Aqueous harvesting of ${}^{88}\mathrm{Zr}$ at a radioactive-ion-beam facility for cross-section measurements

Isotope harvesting is a method of collecting the long-lived radioisotopes that build up during the operation of ion-beam facilities in a way that is useful for subsequent research. As a demonstration of this method for the collection of a group IV metal at a fragmentation facility, the high-energy ${}^{88}\mathrm{Zr}$ secondary beam produced from a 140-MeV/u ${}^{92}\mathrm{Mo}$ primary beam at the National Superconducting Cyclotron Laboratory (NSCL) was stopped in a water target. The setup aimed to mimic the aqueous beam dump that will be implemented at the Facility for Rare Isotope Beams (FRIB). The collected ${}^{88}\mathrm{Zr}$ and accompanying ${}^{88}\mathrm{Y}$ decay daughter were radiochemically extracted from the solution and made into target samples suitable for neutron-capture cross-section measurements. These samples were then irradiated at two reactor facilities, and the ${}^{88}\mathrm{Zr}$ average thermal-neutron-capture cross section (${\sigma }_T$) and resonance integral ($I$) were determined to be ${\sigma }_T=(8.04\pm 0.63)\times {10}^5$ b and $I=(2.53\pm 0.28)\times {10}^6$ b. The ${\sigma }_T$ value agrees well with previous results and $I$, determined for the first time here, was found to be the largest measured resonance integral by two orders of magnitude. The ${}^{88}\mathrm{Y}$ thermal-neutron-capture cross section was determined to be less than $1.8\times {10}^4$ b. This work demonstrates the steps needed to make cross-section measurements with samples produced via aqueous isotope harvesting.

Figure 1

$\gamma$-ray spectrum from a typical bottle containing an aqueous sample recorded 7 hours after being collected at the NSCL. Decay and efficiency corrections have not been applied. In addition to ${}^{88}\mathrm{Zr}$, many other ${}^{92}\mathrm{Mo}$ fragmentation products are observed, including ${}^{90}\mathrm{Nb}, {}^{87m}\mathrm{Y}$, and ${}^{86}\mathrm{Y}$.

Figure 2

(a) photograph of the gold-vapor-deposited titanium entrance window to the cell prior to irradiation, (b) autoradiograph of the window after irradiation, (c) photograph of the backplate of the cell after irradiation, and (d) autoradiograph of the backplate of the cell after irradiation. In (b) and (d), the darker areas indicate locations with higher radioactivity. The cell backplate, which was in contact with the water of the cell, contains about 12 times the radioactivity of the entrance window.

Figure 3

Flowchart depicting the steps taken to isolate ${}^{88}\mathrm{Zr}$ and ${}^{88}\mathrm{Y}$ from the harvested samples for target production.

Figure 4

$\gamma$-ray spectra of (a) ${}^{88}\mathrm{Zr}$ target before irradiation, (b) ${}^{88}\mathrm{Zr}$ target after 10-hour irradiation at MURR and 3.6-day decay, (c) ${}^{88}\mathrm{Y}$ target before irradiation, and (d) ${}^{88}\mathrm{Y}$ target after 10-hour irradiation at MURR and 7.4-day decay. Decay and efficiency corrections have not been applied. The initial ${}^{88}\mathrm{Zr}$ activity was accompanied by ${}^{88}\mathrm{Y}$, which grew in from the decay following chemical separation. After irradiation, the ${}^{88}\mathrm{Zr}$ activity was greatly reduced and a large quantity of ${}^{89}\mathrm{Zr}$ was present. Conversely, no ${}^{88}\mathrm{Y}$ burnup was observed in the ${}^{88}\mathrm{Y}$ targets after irradiation. The initial ${}^{88}\mathrm{Y}$ activity was accompanied by ${}^{85}\mathrm{Sr}$ and a small amount of ${}^{83}\mathrm{Rb}$. After irradiation, ${}^{140}\mathrm{La}$, the radiative capture product on stable ${}^{139}\mathrm{La}$, was also observed.

Figure 5

Neutron energy spectra of MNRC reactor in NTD void location calculated using MCNP5. The main figure displays the full energy range on a logarithmic scale with lethargy-normalized flux. The inset features a more detailed model of the epithermal region on a linear energy scale with the non-normalized flux. More points were used to model this region to allow for a fit (gray solid line) to determine the epithermal-neutron shape factor. Both calculations include the 1-cm-thick Al sample holder.

Figure 6

Typical $\gamma$-ray spectra of a ${}^{88}\mathrm{Zr}$ target (a) before irradiation, (b) after irradiation at MNRC, and (c) after irradiation at MNRC in a Cd-lined container. Decay and efficiency corrections have not been applied. The initial target contained only ${}^{88}\mathrm{Zr}$ and its daughter ${}^{88}\mathrm{Y}$, and after irradiation a decrease in the ${}^{88}\mathrm{Zr}$ and an increase in the ${}^{89}\mathrm{Zr}$ were observed. When the thermal-neutron flux was suppressed with the Cd liner, the ${}^{88}\mathrm{Zr}$ burnup and ${}^{89}\mathrm{Zr}$ production were both reduced.