Oliver, J. F., & Rafecas, M. (2010). Improving the singles rate method for modeling accidental coincidences in high-resolution PET. Phys. Med. Biol., 55(22), 6951–6971.
Abstract: Random coincidences ('randoms') are one of the main sources of image degradation in PET imaging. In order to correct for this effect, an accurate method to estimate the contribution of random events is necessary. This aspect becomes especially relevant for high-resolution PET scanners where the highest image quality is sought and accurate quantitative analysis is undertaken. One common approach to estimate randoms is the so-called singles rate method (SR) widely used because of its good statistical properties. SR is based on the measurement of the singles rate in each detector element. However, recent studies suggest that SR systematically overestimates the correct random rate. This overestimation can be particularly marked for low energy thresholds, below 250 keV used in some applications and could entail a significant image degradation. In this work, we investigate the performance of SR as a function of the activity, geometry of the source and energy acceptance window used. We also investigate the performance of an alternative method, which we call 'singles trues' (ST) that improves SR by properly modeling the presence of true coincidences in the sample. Nevertheless, in any real data acquisition the knowledge of which singles are members of a true coincidence is lost. Therefore, we propose an iterative method, STi, that provides an estimation based on ST but which only requires the knowledge of measurable quantities: prompts and singles. Due to inter-crystal scatter, for wide energy windows ST only partially corrects SR overestimations. While SR deviations are in the range 86-300% (depending on the source geometry), the ST deviations are systematically smaller and contained in the range 4-60%. STi fails to reproduce the ST results, although for not too high activities the deviation with respect to ST is only a few percent. For conventional energy windows, i.e. those without inter-crystal scatter, the ST method corrects the SR overestimations, and deviations from the true random rate are of the order of 1% or less. In addition, in the case of conventional energy window STi results reproduce ST results and therefore the former can be used to obtain the true random rate.
|
Decoster, S., Cottenier, S., Wahl, U., Correia, J. G., Pereira, L. M. C., Lacasta, C., et al. (2010). Diluted manganese on the bond-centered site in germanium. Appl. Phys. Lett., 97(15), 151914–3pp.
Abstract: The functional properties of Mn-doped Ge depend to large extent on the lattice location of the Mn impurities. Here, we present a lattice location study of implanted diluted Mn by means of electron emission channeling. Surprisingly, in addition to the expected substitutional lattice position, a large fraction of the Mn impurities occupies the bond-centered site. Corroborated by ab initio calculations, the bond-centered Mn is related to Mn-vacancy complexes. These unexpected results call for a reassessment of the theoretical studies on the electrical and magnetic behavior of Mn-doped Ge, hereby including the possible role of Mn-vacancy complexes.
|
Colonna, N., Belloni, F., Berthoumieux, E., Calviani, M., Domingo-Pardo, C., Guerrero, C., et al. (2010). Advanced nuclear energy systems and the need of accurate nuclear data: the n_TOF project at CERN. Energy Environ. Sci., 3(12), 1910–1917.
Abstract: To satisfy the world's constantly increasing demand for energy, a suitable mix of different energy sources has to be devised. In this scenario, an important role could be played by nuclear energy, provided that major safety, waste and proliferation issues affecting current nuclear reactors are satisfactorily addressed. To this purpose, a large effort has been under way for a few years towards the development of advanced nuclear systems with the aim of closing the fuel cycle. Generation IV reactors, with full or partial waste recycling capability, accelerator driven systems, as well as new fuel cycles are the main options being investigated. The design of advanced systems requires improvements in basic nuclear data, such as cross-sections for neutron-induced reactions on actinides. In this paper, the main concepts of advanced reactor systems are described, together with the related needs of new and accurate nuclear data. The present activity in this field at the neutron facility n_TOF at CERN is discussed.
|
Freitas, E. D. C., Monteiro, C. M. B., Ball, M., Gomez-Cadenas, J. J., Lopes, J. A. M., Lux, T., et al. (2010). Secondary scintillation yield in high-pressure xenon gas for neutrinoless double beta decay (0 nu beta beta) search. Phys. Lett. B, 684(4-5), 205–210.
Abstract: The search for neutrinoless double beta decay (0 nu beta beta) is an important topic in contemporary physics with many active experiments. New projects are planning to use high-pressure xenon gas as both source and detection medium. The secondary scintillation processes available in noble gases permit large amplification with negligible statistical fluctuations, offering the prospect of energy resolution approaching the Fano factor limit. This Letter reports results for xenon secondary scintillation yield, at room temperature, as a function of electric field in the gas scintillation gap for pressures ranging from 2 to 10 bar. A Large Area Avalanche Photodiode (LAAPD) collected the VUV secondary scintillation produced in the gas. X-rays directly absorbed in the LAAPD are used as a reference for determining the number of charge carriers produced by the scintillation pulse and, hence, the number of photons impinging the LAAPD. The number of photons produced per drifting electron and per kilovolt, the so-called scintillation amplification parameter, displays a small increase with pressure, ranging from 141 +/- 6 at 2 bar to 170 +/- 10 at 8 bar. In our setup, this Parameter does not increase above 8 bar due to nonnegligible electron attachment. The results are in good agreement with those presented in the literature in the 1 to 3 bar range. The increase of the scintillation amplification parameter with pressure for high gas densities has been also observed in former work at cryogenic temperatures.
|
AGATA Collaboration, Doncel, M., Recchia, F., Quintana, B., Gadea, A., & Farnea, E. (2010). Experimental test of the background rejection, through imaging capability, of a highly segmented AGATA germanium detector. Nucl. Instrum. Methods Phys. Res. A, 622(3), 614–618.
Abstract: The development of highly segmented germanium detectors as well as the algorithms to identify the position of the interaction within the crystal opens the possibility to locate the gamma-ray source using Compton imaging algorithms. While the Compton-suppression shield, coupled to the germanium detector in conventional arrays, works also as an active filter against the gamma rays originated outside the target, the new generation of position sensitive gamma-ray detector arrays has to fully rely on tracking capabilities for this purpose. In specific experimental conditions, as the ones foreseen at radioactive beam facilities, the ability to discriminate background radiation improves the sensitivity of the gamma spectrometer. In this work we present the results of a measurement performed at the Laboratori Nazionali di Legnaro (LNL) aiming the evaluation of the AGATA detector capabilities to discriminate the origin of the gamma rays on an event-by-event basis. It will be shown that, exploiting the Compton scattering formula, it is possible to track back gamma rays coming from different positions, assigning them to specific emitting locations. These imaging capabilities are quantified for a single crystal AGATA detector.
|
