During a nuclear reactor’s normal operation, approximately 7-8% of the total heat produced is due to delayed β-decay of the initial fission products, and is known as decay heat. Once a reactor is shut down this decay heat contributes 100% of the heat produced by the fuel. Total absorption γ-ray spectroscopy utilises a near 4π geometry with a high efficiency to collect γ rays produced from a source placed in its centre. This total absorption can be used to determine the β-feeding levels of fission fragments that have large values. Conventional methods using HPGe detectors to determine β-feeding can be affected by the "Pandemonium effect". This occurs due to the low detection efficiency of high-energy γ rays in HPGe detectors. Current decay heat calculations predict lower values than calorimetry measurements and this needs to be addressed. An experiment was carried out in Jyvaskyla to measure the β-feeding levels of key nuclei (86Br, 91Rb and 94Sr) for decay heat calculations using a BaF2 Total Absorption Spectrometer (TAS). This thesis describes the experimental method, the calibration of the TAS and, the analysis procedure to obtain the average mean γ ray and β particle energy for each isotope as well as the β decay strength function. The final results from this work have provided new mean energy values for the β decays of 86Br (Eγ=3822(6)(54)keV Eβ=1670(4)(28)keV) and 91Rb (Eγ=2788(5)(29)keV Eβ=1330(3)(22) keV) showing that the "Pandemonium effect" was present in the previously recorded data and a reduced uncertainty was obtained for the decay of 94Sr (Eγ=1472(9)(15)keV Eβ=826(5)(6)keV). The results have given increased validity to previous TAS measurements by Greenwood et al. and subsequently questions work by Rudstam et al. on the measurements of β-particle and γ-ray spectra of many fission fragments.