensor matrix temperature, the amount of SiPM active area and the crystal type is also analyzed. Measurements carried out with a crystal array of 2 mm pixel size and 10 mm height have shown time resolutions for the entire 288 SiPM two-detector set-up as good as 800 ps full width at half maximum (FWHM).

First results of an ASIC controlled γ-detector based on a SiPM-array and a monolithic LYSO

We present the first results of an MR compatible PET system based on continuous crystals and SiPM detectors, along with a dedicated application-specific integrated circuit (ASIC) for the readout stage. The use of continuous scintillation crystals preserves the spatial distribution of scintillation light for each γ--ray impact, which can be reconstructed with a small number of statistical moments Therefore, it is possible to reduce the number of analogue-to-digital conversion channels. The current ASIC can measure up to 8 moments of the light distribution, providing information about the X and Y photon impact coordinates, photon energy, depth of interaction and other higher moments that can improve the position accuracy of the photon impact. SiPMs exhibit their best performance for reduce active areas, where the dark counts are minimized. An innovate coupling design between crystal and SiPM-array has been made in order to keep the compromise between reduced active areas and efficient collection of the scintillation light. SiPM detectors are also suitable to work in the presence of magnetic fields. Moreover, they could be integrated in a RF coil of the MRI system due to their reduced dimensions [1]. Experimental results show very promising possibilities for the system.

Characterization of protons accelerated from a 3 TW table-top laser system

This project has been funded by Centro para el Desarrollo Tecnologico Industrial (CDTI, Spain) within the INNPRONTA program, grant no. IPT-20111027, by EUROSTARS project E9113, and by the Spanish Ministry for Economy and Competitiveness within the Retos-Colaboracion 2015 initiative, ref. RTC-2015-3278-1.

Detailed requirements for a laser-based proton/ion accelerator for radioisotope production

Background and objectives: Laser-plasma acceleration of protons and ions is often considered a promising technique for compact applications of highly intense beams of multi-MeV particles. A remarkable example is the on-site production of short-lived radioisotopes for medical and preclinical interventions. We study quantitatively the activity of four important PET isotopes which may be obtained by irradiation of suitable target nuclei with laser-accelerated protons and deuterons. These simulations allow for confining the range of useful parameters of a laser-based production system. Methods: We choose a total of ten p- and d-induced reaction channels for the production of F-18, C-11, O-15, and N-13 from suitable target nuclei. We calculate the activity yield as a function of projectile energy starting from the corresponding, known cross sections. In order to simulate typical laser-plasma particle spectra we generate exponentially decaying distributions spread over a wide range, up to a maximum energy between 6 and 16 MeV. From the yield curves and the spectra we obtain the single-shot activation and the total activity after a realistic production time at 100 Hz pulse rate, taking into account saturation effects due to decay during irradiation. Results: We present numerical results for ten reaction channels and six realistic projectile spectra. With single laser shots, the highest activities are generated for O-15 (up to 11 (20) kBq for 16 MeV maximum p (d) energy). After prolonged irradiation at 100 Hz pulse rate, useful quantities of C-11 and O-15 may be obtained from spectra with 10 MeV maximum energies. The production of N-13 and F-18, to the contrary, requires higher energies and/or shot rates. Conclusions: 10 MeV particle energy and 100 Hz pulse rate are realistic benchmarks for a laser-based PET isotope production system. Experimental work to achieve these demanding objectives is in progress.

Analysis of the Statistical Moments of the Scintillation Light Distribution With dSiPMs

In γ-ray detectors, monolithic scintillation crystals offer the possibility of preserving the scintillation light distribution especially when painted black. The statistical moments of this distribution provide accurate information on the three photon impact coordinates, including their depth of interaction (DOI). Digital SiPMs (dSiPMs) return digital information based on pixels about the collected light distribution, since the signal is a digital sum of the trigger bins. In this work we present, for the first time, an accurate analysis of the statistical moments of the light distribution using monolithic painted black crystals and state-of-the-art dSiPMs. Two 32.6 ×32.6 mm2 monolithic LYSO crystals covering the entire photodetectors area have been used in coincidence with 10 mm in thickness. The photosensor tiles were kept at a stable temperature of T = 20 °C. Energy resolution of about 18% was reached in relation to the zeroth moment. The first moment, related to the impact position, determined a spatial resolution of about 3 mm near the crystal center, but quadratically degrading towards the crystal borders. The DOI resolution, measured by means of the second moment, was found to be nearing 4 mm in the crystal center region. The third order moment, the so-called skewness, is related to the degree of truncation and once calibrated minimizes the compression effects. A corrected spatial resolution of about 3 mm was then measured for the entire crystal surface. DOI resolution improved at the crystals center, reaching 3.5 mm, but a degradation towards the borders remained due to truncation of the scintillation light distribution.

A new method for image reconstruction in computed tomography (CT) using QR-Decomposition: Image quality assessment

This paper describes the results obtained with a new method for medical image reconstruction with computed tomography (CT): QR-Decomposition. QR-Decomposition is a model based (MB) algorithm like maximum likelihood expectation maximization (MLEM) but not iterative. It can be classified as a model-based direct reconstruction (MBDiR) algorithm. The QR-Decomposition algorithm takes advantage of the benefits of the MB approach, but only requires a matrix vector multiplication and backward substitution for image reconstruction. Noise power spectrum (NPS) of three dimensional (3D) images is analyzed and compared using QR-Decomposition standard filtered backprojection (FBP) and maximum likelihood expectation maximization (MLEM). 3D CT reconstructed images show that QR-decomposition process achieves competitive advantages compared to FBP and MLEM images reconstructed with the same voxel size.

Improving PET sensitivity with a Compton algorithm

Current PET detectors have a very low sensitivity, of the order of a few percent. One of the reasons is that Compton events are being rejected. In this work we aim to prove that Compton events are a very rich source of additional information that can play a crucial role in the image reconstruction process. With this additional data, the detector sensitivity will be substantially improved and thus, the applied dose on the patient can also be reduced. This could be a really breaking point for PET detector technology as one should be able to obtain better image quality with less patient radiation. By means of Compton cone matching (the Compton cones coming from the same event should be compatible) one should be able to better recognize matching events and discard randoms and even events that have previously suffered scattering within the patients body.

Highly improved operation of monolithic BGO-PET blocks

In PET scanners both scintillation crystals and photosensors are key components defining the systems performance and cost. Original PET systems used BGO or NaI(Tl) scintillators but achieved limited performance due to its slow decay and relatively low light output. Moreover, NaI(Tl) has low stopping power for 511 keV annihilation photons. In this study we report the possibility to reintroduce BGO crystals, and in particular in the form of monolithic blocks, especially suitable for low-dose large-size PET scanners, offering significantly improved sensitivity at a highly reduced cost compared to LYSO type fast scintillators. We have studied the performance of a monolithic BGO block as large as 50 × 50 × 15 mm3 with black-painted lateral walls to reduce lights spread, enabling accurate photon depth of interaction (DOI) measurements. A directional optical layer, called retro-reflector, was coupled to the entrance face bouncing back the scintillation light in the direction of the emission source and, therefore, adding to the light signal while preserving the narrow light cone distribution. Four configurations namely 12 × 12 and 16 × 16 SiPM arrays (3 mm × 3 mm each) as photosensors, with or without a nanopattern treatment at the crystal exit face, have been studied. This structure consisted of a thin layer of a specific high refractive index material shaped with a periodic nanopattern, increasing the scintillation light extraction. The readout returned information for each SiPM row and column, characterizing the X-Y light distribution projections. We have studied the detector spatial resolution using collimated 22Na sources at normal incidence. The DOI resolution was evaluated using collimated gamma beams with lateral incidence. The overall best detector performance was obtained for the 16× 16 SiPM array offering higher readout granularity. We have determined the spatial resolution for 3 separated DOI layers, obtaining the best results for the DOI region near to the photosensor.

Time-of-Flight detection of Al ions from laser produced plasma

Background: The interaction of highly intense laser pulses with solid targets covers a wide range of phenomena over several orders of magnitude in laser intensity. Time-of-flight measurements provide an accurate reconstruction of the ion energy spectra. We report on the observation of aluminium ions from a laser-plasma interaction by a scintillator-based detector with adjustable dynamic range. Methods: Data have been taken at a 30 fs, 200 TW pulsed Ti:Sapphire laser focused on aluminium foils with 1.8 and 12.5 μm thickness. A time-of-flight detector consisting in a plastic scintillator with fibre-optic coupling to a PMT has been mounted 50 cm behind the target. The PMT output pulses have been recorded on a fast oscilloscope. Results: After an initial peak caused by prompt X-rays and relativistic electrons, a second peak has been observed in the time-of-flight spectra after several microseconds. It corresponds to Al ions with energies of the order 1 keV. Quantitative spectra have been reconstructed. A PIC simulation of the plasma expansion gives an explanation for the acceleration of initially thermal ions to the observed energies. Discussion: At the given ion energies the detection mechanism, based on electrostatic charge transfer, is different to the usual ionisation process in scintillators. Additional tests have been performed to corroborate its working principle. At the nominal, focused laser intensity with femtosecond pulses, proton acceleration to MeV energies would be expected, but is excluded by our observations. This detailed study of the interaction kinematics indicates the formation of a cold plasma by a prepulse several picoseconds ahead of the peak intensity.

Design of a Thomson parabola spectrometer for the detection of laser-accelerated protons and ions

Very intense pulses of protons and ions can be produced in laser-plasma interactions at ultra-high energy densities. In a Thomson parabola spectrometer (TPS) the accelerated particles are separated by their mass, charge, and momentum. We present the design of a TPS for the spectral characterization of laser-accelerated protons and carbon ions which will be implemented in a table-top laser setup which is currently under preparation. First estimates of the magnetic and electric fields as well as the particle flight paths necessary for the clear separation of particle momenta have been obtained from well-known equations. We have designed a pair of permanent magnets with a C-shaped yoke to achieve a field of the order of 0.54 T. An exact field map has been obtained from simulations with COMSOL Multi-physics. The same software has been used to simulate the electric field between charged copper plates and the depletion of particles in the entire detector system. Our versatile setup allows for adjusting the desired energy range by variation of the position of the detector plane and the field gradient of the electric field. In the case of protons the low-energy interval ranges from 100 to 1000 keV, and the high-energy interval, from 1 to 10 MeV. Carbon ions can be separated by charge and momentum in both configurations.