Research Activities 1991–2016

RIKEN Accel. Prog. Rep. 50 (2017)

Scientiﬁc pedigree of Chief Scientists of Radiation Laboratory
Hideto En’yo ∗1, ∗2hara summarized the achievements of this project inthe article “Pursuit of Spin Physics Program at RHIC”
in this volume.2)
Besides spin physics, RHIC’s original mission wasto create quark gluon plasma (QGP), the state of theUniverse just after the Big Bang. Y. Akiba, the associate chief scientist of Radiation Laboratory, was rewarded by the Nishina Memorial Prize in 2011 for hisleading role in the measurement of thermal photons,
which showed that the initial temperature reached in
the central Au+Au collision at 200 GeV is approximately 350 MeV, far above the expected transition temperature Tc ∼ 170 MeV, from hadronic phase to quarkgluon plasma.3) This was an important discovery thatassured the creation of QGP at RHIC.
Acceleration of polarized proton at RHIC was an innovative challenge. Protons make precession in magnetic ﬁeld. Once the ratio of the precession frequency
to the ring-revolution frequency becomes integer, theproton spin resonates and the polarization is lost.
“Siberian Snake Magnet” is a technology to overcome
such resonances by ﬂipping the spin of proton in every
turn. The principle methodology is established but thereality was more diﬃcult. The biggest problem was thedepolarization that occurred in the AGS ring. M. Okamura, who once contributed to the design of the snake
magnets in the RHIC main ring, solved the problem ofthe AGS depolarization by using a variable-pitch helical snake magnet.4) Okamura, J. Takano, and others inRadiation Laboratory fabricated such a novel magnet
in a year by taking advantage of a special mobility ofRadiation Laboratory and growth of acceleratorbased science
When Yoshio Nishina passed away in 1951, his laboratory was split into four laboratories. One of them,headed by Fumio Yamazaki, was soon after renamed
as the Radiation Laboratory. Since then, Tastuzo
Hamada, Masayasu Ishihara and Hideto En’yo have
served as the Chief Scientists of the Radiation Laboratory. The ﬁrst volume of APR was issued 50 years
ago when Yamazaki was in charge.
Figure 1 shows the scientiﬁc pedigree of Chief Scientists who are presently engaged in Nishina Center which
was inaugurated in year 2006 when the Superconducting Ring Cyclotron was commissioned. One can see thegrowth of accelerator-based science in RIKEN over 50years since 1966. Apparently, the name of Radiation
Laboratory has persisted for the longest time.
Radiation Laboratory and RIKEN-BNL collaboration
When the author assumed this honorable post inyear 2001, the major mission of Radiation Laboratory
was and still to understand how the spin of nucleon isformed with ingredients (quarks and gluons) throughthe RIKEN-BNL collaboration which modiﬁed Relativistic Heavy Ion Collider (RHIC) to include a capability to accelerate and collide polarized protons. M. Ishi-0.RGHUD/LQDF/DE+.XPDJDL&\FORWURQ/DE+6DNXUDL
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3K\VLFV/DEFig. 1. History of Chief Scientists in Nishina Center. Shaded laboratories are for accelerator-based
science.1)∗1∗2

Chief Scientist of Radiation Laboratory (2001–present)
Director of RIKEN Nishina Center (2009–present)

S-49RIKEN Accel. Prog. Rep. 50 (2017)

Research Activities 1991–2016

RIKEN.
Okamura also developed a laser ion source named
Direct Plasma Injection Scheme. In this, ions are extracted, not by high voltage, but by the initial expansion of plasma diﬀusion. The plasma stays with neutral charge for a long time; one can get rid of the spacecharge eﬀect. His ion source recorded the highest peakcurrent many times.5) His laser ion source technology
was adopted ﬁrst by the NASA Space Radiation Laboratory, and then for the RHIC injector. With thoseachievements, Okamura was head-hunted by BNL andis now a tenure physicist in the Collider-Accelerator
Department.

devices for neutron beams, etc. Many of the group’sworks have been taken over by the RIKEN Center
for Advance Photonics. Especially, T. Otake and her
Neutron Beam Technology Team constructed RANS,RIKEN Accelerator-driven compact Neutron Source,and developed technology “to make invisible visible.”
The ultimate goal is to make available a transportablenon-destructive neutron inspection system for outdoor
use in applications, like a bridge, tunnel, or building.
This is a very active oﬀspring from Radiation Laboratory for societal needs.11)

Radiation Laboratory and chiral symmetry
restoration

RHIC was once the largest heavy-ion collider in theworld. In 2010, however, the Large Hadron Collider(LHC) at CERN started lead-lead collision with energy that was 14 times higher than that at RHIC.
The PHENIX detector, which the Radiation Laboratory was working on, stopped operation in 2016 tometamorphose to the upgraded sPHENIX detector, toaccomplish the latest measurements, that cannot beperformed by the present PHENIX.
In 2015, the Nuclear Science Advisory Committee(NSAC) was requested by the US Department of Energy (DOE) to update their long range plan; the recommendation was to construct an electron-ion collider.12) In the past, all the NSAC recommendations
were achieved in the US contradictory to recommendations by the Japan Science Council. The physicsof proton structure, which was initiated by RIKEN atRHIC, will be the foundation of the future of US nuclear
physics.
Selection for a new Chief Scientist in the ﬁeld of
hadron physics is happening currently. He/She will inaugurate a new era in the history of Radiation Laboratory that has produced streams of new scientiﬁc ﬂow.

Radiation Laboratory for future

When the hadronic matter gets hotter or denser, chiral symmetry must be restored as theoretically predicted by Nambu,6) i.e., hadrons’ mass must change
when they are in diﬀerent vacuum. Although QGP
is created at RHIC, there are no measurements so far
which show a signature of restored symmetry, owing to
the diﬃculty in performing experiments at the very highmultiplicity environment of hot QGP. Complementarymeasurements can be done in dense but cold nuclearmatter. Radiation Laboratory performed KEK E325
experiment at 12 GeV-PS and CERN NA60 experimentat SPS, and had challenged to measure mass of vector
mesons when they ﬂy in dense nuclear matter. Theformer used proton nuclear interaction and measured
electron pairs, while the latter used indium-indium collision and measured muon pairs.
The E325 experiment produces theses by S. Yokkaichi,
K. Ozawa, M. Naruki, R. Muto, F. Sakuma, andT. Tabaru. They succeeded in detecting the mass modiﬁcation of ρ, ω and φ mesons,7) and concluded that themeasured modiﬁcations are consistent with the theoretical model by Hatsuda and Lee.8) On the other hand,
NA60 concluded that the measured spectrum modiﬁcation of ρ meson is explained not by mass shift butby mass broadening.9) The states of nuclear matter forboth experiments are so diﬀerent that the physics picture can be diﬀerent, but it can be admitted that thetwo are not very consistent. To obtain the ﬁnal conclusion, the E16 experiment is under preparation at JPARC. Although the E325 experiment was only the experiment that could measure electron pair decays of φmesons with high statistics. E16 is aggressively seeking
to go beyond.10)
Radiation Laboratory and neutron
From 1999 to 2005, the Radiation Laboratory
hosted the Image Information Research Unit lead by
H. Shimizu. This unit developed a Superconducting
Tunnel Junction detector for Tera Hertz light, optical

S-50

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