Half-life of the $\beta$ decay ${}^{115}$In(9/2${}^{+}$)$\to$${}^{115}$Sn(3/2${}^{+}$)

The half-life of the rare β${}^{-}$ decay of ${}^{115}$In (9/2+) to ${}^{115}$Sn (3/2+) was determined by measuring the subsequent 497.334(22) keV γ-ray emission in a high-purity indium sample. The measurements were carried out by means of ultralow-level γ-ray spectrometry in the HADES underground laboratory, using three different high-purity germanium detectors. The value of the partial half-life for this low-$Q_{\beta }^{-}$ transition was measured to be 4.3(5) × 10${}^{20}$ yr.

Figure 1

Decay scheme of ${}^{115}$In updated with the rare decay branch and with inclusion of ${}^{115}$In${}^{\mathrm{m}}$.

Figure 2

The sandwich spectrometer. Dimensions in millimeters if not otherwise specified.

Figure 3

Relative difference between experimental efficiency and egs4 efficiency at different energies for detectors (a) Ge-4, (b) Ge-6, and (c) Ge-7.

Figure 4

Summed indium spectrum of the three detectors used for qualitative analysis, compared to the summed background (normalized to the measurement time of the indium disc). Bin width 1 keV.

Figure 5

The γ-ray spectra for the indium disc sample, compared to the background spectra, in the energy region of the 497-keV γ line: (a) Ge-4, (b) Ge-6, and (c) Ge-7. Bin width 1 keV.

Figure 6

The γ-ray spectrum of the indium disc compared to the background for detector Ge-7; the bremsstrahlung spectrum from the ground-state-to-ground-state β${}^{-}$ decay is clearly visible.