Design and characterization of magnetic systems in race-track microtrons

During last four or five decades there has been a growing demand in particle accelerators which can provide electron beams in the energy ranging from 2 MeV to 100 MeV with high energy resolution and good dose control. Other important requirements are that the machines must be compact, of low power consumption, low price and relatively low maintenance cost. There is a variety of sectors interested in such particle accelerators ranging from industry (industrial radiography) to nuclear physics experiments. One type of machines that meet all these requirements are the electron accelerators with beam recirculation. Fair representatives of this class of accelerators are race-track microtrons (RTM). These sources of electron beam are the most efficient equipment for applications with a relatively low beam current and medium energies ranging from 2 MeV to 100 MeV. The aim of the present thesis is to perform studies of some aspects of the RTMs. One part of the thesis is devoted to the design and development of magnetic elements with permanent magnets of two RTMs for different applications. The first one, which is currently under construction at the UPC (Universidad Politécnica de Cataluña), is a novel accelerator with the beam energy variable between 6 MeV and 12 MeV for medical applications (Intraoperative Radiation Therapy treatments). The other machine is a 55 MeV RTM for the detection of explosives by means of photonuclear reactions, which is at the stage of tests at the Skobeltsyn Institute of Nuclear Physics (SINP). The magnetic field in the designed magnets is generated by rare earth permanent magnet (REPM) materials. This allows to get quite compact magnetic systems compatible with high vacuum environment. In the thesis the design and magnetic properties characterization of the magnetic system of these RTMs are carried out. The calculations were performed by means of 2D and 3D simulations using the POISSON, FEMM and ANSYS codes. In the case of the UPC RTM the design of the 180º dipoles, extraction magnets and quadrupole magnet are carried out. For the SINP 55 MeV RTM the optimization of the magnetic field shielding with the aim to reduce the stray magnetic field generated by the extraction magnet is presented. The results of the simulations were confirmed by experimental measurements of the magnetic field of the magnet with the optimized magnetic field shielding. In the other part of the thesis some aspects of the beam dynamics in RTM magnetic systems are studied. A detailed analysis of the fringe - field focusing in RTM dipole magnets is carried out. Equations for calculation of the fringe - field effect on electron beam trajectories are derived and are applied for a study of the end magnets of the UPC 12 MeV RTM. A general formalism for describing the longitudinal beam dynamics in RTMs for electron beams with arbitrary energy and end magnets with arbitrary magnetic field profile is also developed. This formalism is used for the calculation of the phase-slip effect in RTMs with low energy injection

A Fast 0.5 T Prepolarizer Module for Preclinical Magnetic Resonance Imaging

We present a magnet and high power electronics for Prepolarized Magnetic Resonance Imaging (PMRI) in a home-made, special-purpose preclinical system designed for simultaneous visualization of hard and soft biological tissues. The sensitivity of MRI systems grows with field strength, but so do their costs. PMRI can boost the signal-to-noise ratio (SNR) in affordable low-field scanners by means of a long and strong magnetic pulse. However, this must be rapidly switched off prior to the imaging pulse sequence, in timescales shorter than the spin relaxation (or T1) time of the sample. We have operated our prepolarizer at up to 0.5 T and demonstrated enhanced magnetization, image SNR and tissue contrast with PMRI of tap water, an ex vivo mouse brain and food samples. These have T1 times ranging from hundreds of milli-seconds to single seconds, while the preliminary high-power electronics setup employed in this work can switch off the prepolarization field in tens of milli-seconds. In order to make this system suitable for solid-state matter and hard tissues, which feature T1 times as short as 10 ms, we are developing new electronics which can cut switching times to ~ 300 μs. This does not require changes in the prepolarizer module, opening the door to the first experimental demonstration of PMRI on hard biological tissues

Calibration and Performance Tests of Detectors for Laser-Accelerated Protons

“©2015 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.”We present the calibration and performance tests carried out with two detectors for intense proton pulses accelerated by lasers. Most of the procedures were realized with proton beams of 0.46-5.60 MeV from a tandem accelerator. One approach made use of radiochromic films, for which we calibrated the relation between optical density and energy deposition over more than three orders of magnitude. The validity of these results and of our analysis algorithms has been confirmed by controlled irradiation of film stacks and reconstruction of the total beam charge for strongly non-uniform beam profiles. For the spectral analysis of protons from repeated laser shots, we have designed an online monitor based on a plastic scintillator. The resulting signal from a photomultiplier directly measured on a fast oscilloscope is especially useful for time-of-flight applications. Variable optical filters allow for suppression of saturation and an extension of the dynamic range. With pulsed proton beams we have tested the detector response to a wide range of beam intensities from single particles 3 x 10(5) to protons per 100 ns time interval.Project funded by the Spanish Ministry of Economy and Competitiveness and co-funded with FEDER's funds within the INNPACTO 2011 program under Grant No. IPT-2011-0862-900000. This work was supported by the Spanish Plan Nacional de Investigacion Cientifica, Desarrollo e Innovacion Tecnologica (I+D+i) under Grant No. TEC 2013-48036-C3-1-R and the Valencian Local Government under Grants PROMETEOII/2013/010 and ISIC 2011/013. The work of A. J. Gonzalez is financed by CSIC with a JAE-Doc contract under Junta de Ampliacion de Estudios program, cofinanced by the European Social Fund.Seimetz, M.; Bellido, P.; Soriano Asensi, A.; García López, J.; Jiménez-Ramos, M.; Fernández, B.; Conde Castellanos, PE.... (2015). Calibration and Performance Tests of Detectors for Laser-Accelerated Protons. IEEE Transactions on Nuclear Science. 62(6):3216-3224. https://doi.org/10.1109/TNS.2015.2480682S3216322462

Noise Analysis in Computed Tomography (CT) Image Reconstruction using QR-Decomposition Algorithm

“©2015 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.”In this paper, the noise of 3D computed tomography (CT) image reconstruction using QR-Decomposition is analyzed. There are several types of image noise that can interfere with the interpretation of an image. Here, the noise introduced by the reconstruction process is studied. In this analysis, condition numbers are calculated with different CT model parameters, three dimensional (3D) CT image reconstruction with simulated and real data are performed, image noise analysis is performed through various image quality parameters and the condition number of the linear system is related with the image quality parameters. Results show the condition number’s dependence on the CT model. Image reconstructions with simulated data show errors significantly below the condition number theoretical bound and image reconstructions with real data show that quality improvements depend strongly on the condition number. This allows a reduction on the number of projections without compromising image quality and places this reconstruction method as a strong candidate for low-dose 3D CT imaging reconstruction.This work was supported in part by the Spanish Goverment grant RTC-2014-2065-2 and the Valencian Local Government grants PROMETEOII/2013/010 and ISIC 2011/013.Iborra Carreres, A.; Rodríguez Álvarez, MJ.; Soriano Asensi, A.; Sanchez, F.; Bellido, P.; Conde Castellanos, PE.; Crespo Navarro, E.... (2015). Noise Analysis in Computed Tomography (CT) Image Reconstruction using QR-Decomposition Algorithm. IEEE Transactions on Nuclear Science. 62(3):869-875. doi:10.1109/TNS.2015.2422213S86987562

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

“©2015 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.”In gamma-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 x 32.6 mm(2) 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 degrees 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 crystal's center, reaching 3.5 mm, but a degradation towards the borders remained due to truncation of the scintillation light distribution.This work was supported in part by the Spanish Plan Nacional de Investigacion Cientifica, Desarrollo e Innovacion Tecnologica (I+D+I) under Grant FIS2010-21216-CO2-01 and in part by the Valencian Local Government under Grants PROMETEOII/2013/010 and ISIC 2011/013.Conde Castellanos, PE.; González Martínez, AJ.; Gonzalez, A.; Hernández Hernández, L.; Bellido, P.; Crespo Navarro, E.; Iborra Carreres, A.... (2015). Analysis of the Statistical Moments of the Scintillation Light Distribution With dSiPMs. IEEE Transactions on Nuclear Science. 62(5):1981-1988. https://doi.org/10.1109/TNS.2015.2473695S1981198862