Journal of Nuclear and Radiochemical Sciences, Vol. 1, No. 1, pp. 39−42, 2000

39

Nuclear Chemistry in Japan
Hisaaki Kudo*
Department of Chemistry, Faculty of Science, Niigata University, 8050 Igarashi 2 no-cho, Niigata 950-2181,
Japan
Received: December 22, 1999
Nuclear chemistry studies in Japan are briefly reviewed. Main topics are concerned with the works related tonuclear fission and discoveries of new radioisotopes. The other works, such as photonuclear reaction, high energy
light ion-induced reaction, intermediate energy heavy ion-induced reaction, and neutron-induced reaction, are justoutlined. The future plan for transactinide elements is also briefly introduced.

1. Introduction
In recent years, the field of nuclear chemistry in Japan has
greatly progressed in accordance with the development ofaccelerators and their experimental facilities.
Researches on nuclear chemistry are mainly presented at the
Symposium on Radiochemistry in Japan. In order to see a
general view of the activities of nuclear chemistry in Japan, thepapers presented at the Symposium on Radiochemistry for thelast ten years are classified into several groups and displayedin Figure 1. Here the researches purely related to nuclei, suchas nuclear reactions and decay properties, are classified as
nuclear chemistry work. On the average, about 25 papers havebeen presented in a year, and almost half of them are related tonuclear fission.
In this paper, nuclear chemistry studies in Japan, including
the future plan for transactinide elements, are briefly summarized.

Figure 1. Number of papers related to nuclear chemistry presented at
the Symposium on Radiochemistry for the last ten years from 1989 to1998.

2. Brief Summary of Present Research
2.1. Nuclear Fission. Nuclear fission has been traditionallystudied for a long time in Japan. Many precise and systematic
measurements have been performed in the recent studies by theuse of various experimental facilities.
The group of Osaka University has measured many shortlived fission products in proton-induced fission of 238U by theuse of a gas-jet transport system together with an conventional*Corresponding author. E-mail: hkudo@sc.niigata-u.ac.jp. FAX:
+81-25-262-6116.

stacked foil method and discussed the fission mechanism fromthe charge and mass distributions of fission products.1,2 They
also carried out the double-velocity and double-energy measurement (2V2E) for thermal-neutron-induced fission of
233,235
U and 239Pu in order to obtain the information of scission
point configurations.3 From the double-velocity data, massesof the primary fragments can be deduced with an assumptionof isotropic emission of neutrons, while the double-energy data
yield the secondary fragment masses. Thus, the number ofneutrons ν− from each fragment can be obtained indirectly.
They estimated the excitation energy of each fragment from ν−and neutron separation energies, and deduced the degree of
deformation at scission assuming that the excitation energy
only comes from the deformation energy of the nascent fragment of a spheroidal shape. They concluded that there exists
two configurations at scission point, one corresponding to a
deformed heavy fragment and the other to a spherical one.
Direct measurement of neutrons in thermal-neutron-induced
fission of 2 3 5 U was carried out by the group of Kyoto
University and Kyoto University Research Reactor Institute.4,5They measured energy and multiplicity of neutrons coincident
with each fission fragment. The obtained energy specta ofneutrons exhibit Maxwellian type distributions from which
nuclear temperatures can be deduced. Combined with theexcitation energy estimated from multiplicity data, one candeduce level density parameters of fission fragments.
The group of Niigata University measured short-lived fission products by the use of an ion guide isotope separator online (IGISOL) which was installed at the Cyclotron andRadioisotope Center of Tohoku University. IGISOL has no
ordinary ion source. Reaction products are stopped in a helium
buffer gas and remaining recoil ions of +1 charge state are
guided by an electric field and extracted to the separator by askimmer system. By the modification suitable for the measurement of fission products,6 isomeric yield ratios for manyfission products were obtained in proton-induced fission oflight actinides.7,8 The obtained isomeric yield ratios were converted to angular momenta of fission fragments and examinedfrom various points of view. They found a good correlation
between angular momenta and excitation energies of fission
fragments. And the incident energy dependences of angularmomenta were found to be different between symmetricallydivided products and asymmetrically divided ones. They alsomeasured the charge distributions of fission products in proton-induced fission of 238U by the use of IGISOL and determined the most probable charges Zp for 40 mass chains.9 The
charge polarization in fission process was examined withrespect to the ground-state Q values. It was reported that thetrend of Zp is well explained by the ground-state Q values andthe existence of deformed shell closure at neutron number of© 2000 The Japan Society of Nuclear and Radiochemical Sciences

40 J. Nucl. Radiochem. Sci., Vol. 1, No. 1, 2000

Kudo

86−88 was suggested experimentally.
Among many studies related to fission phenomena, a seriesof researches performed by the group of Tokyo Metropolitan
University and JAERI is worth to be noticed. They measured
the velocities of complementary fission fragments by a double
velocity time-of-flight (TOF) system with a high resolution in< 2.0 u) and kinetic energy (∆TKE −< 2.5 MeV). In
mass (∆M −the proton-induced fission of 232 Th, there are two kinds ofkinetic energies at the mass region near A=130.10 The lower
energy component is almost the same with those of symmetrically divided fragments and the higher one is corresponding tomore asymmetrically divided fragments. This means that there
are two kinds of elongation at scission, the one corresponding
to an elongated shape and the other to a compact shape. On
the other hand, the incident energy dependence of the yields
corresponding to symmetrically divided products are quite different with those of asymmetrically divided ones. This factmeans that there are two kinds of fission thresholds (fission
barriers). The angular distributions also exhibit two kinds ofthresholds. Nagame et al. performed the experiments which
verify the connection between the two thresholds to the two
scission configurations.11 They measured the incident energy
dependences of the relative yields of the components of two
different kinetic energies and found that the higher thresholdcomponent which leads to symmetric mass division corresponds to the lower kinetic energy component which has elongated shapes at scission, and the lower threshold component
corresponds to the rather compact shape at scission. Thismeans that two different paths are present in fission process.
The same kind of experiments were performed for the systemof protons on 238U and the existence of two deformation pathsin the mass division process of actinides are experimentally
verified.12,13 They have systematically studied the low energy
fission of actinides both by radiochemical method14−17 and by
TOF method18,19 and deduced quite interesting results. From
the radiochemically obtained excitation functions of fission
products, systematic trends of fission barriers were deduced
both for symmetric mass division and for asymmetric mass
division.16 Zhao et al. reported that the degree of deformation~of asymmetrically divided fragments is almost the same (β −1.53) at scission regardless of fissioning nuclei and that of~ 1.65)symmetrically divided fragments is also the same (β −among them except for the fragments of shell influenced symmetric deformation (β −~ 1.43).19 These results indicate a kindof universality of deformation in fission process.
2.2. New Isotopes. In the past ten years, many new isotopes were discovered by the use of various types of on-line
mass separators which were constructed at various institutes inJapan. The separators used are roughly classified into twogroups. One is the so-called isotope separator on-line (ISOL)
which has an ion source, and the other is a recoil separator
which has no ion source and is used for in-flight separation. In
general, the time required for separation by the latter is shorter
than that by the former, whereas the former has a better mass
resolving power than the latter. In this connection, IGISOL isplaced between these two groups. IGISOL was effectively
used for the investigation of mirror nuclei, such as 45V, 47Cr,
49
Mn, and 51Fe, at the Cyclotron and Radioisotope Center of
Tohoku University.20
Many new isotopes of lanthanoides, such as 121La (Ref. 21),161
Sm (Ref. 22), 165Gd (Ref. 22), 166Tb (Ref. 23), and 167,168Tb(Ref. 24), were discovered by the use of a gas-jet coupled
ISOL connected to the tandem accelerator at Japan Atomic
Energy Research Institute (JAERI), Tokai. Mono-oxide ions
are separated for the investigation of neutron-rich isotopes oflanthanoid elements produced in proton-induced fission of
238
U, which is very effective because the cross sections ofheavier elements by 16 mass number are quite small comparedwith the elements of interest.22−24 Recently they have also suc-236

ceeded in the mass separation of americium isotope, Am(Ref. 25).
At Kyoto University Research Reactor Institute, the gas-jet
coupled ISOL was also used for the discoveries of neutron-richlanthanoid isotopes such as 150La (Ref. 26), 152Ce (Ref. 27),
154
Pr (Ref. 28), and 156Pm (Ref. 29), which were produced inthermal neutron-induced fission of 235U.
The ISOLDE type ISOL was constructed at JAERI,
Takasaki and connected to the AVF cyclotron in the TIARA
facility. The separator itself is the same with that at JAERI,
Tokai and neutron deficient isotopes of praseodymium 125Pr
and 127Pr were discovered in the reaction of 92Mo + 36Ar system.30 In this ISOL system, a laser ion source in addition to
ordinary ones can be combined and operated in an element
selective mode.31
In the second group of separators, reaction products are separated in flight from the intense primary beam and practicallyno mass separation is made. The identification of new isotopes
are usually performed by a time-of-flight system and a position
sensitive detector on the basis of time- and position-correlated
decay chains. Two kinds of recoil separators of such a type areused for the search for new isotopes of heavy elements. One is
the gas-filled recoil ion separator (GARIS) at the Institute ofChemical and Physical Research (RIKEN). Recoil ions justejected from the target material have great variety of charge
states. Accordingly, the orbits of the ions with different charge
states will differ under the applied magnetic field for separation. But if a suitable gas is filled in the separator, recoil ionsbehave as if singly-charged ions as a results of many charge
exchanging collisions with the buffer gas. Therefore, the collection efficiency becomes larger. This is the main advantage
of a gas-filled type separator. The high collection efficiency isessential in the case of new isotopes produced with extremely
small cross sections. By the use of GARIS at RIKEN, newisotopes of radon 197Rn (Ref. 32), 196Rn (Ref. 33), and francium
200
Fr (Ref. 32) were discovered.
The other is the recoil mass separator installed at the tandem-booster facility of JAERI (JAERI-RMS). This separator
consists of a quadrupole doublet, the first electric dipole, amagnetic dipole, the second electric dipole, a quadrupole doublet, and an octupole magnet. The characteristic feature of thisseparator is that the anode of the first electric dipole was vertically split into two parts separated in 1 cm, and the primarybeam passed through without hitting the anode surface.34 By
this way, unfavorable backgrounds originating from the scattered beam were greatly suppressed. The new isotopes of thorium 209Th (Ref. 35) and protoactinium 212Pa (Ref. 36) werediscovered by the use of JAERI-RMS.
Extremely neutron-rich light nuclei are being investigatedby the RIKEN projectile-fragment separator (RIPS) for understanding the nuclear stability near the neutron drip line. Up tonow, such nuclei as 10He (Ref. 37), 31F (Ref. 38), 31Ne (Ref.
39), 37Mg (Ref. 39), 38Mg (Ref. 40), 40Al (Ref. 40), and 41Al
(Ref. 40) were discovered.
2.3. Others. The rest of works related to nuclear chemistry
in Japan were just outlined here.
2.3.1. Photonuclear Reaction. Tremendous yield data havebeen accumulated in 30 − 1200 MeV bremsstrahlung induced
reactions with various targets (7Li to 209Bi) by Sakamoto of
Kanazawa University and collaborators. Almost all of thesedata were obtained radiochemically except for a long-lived
beryllium isotope, 10Be, which was measured by accelerator
mass spectrometry. Because of the continuous nature of thebremsstrahlung energy-spectrum, they performed the measurements at small energy steps less than 50 MeV and the obtained
yields were unfolded into cross sections per photon of a specific energy. They discussed the reaction mechanisms of photonuclear reaction from various aspects, such as π emission
channel,41−44 spallation,45−47 fragmentation,48 and recoil proper-Nuclear Chemistry in Japan
49,50

ties of products.
2.3.2. High Energy Light Ion Induced Reaction. Extensive
investigations have been performed at the National Laboratoryfor High Energy Physics (KEK) to obtain production cross sections in spallation reactions with high energy light ions. Target
materials are ranging from Al to Pb.51 Some of radioactive
nuclides of which decay mode are pure beta emission or electron
capture have large production cross sections. It is practically
impossible to estimate their activities by direct measurements.
One of such nuclides is 3H. Noguchi et al. measured production cross sections of 3H with 12 GeV protons for various targets and deduced a simple relation between production crosssections and target mass numbers.52 These cross section data
are useful for radiation protection at high energy hadron accelerator facilities.
2.3.3. Intermediate Energy Heavy Ion Induced Reaction.
Reaction mechanisms with heavy ions are radiochemicallyinvestigated at the energy region around 100 MeV.53−56 In
addition to the limiting behavior of product yields,55 such asrecoil properties of products53 and angular momentum transfer54 are examined, and target fragmentation mechanism isstudied.
2.3.4. Neutron Induced Reaction. From the view point ofradiation protection and astrophysical interests, production
cross sections of long-lived light nuclides have been measured
with semi-monoenergetic neutrons of energies up to 38
MeV.57−59 The measurements of 14C and 26Al were performedby an accelerator mass spectroscopy method.57,58
2.4. Future plan. Syntheses of new isotopes of heavy elements and study of chemical properties of transactinide elements are intended at JAERI and RIKEN in near future by alarge collaboration group in Japan. The scientific goals are (1)systematic understanding of nuclear properties of heavy nuclei,
such as mass, deformation, beta-delayed fission, etc., and (2)clarification of the chemical properties of transactinide elements which are expected to strongly deviate from their lighthomologs because of the relativistic effect of orbital electrons.
The characteristic decay mode in actinide and transactinide
elements is α-decay and the detection efficiency of α particles
is quite large. In the case of even-even nuclei, it is easily possible to obtain masses of parent nuclei from the principal αenergies and known masses of daughter nuclei. In general, theresolution of the α spectra is not so good as that of γ spectra
and the probabilities of branching decays are small, but forsome nuclei, it is possible to deduce Nilson levels frombranching α decays. The other unique decay mode of heavy
nuclei is β (β −, EC)-delayed fission which is considered to beimportant in the process of nuclear syntheses in nature. It isnecessary for β-delayed fission that β decay energy Qβ is larger than the fission barrier height Bf of the nucleus of interest.
The more apart from the β stability line, the larger the Qβ is.
Accordingly, the probability of β-delayed fission is larger for
nuclei far from the β stability line. Therefore, it is very important to synthesize new isotopes of heavy nuclei far from the βstability line and to investigate their decay properties.
To investigate the chemical properties of the transactinide
elements is one of the most important and challenging subjects
in nuclear chemistry. Due to the strong Coulomb field of thehighly charged atomic nucleus, the motions of orbital electrons
near the nucleus are relativistic, and consequently the sphericals and p1/2 orbitals are stabilized. While the diffused d and forbitals are de-stabilized as a result of shielding of the nuclear
charge by the inner shell electrons. Thus, the electron configurations of the heaviest elements may be different from those oflighter homologs, and which makes the strong deviation of thechemical properties of transactinide elements.

J. Nucl. Radiochem. Sci., Vol. 1, No. 1, 2000 41
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