Total Absorption Spectrometers are characterized by high efficiency detection of gamma- ray radiation, which is due to their large volume and nearly 4pi solid angle coverage. This property, used in the study of beta decay of unstable nuclei, allows for the total detection of the deexcitation path of daughter nuclei. This makes total absorption spectroscopy an ideal technique to establish true beta decay feeding patterns. Recent studies show that the measurements of fission products by total absorption spectroscopy are extremely important to understanding the anti-neutrino spectrum emitted from nuclear reactors. The number of reactor anti-neutrino interactions measured by inverse-beta decay detectors is about 6% smaller than the expected number of events, which is named the reactor anti-neutrino anomaly. The anti-neutrino energy spectrum, obtained from the fission product beta-decay schemes, is used to calculate the total anti-neutrino flux emitted by reactor cores and the number of anti-neutrino interactions with the detector matter. The measurements of the beta decay of fission products using the total absorption technique allow verification of the expected number of interacting reactor anti-neutrinos with matter. I will present several results of total absorption spectroscopy measurements of the beta decay of nuclides abundantly produced in the reactor core. The measurements were performed at the Holifield Radioactive Ion Beam Facility (HRIBF) with the Modular Total Absorption Spectrometer (MTAS). The impact of the results on the reconstruction of anti- neutrino spectra will be presented as well.