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No description available.

Le sujet de cette thèse est né de l'observation de phénomènes incompris des mécanismes à l'origine des très faibles efficacités de discrimination neutron/gamma (n/g) dans les scintillateurs plastiques en comparaison à leurs homologues liquides. Le flash lumineux qu'ils génèrent suite à une interaction avec un rayonnement ionisant (majoritairement des protons de recul dans le cas des neutrons et des électrons dans le cas des gamma) présente des caractéristiques temporelles qui dépendent de la nature de la particule détectée (type et énergie). C'est sur la forme du signal que la séparation peut être réalisée (PSD). Les scintillateurs liquides ont largement été étudiés. C'est seulement récemment qu'il a été démontré qu'une séparation nette pouvait être réalisée à l'aide de plastiques spécialement conçus. L'étude de ces systèmes et la compréhension des processus photophysiques en plastique par rapport à leur efficacité de PSD est toujours d'actualité. Ce travail est dédié à la compréhension des phénomènes en amont des émissions de luminescence, juste après l'interaction rayonnement/matière. Dans un premier temps un état de l'art qui s'étend de 1960 à nos jours a été réalisé. Une deuxième partie est consacrée à la caractérisation des matériaux scintillants mis en jeu pour définir leurs propriétés de fluorescence et de scintillation sous rayonnement. La troisième partie se focalise sur la simulation de traces neutrons via un faisceau de protons pour quantifier dans des conditions contrôlées leur dépôt spécifique d'énergie dans notre plastique présentant des capacités de discrimination n/g-. Une quatrième partie est dévolue à la détermination de leur efficacité de PSD en fonction de leurs paramètres de structure moléculaire. Ces travaux ont conduit à la préparation d'un plastique dans notre laboratoire présentant des propriétés de PSD considérées efficaces, ce qui a donné lieu à un dépôt de brevet. Enfin une étude photophysique conclut ce travail à l'aide d'un laser femtoseconde pour simuler de façon purement optique des interactions neutron avec la matière donnant lieu à de la fluorescence retardée. Nous tentons ainsi de définir la nature des transferts d'énergie mis en jeu. Nous concluons enfin sur la corrélation structure/propriétés de discrimination. Ces avancées permettront de guider le potentiel en termes d'applications industrielles et le développement de nouveaux matériaux.Ce travail est dédié à la compréhension des phénomènes en amont des émissions de luminescence, juste après l'interaction rayonnement/matière. Dans un premier temps un état de l'art qui s'étend de 1960 à nos jours a été investigué. Une deuxième partie est consacrée à la caractérisation complète des matériaux scintillants mis en jeu pour définir leurs propriétés de fluorescence et de scintillation sous rayonnement. Une troisième partie est dévolue à la détermination de leur efficacité en termes de discrimination neutron/gamma en fonction de leurs paramètres de structure moléculaire. Ces travaux ont menés à la préparation d'un plastique dans notre laboratoire présentant des propriétés de PSD considérées efficaces, ce qui a donné lieu à un brevet. La quatrième partie se focalise sur la simulation de traces neutrons via un faisceau de protons pour quantifier dans des conditions contrôlées leur dépôt spécifique d'énergie dans notre plastique, présentant des capacités de discrimination neutron/gamma. Enfin une étude photophysique conclut ce travail à l'aide d'un laser femtoseconde pour simuler de façon purement optique des interactions neutron ou gamma avec la matière donnant lieu à de la fluorescence retardée, et tenter de définir ainsi la nature des transferts d'énergie mis en jeu. Nous concluons enfin sur la corrélation structure / propriétés de discrimination. Ces avancées permettront de guider le potentiel en termes d'applications industrielles et le développement de nouveaux matériaux. / This PhD topic was born from misunderstandings and incomplete knowledge of the mechanism and relative effectiveness of neutron and gamma-ray (n/g) discrimination between plastic scintillators compared to liquid scintillators. The shape of the light pulse these materials generate following interaction with an ionizing particle (predominantly recoil protons in the case of neutrons and electrons in the case of gamma-rays) is different in time in a way that depends on the detected particle (nature and energy). It is this fact that enables separation (PSD). The behavior in liquid scintillators has been extensively studied experimentally for practical applications. Only recently has it been shown that a weak separation can also be achieved using specially prepared plastics. The study of this system presents an open field and the understanding of both liquids and plastics with respect to their PSD properties is far from complete. This work is dedicated to exploring the fundamental photophysical phenomena at play in the generation of luminescence emission, following the interaction of ionizing radiation with organic scintillators. For this purpose, firstly a detailed literature review of the state-of-the-art has been conducted extending from 1960 to the present day. Secondly a complete characterization of the main scintillating materials has been conducted to define their fluorescence properties and the characteristics of their scintillation under irradiation. Thirdly a proton beam has been used to simulate recoil protons to quantify under controlled laboratory conditions their specific energy deposition in a plastic scintillator with PSD properties. The fourth part of this thesis is devoted to the study of PSD efficiency of scintillators as a function of their molecular structure. This investigation has led to a plastic scintillator prepared in our laboratory with good PSD properties and a patent submission. Finally, photophysical experiments were performed using a femtosecond laser to simulate, purely optically, neutron/matter interactions giving rise to delayed fluorescence. This technique was chosen so as to examine the fundamental energy transfers involved. Overall this work led to an improved understanding and greater insight about the correlation between the chemical structure of organic scintillating materials and their n/g- discrimination properties. This will guide both applications and future materials development.

L’installation n_TOF au CERN est unesource pulsée de neutrons, unique au monde pour lamesure de données nucléaires sur le spectre enénergie le plus étendu, avec deux lignesactuellement exploitées. Une ligne horizontalealimente l’aire expérimentale 1 (EAR-1) avec unebase de vol de ~185 mètres. La seconde ligne estverticale et alimente l’aire 2 (EAR-2) à ~20 mètresde la cible de productionLa première partie de ce travail de thèse concernela caractérisation du faisceau de neutrons (flux,profil géométrique, spectre en énergie) de lanouvelle ligne EAR-2, particulièrement importantepour la définition des expériences et leur analyse.Une mesure a été réalisée à EAR-2, basée sur desdétecteurs PPAC enregistrant la fission de 235U, àpartir de laquelle nous avons obtenu le profil et leflux sur la gamme en énergie accessible (thermiqueà 200 MeV).La seconde partie de la thèse a pour but l’étudede la fission de 237Np. Cet isotope est abondammentproduit dans les réacteurs nucléaires actuels et estun des constituants des déchets à vie longue. A cetitre on peut le considérer comme une ciblepotentielle pour l’incinération en réacteur rapide.Ceci a motivé des mesures récentes de sa sectionefficace de fission. Cependant des déviationsimportantes sont apparues, en particulier la mesureeffectuée à n_TOF en 2010 est 6% supérieure auxévaluations basées sur les mesures antérieures. Cecia motivé une nouvelle mesure à n_TOF avec uneconfiguration permettant une mesure précise del’efficacité de détection, pour apporter une réponseau problème. Ce travail a permis de mettre enévidence une dépendance de l’efficacité dedétection avec l’élément, résultant des conditions del’électrodéposition. Après application de cettecorrection d’efficacité dans la région 1 à 5 MeV lasection efficace de fission ainsi extraite est 2 à 3%plus petite par rapport à la mesure de 2010,cependant elle reste 3 à 4% plus forte que lesévaluations / The neutron time-of-flight (n_TOF)facility at CERN is a unique worldwide pulsedneutron source to measure the nuclear data over thewidest energy range with two beam lines currentlyexploited. One is horizontal with a ~185 metersflight path, sending neutrons to experimental area-1(EAR-1). The second one is a new line sendingneutrons vertically to experimental area-2 (EAR-2)with a ~20 meters flight path.The first part of this PhD work is dedicated to thecharacterization of the beam (flux, geometricalprofile, energy spectrum) of the new EAR-2 neutronbeam, of the utmost importance for the experimentalproposals and analyses. An experiment was carriedout at EAR-2, based on PPAC detectors looking atfission of 235U, and the beam profile and neutronflux have been obtained for the entire availableenergy range (from thermal to 200 MeV).The second part of the thesis aims to study theneutron-induced fission of 237Np. 237Np isabundantly produced in present nuclear reactor andis one of the major long-lived components ofnuclear waste which can be considered as apotential target of incineration in fast neutronreactors. Consequently its neutron-induced fissioncross section has been measured at differentfacilities. However, significant discrepancies existbetween different experiments. Especially, therecent one performed at n_TOF in 2010 is about 6%higher by comparison to the evaluation data basedon previous experiments. Therefore an experimenthas been performed at n_TOF EAR-1 to measure itsfission cross section, in a configuration allowing anaccurate control of the detection efficiency, aimingto give a definite answer to the puzzle. In this work,we found that the efficiencies for different targetelements are different, even though they havesimilar thickness, reflecting the conditions ofelectrodeposition. After application of thisefficiency correction in the energy range from 1 to 5MeV, the newly extracted fission cross section is 2-3% lower compared with our previous measurementin 2010, however they are 3-4% higher than the

<p>Excess sodium (Na) intake is directly related to hypertension and an increased risk of developing many chronic diseases, but there is currently no method to directly quantify Na retained in the body. Because of this, the locations of Na storage and its exchange mechanisms are not well known. This information is critical for understanding the impact of increased Na intake in modern diets. In order to non-invasively quantify Na in bone and soft tissue, a compact deuterium-deuterium (DD) neutron generator-based <i>in vivo</i> neutron activation analysis (IVNAA) system was developed. MCNP was used to design a custom irradiation assembly to maximize Na activation in hand bone while minimizing dose. In order to test the system, live pigs were used. Two 100% efficient high purity germanium (HPGe) detectors collected Na-24 counts over 24 hours post irradiation. From the pig studies, a two-compartment model of exchange was developed to quantify Na in bone and in soft tissue. The right legs of four live pigs, two on a low Na diet and two on a high Na diet, both for 14 days, were irradiated inside the customized irradiation cave for 10 minutes (45 mSv dose to the leg) and then measured with the HPGe detectors. The spectra were analyzed to obtain the net Na counts at different time points. Analysis shows exponential decrease of Na in the leg during the first one hour of measurement, while the change was minimal at the second hour, and the counts were stabilized at the second and third 2 hour measurements, taken 7 and 21 hours post irradiation. Bone Na and soft tissue Na concentrations were calculated using calibration lines created with bone and soft tissue equivalent Na phantoms as well as the parameters obtained from the two-compartment model. The results show that the difference in bone and soft tissue Na between the pigs on high vs low Na diets was significant. With these results, we conclude that DD neutron generator-based IVNAA can be used to accurately quantify Na in bone and soft tissue <i>in vivo </i>and the system is a potential valuable tool for nutrition studies.</p>

No description available.

No description available.

Using computational deterministic and Monte Carlo methods, I present an analysis of the gamma and neutron signatures emitted from special nuclear material (SNM) in weapons stockpile storage scenarios. My efforts are focused on 1 year old, 25 year old, 50 year old, and 75 year old highly enriched uranium (HEU), and 1 year old, 22.5 year old, and 50 year old weapons grade plutonium (WGPu). HEU gammas are easily shielded, and when reasonably shielded, do not produce a definable signature at low energies; however, using new methods applied in this work that involve analyzing the higher energy, penetrating gammas from HEU, it can be shown that not only the presence of the HEU can be verified, but also the age since separation of the material can be discerned. Through computational modeling, I am able to verify that the novel methods investigated are both unique and effective for HEU detection. In addition, I also present my investigation of similar methods applied to the detection of WGPu. From this work I determined that WGPu age discrimination is more challenging compared to that of HEU (in spite of more radiation per unit mass) due to the high rate of induced gammas from n-gamma interactions taking place within the Pu metal and container; however, I believe that by combining neutron detection with gamma signature verification of WGPu, the approach identified can be successful. I present the neutron signature of the WGPu and how it would be observed in the detector used for material verification. I calculated the detector response for a pre-determined neutron detector design using adjoint calculations in order to determine whether the detector will perform as designed. In addition to developing a new protocol for WGPu detection, I present in this work, a comprehensive source book as a product of this research, detailing the gamma and neutron signatures for both solid and shell configurations of HEU and WGPu. This can serve as a very beneficial guide for anyone interested in modeling SNM, since the many steps needed to obtain this radiation leakage data will save a significant amount of researcher time. The results from my work have contributed to a collaborative effort supporting funded US department of State research towards designing a mobile detection system that can rapidly validate and verify the presence of SNM in weapons pit containers.

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.