Ⅲ-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 51 (2018)

Measurement of excitation functions ofthe 206/207/208 Pb(11 B, x)212 Fr reactions

Francium (Fr) is the heaviest alkali metal with theatomic number 87. It is one of the least-studied elements among the naturally occurring elements because
all its isotopes are short-lived; the half-life of its longestlived isotope, 223 Fr, is only T1/2 = 21.8 min. Owing
to experimental diﬃculties, the chemical properties ofFr have not been studied in detail so far. It is considered that the chemical properties of Fr are similar to
those of its lighter homolog, Cs. However, the chemical properties of Fr cannot be simply deduced from theextrapolation from the lighter homologs because relativistic eﬀects come into play in such a heavy atom asFr.1) Therefore, it is of great interest and importance toclarify the chemical properties of Fr and to elucidate theinﬂuence of relativistic eﬀects on the chemical propertiesof Fr. Recently, Haverlock et al reported on the complex
formation of Fr+ with calix[4]arene-bis(benzocrown-6)(BC6B).2) They reported that Fr+ is more eﬀectivelyextracted with BC6B than Cs+ ; however, the reason
for the selectivity is unclear. We plan to investigate
the stability of the complex formation of Fr with crownethers by systematically varying their cavity size andsubstituent group to understand their inﬂuences on theselectivity in the complex formation and solvent extraction of Fr. In this study, we will use the long-lived isotope 212 Fr (T1/2 = 20 min), which can be produced in
the 206/207/208 Pb(11 B, x)212 Fr reactions. Because no experimental excitation functions are available for thesereactions, we measured the excitation functions of the206/207/208
Pb(11 B, x)212 Fr reactions to optimize the production conditions of 212 Fr.
The excitation functions were measured using thestacked-foil technique. The 206/207/208 Pb targets wereprepared by vapor deposition on 3.1-µm Al foils (> 99%chemical purity). The size of all foils was 10 × 10 mm2 .
The target stacks consist of 20 sets of 206 Pb (99.51%enrichment, 791-µg/cm2 thickness), 207 Pb (99.40%enrichment, 851-µg/cm2 thickness), or 208 Pb (99.59%enrichment, 642 µg/cm2 thickness). The 3.1-µm Al foils
were inserted between the 206/207/208 Pb targets to catch
212
Fr atoms that recoiled out of the targets. Each stack
was irradiated for 10 min with a 100.9-MeV 11 B beam
supplied from the RIKEN AVF cyclotron. The incident
beam energy was determined by time-of-ﬂight measurement.3) The average beam current was measured with a
Faraday cup and found to be 48.6, 48.8, and 50.0 pnA forthe 206 Pb, 207 Pb, and 208 Pb stacks, respectively. Afterthe irradiation, each foil was subjected to γ-ray spectrometry with Ge detectors.
The radioactivity of 212 Fr was determined from its
227.72-keV (γ-ray intensity Iγ = 42.6%) γ line. Figure 1∗1Cross section [mb]Y. Komori,∗1 T. Yokokita,∗1 S. Yano,∗1 and H. Haba∗1600

206/207/208Pb( 11B,x)212Fr206Pb
207Pb
208Pb

400

200

060

70
8090
Beam energy [MeV]

100Fig. 1. Excitation functions of the 206/207/208 Pb(11 B, x)212 Frreactions.

shows the excitation functions measured for the ﬁrst
time for the 206 Pb(11 B, 5n)212 Fr, 207 Pb(11 B,6n)212 Fr,and 208 Pb(11 B,7n)212 Fr reactions. The maximum crosssection for the production of 212 Fr is available in the206
Pb(11 B, 5n)212 Fr reaction around 73.6 MeV.
Based on the measured excitation functions, wewill optimize the production condition of 212 Fr in the206
Pb(11 B, 5n)212 Fr reaction for future chemistry studies of Fr using the multitarget He/KCl-jet transport system.4) In this system, 4 sets of an 864-µg/cm2 206 Pb
target on a 10-µm Be foil are placed in 12-mm-spacingin 129-kPa He and are irradiated with a 11 B beam at
an energy of 86 MeV. The beam energies on the four206
Pb targets are calculated to be in the range of 70–
79 MeV, which covers the peak region of the excitation
function of the 206 Pb(11 B, 5n)212 Fr reaction. The 212 Fr
atoms that recoiled out of the 206 Pb target are thermalized in the He gas, attached to KCl aerosol particles,
and transported through a Teﬂon capillary to a chemistry laboratory, where the solvent extraction of 212 Fr
will be performed. 60 kBq 212 Fr, which is suﬃcient radioactivity to study its solvent extraction behavior, isavailable after the 1-min aerosol collection by assuming
a beam intensity of 300 particle nA, recoil eﬃciency of48%, and gas-jet eﬃciency of 50%.
References
1) V. Pershina et al., Chem. Phys. 395, 87 (2012).
2) T. J. Haverlock et al., J. Am. Chem. Soc. 125, 1126
(2003).
3) T. Watanabe et al., Proc. 5th Int. Part. Accel. Conf.
(IPAC2014), (2014), 3566.
4) H. Haba et al., Advances in Nuclear and Radiochemistry.
Extended abstracts, 333 (2004).

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