Revision of forensics-relevant nuclear data in ${}^{127}\mathrm{Sb} \beta$ decay

With a half-life of 3.85(5) d, the fission product ${}^{127}\mathrm{Sb}$ is potentially useful for the characterization of fallout debris that would be collected for forensic analysis following a hypothetical nuclear event. The $\beta$ decay of ${}^{127}\mathrm{Sb}$ has many characteristic $\gamma$-ray lines, however, the majority of decay strength is concentrated in the three strongest lines near 686, 473, and 784 keV, based on low-precision spectroscopic measurements from decades ago. In a new measurement of ${}^{127}\mathrm{Sb}\beta$ decay, obtained from radiochemical separations of U photofission products, we show that the energies of the 686 and 784 lines are actually shifted lower by 0.8 and 1.1 keV, respectively. Such large discrepancies in the nuclear data inhibit accurate isotopic analyses and have important implications, for example, on the assessment of fission debris that might be collected in the days following a nuclear event. Therefore, a new evaluation of ${}^{127}\mathrm{Sb}\beta$ decay and appropriate revisions to the various nuclear databases are warranted. This paper reports a new set of $\gamma$-ray energies and relative intensities corresponding to ${}^{127}\mathrm{Sb}$ decay.

Figure 1

The HPGe efficiency curve determined from a 5 mL solution of mixed isotope standards.

Figure 2

The two counts of the Sb sample are compared (second count live-time normalized to the first). The second count was recorded 20 h after the first; note the drastic reduction in photopeak intensities for the short-lived isotopes ${}^{99\text{m}}\mathrm{Tc}, {}^{115\text{m}}\mathrm{In}, {}^{112}\mathrm{Ag}$, and ${}^{132}\mathrm{I}$. The Pb x rays, 662 keV ${}^{137}\mathrm{Cs}$ line, and 1461 keV ${}^{40}\mathrm{K}$ line are artifacts of the laboratory environment.

Figure 3

The differences in centroid channels for various $\gamma$-ray lines between the two Sb sample measurements are shown as a function of energy. The red line shows the best fit to the data.

Figure 4

The energy calibration curves obtained from (a) contaminant peaks in the ${\mathrm{Sb}}^1$ spectrum and (b) from the calibration spectrum recorded just before the ${\mathrm{Sb}}^2$ count. In both cases the deviations from linearity are small over the fitted channel ranges.

Figure 5

The spectral width function was obtained by fitting peaks from ${}^{113}\mathrm{Sn}, {}^{85}\mathrm{Sr}, {}^{137}\mathrm{Cs}, {}^{88}\mathrm{Y}$, and ${}^{60}\mathrm{Co}$ in the mixed isotope standard recorded in the calibration spectrum.

Figure 6

The ratios of absolute counts in the strongest ${}^{127}\mathrm{Sb}$ decay peaks between the ${\mathrm{Sb}}^1$ and ${\mathrm{Sb}}^2$ measurements. The red solid line shows the expected ratio based on the evaluated ${}^{127}\mathrm{Sb}$ lifetime and the ${\mathrm{Sb}}^1$ and ${\mathrm{Sb}}^2$ count lengths and dates. The green solid line shows the best fit to the measured data. The dashed lines indicate $1\sigma$ standard deviations.