RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-9. Instrumentation

Separated ﬂow operation of the SHARAQ spectrometer for in-ﬂight
proton-decay experiments†
M. Dozono,∗1 T. Uesaka,∗2 S. Michimasa,∗1 M. Takaki,∗1 M. Kobayashi,∗1 M. Matsushita,∗1 S. Ota,∗1H. Tokieda,∗1 and S. Shimoura∗1
The SHARAQ spectrometer1) is a high-resolution
magnetic spectrometer designed for radioactive isotope (RI) beam experiments at the RI Beam Factory(RIBF). The spectrometer has opened up new experimental opportunities in combination with the highresolution beam line. One interesting example is a new
type of missing-mass spectroscopy with an RI beam
used as a probe. To date, investigations of spin-isospin
properties in nuclei have been strongly promoted by
RI-induced charge-exchange reactions such as (t, 3 He),
(12 N, 12 C), (10 C, 10 B), and (8 He, 8 Be).
We have developed a new ion-optical mode ofSHARAQ operation. The new mode, called the “separated ﬂow mode”, enables the invariant-mass spectroscopy of proton-unbound states with SHARAQ
and thus extends the research ﬁeld of the nuclear
chart toward proton-rich nuclei. In addition, protonunbound nuclei can be used as probe particles. Oneinteresting example is the parity-transfer reaction(16 O, 16 F(0− , g.s.))2) . This reaction has a unique sensitivity to unnatural parity states and can be used asa powerful tool to probe 0− states in a target nucleus.
In the separated ﬂow mode, the SHARAQ spectrometer is used as two spectrometers with diﬀerent magnet
conﬁgurations of “QQD” and “QQDQD” at the sametime (see Fig. 1). The reaction products from the target (S0) are separated and analyzed in either of thesetwo conﬁgurations depending on their mass-to-charge
ratios (A/Z). The particles with A/Z ∼ 1, such as
protons, are analyzed in the “Q1-Q2-D1” conﬁgurationand detected in the S1 focal plane, which is located on
the low-momentum side downstream of the D1 magnet. On the other hand, the heavy-ion products areanalyzed in the “Q1-Q2-D1-Q3-D2” conﬁguration toincrease the resolving power and detected at the S2 ﬁnal focal plane. Therefore, this new technique enables
us to perform coincidence measurements of the protonand heavy-ion pairs produced in the decays of protonunbound states in nuclei. Details of the ion-opticalproperties and the focal-plane detector systems can befound in our published paper.
The separated ﬂow mode was successfully introduced in an experiment with the reaction (16 O, 16 F)
at a beam energy of 247 MeV/u (SHARAQ08 experiment). The outgoing 15 O + p produced in the decayof 16 F were measured in coincidence with SHARAQ.
†∗1∗2

Condensed from the article in Nucl. Instrum. Meth. A 830,233 (2016)
Center for Nuclear Study, University of Tokyo
RIKEN Nishina Center- 204 -Fig. 1. Schematic layout of the SHARAQ spectrometer.
The SHARAQ spectrometer consists of two superconducting quadrupole magnets (Q1 and Q2), one normal
conducting quadrupole magnet (Q3), and two dipolemagnets (D1 and D2) in a “QQDQD” conﬁguration.
In the separated ﬂow mode, the protons and the heavyion products from the target (S0) are separated andmeasured in coincidence at two diﬀerent focal planes(S1 and S2) of SHARAQ. The inset shows the tracking
detector system at S1, which consists of two multi-wiredrift chambers (MWDCs) and two plastic scintillators.

For the relative energy between the 15 O particle andthe proton, a resolution of 100 keV (FWHM) wasachieved, and the 0− g.s. of 16 F (Erel = 0.535 MeV)
was clearly separated from the excited states, the 1−
state at 0.193 MeV (Erel = 0.728 MeV) and the 2−state at 0.424 MeV (Erel = 0.959 MeV). In addition,a high resolution of ∼2 MeV (FWHM) was achieved
for a 16 F kinetic energy of 3,940 MeV. Such an accurate missing-mass measurement combined with aninvariant-mass method gives unique opportunities toexplore little-studied excitation modes in nuclei using
new types of reaction probes with particle-decay channels.
References
1) T. Uesaka, S. Shimoura, and H. Sakai, Prog. Theor.
Exp. Phys. 2012, 03C007 (2012).
2) M. Dozono et al., RIKEN Accel. Prog. Rep. 45, 10(2012).