The Terbium Theragnostic Quartet is a group of four radionuclides (^{149,152,155,161} Tb) with promising characteristics for nuclear medicine. Terbium-152, decaying to ¹⁵²Gd by β⁺ decay and electron capture with T_1/2 = 17.8784(95) h and Q_EC = 3.99(4) MeV, has demonstrated good performance in positron emission tomography (PET) imaging as shown in first-in-human trials. Before routine use of this radionuclide can be considered, the precision and completeness of the nuclear data for this decay needs to be improved, specifically the gamma-ray intensities and beta feeding, to facilitate accurate dosimetry in clinical settings. This thesis presents results of a decay spectroscopy study of ¹⁵²Tb using sources produced by proton-induced spallation of a tantalum target at CERN-ISOLDE, followed by mass-separation using the Isotope Separation On-Line (ISOL) technique. The sources were delivered to Institut Laue-Langevin (ILL), Grenoble, where gamma-ray coincidences were measured using the Fission Product Prompt Spectrometer (FIPPS), supported by parallel electron-gamma spectroscopy measurements of the same samples using the PN1-LOHENGRIN conversion electron spectrometer. Over 1000 transitions are observed in these measurements, with 752 having been placed on the level scheme for the decay, leading to the first identification of 97 high-energy excited states in ¹⁵²Gd. Angular correlations and conversion electron spectroscopy have supported the construction of the level scheme, allowing the assignment of spins and parities to levels, multipolarities to transitions, and the direct measurement of internal conversion coefficients and E0 transition strengths. Analysis of the final level scheme gives beta feeding intensities for each level, revealing significant shifts in the beta strength function compared to the previous evaluated data. The result of this is an 8.9% decrease in the positron emission probability, and a 4.5% increase in the average gamma-ray energy emitted per decay. These results already have significant implications for dosimetry calculations, the precision of which may be further improved by Total Absorption Spectroscopy (TAS) measurements of the same decay, relying on the level scheme described here for the data analysis. First evidence of an electric octupole (E3) transition in this decay is also presented, and will facilitate the completion of systematic reduced transition probabilities (B(E3)) for the gadolinium isotopic chain following the upcoming lifetime measurement of the first J^π = 3⁻ level in ¹⁵²Gd. The project is supported by parallel efforts to characterise an upgraded gamma-gamma coincidence array at the National

Physical Laboratory for primary radionuclide standardisations, using fast-timing CeBr₃ scintillator crystals for clean coincidence gating and lifetime measurements of short-lived excited states.