JAEA-Conf 2011-002 - 日本原子力研究開発機構

JAEA-Conf 2011-002
7. Research in Surrogate Reactions at JAEA
Satoshi Chiba
Advanced Science Research Center, Japan Atomic Energy Agency
Tokai-mura, Naka-gun, Ibaraki-ken 319-1195 Japan
e-mail: chiba.satoshi@jaea.go.jp
A novel method to measure neutron cross sections of unstable or rare nuclei, namely,
the surrogate reaction method, is becoming a unique tool in the field of nuclear data and
nuclear astrophysics. We will describe a status of research in the surrogate method
based on heavy-ion as well as light-ion projectiles carried out at JAEA in collaboration
with other organizations.
1. Introduction
Accurate nuclear data for rare or unstable nuclei are more and more necessitated in
design of next-generation high-burnup reactors and fast reactors acting as transmuters of
long-lived radioactive nuclei contained in nuclear wastes. For these nuclei, direct
measurements using neutrons are extremely difficult to be carried out. Therefore, a lot
of important data still remain unmeasured in the minor-actinide and fission product
reagions. Similary, nuclear data for unstable nuclei at branching points of the s-process
are necessary to assess astrophysical conditions such as density and temperature of the
s-process cite.
Recently, a new method called surrogate method is actively applied to measure
neutron cross section indirectly using available targets. This method utilizes nucleon
transfer reactions or inelastic scattering to populate excited nuclei which correspond to
compound nuclei in neutron-induced reactions on a target nucleus having one-less
neutron. Then, decay branching ratios to fission or capture channel can be determined
in principle. A conceptual drawing of the surrogate method is shown in Fig. 1. This
particular figure explains a way to determine neutron cross sections of 239U which has a
half life of only 23.5 min. Obviously, we cannot conduct direct measurements using
neutrons on a 239U target. Instead, we will prepare in the surrogate method a target of
238U and use a 2 neutron transfer reacction, 238U( 18O, 16O) 240U* reaction, to populate the
same compound nucleus 240U as the desired 239U+n reaction.
Already, US-French collaboration has yielded some interesting results (see, e.g., refs.
[1,2] and references therein). However, physical foundation of the surrogate method is
not established yet. The problem lies in the fact that the spin and parity distributions of
the nuclei populated by the surrogate reactions are not easy to be determined due to the
complexity of relevant reaction mechanisms. Furthemore, the spin-distributions are
(very probably) different from those of the neutron-induced reactions, while the decay
branching ratios are sensitive to the spin-parity values in the energy range of our interest.