About the Cover

Vol. 97 No. 6 (2021)

   Radioisotopes (RIs) in the form of radiopharmaceuticals are being used in diagnostic imaging and radiotherapy. ^99mTc with a half-life (T_1/2 = 6 h) obtained from ⁹⁹Mo (T_1/2 = 66 h) is the most widely used radioisotope in diagnosis worldwide, about 0.9 million diagnoses every year in Japan; ¹³¹I (T_1/2 = 8.02 d) has been used to treat thyroid cancer. Japan imports all of its required ⁹⁹Mo several times per week, and also ¹³¹I. A constant supply of ⁹⁹Mo is a key issue for ensuring the routine application of ^99mTc. Most of ⁹⁹Mo is produced in several aging research reactors around the world, but the supply chain of ⁹⁹Mo is vulnerable, which often causes a shortage of ⁹⁹Mo worldwide. Hence, the development of alternative schemes, although they have challenges of lower reaction rates and lower specific activity of ⁹⁹Mo, is indispensable for the reliable production of ⁹⁹Mo.
   Yasuki Nagai (see the article in this issue pp. 292-323) has proposed a new production scheme for ⁹⁹Mo using accelerator neutrons; neutrons that have a high neutron flux of 2.5×10¹⁴ n/s with a most probable energy of 14 MeV are generated by bombarding a carbon target with 40-50 MeV, 2 mA deuterons. ⁹⁹Mo is produced by irradiating a ¹⁰⁰Mo sample with the neutrons. A longer traveling range of neutrons (no charge) in matter allows the use of a thick sample, and the neutron energy is suitable for producing carrier-free RIs, including therapeutic ones, such as ⁶⁴Cu (T_1/2 = 12.7 h) and ⁶⁷Cu (T_1/2 = 62 h), with a minimum level of impurity RIs. The cover figure (left) shows a schematic of producing accelerator neutrons and RIs. Most accelerator neutrons are emitted in the forward-direction with respect to the deuteron beam direction; this allows the use of an enriched ¹⁰⁰Mo sample with a 4-5 cm thick small surface area for producing ⁹⁹Mo by fully utilizing the generated neutrons (right figure). Nagai and his colleagues have shown that they can solve the problem of a lower reaction rate by using such accelerator neutrons. Further, they challenged the problem of a lower specific activity by employing a thermos-chromatography separation method, and solved it by measuring the diffusion coefficient of ^99mTc in a molten MoO₃ sample and a separation efficiency of ^99mTc from ⁹⁹Mo using a home-made electric furnace. The furnace played an essential role to solve the problem. These fundamental studies have led them to a proposal, named GRAND, for a prototype facility that consists of a cyclotron with a 50 MeV, 2 mA deuteron beam intensity to secure a constant supply of ⁹⁹Mo for domestic use and to promote the use of therapeutic RIs.

Toshimitsu Yamazaki
Member of the Japan Academy