Limits on the Variation of $^{54}$Mn and $^{137}$Cs Nuclear Decay Parameters due to Nuclear Reactor on-off Cycling

An experiment has been conducted to search for possible variation of the nuclear decay lifetime parameters in the electron capture decay of $^{54}$Mn and the beta decay of $^{137}$Cs due to an influence of the reactor $\overline{\nu}$ flux at the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL). The samples were exposed to approximately equal, reactor-on periods of 28 days of anti-neutrino flux at $\sim3\times10^{12}\overline{\nu}\,cm^{-2}\,sec^{-1}$ and reactor-off periods. The variation limits are set by comparing the decay rate parameters as a function of the HFIR on and off periods. Equivalent cross section limits are set on variations assuming a linear relationship between the flux exposure and the induced nuclear decay parameter variation. Measuring the decay rate requires both the detector and environment be stable over the full-time period of the experiment. A High Purity Germanium spectrometer, sensitive to radiation from 3-keV to over 3-MeV, has been built to measure radioactive decay constants to a level of 10$^{-5}\sim10^{-6}$ at a location only 6 meters from the HFIR core. Such accuracy requires an understanding of the background, signal-processing algorithms, and both the double and triple event pile-up in the observed spectrum.