Evaluation of radionuclidic profiles of molybdenum-99 from accelerator transmutation of natural and Mo-100 enriched molybdenum

Objectives Evaluate the profiles of two sources of accelerator produced molybdenum-99 by γ-ray spectroscopy with the use of a High Purity Germanium detector (HPGe).

Methods Enriched Mo (>99% Mo-100) was irradiated in a stack of 10 5-mm sintered disks of 0.5g each, with a total yield of 5.1GBq. Natural Mo metal (10% Mo-100) was irradiated in a stack of 20 12.5-mm disks of about 5g each, with a total yield of 2.84GBq. One disk from each source was used for γ-ray spectroscopy, the others were dissolved and used with our solvent extraction generator technology to produce Tc99m, with samples taken for this study. γ-ray spectroscopy was performed regularly at 15cm, 45cm and 75cm from the detector for a period up to 40 days. HPGe spectra for the dissolution and extraction samples were similarly obtained.

Results The high initial activity of the enriched Mo saturates the HPGe detector at 15cm. Useable spectra was possible only after 8-day decay for 15cm or for the initial sample at 75cm. The spectra show distinct peaks of attributable only to Mo99 (Fig 1A) (peaks at 140, 180, 366, 739, 777, 822 and 960 keV). However, at day 40 (Fig 1B) significant decay and minimization of Mo99 allows visualization of impurities, with peaks of Zr95, Nb95 and Sc46. For natural molybdenum, impurity peaks of Nb96 show up initially but decay rapidly due to shorter half-life (23.35h). At 15 days, Nb96 peaks disappear and peaks for Nb91m, Nb92m, Nb95m and Nb95 are clearly prominent (t1/2= 60.86d, 10.15d, 86.6h, 35.15d, respectively).

Conclusions HPGe γ-ray spectroscopy demonstrates the advantage of using Mo-100 enriched raw material to obtain Mo99 product of high radionuclic purity. Quantitation of impurities from the two sources for compliance to compendial limits, and an efficient extraction process that excludes radionuclidic impurities in the final Tc-99m product are therefore important parameters.

Research Support Non-Reactor Isotope Support Program, Natural Resources Canada and the Winnipeg Health Sciences Centre Foundatio