Efficient Method for Molybdenum-99 Production

Introduction
⁹⁹Mo radioisotope is a precursor of ⁹⁹mTc which is widely used for single photon emission computed tomography (SPECT). Recently, in connection with nonproliferation and ecology issues, non-reactor based methods of ⁹⁹Mo production are attracting considerable attention.
In this paper we considered photoneutron method for ⁹⁹Mo radioisotope production with linear electron accelerator. The proposed approach uses molybdenum target that acts both as a bremsstrahlung converter for incident electron beam and simultaneously as a ⁹⁹Mo producing target via the 100Mo(γ,n)⁹⁹Mo reaction on bremsstrahlung photons [1].

Methods
Using PHITS particle transport code version 2.52 [2] we calculated and compared specific yields of 99Mo isotope for the setup with a single molybdenum target-converter and conventional setup with tungsten bremsstrahlung converter and molybdenum target. The calculations covered the 20-50 MeV energy range of incident electrons with the beam radius of 1.5 mm. For both cases we used the same 100Mo cylindrical target with the diameter of 7.7 mm and length of 20 mm. Tungsten converter was also a cylinder with the diameter of 12 mm. We considered two converter configurations: fixed thickness of 7 mm (2 radiation lengths) for all beam energies and thickness that provides maximum bremsstrahlung efficiency for the given beam energy.

Results
Calculations of the specific yields for setups with and without tungsten converter showed that latter provides higher values of ⁹⁹Mo production for entire energy range. Specific yield of ⁹⁹Mo isotope in this case is 1.8-2.4 times higher than from setup with converter thickness of 2 radiation lengths, and about 1.2-1.3 times higher than from converter with maximum bremsstrahlung efficiency.
Using Monte Carlo simulations we obtained not just a gross yield of ⁹⁹Mo, but also spatial distribution of the produced isotope in the target volume. We obtained location and size of the core of molybdenum production within the target and location of the ‘dead’ zone if the front part of the target where no production occur. This provides the way of the further optimization of the proposed method for ⁹⁹Mo production using combined target-converter. Namely, optimal target geometry without the ‘dead zones’ could significantly improve the specific yield of molybdenum under the same beam conditions.

Conclusions
Using the Monte Carlo simulations we proved that setup with combined molybdenum target-converter has obvious advantages over the conventional ⁹⁹Mo production scheme using linear electron accelerators. Such setup provides better utilization of the bremsstrahlung photons that results in higher specific yield of molybdenum isotope. Even higher rates of molybdenum yield could be achieved by optimizing the target geometry.

References
1. A. Tsechanski, A.F. Bielajew et al. Electron accelerator-based production of molybdenum-99: Bremsstrahlung and photoneutron generation from molybdenum vs. tungsten, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, Vol. 366, 2016, pp. 124-139
2. T. Sato, Y. Iwamoto et al. Features of Particle and Heavy Ion Transport code System (PHITS) version 3.02, J. Nucl. Sci. Technol., 2018