RIKEN Accel. Prog. Rep. 56 (2023)

FEATURE ARTICLE

RIKEN Accel. Prog. Rep. 56 (2023)

Nuclear masses weigh heavily with us

As a self-organizing quantum system, the nucleus
makes every eﬀort to reduce its mass. As a naturalconsequence, the nuclear mass contains all the information on mechanisms that stabilize the nucleus. Thesemechanisms can be traced back using the nuclear massdata.
In 1919, Francis William Aston performed the ﬁrst
successful mass measurement and discovered many isotopes using his mass spectrograph. At that time, themass resolution was approximately 1%. The subsequent century has witnessed incredible developments
in techniques used for measuring nuclear masses. The
most established and famous techniques of precisionmass measurements are the Penning trap method andthe Schottky method using a storage ring.
Unfortunately, neither of the methods can be applied to mass measurements of rare nuclei produced at
the RI Beam Factory (RIBF). This is solely becauseof their short lifetimes of ≫1 s. In the long measurement time (>1 s) of the Penning trap and the Schottky
methods, the short-lived nuclei decay to their daughters, which prevents us from measuring their masses.
The main physics cases in nuclear mass measurements
at RIBF involve the magicity and the r-process nucleosynthesis, both of which require a mass precision of
100 keV, or δm/m ∼10−6 . Since the statics expected
for rare nuclei are as small as ∼102 , a mass resolving
power of 105 or better is mandatory in their studies.
The unprecedentedly fast mass-measurement technique with a mass resolving power as high as m/∆m >105 has been highly desired, but accomplishing it is abig challenge for nuclear physicists.
Scientists at RIKEN, KEK, and CNS have tackled
this problem and have recently succeeded in measuring masses of rare nuclei: The ﬁrst one is the TOFBρ mass measurement using the long OEDO beamline
coupled with the SHARAQ spectrometer. The measurement combining a ﬂight length of 105 m (one path)and the high-resolution magnetic system with a controlled correction of higher-order aberrations enables
an extremely short measurement time of 0.0005 mswith a reasonable resolving power of >104 and a higheﬃciency of ∼100%.
The second is an isochronous mass measurement using the Rare RI Ring (R3). R3 is the world’s ﬁrst
cyclotron-type storage ring with an individual injection
capability. The setup leads to a 0.7-ms measurement
time with a resolving power > 3 × 105 .
The third is the multireﬂection time-of-ﬂight(MRTOF) mass spectrograph combined with a gas
catcher system. This versatile and “portable” device
can be placed at any beamline to accept a wide varietyof nuclei, whose masses are measured in several tens ofms (including the time needed to stop the ions) witha high resolving power of 106 .
The combined use of the abovementioned three techniques with diﬀerent characteristics, shown in Fig. 1,
opens a broad spectrum of research opportunities atRIBF.

Fig. 1. Mass precision and nuclear half-life regions coveredby the three mass measurement methods.

TOF-Bρ mass measurement using OEDOSHARAQThe OEDO-SHARAQ system, initiated collaboratively by CNS, the University, of Tokyo and RIKEN
Nishina Center is promoting the program of direct
mass measurements of radioactive nuclei far from the βstability line. In 2009, the OEDO-SHARAQ program
started with the installation of the SHARAQ spectrometer1) and a dispersion-matched High-Resolution
beamline2) (renamed by OEDO beamline in 2017) into
the downstream of the BigRIPS separator3) at theRIBF facility.
Atomic masses of very short-lived nuclei have been
determined so far by various time-of-ﬂight (TOF)
methods. In our system, the TOF magnetic-rigidity(TOF-Bρ) method, developed at GANIL,4) has been
adopted, and further sophistication in its ion optics
and detector performance has been implemented forthe measurements. The principle of mass determination using the TOF-Bρ method can be simply expressed by the following relation:√( )2Bρ
ctm=− 1,(1)qcLwhere m, q, c, t, and L are the nuclear mass, atomic- S1 -RIKEN Accel. Prog. Rep. 56 (2023)

FEATURE ARTICLE

charge, speed of light in vacuum, TOF, and the ﬂightpath length, respectively. This relation corresponds
to the Lorentz force in a magnetic ﬁeld, and indicatesthat the mass-to-charge ratio (m/q) is determined byTOF as long as the ﬂight-path length and the magnet ﬁeld of the beamline are ﬁxed. However, because
the ﬂight-path length is indeed dependent on the incident condition of the short-lived nuclei, we perform an
event-by-event correction of the ﬂight-path length using the trajectory information obtained at the startingand ﬁnal foci of the beamline. The essential point thathas led to the success of our direct mass measurement
program has been the high-resolution performance ofthe system.
The detector setup for the mass measurements isshown in Fig. 2. The CVD diamond detectors were
installed at F3 and S2 to measure the TOF. The CVD
diamond detector was developed at CNS to achieve
high-resolution timing and high-rate capability.5) The
low-pressure multiwire drift chambers (LP-MWDCs)6)
were used for beam tracking at F3 and S2, and provided beam-trajectory information that is mandatory
for accurate corrections of the ﬂight-path lengths. A
delay-line parallel-plate avalanche counter (DL-PPAC)
installed at S0 measured the Bρ values of the beam
particles. The m/q values of the individual short-lived
nuclei were determined based on the combination ofthese measured quantities on an event-by-event basis.
Additionally, a silicon solid-state detector (SSD) wasused to identify the atomic numbers. The delayed
γ-ray detection system was placed at the end of thebeamline to search for the unknown isomeric states in
the radioactive nuclei.

Fig. 2. Experimental detector setup in the OEDO-SHARAQ
system for the direct mass measurements.

An advantage of the SHARAQ mass measurements
is its accessibility to very short-lived isotopes, whose
lifetimes are less than 1 ms, with its high transmission
eﬃciency, because the TOF of the system is as short as∼500 ns. With this capability, the system is expected
to enable the measurement of nuclear masses on thedripline.

2- S2 -We have successfully measured the atomic massesof very neutron-rich Ca and Ti isotopes towards thedripline by a single setting of the BigRIPS separator
and the SHARAQ spectrometer. These new masses arecovered with the region connecting the emergence ofmagicities at N = 32 and 34 and the island-of-inversion
at N = 40. The work has clearly exhibited the sudden emergence of magicity in 54 Ca.7) Moreover, the results for Ti isotopes reveal an onset of the Jahn-Teller
stability around 62 Ti,8) indicating shell quenching at
N = 40 based on the comparison with the theoreticalmass predictions.9)
These experiments have demonstrated that our experimental technique has capabilities to eﬃciently pin
down the mass irregularities and provide essential information on its onset mechanism based on mass evolution. Recently, the ﬁrst TOF-Bρ mass measurements
after the construction of the OEDO system were performed to access the mass region of two-proton radioactivity.10) The TOF-Bρ method excels in extending the mass data toward the nuclear drip lines. We
will extend our challenge toward the 78 Ni and 100 Sn
regions in the near future.
Isochronous mass measurement with the rare
RI ring
In the decade after the R3 was built, we have successfully conducted its commissioning using a primarybeam, validation of the mass measurement method using unstable nuclei with known masses, and precise
mass measurements. Recently, the mass of 123 Pd, oneof the nuclei near the second peak of the r-process,was successfully determined.11) The impact of the measured mass on the heavy element synthesis was investigated by inputting the new mass value into the rprocess network simulations from which its eﬀect on
the probability of neutron capture and neutron emission after beta decay were evaluated. The compositions observed in the solar system can be reproducedwith the new mass value.
Figure 3 illustrates the mass measurement schemeof the R3. The mass measurements for such nuclei
have been achieved by our bold move to connect a
cyclotron and a storage ring, which have been known
to be essentially incompatible, taking the advantageof the “long” fragment separator. The key point of
this scheme is to know, prior to injection, that thenucleus of interest is produced in the ﬁrst section of theBigRIPS and to inject it into R3. The masses are thendetermined event-by-event by performing isochronous
mass spectrometry (IMS).12,13)
In RIBF, various unstable nuclei are produced by
the in-ﬂight ﬁssion of the uranium beam or the projectile fragmentation of heavy-ion beams such as xenon,krypton, etc. Their extremely low production rate
makes the timing of production completely random.

RIKEN Accel. Prog. Rep. 56 (2023)

FEATURE ARTICLE

SRC

KickersystemTargetPID(Bρ-TOF-dE)
TimingT rignalger sig

missiotransre-PID(dE-E)nIMS at R3

Produced randomly detector
Rare event

Fig. 3.

timeBeamline

Conceptual design and method for measuring the mass of short-lived rare nuclei using R3.

The species of the produced nuclei can be identiﬁed onan event-by-event basis using the Bρ-TOF-dE methodin the F0–F3 section of BigRIPS.14) Subsequently, theidentiﬁed isotope passes though the timing detector
placed at F3. Using the timing and energy-loss information from the detector, the signal to trigger theR3 kicker system is generated only for the isotopesof interest. Synchronization of the kicker excitation
to the arrival of the isotope beam is the key technology enabling the unique combination of a cyclotron
and a storage ring. It should also be noted that theRF signal from the accelerator is utilized to eﬃciently
reduce the trigger rate by ﬁltering the signals corresponding to unnecessary events. Using this setup, we
have realized the isotope-selectable self-triggered injection, called the individual injection, for the ﬁrst time.
The injected isotope is circulated in R3 for certain
turns (typically ∼2000) and extracted after ∼1 ms.
The revolution time is obtained by dividing the ﬂighttime between injection and extraction by the numberof turns. Further, the extracted events are identiﬁed
once again for veriﬁcation (re-PID). The ratio of therevolution times of the nucleus of interest to a reference nucleus, whose mass is well known, is use to accurately determine the mass. It should be noted herethat isochronism of R3 holds only for a single isotope,
the reference nucleus in many cases and the revolution
time of the nucleus of interest has momentum dependence. Thus a small correction is introduced using thevelocity, β, or Bρ determined by the measurements
prior to injection.
The main factor crucial for the precision of thederived mass is the degree of isochronism, including
the magnetic ﬁeld ﬂuctuation. Isochronism is deﬁned by the spread of the revolution time, dT/T, in
the acceptance of the ring. A standard deviation ofdT /T ∼ 2.8 × 10−6 for a high degree of isochronism
has been achieved at R3 for the full momentum acceptance (dp/p ∼ ±0.3%). This leads to a high mass
resolving power of 3.5 × 106 . We have established ascheme that can determine masses of the order of 10−6
with a measurement time of less than 1 ms. Further

eﬀorts are ongoing to increase the mass resolving power
and injection eﬃciency by upgrading the kicker system,15,16) and by ﬁne-tuning the ion optics. In the nearfuture, it is expected to be able to derive the masses ofnuclei in a region that can only be achieved with theR3, i.e., the nuclei having half-lives of <10 ms, andextremely rare that one can be extracted in a day.
MRTOF mass measurement
Three multi-reﬂection time-of-ﬂight mass spectrographs (MRTOF-MS) are in operation at RIBF. Oneof them is called ZD-MRTOF, which is located behind the ZeroDegree spectrometer (ZDS), combinedwith an RF-carpet-type cryogenic He gas catcher(RFGC)17,18) in the SLOWRI project (Fig. 4). Thefast (>100 MeV/nucleon) RIs provided from BigRIPS
are stopped in the He gas and extracted as slow(<10 eV) RI ions from the RFGC using RF ion carpets. They are then guided into an ion trap, accumulated, cooled, and injected into the mass spectrograph(Fig. 5).19) The ions are reﬂected back and forth between the ion mirrors typically for 600 revolutions corresponding to ≈10 ms at a maximum kinetic energyof 2.5 keV. Currently, the maximum mass resolving
power has reached to 106 .
Since the location of the ZD-MRTOF is just in frontof the beam dump of ZDS, mass measurements havebeen conducted symbiotically by re-using the RIs fromother experiments carried out upstream without extra
costs. For example, during the in-beam γ-ray experiments (HiCARI campaign) in winter 2020, more than
70 masses on RIs were measured with the ZD-MRTOF
system (Fig. 6). Among them, three isotope masses
have been measured for the ﬁrst time and mass uncertainties of 11 isotope have been signiﬁcantly improved
from the previous ones.
For Ti and V isotopes, the nonexistence of the N =34 empirical two neutron shell-gaps has been revealed
experimentally with the new precision achieved by ourequipment.20)- S3 -3

RIKEN Accel. Prog. Rep. 56 (2023)

FEATURE ARTICLE

Fig. 4. Photographs of an RF-carpet-type He gas catcher(RFGC) behind ZDS of BigRIPS.

Fig. 6.

Nuclear mass measurements performed with the three MRTOF-MS.

The second MRTOF is located downstream ofthe beamline at the KEK isotope separation system(KISS), which employs a mass spectrograph and a particle identiﬁcation detector for laser spectroscopy. Ingas-cell laser resonant ionization spectroscopies for Osand Pt isotopes have been conducted with the KISSMRTOF.21,22) In addition, a new isotope of 241 U has
also been discovered by a precise mass measurement
using KISS-MRTOF.23)
The third MRTOF, SHE-Mass, is located behind
GARIS-II at E6. It has been used for mass measurements of super-heavy nuclei such as 257 Db.24) In addition, a 9 MBq 252 Cf ﬁssion source has been installed
just in front of the He gas catcher. Even oﬀ-line, massmeasurements on the ﬁssion fragments have continued
and several ﬁrst mass measurements have been performed.25)
We have a plan to increase this versatile andportable instrumentation at RIBF; an MRTOF-MS
is being installed behind GARIS-III, and a plan isunderway to install an MRTOF-MS with a large He
gas catcher behind SD4 at BigRIPS, which will leadto more opportunities to study unexplored nuclear
species.

4Fig. 5. Schematic view of MRTOF-MS (top) and a photograph of the ion mirrors behind ZDS of BigRIPS (bottom).

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FEATURE ARTICLE

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