The Identification of Contaminants by Time Evolution in ⁹⁹Mo Gamma Decay Spectrum

In efforts to differentiate emitted γ-rays with similar emission energies in the analyze of the γ-decay spectra from the ⁹⁹Mo production, an automated program was created. The ⁹⁹Mo production is part of the novel approach to producing medical isotopes, using inverse kinematics, successfully implemented at the Cyclotron Institute, Texas A&M University[1-2]. The ⁹⁹Mo was produced using an accelerated ¹⁰⁰Mo beam impinging on a ⁴He gas cell-target. An ²⁷Al catcher foil, placed after the gas cell, was used to collect the ⁹⁹Mo nuclei, as well as other coproduced nuclides. After irradiation, the γ decays spectra of the foil was measured using HPGe detectors. The photopeak associated with the highest γ-ray intensity of ⁹⁹Mo, located at E­_γ­ = 140.5 keV, was not resolved, due to contributions of γ decays from multiple nuclides. The total activity at E­_γ­ = 140.5 keV can be described as a combination of ⁹⁹Mo (t­_1/2­ = 65.94 h; I­_γ­ = 89.43%), ^99mTc (t­_1/2­ = 6.01 h; I­_γ­ = 89.0%), and ⁹⁰Nb (t­_1/2­ = 14.60 h; I­_γ­ = 66.8%). The new analysis program was used to plot the activities as a function of time, for the 140.5 keV photopeak while considering the activities obtained from other photopeaks associated with ⁹⁹Mo (E­_γ­ = 181 keV) and ⁹⁰Nb (E­_γ­ = 1129 keV). Using the known decay constants of each produced nuclide, the contribution of each nuclide was determined. The activity as a function of time was fitted and contributions from ^99mTc, ⁹⁹Mo, and ⁹⁰Nb were determined. 

[1] G. A. Souliotis et al., A novel approach to medical radioisotope production using inverse kinematics: A successful production test of the theranostic radionuclide ⁶⁷Cu, Appl. Radiat. Isotopes 149, 89 (2019).

[2] J. Mabiala et al., Enhanced production of ⁹⁹Mo in inverse kinematics heavy ion reactions, HINPw6 proceedings, EPJ-woc proceedings, article 08003 (to be published).