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Made available in DSpace on 2014-10-09T12:37:02Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:56:36Z (GMT). No. of bitstreams: 1 04488.pdf: 5615581 bytes, checksum: 5c91eb9ed6f1954cedefb34ecd8cbfc1 (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Doctor of Philosophy / Department of Chemical Engineering / James H. Edgar / Semiconducting icosahedral boron arsenide, B[subscript]12As[subscript]2, is an excellent candidate for neutron detectors and radioisotope batteries, for which high quality single crystals are required. Thus, the present study was undertaken to grow B[subscript]12As[subscript]2 crystals by precipitation from metal solutions (nickel) saturated with elemental boron and arsenic in a sealed quartz ampoule. B[subscript]12As[subscript]2 crystals of 8-10 mm were produced when a homogeneous mixture of the three elements was held at 1150 °C for 48-72 hours and slowly cooled (3°C/hr). The crystals varied in color and transparency from black and opaque to clear and transparent. X-ray topography (XRT), Raman spectroscopy, and defect selective etching confirmed that the crystals had the expected rhombohedral structure and a low density of defects (5x10[superscript]7 cm[superscript]-2). The concentrations of residual impurities (nickel, carbon, etc) were found to be relatively high (10[superscript]19 cm[superscript]-3 for carbon) as measured by secondary ion mass spectrometry (SIMS) and elemental analysis by energy dispersive x-ray spectroscopy (EDS). The boron arsenide crystals were found to have favorable electrical properties (μ = 24.5 cm[superscript]2 / Vs), but no interaction between a prototype detector and an alpha particle bombardment was observed. Thus, the flux growth method is viable for growing large B[subscript]12As[subscript]2 crystals, but the impurity concentrations remain a problem.

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Made available in DSpace on 2014-10-09T12:38:33Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:05:29Z (GMT). No. of bitstreams: 1 06050.pdf: 2669277 bytes, checksum: d5c09b330819048bd8379029a5eabfe4 (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Aujourd’hui les réacteurs d’irradiations technologiques (MTR) ont un rôle de premier plan pour l’industrie du nucléaire. Dans les réacteurs d’irradiation les niveaux de flux neutroniques et photoniques sont très importants. Un des besoins cruciaux des mesures en MTR est la mise en œuvre d’un dispositif de détection de rayonnement, précis, sélectif, fiable et robuste dans les conditions extrêmes de flux neutroniques et photoniques, et de températures élevées. Les semi-conducteurs à grande bande d’énergie interdite tels que le carbure de silicium (SiC) le diamant et le nitrure de gallium (GaN) possèdent des propriétés remarquables en termes de tenue en température et de résistance aux radiations. Cette thèse a pour principal objectif la comparaison des performances des détecteurs de neutrons dont les parties sensibles sont faites de carbure de silicium (SiC) avec celles des détecteurs basés sur le diamant pour la mesure de neutrons en conditions d’irradiation identiques. Pour cela nous avons réalisé les essais d’irradiation dans le réacteur de recherche de type maquette critique MINERVE au CEA Cadarache. Nous avons également testé les capteurs pour la détection des neutrons rapides de 14 MeV afin d’investiguer la future possibilité de mesurer en ligne des flux de neutrons rapides notamment pour les besoins de la fusion nucléaire (projet ITER). L’ensemble de ces travaux contribue à l’amélioration de la fonctionnalité du détecteur de neutrons en SiC, qui peut être augmentée en intégrant le détecteur à une électronique adaptée et aux outils spécifiques pour l’analyse du signal développés dans le cadre de cette thèse. / Nowadays, the material testing reactors (MTR) are playing a crucial role for nuclear industry. The research reactors allow carrying out the research on material damage and nuclear fuel advanced studies. Harsh radiation environment near the nuclear reactor core requires the radiation detectors to be resistant to high radiation level and high temperature. Neutron radiation detector for nuclear reactor applications plays an important role in getting information about the actual neutron flux. Most suitable semiconductors for harsh environment applications are SiC and diamond thanks to their outstanding properties. The aim of this thesis is to compare the ability of these two semi-conductors to detect neutrons. For this purpose, the neutron irradiation tests of detectors were implemented at MINERVE nuclear research reactor at CEA Cadarache. In this work we also studied the response of both materials to 14 MeV neutron beam with the prospect for future applications for fusion facilities. This work helps to improve the SiC-based detector characterization. The functionality of this detector could be enhanced by integrating it with appropriate radiation resistant electronics and tools for the signal analysis which was developed in the frame of this thesis

Instalações para irradiação com nêutrons foram projetadas e construídas, visando o estabelecimento de procedimentos de calibração de monitores e dosímetros de nêutrons. Uma fonte calibrada de ANTPOT.241 AmBe com atividade de ANTPOT.185 GBq, garantiu rastreabilidade às medidas realizadas, e possibilitou irradiações com taxas de dose que variaram entre 9 nSv s POT.-1 e 0,5 muuSv s POT.-1. No arranjo elaborado para irradiações com nêutrons térmicos, com fluxo calibrado, a taxa de dose utilizada foi 50 nSv s POT.-1. A calibração de um espectrômetro de esferas de Bonner permitiu o estabelecimento dos procedimentos de calibração com base em três métodos propostos por normas internacionais, mostrando serem apropriadas às dimensões da sala de irradiação projetada para essa finalidade. A decoração do espectro de nêutrons, a partir da determinação dos parâmetros de calibração para o detector de esferas de Bonner, permitiu determinar o espectro da fonte de ANTPOT.241 AmBe calibrada, com valores de taxa de fluência, taxa de dose equivalente e energia media, que corresponderam satisfatoriamente aos valores calculados esperados, possibilitando o uso de tal detector na dosimetria de área. Para a dosimetria pessoal, foi elaborado um sistema dosimétrico baseado no uso do polímero CR-39 e no ataque eletroquímico do mesmo. Para isso, foram projetadas e construídas uma câmara para o processamento do detector e uma fonte de alta tensão e alta frequência que mostraram-se adequadas às finalidades propostas. O dosímetro pessoal elaborado utiliza também material conversor (n, alfa) e seu principio de detecção possibilitou determinar um fator de calibração relacionando a resposta do detector com a dose recebida para nêutrons térmicos, epitérmicos e rápidos. Os parâmetros utilizados para o ataque eletroquímico foram: solução de KOH 6N, temperatura de 59°C, campo elétrico alternado de 20 kv IND.ppcm POT.-1, frequência de 2,0 kHz; para a detecção de nêutrons térmicos e epitérmicos o tempo de revelação empregado foi de 3 horas e para nêutrons rápidos de 6 horas. O sistema dosimétrico desenvolvido apresentou sensibilidades a nêutrons térmicos, epitérmicos e rápidos dadas pelos valores (1,46 +/- 0,09) 10 POT.4 traços cm POT.-2 mSv POT.-1, (9 +/- 3) 10² traços cm POT.-2 mSv POT.-1 e (26 +/- 4) traços cm POT.-2 mSv POT.-1 respectivamente. As doses mínimas e máximas detectáveis foram respectivamente 0,002 mSv e 0,6 mSv para nêutrons térmicos, 0,04 mSv e 8 mSv para nêutrons epitérmicos e 1 mSv e 12 mSv para nêutrons rápidos. Tendo em vista as implicações das recomendações do ICRP-60, pode-se concluir que o dosímetro pessoal elaborado neste trabalho apresenta a sensibilidade necessária no caso da monitoração de nêutrons térmicos e epitérmicos, mas que exigiria tempos de integração de doses superiores à mensal no caso da monitoração de nêutrons rápidos.O procedimento proposto para ser realizar a dosimetria de nêutrons é baseado no uso conjugado do dosímetro pessoal elaborado e do espectrômetro de esferas de Bonner enquanto dosímetro de área. / A neutron irradiation facility was designed and built in order to establish a procedure for calibrating neutron monitors and dosimeters. A 185 GBq ANTPOT.241 AmBe source in the air provides neutron doses rates between 9 nSv s POT.-1 and 0,5 muuSv s POT.-1. A calibrated 50 nSv s POT.-1 thermal neutron field is obtained by using a specially designed paraffin block in conjunction with the ANTPOT.241 AmBe source. A Bonner multisphere spectrometer was calibrated, using a procedure based on three methods proposed by international standards. The unfolded ANTPOT.241 AmBe neutron spectrum was determined from Bonner spheres data and resulted in a good agreement with expected values for fluence rate, dose rate and mean energy. A dosimetric system based on the electrochemical etching of CR-39 was developed for personal dosimetry. The dosimeter badge using a (n, alfa) converter, the etching chamber and high frequency power supply were designed and built specially for this project. The electrochemical etching (ECE) parameters used were: a 6N KOH solution, 59°C, 20 kV IND.ppcm POT.-1, 2,0 kHz, 3 hours of ECE for thermal and intermediate neutrons and 6 hours for fast neutrons. The calibration factors for thermal, intermediate and fast neutrons were determined for this personal dosimeter. The sensitivies determined for the developed dosimetric system were (1,46 +/- 0,09)10 POT.4 tracks cm POT.-2 mSv POT.-1 for thermal neutrons, (9 +/- 3)10² tracks cm POT.-2 mSv POT.-1 for intermediate neutrons and (26 +/- 4) tracks cm POT.-2 mSv POT.-1 for fast neutrons. The lower and upper limits of detection were respectively 0,002 mSv and 0,6 mSv for thermal neutrons, 0,04 mSv and 8 mSv for intermediate neutrons and 1 mSv and 12 mSv for fast neutrons. In view of the 1990s ICRP recommendations, it is possible to conclude that the personal dosimeter described in this work is sufficiently sensitive to thermal and intermediate neutrons but fast neutron monitoring at radiological protection level would require an integration period of over a month. The proposed dosimetric procedure is based on the conjugated use of the developed personal dosimeter and the Bonner multisphere spectrometer.

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abstract: The reduced availability of 3He is a motivation for developing alternative neutron detectors. 6Li-enriched CLYC (Cs2LiYCl6), a scintillator, is a promising candidate to replace 3He. The neutron and gamma ray signals from CLYC have different shapes due to the slower decay of neutron pulses. Some of the well-known pulse shape discrimination techniques are charge comparison method, pulse gradient method and frequency gradient method. In the work presented here, we have applied a normalized cross correlation (NCC) approach to real neutron and gamma ray pulses produced by exposing CLYC scintillators to a mixed radiation environment generated by 137Cs, 22Na, 57Co and 252Cf/AmBe at different event rates. The cross correlation analysis produces distinctive results for measured neutron pulses and gamma ray pulses when they are cross correlated with reference neutron and/or gamma templates. NCC produces good separation between neutron and gamma rays at low (< 100 kHz) to mid event rate (< 200 kHz). However, the separation disappears at high event rate (> 200 kHz) because of pileup, noise and baseline shift. This is also confirmed by observing the pulse shape discrimination (PSD) plots and figure of merit (FOM) of NCC. FOM is close to 3, which is good, for low event rate but rolls off significantly along with the increase in the event rate and reaches 1 at high event rate. Future efforts are required to reduce the noise by using better hardware system, remove pileup and detect the NCC shapes of neutron and gamma rays using advanced techniques. / Dissertation/Thesis / Masters Thesis Electrical Engineering 2015

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