The two-photon decay in nuclei has been formally theorized for many years and attempted to be measured on numerous occasions. The special case of a 0 + → 0 + transition in nuclei was examined for certain isotopes, and a branching ratio for the two-photon decay determined for each isotopes. Measurements of the branching ratio in nuclei other than this special case had so far proven unsuccessful. Motivated to find the two-photon branching ratio in a case where the transition competes with the single photon transition, we study the 11/2 − 137 Ba isomer. The experiment was performed at the Technische Universität Darmstadt using the available LaBr 3 scintillation detectors. We first study the absorption of various gamma energies by lead and compare the resulting values to a GEANT4 simulation. With an ideal value for lead shield thickness, the experimental setup is built in order to obtain a high two-photon count rate, while suppressing direct Compton scattering between detector pairs and suppressing other background interference. In order to suppress the background, plastic scintillators were placed atop the experimental setup. To treat the daunting level of random coincidences measured with this setup, fine energy and time gates were placed on the processed events in order to limit observation to the region of interest. Throughout the experiment, three different detector pair angles were successfully examined: 72 ◦ , 120 ◦ , and 144 ◦ . With these three angles a partial representation of the angular distribution of the two-photon decay is observed. The branching ratios were measured to be 1.56(23)·10 −6 , 0.55(22)·10 −6 , and 0.70(18)·10 −6 for the angles of 72 ◦ , 120 ◦ , and 144 ◦ respectively, with the values of 72 ◦ and 144 ◦ recorded in Ref.[1]. This experiment therefore shows it is possible to obtain a value for the two-photon branching ratio in the 11/2 − excited state of 137 Ba . A precise determination of this value, and for that of other nuclei, might contribute to solve current fundamental open problems such as restricting the parameters of the equation of state, or accurately determining neutron skin thickness.

The effect of the spatial distribution of a radioactive source on timing measurements has been investigated with particular consideration toward the focal plane of recoil separators. The work conducted during this thesis is a precursor to understand the magnitude of such effects for the upcoming fast timing array (FATIMA) at FAIR. An experiment was undertaken at the University of Jyvaskyla using the K130 cyclotron to accelerate a 36-Ar beam to 190 MeV, directed onto a 106-Cd target, to produce recoils of 138-Gd and 136-Sm via fusion-evaporation reactions. Recoils directed using RITU to the focal-plane DSSSD of GREAT were distributed over the majority of the 124-mm by 40-mm extension of the DSSSD. A new array consisting of eight lanthanum bromide detectors was used to measure the time between coincident prompt-gamma rays emitted following the de-excitation of isomeric recoil states implanted into the DSSSD. Lifetimes were measured to be 213(20) ps and 200(100) ps for the first-excited 2+ states in 138-Gd and 136-Sm, respectively. Positional information, extracted from the DSSSD, was used to correct for the difference in the time-of-flight of gamma rays as they travelled from the implantation position to the lanthanum bromide detectors. When accounted for, the lifetimes were remeasured to be 217(20) ps and 210(90) ps, respectively, showing no significant change in value or error. A method of quantifying the increase in uncertainty of a lifetime measurement due to the spatial distribution of the source and the position of the surrounding detectors, supported by simulation, has been provided to explain these observations. A new technique for extracting the time-walk from each of the CFDs in a multi-detector array has been presented. The new technique offers a reduced complexity in calculations by accounting for the correlated time-walks present in time measurements from different detector-pairs sharing a common CFD. Work towards a technique for extracting lifetimes from time data has been presented. Dubbed the Symmetrised-Convolution Lifetime Measurement (SCLM) method, this technique essentially applies a model-dependent convolution of the prompt-response with nuclear exponential decay on both time spectra, obtained by inverting the start and stop conditions of a TAC, simultaneously and draws parallels to the Mirror Symmetric Centroid Difference method.

Reprocessing nuclear fuel is becoming more viable in the United States due to the anticipated increase in construction of nuclear power plants, the growing stockpile of existing used nuclear fuel, and a public desire to reduce the amount of this fuel. However, a new reprocessing facility in non-weapon states must be safeguarded and new reprocessing facilities in weapon states will likely have safeguards due to political and material accountancy reasons. These facilities will have state of the art controls and monitoring methods to safeguard special nuclear materials, as well as to provide real-time monitoring. The focus of this project is to enable the development of a safeguards strategy that uses well established photon measurement methods to characterize samples from the UREX+3a reprocessing method using a variety of detector types and measurement times. It was determined that the errors from quantitative measurements were too large for traditional safeguards methods; however, a safeguards strategy based on qualitative gamma ray and neutron measurements is proposed. The gamma ray detection equipment used in the safeguard strategy could also be used to improve the real-time process monitoring in a yet-to-be built facility. A facility that had real-time gamma detection equipment could improve product quality control and provide additional benefits, such as waste volume reduction. In addition to the spectral analyses, it was determined by Monte Carlo N Particle (MCNP) simulations that there is no noticeable self shielding for internal pipe diameters less than 2 inches, indicating that no self shielding correction factors are needed. Further, it was determined that HPGe N-type detectors would be suitable for a neutron radiation environment. Finally, the gamma ray spectra for the measured samples were simulated using MCNP and then the model was extended to predict the responses from an actual reprocessing scenario from UREX+3a applied to fuel that had a decay time of three years. The 3-year decayed fuel was more representative of commercially reprocessed fuel than the acquired UREX+3a samples. This research found that the safeguards approach proposed in this paper would be best suited as an addition to existing safeguard strategies. Real-time gamma ray detection for process monitoring would be beneficial to a reprocessing facility and could be done with commercially available detectors.

With the complex composition of the radioisotopes and waste materials, the characterization of the volumetric low-level wastes from CANDU plants is challenging. This study presents a technique to localize and quantify the contaminations presented in the CANDU waste containers. MCNP-based models are developed for an N-type coaxial HPGe detector and a LaBr3 detector to simulate the photon peak information. The simulated efficiency and the experimental count rates are combined to estimate the activity of unknown waste samples. During the spectrum collection of a 4L Marinelli beaker source and 1-quart waste samples, the MCNP algorithm showed better accuracy in activity estimation than the Mirion ISOCS/LabSOCS software. With further development, this method has the potential to outperform the popular commercial software in estimating activity for volume sources with complex geometry and uneven distribution. The multi-detector array models with hotspot designs are also studied in this work to provide real-time information about the location and activity of the contamination inside the 2.2 m3 industrial low-level waste containers. The on-site measurements show promising results as the position of the contamination was able to be located within a volume of 61×40×34 cm. Overall, this technique has good potential to be utilized in the nuclear industry for large-volume low-level waste analysis. / Thesis / Master of Science (MSc)