Branching ratio to the 803 keV level in ${}^{210}\mathrm{Po}\alpha$ decay

Precise knowledge of the branching ratio in the α decay of ${}^{210}\mathrm{Po}$ is important for accurate measurement of the ${}^{209}\mathrm{Bi}(n,\gamma ){}^{210\mathrm{g}}\mathrm{Bi}$ cross section, the reaction involved in the termination of the astrophysical $s$ process. The branching ratio was determined from independent measurements of α and γ spectra of bismuth samples simultaneously irradiated by neutrons near the core of the Soreq research reactor (IRR1). The branching ratio was found to be $\left(1.15\pm 0.09\right)\times {10}^{-5}$, consistent with the results of several measurements performed six decades ago. As a by-product value the ${}^{209}\mathrm{Bi}(n,\gamma ){}^{210\mathrm{g}}\mathrm{Bi}$ thermal cross section was measured to be $21.6\pm 1.1$ mb.

Figure 1

The processes involving neutron capture of ${}^{209}\mathrm{Bi}$. The branching ratio of ${}^{210}\mathrm{Po}\alpha$ decay to the 803 keV state is taken from Ref. [16].

Figure 2

γ spectrum from measurement of irradiated 1-mm-thick bismuth target in a low background shielded HPGe. Background spectrum normalized to the same live time as the bismuth spectrum is also shown. A zoom-in on the 803 keV peak is shown in the inset.

Figure 3

Example of spectrum obtained from the irradiated thin bismuth sample (red line) taken 40 days after the irradiation. The spectrum of the calibrated triple α source is also shown (black line). The vertical dashed line represents the threshold used to distinguish between events originating from α and β particles.

Figure 4

${}^{210}\mathrm{Po}$ activity from α measurements of the irradiated bismuth foil as a function of time since irradiation. The solid blue line is a fit using Eq. (4).

Figure 5

The result of the present measurement is compared with the literature [6, 7, 8, 9, 10, 11, 12, 13, 14, 15] and the most recent nuclear data evaluation [16, 25, 26]. The solid squares ([6, 7, 8, 9, 10, 11, 12, 13], and the present work) are the experimental results of the direct measurements of the 803 keV γ ray. The solid triangles are the experiments based on direct measurement of the weak ${\alpha }_2$ branch [14, 15]. The open squares [16, 25] and the open triangle [26] are the nuclear data evaluations of the γ-ray and α-ray experiments, respectively. See Table 2 and text for details.