The production of the exotic neutron-rich ion beams from photofission of the actinide targets in an IGISOL facility will be studied via an experimental program that will take place at the Extreme Light Infrastructure - Nuclear Physics (ELI-NP) facility. Geant4 simulation toolkit was used for optimizing the target configuration in order to maximize the rate of released photofission fragments from targets placed in a cell filled with He gas

Anh, Le Tuan

Balabanski, Dimiter

Constantin, Paul

Cuong, Phan Viet

Ha, Nguyen Hong

Khue, Pham Duc

Mei, Bo

Thanh, Kim Tien

Thao, Ho Thi

Vinh, Nguyen The

Vietnam Academy of Science and Technology: Journals Online

Communications in Physics, Vol. 29, No. 3 (2019), pp. 277-284
DOI:10.15625/0868-3166/29/3/13831
SIMULATION FOR OPTIMIZING THE DESIGN OF CRYOGENIC STOPPING
CELL FOR THE IGISOL FACILITY AT ELI-NP
LE TUAN ANH1,† PHAN VIET CUONG2, P. CONSTANTIN3, B. MEI3,
D. L. BALABANSKI3, NGUYEN HONG HA4, HO THI THAO4, KIM TIEN THANH4
NGUYEN THE VINH5, PHAM DUC KHUE6 AND HOANG HUU DUC7
1Graduate University of Science and Technology, Vietnam Academy of Science and Technology
2Research and Development Center for Radiation Technology, Vietnam Atomic Energy Institute
3Extreme Light Infrastructure – Nuclear Physics, “Horia Hulubei” National Institute for Physics
and Nuclear Engineering, Str. Reactorului 30, 077125 Bucharest Magurele, Romania
4Centre of Nuclear Physics, Institute of Physics, Vietnam Academy of Science and Technology
5Vietnam Atomic Energy Institute
6Institute for Nuclear Science and Technology
7Centre for Technology Environmental Treatment, Ministry of Defence, Hanoi, Vietnam
†E-mail: letuananh.nuclphys@gmail.com
Received 20 May 2019
Accepted for publication 16 July 2019
Published 15 August 2019
Abstract. The production of the exotic neutron-rich ion beams from photofission of the actinide
targets in an IGISOL facility will be studied via an experimental program that will take place at the
Extreme Light Infrastructure - Nuclear Physics (ELI-NP) facility. Geant4 simulation toolkit was
used for optimizing the target configuration in order to maximize the rate of released photofission
fragments from targets placed in a cell filled with He gas.
Keywords: ELI-NP, Photofission, Gas cell, IGISOL, Radioactive ion beam.
Classification numbers: 23.90.+w, 25.85.Jg.
I. INTRODUCTION
Extreme Light Infrastructure (ELI) is one of the 48 infrastructures of the European Strategy
Forum for Research Infrastructure (ESFRI) [1] . ELI-NP facility, which is one of the three labo-
ratories of the ELI [1, 2], has the mission to promote nuclear physics studies with a laser-driven
c©2019 Vietnam Academy of Science and Technology
278 SIMULATION FOR OPTIMIZING THE DESIGN OF CRYOGENIC STOPPING CELL ...
electron, proton or heavy-ion beams, and especially with a brilliant γ beam. This γ beam is ob-
tained by Compton backscattering (CBS) of a laser beam on an intense electron beam accelerated
by a linear accelerator [3]. This gamma beam will be highly polarized (>99%) and have a high
spectral density of up to 4.104 photons/s/eV in the energy range of 0.2-19.5 MeV with a bandwidth
of 0.3-0.5%.
Because of having energy range which covers the whole Giant Dipole Resonant of Uranium
and Thorium isotopes [4], the ELI-NP gamma beam is suitable for the production of the exotic
neutron-rich photofission fragments.
To form the radioactive beam from photofission fragments, an ion-guide isotope separation
on-line (IGISOL) facility will be constructed at ELI-NP. The gamma beam impinging on Uranium
thin foils placed in the center of a Cryogenic Stopping Cell (CSC) will induce photofission. The
ions diffusing into the gas from the thin foils will, then, be drifted out by a strong DC field in the
orthogonal direction. When these ions reach close to the cell wall, a resonant RF fields will push
them towards the exit nozzle where they are taken out in a supersonic jet by a gas flow.
In this paper, simulations with Geant4 toolkit were performed for optimizing some basic
parameters for the development of CSC prototype for IGISOL facility at ELI-NP.
II. LASER COMPTON BACK SCATTERING AT ELI-NP
Fig. 1. Energy-angle correlation for two gamma beams: a broad beam up to 18.5 MeV
collimated below 0.7 mrad (blue) and a pencil beam up to 12.9 MeV collimated below
0.09 mrad (red) [5].
The ELI-NP gamma beam will be produced through Compton Backscattering of a laser
beam on an intense accelerated electron beam. CBS can be considered as a ”photon accelerator”.
More details of CBS were presented in [3]. The energy which a photon with the initial energy
EL obtained after scattering off a relativistic electron with the kinetic energy Te is approximately
calculated as:
E(θ ,Te) =
4γ2eEL
(1+δ 2/4+a20p/2)+ γ2e θ 2
(1)
LE TUAN ANH et al. 279
where θ is the photon scattering angle and γe = 1+ Temec2 is Lorentz factor of accelerated electron.
ELI-NP gamma beam uses green laser with EL=2.4 eV, the laser incident angle δ = 7.5o and laser
parameter a0p = 0.041 [5]. Eq.(1) implies that by using a suitable collimator placed at certain θ
angle, one can select the energy of gamma beam. The maximum energy which a scattered photon
can gain is achieved in head-on collisions. With the above set of parameters, the maximum energy
is given by:
Eγmax(Te) = 9.55eV (1+
Te
mec2
)2 (2)
where me is the rest mass of the electron and c is the speed of light.
Figure 1 presents the two types of γ beams that will be produced at ELI-NP by using suitable
collimator. This figure is obtained through Geant4 simulation. The broad beam marked by blue
dots has the energy range from 10 MeV to 18.5 MeV. This broad beam, which obtained by setting
Te = 720 MeV and collimating the beam below 0.7 mrad, will be used for photofission.
III. THE IMPLEMENTATION OF GEANT4 CODE FOR OPTIMIZING THE DESIGN
OF GAS CELL AT ELI-NP
Geant4 Monte Carlo simulation framework [6] is dedicated to the simulation of particles
through matter. In our work, the Geant4 code was implemented to help in giving a conceptual
design of future CSC, as well as studying setup for other future experiments at ELI-NP.
III.1. Simulation of photofission process in Geant4
Fig. 2. The 238U photofission cross sections measured
by Caldwell et al. [4], Ries et al. [7], and Csige et
al. [8], as a function of the incident photon energy. The
full line indicates calculations by the parametrization
developed by our work in [9].
To describe a process in Geant4,
two mandatory modules must be imple-
mented. The first one controls the cal-
culation of reaction cross-section, while
the other determines the final states of
out-going particles and residual nucleus.
For the first module implementa-
tion, the 238U photofission cross-section
was calculated by the parametrization
obtained by using the experimental data
measured by Caldwell et al. [4], Ries et
al. [7], and Csige et al. [8]. Fig-
ure 2 shows the comparison between
the parametrization and experimental
data. More details for this empiri-
cal parametrization were presented in
our publication [9]. This parametriza-
tion is implemented into a new class
which inherited from Geant4 class
G4VCrossSectionDataSet.
The second module controls which particles will be created and their kinematics. This
module includes two parts. The first part relates to what kind of fragments and particles will be
created in the photofission. The parametrization presented in our publication [9] was implemented
280 SIMULATION FOR OPTIMIZING THE DESIGN OF CRYOGENIC STOPPING CELL ...
into Geant4 for this job. Figure 3 shows the yields calculated by the parametrization (the red line)
in comparison with experimental data measured by Donzaud et al. [10] and Pellereau et al. [11].
The second part of the second module generates the kinematics of photofission fragments using
total kinematics energy data from [12, 13] and energy and momentum conservation laws.
III.2. Ion stopping process in Geant4
Fig. 3. Comparison between the mass yields
measured in two experiments at GSI via the vir-
tual photon induced fission of 238U [10, 11] and
those calculated by the parametrization developed
by our work in [9].
After being generated, the photofis-
sion fragments propagate inside the 238U
target and lose their kinetic energy. Some
of the fragments will lose all their kinetic
energy and stop inside the foils. Mean-
while, the others will be released into the
gas, and continue to be slowed down by
the gas. The transport of fragments in-
side target and gas is handled by Geant4
classes for low energy electromagnetic in-
teractions, including the electronic and nu-
clear ion stopping and multiple scattering
effects.
The energy loss by ionization is de-
scribed by the well-known Bethe–Bloch
formula in which the ionic charge q is as-
sumed to be constant during stopping pro-
cess. This assumption is satisfactory for the
ions which have large velocity and low nu-
clear charge because all of their electrons will be quickly stripped out. In general, however, q
fluctuates during ion stopping process in matter due to the competition between ionization and
electron capture processes [5]. Geant4 uses the ionic effective charge formalism from Ziegler and
Manoyan [14] to describe the evolution of q. In our work, another q-parametrization developed
by Schiwietz and Grande [15] is implemented into Geant4 in order to have a comparison with
Ziegler-Manoyan q-parametrization.
IV. TARGET GEOMETRY OPTIMIZATION
The future CSC will be installed at two considered locations at distance D =7 m and 40m
from the γ origin. Figure 4 shows the target configuration inside the gas cell. The γ beam prop-
agates along the positive z-axis. The tilting foils are placed along the γ beam. The fragments
from photofission of 238U released from these foils will be slowed down transversally in the gas
and drifted by a direct current (DC) field. With the fixed number of foils N, the release rate, Nr,
depends on the transversal size A, the tilting angle a and the foil thickness t.
The transversal size A should be set equal to the beam spot size for optimizing the number
of photofission occurring in 238U foils. The beam spot size, and then A, can be estimated as
follows:
A = 2Dθ = 4D
√
El/Eth−EL/Eγmax (3)
LE TUAN ANH et al. 281
where EL = 2.4 eV is the laser photon energy and Eth is the energy threshold. In the case of ELI-
NP range of γ-ray emission angle θ < 1 mrad , the small angle approximation tan(θ)≈ sin(θ)≈ θ
is used. In the rest of this work, the maximum fragment release rate Nr is optimized for Eth = 12
MeV and Emax = 17 MeV.
Fig. 4. The yz-plane of the target geometry inside the gas cell. The gamma beam propa-
gates along the z axis and the DC field drifts ions along the x axis.
The transversal size A, the tilting angle a, and the number of foils N affect the total length
Lt as following:
Lt =
NA
tan(a)
+(N−1)s (4)
where s is the inter-foil distance. The value s should be zero so that the number of foils N gets the
maximum value. Thus, Eq. (4) is rewritten:
Lt =
NA
tan(a)
(5)
Because of the space constraints at the first CSC location, the target length is fixed at its maximum
value Lt = 1 m [16]. Meanwhile, Lt = 2 m is chosen for the second location.
Fig. 5. The dependence on the 238U foil thick-
ness t of the photofission rate with black circles
and of the fragment release rate using different q-
parameterizations: Schiwietz- Grande [15] in red
and Ziegler-Manoyan [14] in blue. The maximum
release rate was found to be in the range of 106 to
107 ions/s.
The dependence of photofission rate
and release rate on the foil thickness are
presented in Fig. 5 for Lt = 1 m, A = 6 mm
and a = 10˚, leading to N = 30. The
black circles stand for photofission rate,
while the squares are for release rate. The
shape of the photofission rate implies that
the photofission rate increases proportion-
ally with the increase of t.
The release rate Nr, however, in-
creases quickly and reaches saturation af-
ter a certain foil thickness. This means
that any increase above this value leads
to an increase of the mass of 238U used,
without a gain in the rate of released frag-
ments. Hence, the background related to
pair production γ → e+e− in the target
foils would increase. Saturation is met
with t > 1µm for the Schiwietz-Grande
q-parameterization [15] expressed by blue
282 SIMULATION FOR OPTIMIZING THE DESIGN OF CRYOGENIC STOPPING CELL ...
squares and t > 2µm for the Ziegler-Manoyan q-parameterization [14] marked with red squares
in Fig. 5. The optimal foil thickness is t ≈ 2µm. This value remains approximate when the other
target geometry parameters change [16].
The release rate depends weakly on the tilting angle a. A series of simulations were done
by changing the value of a, and the results showed that the distribution in Fig. 5 goes down by 2%
and increases by 5% when a is changed by 10˚, respectively. However, the tilting angle may relate
to the loss of released fragments by hitting neighboring foils. This loss increases fast at large a:
from 1% at 5˚, to 3.5% at 15˚, to 24% at 45˚. The 3.5% efficiency loss is considered acceptable,
i.e. a = 15˚ is the optimal choice.
There is another parameter which also affects the release rate. This parameter is the backing
thickness B. The backing is the thin layers of graphite covering the 238U foil for supporting. When
the fragments travel inside the graphite layer, some of them will lose energy and stay inside the
backing layer. This leads to a decrease of the fragments entering the gas. The level of loss depends
on the thickness of the backing layer. To optimize the backing thickness, the quantity PB(%)
is used:
PB =
Number of ions lost in the backing layers
Number of ion released from 238U foils
(6)
Figure 6 shows the dependence of the loss fraction PB on backing thickness for both
Schiwietz-Grande and Ziegler-Manoyan q-parameterizations. If PB = 5% marked by dash line
is acceptable, then the optimal value for backing thickness can be in the range 0.4 - 0.9 µm.
Fig. 6. The dependence of PB on the backing foild thickness.
V. STOPPING LENGTH OF RELEASED FRAGMENTS IN GAS
After releasing out of the target foils, the photofission fragments travel and stop in the He
gas. Studying the stopping length will help to optimize the width of the CSC, i.e. the param-
eter d in Fig. 4. The maximum extraction efficiency of ion in He gas has been observed [17]
in the temperature range between 60 K and 90 K. The gas pressure values below 300 mbar are
LE TUAN ANH et al. 283
considered, accordance with expected limitations of RF carpet functionality [18]. The stopping
length, L, depends on the He gas configurations. Three sets of temperatures and pressures of He
gas are used for studying L: A (T = 90 K, P = 100 mbar), B (T = 80 K, P = 200 mbar), C (T
= 70 K, P = 300 mbar). The corresponding density of these three sets are ρA = 0.053 mg/cm3,
ρB = 0.120 mg/cm3, and ρC = 0.206 mg/cm3, respectively.
Fig. 7. Stopping length for various densities of the He gas: ρA = 0.053 mg/cm3 (blue
triangles), ρB = 0.120 mg/cm3 (red circles) and ρC = 0.206 mg/cm3 (black squares).
Figure 7 shows the fragment stopping length distributions corresponding to each of the
above gas parameter sets. The maximum stopping length, Lmax, which is the path-length at which
95% of the fragments have stopped, is used for determining the width of CSC. Lmax = 43.7 cm is
found for the parameter set A. Meanwhile, Lmax = 19.4 cm, and Lmax = 11.3 cm are found for B
and C, respectively. In all cases, the following relationship is found:
ρLmax = 2.33 mg/cm2 (7)
The width of the CSC can be slightly set above d ≈ 2Lmax. For instance, if the cell operates at
100 mbar and 90 K, its width would be d = 88 cm.
VI. CONCLUSIONS
An implementation of Geant4 simulation toolkit was used for optimizing the target geome-
try of CSC at ELI-NP. The optimal value for 238U foil thickness is t ≈ 2 µm. The tilting angle has
a small effect on the release rate, but it has an impact on the loss of released fragment by hitting
neighboring foils. The optimal value is found to be 15˚ for a. The presence of graphite backing
layers introduces the loss of fragments by stopping inside these layers. The backing thickness
should be chosen in the range from 0.4 µm to 0.9 µm to hold PB ≈ 5%. The maximum release rate
was found to be in the range of 106 to 107 photofission fragments per second. The width of the
CSC should be slightly longer than the value 2Lmax, i.e. depending on the He gas configurations,
the parameter d can be determined.
284 SIMULATION FOR OPTIMIZING THE DESIGN OF CRYOGENIC STOPPING CELL ...
ACKNOWLEDGMENT
This work was supported by the Extreme Light Infrastructure Nuclear Physics Phase II, a
project co-funded by the Romanian Government and the European Union through the European
Regional Development Fund’s Competitiveness Operational Programme (1/07.07.2016, COP, ID
1334). Phan Viet Cuong and Le Tuan Anh acknowledge the support from the Vietnam Academy
of Science and Technology under Grant No. VAST. CTVL.03/17-18.
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Simulation for Optimizing the Design of Cryogenic Stopping Cell for the IGISOL Facility at ELI-NP

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