Speaker
Description
Modern HPGe detector arrays such as GRIFFIN and TIGRESS at TRIUMF/ISAC allow for extremely high precision $\gamma$-ray branching ratio measurements - as low as $10^{-7}$ depending on counting rate and measurement time - allowing for examination of rare decay branches. A promising avenue for pursuing these high precision measurements at TRIUMF is local production of high activity sources using the TR13 medical cyclotron, located within walking distance of both ISAC halls. We have initiated a series of experiments to study the rare decays of $^{93m}$Mo ($t_{1/2} = 6.85$ hr) and $^{92m}$Nb ($t_{1/2} = 10.15$ d) using MBq activity sources produced on the TR13 via proton bombardment of niobium foils.
For $^{93m}$Mo, our goal is to measure the rare $21/2^+_1 \rightarrow 9/2^+_1$ $E6$ (electric hexacontatetrapole) IT decay branch. Only one $E6$ decay branch has been previously observed, from $^{53m}$Fe decay [1,2]. Despite the presence of significant summing and time-random background, there is some preliminary evidence of this extremely rare decay branch in our data. Additional data will be collected throughout 2026, to improve statistics and evaluate systematic uncertainties.
We have also obtained high statistics $^{92m}$Nb $\rightarrow$ $^{92}$Zr decay data, allowing measurement of the $B(E2; 2^+_2 \rightarrow 0^+_2)$ value in $^{92}$Zr for the first time, as well as improved limits for other weak transitions. These are important indicators as to whether $^{92}$Zr exhibits shape coexistence similar to $^{94}$Zr and other neutron rich zirconiums. We are also searching for direct population of $0^+$ states in $^{92}$Zr in order to definitively assign the spin of the $^{92m}$Nb parent, which may be relevant to the survival of $^{92}$Nb in astrophysical scenarios. The current status of our data analysis and future prospects for local source production experiments at TRIUMF will be discussed.
[1] J. N. Black et al., Physical Review Letters 26 451-454 (1971).
[2] T. Palazzo et al., Physical Review Letters 130 122503 (2023).