Theoretical analysis of the effect of target-thickness fluctuations on reaction-rate variability for proton-induced nuclear reactions on enriched Mo targets

Cyclotron production of 99mTc through the 100Mo(p,2n)99mTc reaction1 is being actively investigated as an alternative to reactor-based approaches. A challenge facing cyclotron pro-duction of clinical-quality 99mTc is that proton bombardment of Mo targets results in production of a number of additional Tc and non-Tc isotopes through various reaction channels.2,3 While non-Tc products can be chemically re-moved, other Tc radioisotopes cannot and will therefore degrade radionuclidic purity and contribute to patient radiation dose.5 The radionuclidic purity of cyclotron-produced 99mTc depends on the nuclear cross section governing each reaction channel, the proton current and energy distribution, duration of bombardment, target thickness and isotopic composition. Although conditions that minimize dose from radioactive Tc impurities have been identified,5 cyclotron performance and thus irradiation conditions may randomly fluctuate between and/or during production runs. Fluctuations of certain parameters, for example the total number of bombarding protons, are expected to have little influence on radionuclidic purity, whereas fluctuations in beam energy, target thickness and isotopic composition may dramatically affect the relative amounts of 93gTc, 94gTc, 95gTc, and 96gTc impurities. It is critical to quantify relationships between potential fluctuations and the reproducibility and consistency of the radionuclidic purity of cyclotron-produced 99mTc to guide development and optimization of target preparation, irradiation, and processing techniques. The purpose of this work is to present a mathematical formalism for quantifying the relation-ship between random fluctuations in Mo target thickness and variability of proton-induced nuclear reaction rates for enriched Mo targets. In this study, we use 96gTc as an example of impurity which can potentially contribute to increased patient dose for patients injected with cyclotron-produced 99mTc.4 Herein, we apply the developed formalism to both the 96Mo(p,n)96gTc and the 100Mo(p,2n)99mTc reaction channels, however, the same approach can be applied to any reaction channel of interest