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Demonstration of an Approach to Precisely Measure Long-Lived Gamma-ray Branching Ratios for Beta Emitters


Measurements of γ-rays are a reliable way to identify the quantities of different long-lived radioactive isotopes present in a sample. Many of the radioactive nuclei of interest emit γ-rays at characteristic energies that can be detected with high energy resolution and limited background noise using high purity germanium (HPGe) detectors. However, these γ-rays are only emitted in a fraction of the decays, and this fraction (known as the γ-ray branching ratio) must be known accurately to determine the total number of atoms present.

In the following work, I discuss, in detail, the development of a general method for accurately and efficiently determining the γ-ray branching ratios of fission products through measurement of the emitted β particles, conversion electrons, and γ-rays with extreme precision. I then apply this technique to long-lived fission products that have branching ratios that either have large uncertainties (147Nd) or that have only been precisely measured once (144Ce).

The steps to measure γ-ray branching ratios are: (1) design, create, and simulate a highly-efficient gas counter through a collaboration between the University of California Irvine (UCI) and Lawrence Livermore National Laboratory (LLNL), (2) produce radioactive test sources using the Texas A&M Nuclear Reactor to identify any potential challenges or systematic effects, (3) run experiments at the Californium Rare Ion Breeder Upgrade (CARIBU) facility at Argonne National Laboratory (ANL) to collect isotopically-pure samples on thin foil backings, (4) perform β spectroscopy using the custom-made gas counter, and (5) perform γ-ray spectroscopy using the precisely calibrated high-purity germanium (HPGe) detector at Texas A&M University. Through the efforts of these participating institutions, γ-ray branching ratios are measured with a precision of ~1%.

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