III-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Production cross sections of 225 Ac in the 232 Th(14 N, xnyp) reactions at116 and 132 MeV/nucleon
X. Yin,∗1 A. Nambu,∗1 Y. Komori,∗1 D. Mori,∗1 S. Oshikiri,∗1,∗2 H. Kato,∗1,∗2 A. Hino,∗2 and H. Haba∗1
Ac (T1/2 = 10.0 d) is one of the most promisingalpha-particle-emitting radionuclides for targeted radionuclide therapy.1) However, the current global availability of 225 Ac is too small to support large clinical trials, and a stable supply system for 225 Ac has
not yet been established in Japan even at the basic research scale of 100 MBq. A spallation reactionof 232 Th with high-energy protons is expected to bea potential production route for 225 Ac.2) At RIKEN,radionuclides of a large number of elements, calledmultitracer, have been produced by the spallation ofmetallic targets such as natTi, natAg, and 197 Au irradiated with a 135 MeV/nucleon 14 N beam from theRIKEN Ring Cyclotron (RRC).3) In this work, we investigated the feasibility of 225 Ac production via the232
Th(14 N, xnyp)225 Ac reaction for the future domestic supply of 225 Ac. We also investigated the production of 225 Ra (T1/2 = 14.9 d) because it is useful as an225
Ac/225 Ra generator to produce high-radionuclidicpurity 225 Ac.2)
A 14 N7+ beam was extracted from the RRC. Three
metallic 232 Th foils (69 mg/cm2 ), two 27 Al plates(415 mg/cm2 ), and another three 232 Th foils wereplaced in this order from the upstream side of the beamin the multitracer production chamber.3) The targets
were irradiated for 1 h with a 20-pnA-intensity beam.
225Fig. 1. Radioactive decay curve of the 440.5-keV γ-line of
213
Bi on the decay chain of 225 Ra → 225 Ac → 221 Fr→ 227 At → 213 Bi → · · · . The solid curve indicatesthe fitting result using the two-body successive decayequation (225 Ra → 225 Ac → · · · ).
∗1
∗2

RIKEN Nishina Center
RI Research Department, FUJIFILM Toyama ChemicalCo., Ltd.

Fig. 2. Cross sections of the 232 Th(14 N, xnyp)225 Ac, 225 Ra
reactions in comparison with the phits calculations.

After the irradiation, the second foil of each set of three232
Th foils was subjected to γ-ray spectrometry withGe detectors to determine the production cross sections of 225 Ac and 225 Ra. The 27 Al plates were used
as beam-energy degraders. The beam energies on themeasured 232 Th targets were calculated to be 132 and116 MeV/nucleon using the stopping power model4) inthe lise++ program.5)
The radioactivities of 225 Ac and 225 Ra at the end of
the irradiation were determined by following the activity of 213 Bi (T1/2 = 45.59 min), which was in radioactive equilibrium as the great granddaughter of225
Ac. Figure 1 shows a typical decay curve of the440.5-keV γ-line of 213 Bi. The two-body successive decay equation (225 Ra → 225 Ac → · · · ) was applied to fit
the decay curve after subtracting the small contribution of the 440.4-keV γ-ray of 228 Ac, which originallyexisted in the 232 Th target as the granddaughter of232
Th. Some short-lived parents of 225 Ac and 225 Ra
were produced in the reactions; therefore the measured cross sections of 225 Ac and 225 Ra are cumulativefor electron-capture decay and β − decay, respectively.
The cross sections of the 232 Th(14 N, xnyp)225 Ac,225 Ra
reaction are shown in Fig. 2. The cross sections of225
Ac are larger than those of 225 Ra by a factor of
5. The experimental results were compared with those
calculated by the Particle and Heavy Ion Transport
code System (phits).6) The phits code reproduces thecross sections of 225 Ac, while it overestimates thoseof 225 Ra by a factor of 4. The production yield of225
Ac was tentatively evaluated to be 3.3 MBq/pµA·h- 162 -1

III-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 54 (2021)

at 132–80 MeV/nucleon by normalizing the phits calculations to the experimental cross sections. Basedon our typical experimental conditions (incident beamenergy: 132 MeV; beam intensity: 1 pµA; target thickness: 4.5 g/cm2 ; irradiation time: 2 d), approximately
150 MBq of 225 Ac can be produced at the end of theirradiation. In the near future, we will measure thecross sections of 225 Ac and 225 Ra at lower energies of80 and 100 MeV/nucleon to evaluate their yields morereliably.
References

1) G. Maryline et al., Radiochim. Acta 107, 1065 (2019).
2) A. K. H. Robertson et al., Inorg. Chem. 59, 12156(2020).
3) H. Haba et al., Radiochim. Acta 93, 539 (2005).
4) J. F. Ziegler, J. P. Biersack, U. Littmark, The Stopping
and Range of Ions in Solid (Pergamon Press, New York,1985).
5) O. B. Tarasov et al., Nucl. Instrum. Methods Phys. Res.
B 266, 4657 (2008).
6) Y. Iwamoto et al., J. Nucl. Sci. Technol. 54, 617 (2017).

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