Recoiled and Sputtered Radioactive Impurities in 11 MeV Proton-based F-18 Production

One of the quality control measures in F-18 radionuclide production concerns the impurities which might be present in the proton-irradiated enriched H₂¹⁸O target. In this investigation, proton irradiations of enriched water targets were theoretically simulated by the Stopping and Range of Ions in Matter (SRIM) 2013 codes, followed by experimental measurements. First, the SRIM-calculated data were employed to understand the origins of the recoiled and sputtered species. Using a portable Gamma Ray Spectroscopy System, recoiled and sputtered radioactive impurities were measured in the enriched water target following F-18 production using 11 MeV proton beams. The experimental results indicate that Havar window-originated Co-56 radionuclide and silver body-originated Ag-110m radioisotope were identified in the post-irradiated enriched H₂¹⁸O target. In addition, after passing the ¹⁸F solution through a column containing Quaternary Methyl Acetate (QMA) resin, none of the radionuclidic impurities were any longer observed to any great extent in the sample.

Cyclotron; F-18 radionuclide; Gamma ray spectroscopy; Radionuclidic impurity; SRIM

atoms, depending on the proton energy. This occurrence could impact on quality control assessment, and from a broader perspective it could eventually give rise to patient safety concerns.

A previous investigation discovered that during routine production of F-18 radionuclide using 11 MeV proton beams, Co-56 radionuclide was generated via ⁵⁶Fe(p,n)⁵⁶Co nuclear reaction as the proton beam passed through the Havar window (Kambali et al., 2016). For higher proton energy (16.5 MeV), Bowden et al. (2009) observed other radionuclides such as V-48, Cr-51, Mn-52, Mn-54, Co-57, Co-58, Tc-95m, Tc-96, Cd-109, Re-183 and Re-184 in the Havar foil. Using Nb foil as the window separating the cyclotron vacuum chamber and enriched water target, Köhler et al. (2013) identified further radionuclides, including Zr-89, Nb-92m, Mo-93m, Tc-95, and Tc-96, when they irradiated the enriched water with 18 MeV protons.

Radionuclides generated during proton bombardment of Havar or Nb windows have previously been found to contaminate the enriched water target (Bowden et al., 2009; Hjelstuen et al., 2011; Köhler et al., 2013;). This work aims to comprehensively identify the recoiled and sputtered radionuclidic impurities during F-18 production using a relatively low (11 MeV) proton accelerating cyclotron. For the first time, the properties of the Havar window- and silver body-originated recoils are discussed in terms of their respective absorbed and kinetic energy after they experience collisions with the incoming proton beams, as well as their collisions with individual H and O atoms in the enriched water. In addition, the recoiled and sputtered yields are also highlighted based on the Stopping and Range of Ions in Matter (SRIM) calculated results.

A theoretical and experimental study of recoiled and sputtered radionuclidic impurities has been conducted. The SRIM codes have been employed to study the recoil distributions, kinetic energy and sputtering yields of individual Havar atoms, a silver body and an enriched H₂¹⁸O target. Based on the SRIM-calculated results, the Co, Cr, Fe and Ni recoil yields increase near the far side of the Havar surface (at a distance of 50 µm), which directly touches the enriched water target. These atoms have the highest probabilities of being recoiled off the Havar surface and then falling into the enriched water target or even hitting the silver body, which could result in further sputtering of the Ag atoms. The experimental data indicate that Havar window-originated ⁵⁶Co atoms and silver body-originated ^110mAg atoms are identified in the post-irradiated water target, although a significant presence in the ¹⁸F solution is no longer observed after passing it through a QMA column. The impurities remain detectable in the ¹⁸F solution after a 3-day cooling period. Future work will concentrate on the measurement of non-radioactive recoiled and sputtered atoms in the enriched water target, as well as adsorption of the impurities.

This research project is supported by The World Academy of Sciences (TWAS) under the Principal Investigator’s Research Grant Number 15-020 RG/PHYS/AS_I – FR3240287075, as well as the Indonesian National Nuclear Energy Agency (BATAN). Moreover, kind support from the Dharmais Cancer Hospital has also greatly contributed to the success of the project. Technical support from the cyclotron technicians and staff at Dharmais Cancer Hospital in Jakarta is also gratefully acknowledged.

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