Conceptual designing of a slow positron beam system using Simion simulation program
Nuclear Science and Technology, Vol.9, No. 2 (2019), pp. 45-51
Conceptual designing of a slow positron beam system using
Simion simulation program
Cao Thanh Long, Huynh Dong Phuong, Nguyen Trung Hieu, Tran Quoc Dung
Center for Nuclear Techniques, 217 Nguyen Trai Street, District 1, Hochiminh City
(Received 30 October 2019, accepted 19 November 2019)
Abstract: The slow positron beam system is an important device in the study of positron physics and
techniques, especially in materials research. This paper presents obtained results in studying and
applying a charged particles trajectory simulation program – Simion for building feasible design
models for the system. The built models with different designs based on reference designs of some
typical systems include a straight-shaped model, a 500 bent-shaped model and a 900 bent-shaped
model. Some positron beam trajectory calculation tests have been performed for comparison between
the models. From the tests, the 500 bent-shaped model has been proposed as a conceptual design for
building a real slow positron beam system in the future in Vietnam.
Keywords: Slow positron beam system, simulation program, Simion, conceptual design.
positrons with low energy of several eV. The
positron beam is then pre-accelerated by a pre-
accelerator and guided to an energy filter to
separate the slow monoenergetic positrons out
of the original beam. The separated slow
monoenergetic beam is then guided through an
accelerator to be accelerated to necessary high
energy depending on research purposes,
directed to the sample chamber and interacts
with the investigated sample.
I. INTRODUCTION
Positron annihilation techniques play an
important role in the study of micro-defect of
materials, nanostructures, porous materials,
surface analysis, etc. [1]. However, the study
of surface structure, layers or interface regions
cannot be performed with traditional isotope
positron sources because the energy of the
positrons emitted from the sources varies in a
wide range (positrons from the isotope source
with high energy go very deeply into the
sample, which reduces the chance of positron
interaction as well as the formation of
positronium on the material surface) [2]. Slow
positron beam systems have been developed to
solve that problem. They are applied widely in
materials science, physics of solid state,
condensed matter and surface [2]. In general,
the slow positron beam systems in the world
have different designs but follow a general
operating principle. A fraction of high energy
positrons emitted from a radioactive source are
slowed down (moderated) to become slow
Center for Nuclear Techniques (CNT)
has been using positron annihilation techniques
including positron annihilation lifetime and
Doppler broadening spectroscopy for studying
some metal material properties, carbon
nanotubes, and zeolite. However, the main
22
positron sources used for these studies are Na
with continuous energy spectrum that limits the
study of surface properties of materials.
Therefore, the need to build a slow positron
beam system at CNT in the future to perform
such studies is essential. To ensure the
construction feasibility of the system, necessary
©2019 Vietnam Atomic Energy Society and Vietnam Atomic Energy Institute
CONCEPTUAL DESIGNING OF A SLOW POSITRON BEAM SYSTEM USING …
work in this early stage which needs to be done
is to study and use an appropriate charged
particles trajectory simulation program to
conceptually design the system. We have used
Simion as the main tool for the purpose of
conceptual designing. Among typical simulation
programs, Simion has been widely used to
model ion optics problems including calculating
and simulating electrostatic fields, magnetic
fields and trajectories of charged particles flying
through these fields. The program is highly
interactive and has been used effectively in
many research projects on designing and
building slow positron beams at the Institute of
Radiation Physics, Helmholtz-Centre Dresden-
Rossendorf (Germany), Lawrence Livermore
National Laboratory (USA), University of Bath
(UK) and in other countries such as Romania,
Israel, China [3-6]. Besides that, some initial
simulation tests for the reference design of the
SPONSOR system from the Institute of
Radiation Physics, Helmholtz-Centre Dresden-
Rossendorf were performed successfully at
CNT in 2017. The good agreement between our
simulated results and experimental data from the
SPONSOR system demonstrated that Simon can
be used well for conceptual designing of a slow
positron beam system [7].
particles in these fields when introducing the
electrode configuration with voltage and initial
conditions of the particles [8]. Simion is
intended to provide direct and highly interactive
methods for simulating a wide variety of general
ion optics problems such as modeling ion source
and detector optics, ion traps, quadrupoles, etc.
Electrostatic and magnetic fields can be
modeled as boundary value problem solutions
of a partial differential equation called the
Laplace equation (or the Poisson equation). The
specific method used within Simion to solve this
equation is an over-relaxation technique, which
is a finite difference method to obtain a best
estimate of potentials for each point within the
fields. After the electrostatic and magnetic fields
have been obtained, a standard fourth-order
Runge-Kutta method is used for numerical
integration of the charged particle trajectory in
three dimensions based on the particle definition
parameters provided by users. In particular,
Simion provides functions of extensive support
in the definition of geometry, data logging, and
visualization. One of the most useful features of
Simion is the capability of using user programs
inside the program. This feature allows users to
directly utilize a variety of programming
languages including Lua, C++, C, PRG, etc. to
flexibly and efficiently extend simulation
capabilities of the program.
This paper presents results of applying
Simion program to build some design models
for the system based on design principles of
some built systems in the world and propose a
feasible conceptual design of the system that
can be used to prepare for the stage of detailed
engineering design and construction of the
system in the future.
B. Methods
According to several reference system
designs, we have successfully built some
simulation models that can be possible designs
for the system by using Simion. Due to
application limitations of the program, the
design models are only used to simulate the
operating principle of slow positron beam
systems rather than specify detailed
engineering designs. Three main individual
simulation models of the slow positron beam
system have been built including:
II. CONTENTS
A. Overview of Simion program
Simion is a software package used
primarily to model electrostatic and magnetic
fields and calculate trajectories of charged
46
CAO THANH LONG et al.
- The straight-shaped model using the
ExB filter (figure 1).
- The 500 bent-shaped model based on the
design of the SPONSOR system [9, 10] (figure 2).
- The 900 bent-shaped model based on
the design of a slow positron beam system
from Martin Luther University Halle-
Wittenberg, Germany [11] (figure 3).
Fig. 1. The straight-shaped model using ExB filter.
Fig. 2. The 500 bent-shaped model.
Fig. 3. The 900 bent-shaped model.
Some positron trajectory calculation tests
selected model has been proposed as a feasible
conceptual design for the future system. Tables
of design parameters for the proposed conceptual
design include geometry parameters, electric
parameters and design parameters of coils
creating magnetic fields.
have been performed for each model in order to
investigate the influence of design parameters of
each model on the quality of positron beam
obtained on targets, thereby determining which
model was the most reasonable design. The
47
CONCEPTUAL DESIGNING OF A SLOW POSITRON BEAM SYSTEM USING …
uniform distribution source with a radius of 2
mm for each model. The initial kinetic energy
of the beam was 3 eV. The voltages that were
applied to the pre-accelerator and the
accelerator of each model were 27 V and 30
kV, respectively. The simulation results are
shown below in table I, figure 4 and figure 5.
C. Results and Discussions
Case 1: Simulation of the trajectory of a
monoenergetic positron beam
Simion has been used to calculate the
trajectories for 1000 monoenergetic positrons
in a beam emitted isotropically from a circular
Table I. The simulation results for trajectory of a monoenergetic positron beam
Straight-shaped
model
500 bent-shaped
model
900 bent-shaped
model
Total number of positrons from
source
1000
789
1000
807
1000
795
Total number of positrons
coming to target
The ratio of positron number
coming to target to total positron
number
78.9%
80.7%
79.5%
Beam radius on target
2.75 mm
2.39 mm
2.79 mm
Fig. 4. Spatial distribution of positron beam on the target of the straight-shaped model (left), the 500 bent-
shaped model (middle) and the 900 bent-shaped model (right)
(FWHM = 5.50 eV)
(FWHM = 1.34 eV)
(FWHM = 5.71 eV)
Fig. 5. Energy distribution of positron beam on the target of the straight-shaped model (left), the 500 bent-
shaped model (middle) and the 900 bent-shaped model (right)
48
CAO THANH LONG et al.
From the simulation results, it has been
model was more mono-energetic than those of
the others.
found that the quality of positron beam
obtained on the targets is influenced by the
model designs. The models with the different
designs do not have completely identical
uniform magnetic fields along their axes,
resulting in different qualities of obtained
positron beam on the targets.
Case 2: Simulation of the trajectory of a
monoenergetic positron beam in case that a
solenoid deviated from its original position
Another trajectory calculation test has
been done for each model with the same
monoenergetic positron beam in case that the
solenoid surrounding the accelerator of each
model deviated from its original position.
Distributions of positron beam on the target of
each model have been investigated for
comparison. We have simulated two situations
that the solenoid deviations were 1 cm and 2
cm. The simulation results are shown below in
figure 6, figure 7 and figure 8.
The results of the beam radius of
positron beam on the targets show that
obtained positron beam in case of using the 500
bent-shaped model has better convergence
compared to the others. Furthermore, full width
at half maximum (FWHM) of the energy
distribution of positron beam on the target for
the 500 bent-shaped model was smaller than
those of the others. It has shown that the
obtained positron beam of the 500 bent-shaped
Fig.6. Spatial distribution of positron beam on the target of the straight-shaped model in case of solenoid
deviation of 0 cm (left), 1 cm (middle) and 2 cm (right).
Fig.7. Spatial distribution of positron beam on the target of the 500 bent-shaped model in case of solenoid
deviation of 0 cm (left), 1 cm (middle) and 2 cm (right).
49
CONCEPTUAL DESIGNING OF A SLOW POSITRON BEAM SYSTEM USING …
Fig. 8. Spatial distribution of positron beam on the target of the 900 bent-shaped model in case of solenoid
deviation of 0 cm (left), 1 cm (middle) and 2 cm (right).
The comparison results have shown that
the solenoid deviation influenced the quality of
the positron beam that comes to the targets. For
the straight-shaped model, there were a few
positrons obtained on the target in case of
solenoid deviation of 1 cm. In the case of a
solenoid deviation of 2 cm, we have even got
no positrons coming to the target. For the 900
bent-shaped model, the obtained positron
beam has deviated much more from the target
center compared with that of the 500 bent-
shaped model. The beam has even distorted its
shape when increasing the solenoid deviation
to 2 cm. Therefore, we have concluded that
the 500 bent-shaped model in this simulation
case would be the optimal model compared
with the others.
modeling and simulation of the trajectory of
positron beam flying through electrostatic
fields and magnetic fields of a slow positron
beam system. Research of searching for other
simulation programs to combine with Simion
should be done to further optimize the
conceptual design.
REFERENCES
[1]. P. K. Pujari, K. Sudarshan and D. Dutta (Ed.),
"11th International Workshop on Positron and
Positronium Chemistry (PPC-11)", Journal of
Physics: Conference Series, Volume 618,
Conference 1, 2015.
[2]. P. G. Coleman (Ed.), “Positron Beams and
their
applications”,
World
Scientific,
Singapore, 2000.
[3]. F. A. Selim, A.W. Hunt, J.A. Golovchenko, R.
H. Howell, R. Haakenaasen, K.G. Lynn,
III. CONCLUSIONS
“Improved
source
and
transport
of
We have come up with the selection of
the 500 bent-shaped model as a conceptual
design for our slow positron beam system
based on simulation test results and
consideration of the design feasibility of the
simulation models. The proposed model can be
a good basis for detailed engineering design
and construction of the system in the future.
The study results have also demonstrated that
Simion program is a very suitable tool for
monoenergetic MeV positrons”, Nuclear
Instruments and Methods in Physics Research
B 171 (2000), 182-188, 2000.
[4]. S. May-Tal Beck, D. Cohen, E. Cohen, A.
Kelleher, O. Hen, J. Dumas, E. Piasetzky, N.
Pilip, G. Ron, I. Sabo-Napadensky, R Weiss-
Babai, “Design of the Slow POsitron faciliTy
(SPOT) in Israel”, 13th International
Workshop on Slow Positron Beam Techniques
and Applications, 2014.
50
CAO THANH LONG et al.
[5]. C. K. Cheung, P. S. Naik, C. D. Beling, S. Fung,
[9]. Wolfgang Anwanda, Gerhard Brauer, Maik
Butterling, Hans-Rainer Kissener, Andreas
Wagner, “Design and Construction of a Slow
Positron Beam for Solid and Surface
Investigations”, Defect and Diffusion Forum
Vol. 331, 2012.
H. M. Weng, “Performance of a slow positron
beam using a hybrid lens design”, Department
of Physics, University of Hong Kong, Pokfulam
Road, Hong Kong, PR China, 2006.
[6]. Xu Hong-Xia, Liu Jian-Dang, Gao Chuan-Bo,
Weng Hui-Min, Ye Bang-Jiao, “SIMION
simulation of a slow pulsed positron beam”,
Department of Modern Physics, University of
Science and Technology of China, Hefei
230026, China, 2012.
[10].Wolfgang Anwanda, Gerhard Brauer,
Hans-Rainer
Kissener,
“Magnetically
guided slow positron beam for defect
studies”, Positron Group of TU Dresden at
Research Centre Rossendorf, Dresden,
Germany, 1994.
[7]. Cao Thanh Long, Nguyen Trung Hieu, Tran
Quoc Dung, Huynh Dong Phuong, “Some
initial results of simulating a positron beam
system by using SIMION”, Nuclear Science
and Technology, Vol.7, No. 3, 17-24, 2017.
[11].
R. Krause-Rehberg, “Simple design for a
continuous magnetically guided positron
beam – and – News from the EPOS project”,
Institute of Physics, Martin Luther
University Halle-Wittenberg, Germany,
2010.
[8]. David J. Manura, David A. Dahl, “SIMION
Version 8.0/8.1 User Manual”, Document Revision
5, Scientific Instrument Services Inc., 2011.
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