Ⅱ-9. Instrumentation

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

Development of light collection system for collinear laser spectroscopy
M. Tajima,∗1 A. Takamine,∗1 H. Iimura,∗1 M. Wada,∗2 and H. Ueno∗1
The laser spectroscopy of atomic transitions can aidin clarifying the nuclear structures in ground and isomeric states. We are preparing for the collinear laser
spectroscopy (CLS) of rare isotopes (RIs) at the upcoming thermalized RI beam facility using an He gas
catcher, referred to as SLOWRI.1) Barium isotopeswere measured as a ﬁrst oﬄine demonstration of ourCLS setup.2) Thereafter, oﬄine measurements of stable Zr isotopes were performed.3) An unstable refractory element isotope such as a radioactive Zr isotope isa candidate for an online experiment in the intermediate mass region. As the intensity of exotic Zr isotopes
is limited in the online experiment, the detection efﬁciency should be improved for precise measurement.
This report describes a next step to improve the lightcollection eﬃciency.
We have performed CLS by changing the velocity,that is, Doppler tuning, of the ion beam using postacceleration electrodes in the region where a photomultiplier tube (PMT) is placed as a ﬂuorescence monitor. Figures 1(a) and (b) depict the top and side viewsof the primary design of the electrodes, respectively.
Ion beams and the spectroscopy laser pass collinearlythrough them (from left to right). A post-acceleration
voltage up to ±3 kV is applied on a long electrode at
the center, divided by each resistor connected to eachthin electrode, and grounded at both ends. A sheet ofmetal mesh is attached on both sides at the center. It(e)Cylindrical mirror

avoids optical pumping outside of the region and enables the spectroscopy laser to be locked. Figure 1(e)presents a schematic around the center electrode. The
PMT is placed in front of the mesh and a cylindricalmirror is placed on the opposite side, so that light fromthese two open sides can be detected.
We constructed a new pair of mirror-shaped electrodes to improve the light collection eﬃciency. Figure 1(c) depicts the inner surfaces of the new electrodes:
one is spherical with a hole at the center (left) andthe other is ellipsoidal (right); these were originally designed by M. Baba4) for molecular spectroscopy. They
are made of aluminum, and the inner surfaces are polished. Figure 1(d) presents an integrated view with
the new pair of electrodes at the center. Figure 1(f)presents a schematic where blue and magenta dashedlines correspond to the ellipsoidal and spherical surfaces, respectively. The center of the spherical surface
is one of two simultaneous focal points of the ellipsoidal
surface. If a point-like light source is placed at thecenter, reﬂected light on the spherical surface returns
and re-passes the spherical center, and then re-reﬂected
light on the ellipsoidal surface focuses onto the otherfocal point, which is observed using the PMT throughthe hole. It allows 90% of solid angle coverage for thepointlike source. In CLS, however, 40% of coverage isestimated when the interaction region is 4 cm in length.
We performed the CLS of 90 Zr+ this new apparatus.
The overall eﬃciency is improved by a factor of 2.3 over
the original detection setup. The absolute value is estimated at more than 4 × 10−6 , though it is necessary
to determine the exact number of ions that contributeto the optical detection. So far, a resonance peak wasobserved for the intensity of down to 103 ions per bunchof repetition of 2 Hz, as depicted in Fig. 1(g). Further
improvement such as one-to-one coincidence with ion
detection is necessary to reduce the background rate
for the exotic RIs with intensity lower than 100 pps.
Further, the reﬂectivity of the surface of the electrodein this wavelength region, which is currently estimatedat less than 50%, can be improved up to 60% by polishing.5)
ReferencesFig. 1. Post-acceleration electrodes: (a) top and (b) sideview of the old ones, (c) inner surfaces, and (d) assembled view of the new ones. Schematics of the (e) previousand (f) new design of the center electrode. (g) Observed
spectrum of 90 Zr+ with 103 ions per bunch and Voigt
ﬁtting.
∗1∗21) M. Wada et al., Nucl. Instrum. Methods Phys. Res. B
204, 570 (2003).
2) M. Tajima et al., RIKEN Accel. Prog. Rep. 53, 15(2020).
3) M. Tajima et al., RIKEN Accel. Prog. Rep. 55, 90(2022).
4) M. Baba et al., J. Chem. Phys. 131, 224318 (2009).
5) K. Minamisono et al., Nucl. Instrum. Methods Phys.
Res. A 709, 85 (2013).

RIKEN Nishina Center
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