No description available.

Thesis (MSc)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Nine 100x100x600 mm3 plastic scintillators, formerly built for the neutron time of ight measurements at iThemba LABS, were refurbished. The position resolution of these detectors was determined using muon cosmic rays and coincident measurement techniques. Average position resolution of 4.28 cm (FWHM) was found. In order to predict the time spectrum of the large-volume detector Monte Carlo simulations have been performed. These simulations aimed at anticipating the separation of statistical neutrons, prompt gamma rays and uncorrelated gamma rays from the fast neutrons emitted in the reaction of interest. One of the neutron detectors was tested using fast neutrons from the 232Th( ,xn) reaction at 42 MeV. Statistical neutrons from fusion evaporation reactions were produced in 152Sm(12C,xn) fusion evaporation reaction. Coincidences between neutrons and gamma rays were successfully identi ed. Prompt gamma rays and uncorrelated gamma rays were also identi ed. / AFRIKAANSE OPSOMMING: Nege 100x100x600 mm3 plastiese scintillators, wat aanvanklik gebou was vir neutron vlugtyds meetings by iThemba LABS, was hernu. Die posisie resolusie van die detektore was bepaal deurmiddel van muon kosmiese straling en koïnsidensie meet tegnieke. Posisie resolusie van 4.28 cm (FWHM) was verkry. Monte Carlo simulasies is gebruik om die posisie resolusie van'n groot volume detektor te voorspel. Hierdie simulasies is daarop gemik om onderskeid te maak tussen statistiese neutrone, gelyktydige gamma strale en ongekorreleerde gamma strale vanaf vinnige neutrone in die reaksie van belang uitgestraal word. Een neutron detektor was getoets deur gebruik te maak van vinnige neutrone wat uit die reaksie 232Th( ,xn) by 42 MeV ontstaan. Statistiese neutrone vanaf splitsings verdampingsreaksies, gelyktydige gamma strale en ongekorreleerde gamma strale was geidenti seer. Statistiese neutrone van samesmelting verdamping reaksies was geproduseer in die reaksie 152Sm(12C,xn). Toeval tussen neutrone en gamma strale was suksesvol geïdenti seer, gevra gamma strale en ongekorreleerd gamma strale was ook geïdenti seer.

Thesis (PhD)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: The objective of this research was to develop a neutron detection system that incorporates a mass spectrometer to measure high neutron flux in a nuclear reactor environment. This system consists of slow and fast neutron detector elements for measuring fluxes in those energy regions respectively. The detector should further be capable of withstanding the harsh conditions associated with a high temperature reactor. This novel detector which was initially intended for use in the PBMR reactor has possible applications as an in-core neutron and indirect temperature-monitoring device in any of the HTGR. Simulations of a generic HTGR core model were performed in order to obtain the neutron energy spectrum with emphasis on the behavior of three energy regions, slow, intermediate and fast neutrons within the core at different temperatures. The slow neutron flux which has the characteristic of a Maxwell- Boltzmann distribution were found to shift to larger values of neutron flux at higher energies as the fuel temperature increased, while fast neutron flux spectra remained relatively constant. In addition, the results of the fit of the slow neutron flux with a modified Maxwell-Boltzmann equation confirmed that in the presence of the neutron source, leakage and absorption, the effective neutron temperatures is above the medium temperatures. From these results, it was clear that the detection system will need to monitor both slow and fast neutron flux. Placing neutron detectors inside the reactor core, that are sensitive to a particular energy range of slow and fast neutrons, would thus provide information about the change of temperature in the fuel and hence act as an in-core temperature monitor. A detection mechanism was developed that employs the neutron-induced break-up reaction of 6Li and 12C into α-particles. These materials make excellent neutron converters without interference due to γ-rays, as the contributions from 6Li(γ,np)4He and 12C(γ,3α) reactions are negligible. The mass spectrometer measures the 4He partial pressure as a function of time under high vacuum with the help of pressure gradient provided by a high-vacuum turbomolecular pump and a positive-displacement fore-vacuum pump connected in series. A cryogenic trap, which contains a molecular sieve made of pellets 1.6 mm in diameter, was also designed and manufactured to remove impurities which cause a background in the lighter mass region of the spectrum. The development and testing of the high flux neutron detection system were performed at the iThemba Laboratory for Accelerator Based Sciences (LABS), South Africa. These tests were carried out with a high energy proton beam at the D-line neutron facility, and with a fast neutron beam at the neutron radiation therapy facility. To test the principle and capability of the detection system in measuring high fluxes, a high intensity 66 MeV proton beam was used to produce a large yield of α-particles. This was done because the proton inelastic scattering cross-section with 12C nuclei is similar to that of neutrons, with a threshold energy of about 8 MeV for both reactions. Secondly, the secondary fast neutrons produced from the 9Be(p,n)9B reaction were also measured with the fast neutron detector. The response of this detection system during irradiation was found to be relatively fast, with a rise time of a few seconds. This is seen as a sharp increase in the partial pressure of 4He gas as the proton or neutron beam bombards the 12C material. It was found that the production of 4He with the proton beam was directly proportional to the beam intensity. The number of 4He atoms produced per second was deduced from the partial pressure observed during the irradiation period. With a neutron beam of 1010 s−1 irradiating the detector, the deduced number of 4He atoms was 109 s−1. When irradiation stops, the partial pressure drops exponentially. This response is attributed to a small quantity of 4He trapped in the present design. Overall, the measurements of 4He partial pressure produced during the tests with proton and fast neutron beams were successful and demonstrated proof of principle of the new detection technique. It was also found that this system has no upper neutron flux detection limit; it can be even higher than 1014 n·cm−2·s−1. The lifetime of this detection system in nuclear reactor environment is practically unlimited, as determined by the known ability of stainless steel to keeps its integrity under the high radiation levels. Hence, it is concluded that this high flux neutron detection system is excellent for neutron detection in the presence of high γ-radiation level and provides real-time flux measurements. / AFRIKAANSE OPSOMMING: Die doel van hierdie navorsing was om ’n neutrondetektorstelsel te ontwikkel wat hoë neutronvloed binne in ’n kernreaktor kan meet. Die stelsel bevat twee aparte detektorelemente sodat die termiese sowel as snelneutronvloed gemeet kan word. Die detektor moet verder in staat wees om die strawwe toestande, kenmerkend aan ’n hoë temperatuur reaktor, te kan weerstaan. Die innoverende detektorstelsel, oorspronklik geoormerk vir gebruik in die PBMR reaktor, het toepassingsmoontlikhede as in-kern neutron- sowel as indirekte temperatuurmonitor. Simulasies van ’n generiese model van ’n HTGR reaktorkern is uitgevoer ten einde die neutronenergiespektrum in die kern by verskillende temperature te bekom met klem op die gedrag van neutrone in drie energiegroepe: stadig (termies), intermediêr en snel (vinnig). Daar is bevind dat die stadige neutrone, wat ’n Maxwell-Boltzman verdeling toon, in intensiteit toeneem en dat die piek na hoër energie verskuif met toename in temperatuur, terwyl die vinnige neutronspektrum relatief onveranderd bly. ’n Passing van die stadige spektrum op ’n gemodifiseerde Maxwell-Boltzmann verdeling het bevestig dat die effektiewe neutrontemperatuur weens die teenwoordigheid van bronterme, verliese en absorpsie, hoër as die temperatuur van die medium is. Hierdie resultate maak dit duidelik dat die detektorstelsel beide die stadige sowel as die vinnige neutronvloed moet kan waarneem. Deur detektorelemente wat sensitief is vir die onderskeie spekrale gebiede in die reaktorhart te plaas, kan informasie bekom word wat tot in-kern temperatuur herleibaar is sodat die stelsel inderdaad as indirekte temperatuurmonitor kan dien. Die feit dat alfa-deeltjies geproduseer word in neutron-geïnduseerde opbreekreaksies van 6Li en 12C is as die basis van die nuwe opsporingsmeganisme aangewend. Hierdie materiale funksioneer uitstekend as neutron-selektiewe omsetters in die teenwoordigheid van gamma-strale aangesien laasgenoemde se bydraes tot helium produksie via die 6Li(γ,np)4He en 12C(γ,3α) reaksies, weglaatbaar is. Die massaspektrometer meet die tydgedrag van die 4He parsiële druk binne ’n hoogvakuum wat met behulp van ’n seriegeskakelde kombinasie van ’n turbomolekulêre en positiewe-verplasingsvoorpomp verkry word. ’n Koueval met ’n molekulêre sif, bestaande uit 1.6 mm diameter korrels, is ontwerp en vervaardig om onsuiwerhede te verwyder wat andersins as agtergrond by die ligter gedeelte van die massaspektrum sou wys. Die ontwikkeling en toetsing van die hoëvloed detektorstelsel is te iThembaLABS (iThemba Laboratories for Accelerator Based Sciences) gedoen. Dit is uitgevoer deur gebruik te maak van die hoë energie protonbundel van die D-lyn neutronfasiliteit asook van die bundel vinnige neutrone by die neutronterapiefasiliteit. Om die beginsel en vermoë te toets om by ’n hoë neutronvloed te kan meet, is van die intense 66 MeV protonbudel gebruik gemaak om ’n hoë opbrengs alfa-deeltjies te verkry. Dit is gedoen omdat die reaksiedeursnit vir onelastiese verstrooiing van protone vanaf 12C kerne soortgelyk is aan die van neutrone, met ’n drumpelenergie van 8 MeV vir beide reaksies. Tweedens is die sekondêre vinnige neutrone afkomstig van die 9Be(p,n)9B reaksie ook met die neutrondetektor gemeet. Daar is bevind dat die reaksietyd van die deteksiestelsel tydens bestraling relatief vinnig is, soos gekenmerk deur ’n stygtyd van etlike sekondes. Laasgenoemde manifesteer as ’n toename in die parsiële druk van die 4He sodra die proton- of neutronbundel op die 12C teiken inval. Daar is verder bevind dat die 4He produksie direk eweredig aan die bundelintensiteit is. Vir ’n neutronbundel van nagenoeg 1010 s−1, invallend op die neutrondetektor, is vanaf die gemete parsiële druk afgelei dat die produksie van 4He atome sowat 109 s−1 beloop. In die geheel beoordeel, was die meting van die 4He parsiële druk tydens die toetse met vinnige protone en neutrone suksesvol en het dit die nuwe meetbeginsel bevestig. Dit is verder bevind dat die meetstelsel nie ’n beperking op die boonste neutronvloed plaas nie, maar dat dit vloede van selfs hoër as 1014 s−1 kan hanteer. Die leeftyd van die detektorstelsel in die reaktor is prakties onbeperk en onderhewig aan die bevestigde integriteit van vlekvrystaal onder hoë bestraling. Die gevolgtrekking is dus dat die nuwe detektorstelsel uitstekend geskik is vir die in-tyd meting van ’n baie hoë vloed van neutrone ook in die teenwoordigheid van intense gammabestraling.

<p>When the heavy hydrogen isotopes deuterium (D) and tritium (T) undergo nuclear fusion large amounts of energy are released. At the Joint European Torus (JET) research is performed on how to harvest this energy. Two of the most important fusion reactions, d+d→³He+n (E_n = 2.5 MeV) and d+t→⁴He+n (E_n = 14 MeV), produce neutrons. This thesis investigates how measurements of these neutrons can provide information on the fusion performance.</p><p>The Magnetic Proton Recoil (MPR) neutron spectrometer has operated at JET since 1996. The spectrometer was designed to provide measurements on the 14 MeV neutron emission in DT operation, thereby conveying information on the state of the fuel ions. However, a majority of today’s fusion experiments are performed with pure D fuel. Under such conditions, the measurements with the MPR were severely hampered due to interfering background. This prompted an upgrade of the instrument. The upgrade, described in this thesis, included a new focal plane detector, a phoswich scintillator array, and new data acquisition electronics, based on transient recorder cards. This combination allows for pulse shape discrimination techniques to be applied and a signal to background of 5/1 has been achieved in measurements of the 2.5-MeV neutrons in D experiments. The upgrade also includes a new control and monitoring system, which enables the monitoring and correction of gain variations in the spectrometer’s photo multiplier tubes. Such corrections are vital for obtaining good data quality.</p><p>In addition, this thesis describes a new method for determining the total neutron yield and hence the fusion power by using a MPR spectrometer in combination with a neutron emission profile monitor. The system has been operated at JET both during DT and D experiments. It is found that the systematic uncertainties are considerably lower (≈6 %) than for traditional systems. For a dedicated system designed for the next generation fusion experiments, i.e, ITER, uncertainties of 4 % could be attained.</p><p>Neutron spectroscopy can also be an important tool for determining the neutron emission from residual tritium in D plasmas. This information is combined with other measurements at JET in order to determine the confinement of the 1 MeV tritons from the d+d→t+p reactions.</p>

Neutron are emitted from a deuterium plasma with energies around 2.5 MeV. The neutron spectrum is intimately related to the ion velocity distribution of the plasma. As a consequence, the analysis of neutron energy spectra can give information of the plasma rotation, the ion temperature, heating efficiency and fusion power. The upgraded magnetic proton recoil spectrometer (MPRu), based on the thin-foil technique, is installed at the tokamak JET. The upgrade of the spectrometer was done to allow for measurements of deuterium plasmas. This thesis describes the hardware, the data reduction scheme and the kind of fusion plasma parameters that can be estimated from the data of the MPRu. The MPRu data from 3rd harmonic ion cyclotron resonance and beam heating are studied. Other neutron spectrometer techniques are reviewed as well, in particular in the aspect of suitability for neutron emission spectrometry at ITER. Each spectrometer technique is evaluated using synthetic data which is obtained from standard scenarios of ITER. From this evaluation, we conclude that the thin-foil technique is the best technique to measure, e.g., the ion temperature in terms of time resolution.

When the heavy hydrogen isotopes deuterium (D) and tritium (T) undergo nuclear fusion large amounts of energy are released. At the Joint European Torus (JET) research is performed on how to harvest this energy. Two of the most important fusion reactions, d+d→3He+n (En = 2.5 MeV) and d+t→4He+n (En = 14 MeV), produce neutrons. This thesis investigates how measurements of these neutrons can provide information on the fusion performance. The Magnetic Proton Recoil (MPR) neutron spectrometer has operated at JET since 1996. The spectrometer was designed to provide measurements on the 14 MeV neutron emission in DT operation, thereby conveying information on the state of the fuel ions. However, a majority of today’s fusion experiments are performed with pure D fuel. Under such conditions, the measurements with the MPR were severely hampered due to interfering background. This prompted an upgrade of the instrument. The upgrade, described in this thesis, included a new focal plane detector, a phoswich scintillator array, and new data acquisition electronics, based on transient recorder cards. This combination allows for pulse shape discrimination techniques to be applied and a signal to background of 5/1 has been achieved in measurements of the 2.5-MeV neutrons in D experiments. The upgrade also includes a new control and monitoring system, which enables the monitoring and correction of gain variations in the spectrometer’s photo multiplier tubes. Such corrections are vital for obtaining good data quality. In addition, this thesis describes a new method for determining the total neutron yield and hence the fusion power by using a MPR spectrometer in combination with a neutron emission profile monitor. The system has been operated at JET both during DT and D experiments. It is found that the systematic uncertainties are considerably lower (≈6 %) than for traditional systems. For a dedicated system designed for the next generation fusion experiments, i.e, ITER, uncertainties of 4 % could be attained. Neutron spectroscopy can also be an important tool for determining the neutron emission from residual tritium in D plasmas. This information is combined with other measurements at JET in order to determine the confinement of the 1 MeV tritons from the d+d→t+p reactions.

Cette thèse de doctorat porte sur ma recherche doctorale sur la diffusion moléculaire sur des surfaces de matériaux carbonés. Ces travaux de recherche ont été effectuées sous forme d'études de spectroscopie neutronique et d'hélium. Des modèles théoriques ont été développés pour l'analyse et l'interprétation des données expérimentales.Dans une première partie, la méthode de croissance épitaxiale d'une couche de graphène sur une surface (111) d'un cristal de nickel est décrite. Basés sur des études de spectroscopie à écho de spin d'hélium, des modèles d'adsorption et de diffusion d'eau et de benzène sur la surface de graphène sont ensuite élaborés. L'objectif est de décrire précisément la structure de l'adsorbât et la diffusion moléculaire sur la surface.Dans une deuxième partie des études portant sur la diffusion d'hydrogène moléculaire adsorbé dans un aérogel de carbone, dans un carbone poreux, et dans un graphite exfolié sont présentés. Les résultats expérimentaux de spectroscopie neutronique temps-de-vol nous permettent d'établir le rapport entre la mobilité des molécules d'hydrogène et les propriétés spécifiques aux matériaux de carbone. / This thesis presents my PhD work about molecular diffusion on surfaces of carbon materials. The main research has been undertaken in the form of neutron and helium spectroscopy studies and theoretical models have been developed for an interpretation of experimental data.In the first part, the growth procedure of an epitaxial graphene layer on the (111) surface of a nickel crystal is described and the adsorption and diffusion of water and of benzene on the graphene surface are discussed. Results from helium spin-echo spectroscopy studies are presented with the aspiration to obtain a detailed qualitative and quantitative description of the structure of the adsorbate and the molecular diffusion on the surface.In the following chapters, the diffusion of molecular hydrogen adsorbed in carbon aerogel, in a novel porous carbon D-96-7, and in exfoliated graphite is discussed, based on results from neutron time-of-flight spectroscopy. The aim is a detailed understanding of the connection between porosity, surface chemistry, and the molecular diffusion.

Ce travail de thèse porte sur le développement de techniques de spectroscopie neutronique dans les champs de rayonnement mixte pour des applications liées aux réacteurs de recherche à neutrons rapides, en particulier l’installation MASURCA.La première partie est consacrée à l'étude des configurations expérimentales spéciales de MASURCA dans lesquelles un canal radial est construit pour extraire un faisceau continu de neutrons d'énergie intermédiaires et rapides, adaptable à différents besoins. Exploiter MASURCA en tant qu'installation de faisceau de neutrons ouvrirait de nouvelles possibilités d'expériences telles que des expériences de protection et de transport de neutrons rapides, la production de champs neutroniques standards (de référence), le développement et étalonnage de systèmes de détection des neutrons rapides, etc.La deuxième partie de la thèse est dédiée au développement d’une capacité de spectrométrie neutronique rapide pour la caractérisation en ligne de la distribution d'énergie neutronique. Différents types de détecteurs sont comparés. Le meilleur compromis pour ce spectromètre est un système combinant des compteurs proportionnels et des scintillateurs organiques. Ce système est capable de couvrir le domaine énergétique entre 10 keV et 10 MeV. Le scintillateur organique sélectionné est un monocristal de stilbène obtenu par un procédé “solution-grown” développé récemment. Au bilan, on conclut qu'un spectromètre à neutrons basé sur le stilbène de type “solution-grown” serait adapté à une utilisation dans MASURCA et dans d'autres champs de rayonnement mixte et qu'il serait plus performant que les systèmes de détection traditionnels. / This doctoral thesis work is focused on the development of neutron spectroscopy techniques in mixed radiations fields for fast research reactor applications, especially for the MASURCA facility.The first part of the thesis is dedicated to the study of special MASURCA configurations in which a radial channel is built to extract a continuous beam of intermediate-to-fast energy neutrons, tailorable to meet different needs. Operating MASURCA as a neutron beam facility would open up new possibilities of experiments, such as fast neutron attenuation and shielding experiments; measurements in standard (reference) fast neutron fields, development and calibration of fast neutron detection systems etc.The second part of the thesis is dedicated to the development of fast neutron spectrometric capabilities for the on-line characterization of the neutron energy distribution. Different candidate detector systems are compared. A “best compromise” spectrometer is shown to be a system combining proportional counters and organic scintillators. Such a system would be able to cover the neutron energy domain between 10 keV and 10 MeV. The selected organic scintillator is a stilbene single crystal obtained by a recently developed solution-grown process. Overall, it is concluded that a neutron spectrometer based on a solution-grown stilbene detector would be suitable for use in MASURCA and in other mixed radiations fields, and would perform better than traditional detector systems.

Significant discrepancies had been identified by many research groups world wide between calculated and measured neutron doses from the ⁷Li(p,n) accelerator based neutron source, and therefore precise characterization of the source was needed. In this work neutron spectra from the ⁷Li(p,n) source were measured with a ³He ion chamber in the incident proton energy range from 1.95 to 2.3 MeV. The ³He detector is hypersensitive to slow neutrons, so a time-of-flight based slow neutron rejection acquisition system was built and tested. The system is based on an electrostatic proton chopper and an acquisition system working on coincidence mode. The response function of the ³He was extended down to 30 keV neutron energy and the collected neutron spectra were unfolded using two methods: van Cittert iterative algorithm with Jansson constraint, and a regularized constrained inversion. Theoretical neutron spectra emitted by the ⁷Li(p,n) source were calculated and compared with experimental unfolded spectra, as well as with results of the Monte Carlo simulations of the lithium target assembly and the walls of the experimental area. Using fluence to kerma conversion coefficients, the neutron dose was calculated and compared with results obtained from an independent experiment using the microdosimetric technique with a tissue equivalent proportional counter. Total neutron yield of the ⁷Li(p,n) reaction was measured using induced activity of ⁷Be. Results showed a negative energy offset of the incident proton beam between 50 and 58 keV with respect to the generating voltmeter indication of the accelerator terminal. Shapes of the measured neutron spectra showed significant moderation originating from neutron scattering on the lithium target assembly and walls of the experimental area. When accounting for this offset, neutron yields showed an agreement with calculated values within 22% for 1.95 MeV and within approximately 7% for higher proton energies. / Thesis / Doctor of Philosophy (PhD)

No description available.