Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 51 (2018)

Development of α-ToF detector for correlation measurement ofatomic masses and decay properties of superheavy nuclides
T. Niwase,∗1,∗2 M. Wada,∗3,∗1 P. Schury,∗3 Y. Ito,∗1,∗6 M. Rosenbusch,∗1 D. Kaji,∗1 K. Morimoto,∗1 H. Haba,∗1S. Ishizawa,∗1,∗4 K. Morita,∗1,∗2 and H. Wollnik∗5

Incoming Ions

Wire grid

Impact plate
Material coated
SSD (S-3590)

Trajectory ofSecondary Electron
α/SFSignaling signal was greater than 90% for 5-MeV α-particles
with both coating materials when −2100 V was applied
to the impact plate. The lower panel of Fig. 2 shows
the correlation mapping of ToF and α-ray energy. Thestart signal of ToF was made by the triangle roof ToFdetector7) and the stop signal was the timing signal of
the α-ToF. The vertical axis indicates the α-ray energy measured by the α-ToF. The distance between
the triangle-roof ToF and the α-ToF was 27 cm.
We conﬁrmed that the correlation between the timing signal and the decay energy can be measured using the α-ToF. We will use the α-ToF for the massmeasurement experiment of SHN using the GARISII+MRTOF setup, which is scheduled for FY20182019.
Coincident signal / Alpha signal

The atomic mass is a unique quantity for each nucleus. Precise mass measurement allows us to identify
the atomic number as well as the mass number of anucleus. Recently, we measured the masses of fusionevaporation products1–4) provided from GARIS-II5) byusing an MRTOF mass spectrograph.6) We plan tomeasure the masses of hot-fusion superheavy nuclei(SHN) to identify the nuclides. The expected event
rate is of the order of one event per day. We should
accurately distinguish a true event from a large number of background events which might have originatedfrom scattered ions or molecular ions. For this purpose, we have developed an α-ToF detector. The timecorrelation between a time-of-ﬂight (ToF) signal andsuccessive α-decay signals can discriminate such background events.

The α-ToF detector is made of a commercial MagneToF detector (ETP 14DM572) and a Si PIN diode(Hamamatsu S-3590), as shown in Fig. 1. When aheavy ion is incident on the impact plate of the MagneToF, secondary electrons are emitted from the impact plate and the electrons are isochronously transported by a magnetic ﬁeld and ampliﬁed by an electron multiplier to provide a timing signal of the ion.
We replaced the impact plate with an Au+MgO-or
Au+Al2 O3 -coated Si PIN diode.
We tested the detector by using an 241 Am alphasource, and results are shown in Fig. 2. The upper
panel shows the count-rate ratio of the coincident timing signal to the α-ray signal. The eﬃciency of the tim-∗3∗4∗5∗6   

RIKEN Nishina Center
Department of Physics, Kyushu University
KEK, Wako Nuclear Science Center
Department of Physics, Yamagata University
Department of Chemistry and Biochemistry, New Mexico
State University
Department of Physics, McGill University- 157 -Energy [MeV]Fig. 1. Schematic of the α-ToF detector. The impact plate
is appoximately 3 mm thick.

∗2



Applied Voltage [V]ToF
signal∗1

ToF [ns]Fig. 2. Count-rate ratio of timing signals coincident withthe α-ray signal (upper panel). Correlation mapping ofToF and energy (lower panel).

References
1) P. Schury et al., Phys. Rev. C 95, 011305(R) (2017).
2) S. Kimura et al., arXiv: 1706.00186.
3) Y. Ito et al., Phys. Rev. Lett. 120, 152501 (2018).
4) M. Rosenbusch et al., arXiv: 1801.02823.
5) D. Kaji et al., Nucl. Instrum. Methods Phys. Res. B
317, 311 (2013).
6) P. Schury et al., Nucl. Instrum. Methods Phys. Res. B
335, 39 (2014).
7) K. Morimoto et al., RIKEN Accel. Prog. Rep. 46, 191(2013).