Ⅱ-1. Nuclear Physics
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

Yield investigation of neutron-rich W isotopes produced bymulti-nucleon transfer reactions of 136 Xe + nat Ir
M. Mukai,∗1,∗2 Y. Hirayama,∗3 Y. X. Watanabe,∗3 P. Schury,∗3 A. Andreyev,∗4 S. C. Jeong,∗3 H. Miyatake,∗3T. Niwase,∗3 M. Reponen,∗5 M. Rosenbusch,∗3 H. Ueno,∗2 and M. Wada∗3Nuclear charge radius is a quantity sensitive tochanges in nuclear structure, such as shape transition,
shell closure, and proton-neutron pairing. The nuclei
around Z = 72–78 are known as transitional nuclei,
which exhibit a multitude of nuclear shapes dependingon the neutron and proton numbers.1) In the W isotopic chain, nuclear shape evolution has been investigated up to 190 W in terms of the properties of the ﬁrst
2+ , 4+ excited states2) and K-isomers3) via γ-ray spectroscopy using relativistic ions produced in fragmentation reactions. However, laser spectroscopic dataobtained using low-energy isotope separator on-line(ISOL) beams are lacking, especially in the neutronrich side of this nuclear region,4) due to the refractory nature of these elements. Thus, to investigate theground state shape evolution with the help of theoretical calculations, we are planning to study the change
in nuclear mean-square charge radii of neutron-rich
188, 190
W via in-gas-cell laser ionization spectroscopy5)
at KISS.6) As a preliminary eﬀort, we report the measured extraction yields of 188–190 W isotopes at KISS.
The KISS facility at which we conducted the measurement is an argon-gas-cell-based laser ion source
combined with an ISOL, designed to produce otherwise
diﬃcult-to-access nuclides by multi-nucleon transfer(MNT) reactions. Neutron-rich W isotopes were produced in MNT reactions using an 136 Xe beam (30 particle nA, 10.75 MeV/nucleon) impinging upon a nat Ir(22.7 mg/cm2 ) target. The target-like fragments werestopped and neutralized in a gas cell pressurized to75 kPa with puriﬁed Ar gas. The neutralized W
atoms were transported by gas ﬂow and ionized byelement-selective laser resonance ionization at the gascell exit. The W atoms were resonantly excited bylaser irradiation at λ1 = 260.7165 nm (5d4 6s2 5 D0 →J= 1◦ ),7) followed by non-resonant excitation to continuum states by laser irradiation at λ2 = 308 nm
from an XeCl laser. The post-accelerated ions (E =20 keV/q) were mass separated using a dipole magnet
with a mass resolving power of m/Δm∼900. After being thermalized in a gas cell cooler-buncher8) (GCCB),
the ions were analyzed in a multi-reﬂection time-ofﬂight (MRTOF) mass spectrograph for isobaric identiﬁcation and ion counting. At a typical mass resolving
power of m/Δm∼500,000,9) the overall eﬃciency of∗1
∗2
∗3
∗4
∗5MRTOF+GCCB was 2%.
Figure 1 shows time-of-ﬂight spectra for A/q = 95
ions measured with (black) and without (red) resonant laser excitation of 190W. An enhancement of190
W ion counts accompanying resonant laser irradiation can be clearly observed. In addition to laser
ionized 190W, we observed intense isobaric contaminants consisting of 190Re, 190Os, 190Ir, 190Pt, and190
Au, which are produced with much higher yields
than the more neutron-rich 190W; whether they are
non-resonantly ionized alongside the 190W or the resultof a non-neutralized fraction in the argon gas remains
uncertain. The recent improvements in MRTOF performance9) at KISS allowed for the successful counting of 190W2+ ions in spite of these intense isobariccontaminants. A similar situation was encountered inmeasurements of 188, 189W. The measured extraction
yields of 188, 189, 190W were 65, 5.6, and 1.6 pps, respectively. These yields are suﬃcient to perform future
in-gas-cell laser ionization spectroscopy.
2V



3W ,U

5H



:$X&RXQWVQV−

WLPHRIIOLJKW QVFig. 1. Measured time-of-ﬂight spectra with resonant excitation laser ON (black) and OFF (red) for 190 W.

References
1) R. F. Casten, Nucl. Phys. A 443, 1 (1985).
2) N. Alkhomashi et al., Phys. Rev. C 80, 064308 (2009).
3) G. J. Lane et al., Phys. Rev. C 82, 051304(R) (2010).
4) P. Campbell et al., Prog. Part. Nucl. Phys. 86, 127(2016).
5) M. Mukai et al., Phys. Rev. C 102, 054307 (2020).
6) Y. Hirayama et al., Nucl. Instrum. Methods Phys. Res.
B 412, 11 (2017).
7) NIST
AtomicSpectra
Database,https://physics.nist.gov/asd .
8) Y. Ito et al., JPS Conf. Proc. 6, 030112 (2015).
9) P. Schury et al., in this report.

RIKEN Nishina Center
Graduate School of Engineering, Nagoya University
Wako Nuclear Science Center (WNSC), IPNS, KEK
School of Physics, Engineering and Technology, Universityof York
Department of Physics, University of Jyväskylä- 29 -