Assessment of Production Reactions to Achieve High Radiological Purity Tb-155

Introduction: Terbium-155 (¹⁵⁵Tb) is a novel Auger electron emitter with promising application in cancer therapy. It is also useful for simultaneous SPECT imaging^[1]. In addition, a companion isotope, ¹⁵²Tb, is a useful PET imaging^[2] radionuclide, thus making a potentially important theranostic pair. 

A current issue is that a potentially high yield production of ¹⁵⁵Tb using the reaction ¹⁵⁶Gd(p,2n)¹⁵⁵Tb^[3] can have impurities from ¹⁵⁴Tb and ¹⁵⁶Tb via the (p,3n) and (p,n) reactions, as well as, production on the ¹⁵⁵Gd and ¹⁵⁷Gd components of the enriched ¹⁵⁶Gd target. Hence, alternative high yield reactions induced by ³He and ⁶Li beams are being studied at the Argonne superconducting LINAC, ATLAS (Argonne Tandem Linear Accelerator System). The advantage of these reactions is that they both are predicted to have large cross sections to the isotope ¹⁵⁵Dy which is located between stable (¹⁵⁶Dy) and very long-lived ¹⁵⁴Dy, as shown qualitatively in Figure 1. The PACE4^[4]simulations of the impurity of ¹⁵⁶Tb via the direct production is very small for both ³He and ⁶Li beams as shown in Table I.  Hence, radiologically very pure ¹⁵⁵Tb may be produced as the daughter of ¹⁵⁵Dy. A recent publication^[5] by a group at the Kurchatov Institute demonstrated the feasibility of this concept.

Methods: Various reactions for producing ¹⁵⁵Tb are being simulated with available nuclear model codes EMPIRE^[6], PACE4^[4], and TALYS/TENDL^[7]. It is expected that the reactions ¹⁵⁶Gd(³He,4n)¹⁵⁵Dy  ¹⁵⁵Tb and ¹⁵³Eu(⁶Li,4n)¹⁵⁵Dy  ¹⁵⁵Tb will yield high radiological purity while still having adequate production rates. The novelty of this production method lies in ¹⁵⁵Dy's unique position between two essentially stable isotopes (¹⁵⁴Dy and ¹⁵⁶Dy) so that the impurities ¹⁵⁴Tb and ¹⁵⁶Tb are predicted to be minimal. Additionally, ¹⁵⁵Dy decays via electron capture exclusively with the relatively short half-life of 9.9 hours, to the desired product ¹⁵⁵Tb with half-life 5.3 days. Preliminary gamma decay spectra from the ³He and ⁶Li reactions have been measured and indicate the validity of these possible reactions. The model-dependent excitation functions for these possible production reactions will be presented. The corresponding target thicknesses that yield the best overall yields and ¹⁵⁵Tb purity will be predicted based on these simulations. Plans are in progress to measure the yields, excitation functions, and radiological purity of ¹⁵⁵Tb produced at ATLAS using the ³He and ⁶Li induced reactions. Thermal simulations of the beam heating of targets of the optimized thicknesses for the various beams will also be assessed. Target cooling concepts that could be used with high power beams for production of ¹⁵⁵Tb are being developed. 

Results: Initial gamma spectra from ³He and ⁶Li beams on targets of ¹⁵⁶Gd and ¹⁵³Eu, respectively, have demonstrated the production of ¹⁵⁵Tb.  By using the reaction ¹⁵⁶Gd(³He,4n)¹⁵⁵Dy  ¹⁵⁵Tb or the corresponding ⁶Li induced reaction of ¹⁵³Eu(⁶Li,4n)¹⁵⁵Dy  ¹⁵⁵Tb relatively high radiological purity of ¹⁵⁵Tb is expected. And, as shown in [5] even higher purity is feasible via fast chemical separation of the ¹⁵⁵Dy (half-life 10 hours) from the production targets.

Conclusions: This work suggests that the production of research quantities of pure ¹⁵⁵Tb using ATLAS is feasible. Hence, high purity ¹⁵⁵Tb in quantities required for pre-clinical applications will support the studies required to demonstrate the therapeutic effectiveness of this promising new theranostic Auger-electron emitter. Bringing this work out of the simulation environment and into the laboratory experiments will provide the ever-growing world of Auger electron therapy with an easily producible, purity reliable, research isotope. 

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