Background: Recent studies of the β decay of fission fragments by total absorption spectroscopy have found less decay energy attributed to leptons than previously believed, with studies of ${}^{105}\mathrm{Mo}$ and ${}^{105}\mathrm{Tc}$ being particularly impactful for reactor decay heat. These results are also crucial for interpreting measurements of reactor antineutrino energy spectra.

Purpose: To obtain complete $\beta$-decay patterns of ${}^{105}\mathrm{Mo}$, ${}^{105}\mathrm{Tc}$, ${}^{105}\mathrm{Ru}$, and ${}^{105}\mathrm{Rh}$. Determine the impact on reactor decay heat for all four isotopes and the impact on ${\overline{\nu }}_e$ emission calculation for ${}^{105}\mathrm{Mo}$ and ${}^{105}\mathrm{Tc}$.

Method: The complete $\beta$-decay intensities of ${}^{105}\mathrm{Mo}$, ${}^{105}\mathrm{Tc}$, ${}^{105}\mathrm{Ru}$, and ${}^{105}\mathrm{Rh}$ were measured with the Modular Total Absorption Spectrometer (MTAS) at Argonne National Laboratory. A beam of radioactive isobars with mass number A = 105 was produced by the CARIBU source and delivered to MTAS. Specifically, for ${}^{105}\mathrm{Mo}$, a multireflection time-of-flight device was employed to isolate a pure beam of ${}^{105}\mathrm{Mo}$ nuclides before delivery to MTAS. Highly-efficient silicon $\beta$-trigger has substantially aided the purification of the collected spectra.

Results: The mean $\beta$ and $\gamma$ energies of ${}^{105}\mathrm{Mo}$ and ${}^{105}\mathrm{Tc}$ are found to be consistent with previous TAS measurements. Differences are found in the extracted $\beta$-feeding intensities in commonly used evaluations of ${}^{105}\mathrm{Mo}$ and ${}^{105}\mathrm{Tc}$, which results in substantial differences in reactor antineutrinos detectable via inverse $\beta$ decay. Improved half-lives for ${}^{105}\mathrm{Ru}$ and ${}^{105}\mathrm{Mo}$ are also determined.

Conclusions: The number of reactor antineutrinos detectable via inverse $\beta$ decay for ${}^{105}\mathrm{Mo}$ is found to decrease by 86%, while a 37% increase is found for ${}^{105}\mathrm{Tc}$. An improved precision in the half-lives of ${}^{105}\mathrm{Ru}$ and ${}^{105}\mathrm{Mo}$ has been determined to be 4.4644(13) h, and 35.5(8) s, respectively.

Close

PrevNext

View figure in article

• Beta decay
• Energy levels
• Nuclear reactors
• Nuclear structure & decays
• 90 ≤ A ≤ 149