Measurements of the ${}^{210g}\mathrm{Bi}$ to ${}^{210m}\mathrm{Bi}$ activation ratio for the ${}^{209}\mathrm{Bi}$($n$,γ) reaction with thermal and epithermal neutrons at the Soreq IRR1 reactor

Irradiations of bismuth samples with thermal and epithermal neutrons have been performed for a comparison of activation to the ${}^{210g}\mathrm{Bi}$ ground state and to the ${}^{210m}\mathrm{Bi}$ metastable state. The bismuth irradiations were conducted at the Soreq IRR1 research reactor at a near-core location with integrated neutron flux of approximately $2\times {10}^{18}\mathrm{neutrons}/\mathrm{c}{\mathrm{m}}^2$ and a gold cadmium ratio of 3.1. Two nearly identical bismuth samples were irradiated, one without and one with a cadmium shield, to enable a comparison between thermal and epithermal neutron capture cross sections. Subsequent gamma spectrometry indicates nearly equal levels of activation to the ground and to the metastable states of ${}^{210}\mathrm{Bi}$, both with thermal and epithermal neutrons. Absolute thermal cross sections and resonant integrals were deduced by including previously reported bismuth activation measurements performed conjointly with gold foil activation for normalization. A comparison of the present results with existing data is presented. It is argued that the bismuth resonances lie in the neutron energy range relevant for stellar temperatures, and therefore the neutron capture cross section ratio $\left({\sigma }_g/{\sigma }_m\right)$ measured for the epithermal neutrons is relevant also for stellar ${}^{210}\mathrm{Bi}$ production. This is of importance since bismuth lies at the end of the chain for the s‐process reaction for nucleosynthesis in stars. Precise knowledge of bismuth activation to the ${}^{210m}\mathrm{Bi}$ metastable state is also vital for the planning of GEN-IV reactors given the 3-million-yr half-life of ${}^{210m}\mathrm{Bi}$.

Figure 1

Decay scheme following neutron capture on ${}^{209}\mathrm{Bi}$. Energy level data from [10, 11].

Figure 2

Gamma spectra for irradiated bismuth samples without cadmium shield (blue, above) and with cadmium shield (red, below), acquired 2.23 yr after irradiation, showing the 803 line from ${}^{210g}\mathrm{Bi}\stackrel{\beta }{\to } {}^{210}\mathrm{Po}\stackrel{\alpha }{\to } {}^{206*}\mathrm{Pb}$ decay, and 265.6 keV and 304.6 keV lines from ${}^{210m}\mathrm{Bi} \stackrel{\alpha }{\to } {}^{206*}\mathrm{Tl}$ decay.

Figure 3

Measured 803 keV gamma line with fit, (a) bismuth sample with no cadmium shield, (b) bismuth sample with cadmium shield.

Figure 4

Measured 265.6 keV gamma line with fit, (a) bismuth sample with no cadmium shield, (b) bismuth sample with cadmium shield.

Figure 5

Measured 304.6 keV gamma line with fit, (a) bismuth sample with no cadmium shield, (b) bismuth sample with cadmium shield.

Figure 6

Ratio of $({\sigma }_{\mathrm{g}}/{\sigma }_{\mathrm{m}})$ for the reaction ${}^{209}\mathrm{Bi}$(n,γ)${}^{210}\mathrm{Bi}$ as determined by: Evaluation studies [9, 43, 44, 45, 46, 47], Activation measurements (this work and Borella et al. [1]), Ratios at individual resonances (Borella [3]) TOF measurements (Saito [40]) and MACS(Shor et al. [8]). Results of this work consistent with $({\sigma }_{\mathrm{g}}/{\sigma }_{\mathrm{m}})\approx 1$ at thermal and bismuth resonance energies.

Figure 7

Overlap of ${}^{210}\mathrm{Bi}$ resonances with Maxwell-Boltzmann(MB) distribution at $kT=30\mathrm{keV}$ and reactor epi-thermal $1/{\mathrm{E}}_{\mathrm{n}}$ distribution (MB and $1/{\mathrm{E}}_{\mathrm{n}}$ at arbitrary scales).

Figure 8

Measurements of bismuth MACS at 30 keV for partial cross section to the ${}^{210}\mathrm{Bi}$ ground state and total cross section. (* Total cross section for this work is based on previous measurement for the ground state [8], and current measurement of ${\left({\sigma }_{\mathrm{g}}/{\sigma }_{\mathrm{m}}\right)}_{\mathrm{epi}}=0.95\pm 0.12$, and assuming same ratio for MACS.)