Decay of ${}^{58}\mathrm{Co}^m$

We have studied some of the nuclear decay properties of ${}^{58}\mathrm{Co}^m$. The half-life is found to be ($8.853\pm 0.023$) hours, which is significantly different from the evaluations found in the literature. The absolute intensity of the 24.9-keV $\gamma$ ray emitted in the internal transition decay is ($0.0387\pm 0.0016$)%, in agreement with the calculated yield of a transition of $M3$ multipolarity. We observe $\gamma$-ray photons from a previously unreported branch for electron capture decay, with an intensity of ($0.00120\pm 0.00005$)%, consistent with expectations from the calculation of log $ft$.

Figure 1

Sections of two $\gamma$-ray spectra of 3-h duration, from sample CO-3, taken with a LEPS detector. The midpoint times of the acquisition intervals were 0.15 and 3.82 d after irradiation. The later count is scaled by a factor of 0.1. The main features of the spectra are the 24.9-keV photon associated with ${}^{58}\mathrm{Co}^m$ decay and the Pt $K$ $x$ rays from fluorescence of the sample substrate, induced mainly by radiation from the decay of ${}^{58}\mathrm{Co}^g$.

Figure 2

A section of the first 70-minute $\gamma$-ray spectrum of sample CO-4, taken with a COAX HPGe detector. The midpoint of the acquisition interval was 0.11 day after irradiation. The main features of the spectrum are the photon lines associated with the decays of ${}^{57}\mathrm{Co}$ and ${}^{58}\mathrm{Co}^g$, and the annihilation peak at 511 keV. The peak at 1322 keV is due to summing of annihilation radiation with the 810.8-keV photon from ${}^{58}\mathrm{Co}^g$ decay. The 1265.7-keV peak, decaying with a 9-hr half-life, is discussed in the text.

Figure 3

The decay curve constructed from the 24.9-keV photopeak data taken with the LEPS detector (Fig. 1). Vertical one-$\sigma$ error bars on the data points are smaller than the size of the markers unless explicitly shown. Data are consistent with a ${}^{58}\mathrm{Co}^m$ half-life of ($8.847\pm 0.023$) hours. The weighted least-squares fit to the data is shown as a solid line.

Figure 4

The decay curve constructed from the 810.8-keV photopeak data taken with the COAX HPGe detector (Fig. 2). Vertical one-$\sigma$ error bars on the data points are smaller than the markers. Holding the half-life of ${}^{58}\mathrm{Co}^g$ fixed at 70.86 days, a least-squares fit of a growth-and-decay function to the data (solid line) is consistent with a ${}^{58}\mathrm{Co}^m$ half-life of ($8.942\pm 0.070$) h.

Figure 5

The decay curve constructed from the 1265.7-keV photopeak data taken with the COAX HPGe detector (Fig. 2). Vertical error bars are one $\sigma$. Data are consistent with a half-life of ($8.73\pm 0.55$) h, supporting an assignment to the decay of ${}^{58}\mathrm{Co}^m$. The weighted least-squares fit to the data is shown as a solid line.

Figure 6

Mass-chain decay scheme information for $A=58$. Only the ground state of ${}^{58}\mathrm{Ni}$ is energetically accessible to potential ${\beta }^{-}$ decay from ${}^{58}\mathrm{Co}^m$, but five states of ${}^{58}\mathrm{Fe}$ are accessible to potential EC/${\beta }^{+}$ decays. The observed $\gamma$ transition is placed in the ${}^{58}\mathrm{Fe}$ level scheme. The $\gamma$ transition depopulating the 811-keV state, which also arises in the decay of ${}^{58}\mathrm{Co}^g$, is not shown.