Ⅲ-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 56 (2023)
RIKEN
Accel. Prog. Rep. 56 (2023)

Radioactivity calibrations of 225 Ac and 211 At
K. Arai,∗1 M. Imamura,∗1 D. Mori,∗1 H. Ishizu,∗1 Y. Sato,∗2 H. Haba,∗3 X. Yin,∗3 Y. Wang,∗3 and Y. Wakitani∗1225

Ac and 211 At are short half-life α-emitters thathave attracted attention as radiopharmaceuticals. The
minimum requirements for radiopharmaceuticals, the radioactivity of most preparations is speciﬁed as 90–110%.
To conﬁrm of fulﬁll the speciﬁcation, the calibration of
measuring instruments, and thus standard sources withsmall uncertainties to the best extent possible is essential. However, certain issues plague the radioactivity
calibration. First, 213 Po, which is one of the progenies
of 225 Ac, interferes with the measurement. The shorthalf-life of 3.70 µs1) of 213 Po causes counting loss owing to the β-rays emitted from its parent 213 Bi whenmeasured using a commercial liquid scintillation counter(LSC) with dead time of a few µs. Thus, a correction for
counting eﬃciency is necessary. Second, 211 At is highly
volatile; however, there is minimal knowledge on suitablesamples for the LSC measurement. In addition, 207 Bi,
which is one of the progenies of 211 At, is not in radiativeequilibrium; thus, the contribution of 207 Bi and its uncertainty on the calibration must be evaluated. In thisstudy, our radioactivity calibration methods for 225 Ac
and 211 At were examined to solve the above problems
through comparisons with the results obtained from theNational Institute of Advanced Industrial Science andTechnology (AIST).
The commercial LSC, Tricarb 4810TR (PerkinelmerInc.) and the custom-build LSC with variable dead time,
LSC0001 were used in the radioactivity measurementsof 225 Ac and 211 At. To correct for the counting loss of213
Po in an 225 Ac solution, the dead time of LSC0001
was set to 100 µs, and a correction factor was calculated
from the ratio of the count rate measured by LSC0001
to that measured by 4810TR. The radioactivity was calculated by the CIEMAT-NIST method2) for both Japan
Radioisotope Association (JRIA) and AIST.
211
At was produced in the 209 Bi(α, 2n)211 At reaction at the RIKEN AVF cyclotoron.3) 211 At was dissolved with 1.2% ascorbic acid in 5/15 M phosphatebuﬀer or 1 M sodium hydroxide aqueous solution;4,5)
the sodium hydroxide solution was selected to suppress
loss of volatile 211 At. The contribution of 207 Bi to themeasurement at 24 h after distillation was estimated tobe approximately 0.1%. Therefore, measurements wereperformed using Tricarb 4810TR within 2 half-lives after dry distillation to minimize the contribution of 207 Bi
as low as possible. After 211 At had decayed suﬃciently(above 10 days), the contribution of 207 Bi was evaluatedthrough re-measurements. The radioactivity of 211 At
was also calculated by the CIEMAT-NIST method.2)
The radioactivities of 225 Ac and 211 At were measured∗1∗2∗3R&D Section, Japan Radioisotope Association
National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology
RIKEN Nishina Center

to be 318.9 ± 4.6 kBq/g (k = 2) and 3.155 ± 0.064 kBq/g(k = 2), respectively, and they are shown in Fig. 1.
Most of the uncertainty components were uncertainties
in the calculations by the CIEMAT-NIST method. Inaddition, the radioactivities of 225 Ac and 211 At by AIST
were 317.1 ± 4.8 kBq/g (k = 2) and 3.143 ± 0.025 kBq/g(k = 2), respectively, and both were consistent with ourdata within the uncertainty range.

Fig. 1. Intercomparison result of (A) 225 Ac and (B) 211 At
radioactivity.

In summary, the radioactivities of 225 Ac and 211 At
were measured and compared with those by AIST. Consistent results were obtained for both nuclides. JRIA
is an accredited calibration laboratory related radiation
and radioactivity in Japan Calibration Service System(JCSS). Accredited calibration laboratories, which satisfy the requirements of the measurement act can issue
calibration certiﬁcate with the “JCSS” logo. Furthermore, we are ready to calibrate 225 Ac radioactivity forJCSS calibration. In the future, where 211 At is scheduled to applied for JCSS calibration nuclide and is already available for a general calibration.
References
1) M. -M. Bé et al., Table of Radionuclides (Bureau International des Poids et Mesures, Sèvres, 2010), Vol. 5, p. 59.
2) A. G. Malonda, Free Parameter Models in Liquid Scintillation Counting (Centro de Investigaciones Energéticas,Medioambientales y Tecnológicas, Madrid, 1999).
3) Y. Wang et al., RIKEN Accel. Prog. Rep. 53, 192 (2019).
4) A. Toyoshima and A. Shinohara., Radioisotopes 67, 461(2018).
5) T. Watabe et al., J. Nucl. Med. 60, 1301 (2019).

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