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>Magister Scientiae - MSc / The Physics Department at the University of the Western Cape and the Environmental Physics group at iThemba labs have been conducting radiometric studies on both land and water. In this study a 7.5 cm X 7.5 cm NaI (Tl) detector was used to study activity concentrations of primordial radionuclides in soil and sand samples. The detector and the sample were placed inside a lead castle to reduce background in the laboratory from the surroundings such as the wall and the floor. The samples were placed inside a 1 L Marinelli beaker which surrounds the detector for better relative efficiency as almost the whole sample is exposed to the detector. Additional lead bricks were placed below the detector to further reduce the background by 20%. The NaI detector is known to be prone to spectral drift caused by temperature differences inside and around the detector. The spectral drift was investigated by using a ¹³⁷Cs source to monitor the movements in the 662 keV peak. The maximum centroid shift was about 4 keV (for a period of 24 hours) which is enough to cause disturbances in spectral fitting. There was no correlation between the centroid shift and small room temperature fluctuations of 1.56 ºC. A Full Spectrum Analysis (FSA) method was used to extract the activity concentrations of ²³⁸U, ²³²Th and ⁴⁰K from the measured data. The FSA method is different from the usual Windows Analysis (WA) as it uses the whole spectrum instead of only putting a ‘window’ around the region of interest to measure the counts around a certain energy peak. The FSA method uses standard spectra corresponding to the radionuclides being investigated, and is expected to have an advantage when low-activity samples are measured. The standard spectra are multiplied by the activity concentrations and then added to fit the measured spectrum. Accurate concentrations are then extracted using a chi-squared (χ²) minimization procedure. Eight samples were measured in the laboratory using the NaI detector and analyzed using the FSA method. The samples were measured for about 24 hours for good statistics. Microsoft Excel and MATLAB were used to calculate the activity concentrations. The ²³⁸U activity concentration values varied from 14 ± 1 Bq/kg (iThemba soil, HS6) to 256 ± 10 Bq/kg (Kloof sample). The ²³²Th activity concentration values varied from 7 ± 1 Bq/kg (Anstip beach sand) to 53 ± 3 Bq/kg (Rawsonville soil #B31). The ⁴⁰K activity concentration values varied from 60 ± 20 Bq/kg (iThemba soil, HS6) to 190 ± 20 Bq/kg (Kloof sample). The χ² values also varied from sample to sample with the lowest being 12 (Anstip beach sand) and the highest (for samples without contamination of anthropogenic nuclei) being 357 (Rawsonville soil #B28). A high χ² value usually represents incomplete gain drift corrections, improper set of fitting functions, proper inclusion of coincidence summing or the presence of anthropogenic (man made) radionuclei in the source [Hen03]. Activity concentrations of ⁴⁰K, ²³²Th and ²³⁸U were measured at four stationary points on the Kloof mine dump. The fifth stationary point was located on the Southdeep mine dump. These measurements were analysed using the FSA method and fitting by "eye" the standard spectra to the measured spectra using Microsoft Excel. These values were then compared to values obtained using an automated minimization procedure in MATLAB. There was a good correlation between these results except for ²³²Th which had higher concentrations when MATLAB was used, where 16 Bq/kg was the average value in Excel and 24 Bq/kg was the average value in MATLAB.

Within cosmology, there are two entirely independent pillars which can jointly drive this field towards precision: Astronomical observations of primordial element abundances and the detailed surveying of the cosmic microwave background. However, the comparatively large uncertainty stemming from the nuclear physics input is currently still hindering this effort, i.e. stemming from the 2H(p,γ)3He reaction. An accurate understanding of this reaction is required for precision data on primordial nucleosynthesis and an independent determination of the cosmological baryon density. Elsewhere, our Sun is an exceptional object to study stellar physics in general. While we are now able to measure solar neutrinos live on earth, there is a lack of knowledge regarding theoretical predictions of solar neutrino fluxes due to the limited precision (again) stemming from nuclear reactions, i.e. from the 3He(α,γ)7Be reaction. This thesis sheds light on these two nuclear reactions, which both limit our understanding of the universe. While the investigation of the 2H(p,γ)3He reaction will focus on the determination of its cross- section in the vicinity of the Gamow window for the Big Bang nucleosynthesis, the main aim for the 3He(α,γ)7Be reaction will be a measurement of its γ-ray angular distribution at astrophysically relevant energies. In addition, the installation of an ultra-low background counting setup will be reported which further enables the investigation of the physics of rare events. This is essential for modern nuclear astrophysics, but also relevant for double beta decay physics and the search for dark matter. The presented setup is now the most sensitive in Germany and among the most sensitive ones worldwide.

Within cosmology, there are two entirely independent pillars which can jointly drive this field towards precision: Astronomical observations of primordial element abundances and the detailed surveying of the cosmic microwave background. However, the comparatively large uncertainty stemming from the nuclear physics input is currently still hindering this effort, i.e. stemming from the 2H(p,γ)3He reaction. An accurate understanding of this reaction is required for precision data on primordial nucleosynthesis and an independent determination of the cosmological baryon density. Elsewhere, our Sun is an exceptional object to study stellar physics in general. While we are now able to measure solar neutrinos live on earth, there is a lack of knowledge regarding theoretical predictions of solar neutrino fluxes due to the limited precision (again) stemming from nuclear reactions, i.e. from the 3He(α,γ)7Be reaction. This thesis sheds light on these two nuclear reactions, which both limit our understanding of the universe. While the investigation of the 2H(p,γ)3He reaction will focus on the determination of its crosssection in the vicinity of the Gamow window for the Big Bang nucleosynthesis, the main aim for the 3He(α,γ)7Be reaction will be a measurement of its γ-ray angular distribution at astrophysically relevant energies. In addition, the installation of an ultra-low background counting setup will be reported which further enables the investigation of the physics of rare events. This is essential for modern nuclear astrophysics, but also relevant for double beta decay physics and the search for dark matter. The presented setup is now the most sensitive in Germany and among the most sensitive ones worldwide. / Innerhalb der Kosmologie gibt es zwei völlig unabhängige Ansätze, die gemeinsam die Präzision in diesem Gebiet weiter vorantreiben können: Astronomische Beobachtungen der primordialen Elementhäufigkeiten und die detaillierte Vermessung des kosmischen Mikrowellenhintergrunds. Dieses Vorhaben wird derzeit allerdings noch durch die vergleichsweise große Unsicherheit des kernphysikalischen Inputs verhindert, vor allem bedingt durch das limitierte Verständnis der 2H(p,γ)3He-Reaktion. Eine präzise Vermessung dieser Reaktion ist sowohl für die Präzisionsdaten zur primordialen Nukleosynthese erforderlich, als auch für die damit einhergehende unabhängige Bestimmung der kosmologischen Baryonendichte. Des Weiteren ist unsere Sonne ein exzellent geeignetes Objekt, um unser theoretisches Verständnis über die Physik von Sternen mit experimentellen Messungen abgleichen zu können. Während wir heutzutage in der Lage sind, solare Neutrinos in Echtzeit auf der Erde messen können, mangelt es noch an der theoretischen Vorhersagekraft von solaren Neutrinoflüssen. Auch hier ist die Präzision (erneut) begrenzt durch das limitierte Verständnis der beteiligten Kernreaktionen, vor allem bedingt durch mangelnde Kenntnis über die 3He(α,γ)7Be-Reaktion. Die vorliegende Arbeit beleuchtet diese zwei Kernreaktionen, die beide unser Verständnis des Universums auf verschiedene Weise einschränken. Während sich die Untersuchung der 2H(p,γ)3He-Reaktion auf die Bestimmung ihres Wirkungsquerschnitts in der Nähe des Gamow-Fensters für die Urknall-Nukleosynthese konzentriert, ist das Hauptanliegen für die 3He(α,γ)7Be-Reaktion eine Messung der Winkelverteilung der dabei emittierten γ-Strahlung bei astrophysikalisch relevanten Energien. Darüber hinaus wird über die Installation eines Messaufbaus zur Untersuchung niedriger Aktivitäten berichtet, das sich durch seine äußerst geringe Untergrundzählrate auszeichnet. Bedingt durch seine hohe Sensitivität kann dieser Aufbau in Zukunft bedeutende Beiträge für die moderne nukleare Astrophysik leisten und ist darüber hinaus beispielsweise auch relevant für die Untersuchung von Doppel-Betazerfällen oder die Suche nach dunkler Materie. Der präsentierte Aufbau ist nun der Sensitivste seiner Art in Deutschland und gehört zu den Sensitivsten weltweit.