JAEA-Conf 2011-002 - 日本原子力研究開発機構

JAEA-Conf 2011-002
equilibrium emission of light complex particles (deuteron, triton, 3 He and α particle) in nucleon-induced reactions.
To describe the dynamical processes in these reactions we used both a phase space statistical approach, with the
exciton model (EM) [8,9,10] and the Kalbach systematics [11], and a microscopic simulation approach, with the
quantum molecular dynamics (QMD) model [12,13,14] complemented by a surface coalescence model (SCM) as
described by Watanabe and Kadrev [15]. Neutron experimental data considered in this work are from preliminary
results of the measurements conducted at TSL by Bevilacqua et al. [16]; proton induced data, retrieved from the
EXFOR database, are from Piskor-Ignatowicz [17] and from experiments by Cowley et al. [18].
2. Materials and methods
2.1 Exciton model and Kalbach systematics
We focused our study on the dynamical processes in the production of light charged particles (proton,
deuteron, triton, 3 He and α particle) in nucleon-induced reactions. The two-component EM [8,9,10] describes the
time evolution of the nuclear state; this description is given by the total energy of the system and the total number
of particles above the Fermi surface and corresponding holes below it. The EM does not include direct-like
mechanisms as the nucleon transfer (NT) and the knock-out (KO) of preformed clusters. These mechanisms are
playing a relevant role in the production of light complex particles in the pre-equilibrium emission region. To
account for these direct-like mechanisms, Kalbach [11] proposes a phenomenological model based on
experimental proton and neutron-induced data, with energies respectively up to 90 MeV and up to 63 MeV.
TALYS-1.2 [19] is a code developed to analyze and predict nuclear reactions involving neutrons, photons and
light charged particles for energies up to 200 MeV. The two component EM complemented by the Kalbach
systematics is the default model used by TALYS to calculate nucleon induced DDX for light charged particles
production. TALYS allows to scale the contribution to the DDX of the NT and KO direct-like mechanisms
described by Kalbach. These are the Cstrip parameter (NT) and the Cknock parameter (KO). Their value can vary
between 0 (no contribution) to 10; the default value is 1, corresponding to the original Kalbach prescription.
2.2 Quantum molecular dynamics and surface coalescence model
The QMD model [12,13,14] is a semiclassical simulation method that gives a microscopic description of
the time evolution of nucleon many-body system. Each nucleon propagates in the nuclear mean field formed by
all other nucleons and interactions among nucleons are described by stochastic two-body collisions. In the
original QMD simulation method the nucleon many-body system evolves for a given time, of the order of 10 -22 s,
after the first interaction between the incident neutron and the target nucleus; at the end of this evolution time,
emitted single (proton, neutron) and complex particles are identified according to a specified set of rules.
However, this method underestimates the pre-equilibrium production of light complex particles. To account for
this underestimation Watanabe and Kadrev [15] proposes a modification of the QMD model, including a surface
coalescence effect. In this description, they assume that cluster formation occurs in low-density region of the
nucleon many-body system, i.e. on the surface of the composite system. Here, when a leading nucleon reaches an
a priori defined boundary region, the time evolution of the system is suspended and condition for the formation
of a cluster in the phase space is checked. If this condition is positively verified, then a kinetic energy condition is
checked. If the kinetic energy of the cluster and the Coulomb barrier tunneling allow it, then a cluster particle is
emitted; otherwise, only the leading nucleon is emitted as single particle. The simulation then resumes and the
system evolves until a next leading nucleon will reach the boundary region or until the given evolution time is
completed. The generalized evaporation model is used to describe particle emission when the compound system
reaches thermal equilibrium. In our work, we used a modified version of the JQMD [12,15] code. A complete
description of the method is given by Watanabe and Kadrev [15].
3. Results and discussion
Bevilacqua et al. [16] presented preliminary DDX for the production of light charged particles, at
several angles in the laboratory system, in the interaction of 175 MeV QMN with Fe and Bi. Experimental proton
production is reproduced by default TALYS calculations with the two-component exciton model, whereas
production of light complex particles is largely overestimated by the TALYS code in the pre-equilibrium
emission energy region. In Figure 1, DDX for production of deuteron, triton, 3He and α particle from Bi at