Production of Mg-28 for medical applications by spallation

Magnesium-28 is the only radioactive isotope of magnesium with a half-life long enough to be useful as a tracer. It is utilized in studies of cell growth and reproduction, tumorigenicity, immunological surveillance, hormone regulation, and nutrition. It has been produced at BNL in the past at the graphite reactor by the two-step reaction Li-6 (n,t)α; Mg-26(t,p)Mg-28 and later directly with Mg-26 irradiated by 3.4 MeV tritons from a Van de Graaff accelerator. However, much higher specific activity is attainable at the BLIP facility with proton spallation reactions. KCl has been chosen as the target material because it is inexpensive, easily dissolved, and spallation on both K and Cl can give Mg-28. The KCl target is a 25 g pressed pellet 0.8 cm thick encapasulated in a stainless steel container and is typically irradiated over a weekend at 191 MeV. After irradiation, the pellet is dissolved in water containing Mg and Be carriers. This Be is precipitated by the addition of a pH 10 buffer solution. Then, other metal impurity ions are extracted from the filtrate by adding sodium tartrate and 8-hydroxyquinoline in chloroform and discarding the organic phase. Magnesium-28 is extracted from the KCl solution by mixing with portions of 0.1% oxine in chloroform and n-butylamine. Finally, the organic phase is destroyed with nitric acid and peroxide, and the product is dissolved in pH 4 saline. The overall chemical yield is 93%. The typical saturation activity at EOB is 4.9 mCi with a thick target yield of 5.1 μCi/μAh. The specific activity at EOB is 98 μCi/μg compared to 0.4 μCi/μg from the (t,p) reaction. The radiopurity of the spallation product is excellent, with no impurity gamma emitters detected at the limit of sensitivity (<0.1%)