High-resolution fission studies with the planned GBS facility at ELI-NP

ibutions, following the decay of states in 
the different minima of the PES in the region 
of the light actinides. An important goal is to 
resolve the so far unobserved fine structure of 
the isomeric shelf by decomposing it into 
individual transmission resonances, and to 
observe the predicted nucleon clusterization 
phenomena in SD and HD states of the 
actinides. The polarized γ beams provide an 
excellent opportunity to study the space 
asymmetry of the angular distribution of the 
fission fragments and the correlation between 
the space asymmetry and the asymmetry of the 
fission process [21,22]. 
Another topic, which will be covered by 
the photo-fission programme at ELI-NP, are 
studies of exotic fission modes, such as ternary 
fission, Pb radioactivity and collinear cluster 
tripartition. Ternary fission has been studied so 
far in neutron-induced and spontaneous fission 
experiments, while ternary photo-fission has 
never been observed, due to the low cross 
section and the limited intensities of hitherto 
available photon beams. Fission studies at ELI-
NP will be done with highly polarized beams, 
fixing the geometry of the process, which is 
advantageous for detailed studies. Ternary 
particles, being released close to the scission 
point, provide information about the neck 
formed between the two heavy fission 
fragments, and about fission dynamics. Hence, 
it is very interesting to measure light-particle 
emission in coincidence with fission fragments. 
In order to make these measurements 
possible, two new detector arrays are being 
developed based on existing, well-understood 
cutting-edge technologies [21,22]. These 
detectors will be shortly discussed in the 
sections below, along with presenting some 
test experiments demonstrating their 
performance. 
III. DETECTOR ARRAYS FOR THE 
PHOTO-FISSION STUDIES AT ELI-NP 
For the photo-fission studies with the 
GBS facility at ELI-NP, two detector arrays are 
under development: ELITHGEM and ELI-
BIC. The arrays will be positioned at the 
closest possible distance from the -beam 
production point. 
A. The ELI-THGEM Array: Measurements 
of fission cross sections 
A multi-target detector array, 
ELITHGEM is under development, consisting 
of position sensitive detector modules based on 
the THick Gas Electron Multiplier (THGEM) 
technology [23]. The array consistes of twelve 
THGEM boards inside a low-pressure gas 
chamber, coupled with a transmission mesh 
and a segmented delay-line read-out electrode 
providing a true pixelated radiation 
localization. It will be used for measurements 
of fission cross sections and fission fragment 
angular distributions as a function of the 
D. CHOUDHURY
et al. 
33 
photon energy. The experiments will be carried 
out as a function of the -beam energy. It will 
be possible to map fission resonances with the 
array, which can be a day-one experiment 
within this research program. This detector 
array covers almost a full solid angle (around 
80% of 4π) and has an angular resolution of 
about 5°. A set of ten target foils will be placed 
in the centre of the array and tilted at 45 with 
respect to the beam direction. A performance 
test of one THGEM detector unit was carried 
out using spontaneous fission of 
252
Cf, at MTA 
Atomki, Hungary. The results demonstrated 
good position sensitivity and time stability of 
the detector unit [22]. 
B. The ELI-BIC Arrays: Measurements of 
the properties of the fission fragments 
For studies of the fission fragment 
characteristics, such as energy, mass and 
angular distributions, a highly-efficient four-
fold array, ELI-BIC, of Frisch-grid twin Bragg 
ionization chambers (BIC) [24,25], is designed 
and developed. A thin (100-200 µg/cm
2
) 
elliptical target foil of dimensions 3 cm 0.5 
cm (major and minor axis, respectively), and 
tilted at 10 with respect to the -beam 
direction, will be placed in the centre of the 
cathode of each BIC. The target dimension is 
based on the simulated beam spot dimensions 
using the beam parameters with 0.5% photon 
energy bandwidth [26,27]. The multi-target 
set-up will increase the photo-fission yield at 
deep sub-barrier energies with very low cross 
sections. 
Each BIC of the ELI-BIC array will be 
coupled to eight ΔE-E detectors, consisting of 
an ionization chamber for registration of the 
ΔE signal and a double-sided silicon strip 
detector (DSSD) for the E signal. The array of 
these eight ΔE-E telescopes covers in total 
The array will be used 
for the identification of ternary fission particles 
and the study of their correlations with the 
properties of the fission fragments. 
IV. PERFORMANCE TESTS OF ONE BIC 
WITH ONE E-E DETECTOR 
So far, several tests have been performed 
already to demonstrate the sensitivity and 
performance of the above mentioned detector 
assemblies. Below we report some first-stage 
results of two test experiments. 
A. In-beam experiment performed at the 
cyclotron accelerator in Debrecen 
The experiment was carried out at the 
MGC-20 cyclotron facility of MTA Atomki 
(Debrecen) employing the 
9
Be(p, n) reaction 
to produce 18 MeV neutrons. The neutrons 
were then thermalized by paraffin blocks for 
the 
235
U(nth, f) reaction. A 230 ug/cm
2
 thick 
target foil was placed in the center of the BIC 
cathode. One BIC was coupled with one E-E 
telescope. Signals were collected from both 
anodes and grids of the BIC for the binary 
fission fragments, the anode and cathode of the 
ionization chamber and the two sides of the 
DSSD for the detection of the energy loss E 
and the remaining energy E of light charged 
particles, respectively. 
Signals from the BIC and E electrodes, 
after pre-amplification, were amplified and 
shaped by using normal analog data acquisition 
set-up using amplifiers and discriminators. The 
DSSD data was acquired using analog read-out 
with multiplexer (MUX) interfaces. The data 
was recorded for two days, using an online 
acquisition software developed at MTA 
Atomki. Offline analysis of the data was 
carried out using a ROOT-based [28] analysis 
program to determine the various properties 
(mass, kinetic energy and angular distribution) 
of the fission fragments. The total kinetic 
energy (TKE) vs. mass distribution of the 
fission fragments is shown in Fig. 1. The mass 
HIGH-RESOLUTION FISSION STUDIES WITH THE PLANNED GBS FACILITY AT ELI-NP 
 34 
distribution of the light and heavy fragments 
for the so-called cold fission events, with total 
kinetic energy of TKE>180 MeV and thus very 
low fragment excitation energies, are shown in 
Fig. 2. At ELI-NP, the data from the BIC will 
be acquired using a digital acquisition system 
consisting of 14-bit 500 MS/s waveform 
digitizers facilitating both triggerless as well as 
triggered waveform processing, and better 
sensitivity of the array. 
Fig. 1. TKE-mass correlation of fission fragments produced from 
235
U(nth, f). 
Fig. 2. Pre-neutron mass distribution of fission fragments produced from 
235
U(nth, f). 
B. Performance test using a spontaneously 
fissioning 
252
Cf source 
Due to the moderate neutron flux and 
the very small branching ratio of ternary to 
binary fission, light charged particle 
accompanied fission could not be studied in 
detail. Thus, a 
252
Cf spontaneous fission 
source with a specific activity of ~30 Bq was 
placed in the cathode of the BIC to test the 
capability of the dE-E array, using the same 
settings of the front-end electronics as in the 
in-beam experiment. 
D. CHOUDHURY
et al. 
35 
Preliminary results show a very distinct 
dE-E correlation of the ternary particles (Fig. 
3), measured by one E-E telescope. Energy 
calibration still has to be performed 
considering the results of on-going GEANT4 
simulations: in the present BIC detector 
configuration and geometry, conventional α 
sources cannot be used for energy calibration 
of the dE-E array since alpha particles with a 
kinetic energy of E=4-5 MeV are fully 
stopped withing the range of the Bragg 
section of the array. The shown E-E 
correlations, where the lines represent fits to 
guide the eye, are expected to correspond to α 
and tritons, respectively. A proper energy 
calibration and more statistics are needed for 
confirmation. However, the current result 
clearly indicates that the newly designed ΔE-
E detector array is very promising for the 
efficient identification and study of ternary 
fission particles, and hence, ternary photo-
fission studies at ELI-NP. 
Fig. 3. Uncalibrated ΔE-E correlation of the light charged particles released in fission. 
V. CONCLUSIONS AND OUTLOOK 
The high-intensity, narrow-bandwidth, 
tunable, highly-polarized and highly-
focussed gamma beams at ELI-NP will 
enable high-resolution photo-fission studies 
in actinide nuclei, in the low cross section 
sub-barrier region. Two detector arrays, have 
been designed for such studies. Performance 
tests of the produced prototypes of the 
instruments demonstrate that the expected 
performance is achieved. Necessary DSSD 
detectors and corresponding electronics have 
been procured and tested. The construction 
of the arrays is in progress. 
The detectors will be further tested with 
fission sources and used in-beam at existing 
facilities. The array will be ready for day-one 
experiments at ELI-NP for measurements of 
fission fragment charateristics of light actinide 
nuclei. The study of fission barriers in 
234,238
U 
and 
230,232
Th by high-resolution measurements 
of transmission resonance in the sub-barrier 
energy region of 5–6 MeV will be aimed at as 
day-one flagship experiments at ELI-NP. 
The distant vision for photo-fission 
exeriments at ELI-NP, beyond day-one, 
includes the coupling the fission arrays with 
the gamma- and neutron detector arrays called 
HIGH-RESOLUTION FISSION STUDIES WITH THE PLANNED GBS FACILITY AT ELI-NP 
 36 
ELIADE [27] and ELIGANT [28], 
respectively, which are developed under other 
experimental programmes with the GBS at 
ELI-NP. This coupling of detectors will enable 
high-precision measurements of photon-
induced prompt-fission gamma-rays and 
neutron spectra, as well as -decay 
spectroscopy of excited fission fragments. The 
study of photo-fission, photo-neutron and 
neutron-fission correlations will become 
possible using the time-of-flight technique. 
ACKNOWLEDGEMENTS 
The authors acknowledge the support 
from the Extreme Light Infrastructure Nuclear 
Physics (ELI-NP) Phase II, a project co-
financed by the Romanian Government and the 
European Union through the European 
Regional Development Fund - the 
Competitiveness Operational Programme 
(1/07.07.2016, COP, ID 1334) ) and by the 
Hungarian NKFI (OTKA) Foundation No. 
K124810. 
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