II-9. Instrumentation

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Oﬄine ion source for laser spectroscopy of RI at SLOWRI†
M. Tajima,∗1 A. Takamine,∗1 M. Wada,∗2 and H. Ueno∗1# of ions / pulse# of ions / pulse

The collinear laser spectroscopy of radioactive iso- (a)tope (RI) beams is a powerful technique used to diTarget
RFQ
rectly measure the nuclear properties of ground or isomeric states. Isotope shift measurement is planned usRFCPCylindricaling RI ion beams supplied from the universal slow RIelectrodes
QPIG
beam facility (SLOWRI) at RIBF. The main targets are
medium-mass nuclei of refractory elements, for which
experimental studies are insuﬃcient. To achieve highprecision, it is essential to determine a reference frequency with well-studied isotopes for the planned spectroscopy apparatus to minimize systematic uncertain- (b)x103ties. In this work, an ion source combining the laser
500
ablation of solid targets in helium gas and a radio fre400
quency (RF) ion guide system with an RF carpet1) was300
constructed for the reference measurement of isotope
200shifts.
100Figure 1(a) shows a sketch of the ion source. It0consists of cylindrical DC electrodes, an RF carpet,
899091
92
9394
95
96
97m/q
a quadrupole ion beam guide, and an RF quadrupole(c)(RFQ). A solid target for laser ablation was ﬁxed on
the surface of the cylindrical DC electrodes. Helium
80000
gas was continuously introduced into the ﬁrst cham60000
ber via a piezo valve while the second and the third40000
chambers were evacuated. The pressure of the ﬁrst20000
chamber was measured using a capacitance manome0ter and stabilized by controlling the piezo valve with181 182 183 184 185 186 187 188
m/q
proportional-integral-derivative (PID) feedback. A Qswitched Nd:YAG laser (532 nm, ∼5 ns width) was inFig. 1. (a) Sketch of the ion-source system. (b) Mass specstalled outside the chamber. The laser light was fotrum of Zr+ . (c) Mass spectrum of W+ .
cused using a lens, and the light passed through a view
port normal to the ablation target surface. The laser
power was 25 mJ per pulse at maximum, and it could
system was studied separately using a Cs+ emitter,be varied using a manual attenuator. The spot size wasand the transport eﬃciency was approximately 60–80%.
≤1 mm, which corresponds to a maximum ﬂuence of
The laser-intensity threshold of ion formation was 1–
3.2 J/cm2 . The lens was moved using XY linear stages
1.3 J/cm2 , and weak dependence on the repetition rate
remotely controlled via a labview program on a stepof laser ablation was observed. Consequently, 105 –107
by-step basis such that the laser spot moved randomly
singly charged ions per laser pulse, consistent with theon the surface of the target. This ion source systemnatural abundance, were successfully observed for metal
was connected to a test beamline through an insulatargets of Ni, Ag, Zr, Ta, W, and a barium compoundtion ﬂange. The ion source could be biased to 10 kV to(BaF2 ). The energy spread of the extracted ions wasextract ion beams to the downstream beamline, which
evaluated to be <1 eV via a comparison of the width ofwas grounded. A dipole magnet with movable slits was
the obtained mass spectra with trajectory simulations
placed in front of and behind the magnet to observeassuming various initial temperatures, although direct
mass spectra.
measurements are needed for conclusive evidence.
Figures 1(b) and (c) show the observed mass specTherefore, singly charged ion beams of one selectra when solid targets of the refractory elements Zr andtive isotope including refractory elements are available
W were used for laser ablation, respectively. The rafor the reference measurement of collinear laser spectio of intensity among the isotopes was consistent withtroscopy. Background reduction will be possible using anatural abundance. The performance of the ion guide
time gate in coincidence with the pulsed ablation laser.
†∗1∗2

Condensed from the article in Nucl. Instrum. Methods Phys.
Res. B 486, 48 (2021)
RIKEN Nishina Center
Wako Nuclear Science Center (WNSC), IPNS, KEK

References

1) M. Wada et al., Nucl. Instrum. Methods Phys. Res. B
204, 570 (2003).

- 95 -1