Measured bremsstrahlung photonuclear production of 99Mo (99mTc) with 34 MeV to 1.7 GeV electrons
Introduction
The world's supply of technetium-99 m (99mTc) is at risk, and alternate methods for production must be found to support the demands of diagnostic nuclear medicine. 99mTc is the most widely used medical radioisotope used today, accounting for approximately 80% of all nuclear imaging procedures (Sabel'nikov et al., 2006). 99mTc (6 h) is most commonly supplied as the daughter product from beta decay of 99Mo (66 h), delivered in commercial generator systems. Over 90% of the world's supply of 99Mo is produced at five nuclear reactors, each of which is over 45 years old. The short half life of the parent 99Mo (66 h) requires continuous resupply to meet the medical industry's needs. In 2007 the unanticipated closure of a single reactor facility in Canada decreased isotope stocks in North American hospitals by 80%, causing the cancellation of 50,000 medical procedures in a matter of weeks (Ruth, 2009). This was followed in 2008 by the unexpected failure of another major supplier reactor in the Netherlands. Smaller facilities in Belgium and France are scheduled to cease operation in the next few years, and the National Research Universal (NRU) reactor in Chalk River, Ontario, is scheduled for shutdown in 2016 (Lougheed, 2012; Bénard, 2014). In recent years a great deal of international effort has therefore been made to evaluate alternate methods for 99Mo and 99mTc production, summarized by the International Atomic Energy Agency (IAEA report, 2010).
Numerous approaches have been explored for alternate means of 99mTc production, either directly or as the daughter decay product of 99Mo (Bénard, 2014; TRIUMF report, 2008; Wolterbeek et al., 2014; Cutler and Schwarz, 2014; IAEA report 2013; Nagai 2014, Pillai et al., 2013; Guérin et al., 2010). Some promising methods are being adopted by commercial companies, including the use of low enrichment uranium (LEU), e.g. at SHINE Medical Technologies (SHINE, 2014), and photonuclear activation of molybdenum, e.g. at NorthStar Medical Radioisotopes (NorthStar, 2014). Any evaluation of the merits of a production strategy must include both evaluation of the available accelerator technology, and the subsequent techniques for efficient and high purity 99mTc extraction and conversion to labeled compounds of medical utility. Photonuclear production has been demonstrated as a viable option, with source photons from Bremsstrahlung (Avagyan et al., 2014, Bennett et al., 1999, Dzhilavyan et al., 2011, Dikiy et al., 2004a, Dovbnya et al., 2011; Sabel'nikov et al., 2006; Starovoitova et al., 2014) or Compton sources (Habs and Köster, 2011). This method would provide a source of 99mTc (half life 6 h, the primary decay product from 99Mo) without the security concerns surrounding high enrichment uranium (HEU) targets, or the need for dedicated nuclear reactor facilities. However the method does suffer in that bulk amounts of molybdenum are required, and efficient separation of the 99mTc can be problematic (Dash et al., 2013). Published yield data is scarce, and limited to low energies (below about 50 MeV). Potential for new developments in high energy electron accelerators, including laser plasma accelerators (Leemans et al., 2006), raise the need for the experimental determination of yields at high energy.
The primary goal of this work is to investigate the feasibility and yield for producing 99Mo using high energy electron beams using the laser plasma accelerator facility at the Lawrence Berkeley National Lab. While the use of the photonuclear reaction on molybdenum targets has been demonstrated at low energy, both with natural Mo and enriched 100Mo, there is very little data on reaction yields at high energy (>50 MeV). Our experiment used gamma rays produced from stopping high energy electrons in a natural enrichment molybdenum metal target. The yield for 99Mo produced was measured as a function of initial electron energy from 34 MeV to 1.7 GeV. In addition to the overall yield results, the depth distribution of the product radioisotopes was measured. This information can provide the basis for determining what energy and current of accelerators are needed to provide sufficient 99Mo to serve the need for 99mTc, and what target material and geometry is required. The purpose of this experiment was not to define an idealized system for 99Mo/99mTc production, but rather to provide experimental data which can be used to evaluate the photonuclear approach to production of this vital nuclear medicine radioisotope. The results obtained, over a broad energy range, provide an experimental basis for optimizing the activation energy, with calculations within the measured energy range presented. The work presented here outlines the methodology used, tests with activation of natural copper, and the molybdenum activation vs. energy.
Section snippets
Methods
High-energy electrons colliding with target material produce a broad spectrum of bremsstrahlung gamma rays, which then produce 99Mo primarily by the 100Mo(γ,n)99Mo reaction over the giant dipole resonance energies from approximately 10–25 MeV. Other contributions to the 99Mo yield from electron irradiation include 100Mo(γ,p)99g,mNb–99Mo, as well as secondary beam reactions in thick targets such as 98Mo(n,γ)99Mo (if 98Mo present in the target). In this experiment the yield for 99Mo is determined
65Cu(γ,n)64Cu
In order to validate the methodology for determining the yield using the charge weighted mean energy impacting each target after the magnetic spectrometer, the yield of 64Cu from irradiation of natural isotopic copper was measured. Natural copper blocks in a 1″ cube (two 25.4 mm square, 12.7 mm thick blocks back to back) were irradiated to test both the absolute calibration of the charge measurement system against previous measurements and the energy response of the yield against Monte Carlo
Discussion
Several pertinent conclusions can be drawn from the presented results. The copper activation confirms the methodological approach of using the magnetic spectrometer as an energy selector for the activation. The 64Cu yields using the beam spread over the face of the target are comparable to our previous measurements using direct beam irradiation (Nakamura et al., 2011), albeit with a very different energy distribution. Using the same technique for the molybdenum irradiation gives yields
Acknowledgments
The corresponding author was supported through grant funding from Minnesota State University, Mankato. The authors gratefully acknowledge the assistance of Cs. Toth, D.E. Mittelberger, R. Donahue, and A. Smith of LBNL. This work was performed at Lawrence Berkeley National Laboratory under Contract no. DE-AC02-05CH11231 with the U.S. Department of Energy.
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