The gamma two-step cascade method at dalat nuclear research reactor

Nuclear Science and Technology, Vol. 4, No. 1 (2014), pp. 57-61 
The gamma two-step cascade method 
at Dalat Nuclear Research Reactor 
Vuong Huu Tan
1
, Pham Dinh Khang
2
, Nguyen Nhi Dien
3
, Nguyen Xuan Hai
3
, 
Tran Tuan Anh
3*
, Ho Huu Thang
3
, Pham Ngoc Son
3
, Mangengo Lumengano
4
1)Vietnam Agency for Radiation and Nuclear Safety, 113 Tran Duy Hung, Hanoi, Vietnam 
2) Vietnam Atomic Energy Institute, 59 Ly Thuong Kiet, Hanoi, Vietnam 
3) Nuclear Research Institute, 01 Nguyen Tu Luc, Dalat, Vietnam 
4) Agostinho Neto University, Av, 4 Fevereiro, 71 Ingombotas, Luanda, Angola 
*Email: ndsdalat@vnn.vn 
(Received 7 March 2014, accepted 13 March 2014) 
Abstract: The event-event coincidence spectroscopy system was successfully established and operated 
on thermal neutron beam of channel N0. 3 at Dalat Nuclear Research Reactor (DNRR) with resolving 
time value of about 10 ns. The studies on level density, gamma strength function and decay scheme of 
intermediate-mass and heavy nuclei have been performed on this system. The achieved results are 
opening a new research of nuclear structure based on (n, 2) reaction. 
Keywords: event-event coincidence, thermal neutron beam, nuclear structure. 
I. INTRODUCTION 
The nuclear parameters obtained from 
intensities of two-step cascades have 
considerably higher reliability than those 
obtained within known methods due to 
unsuccessful relation between the experimental 
spectra and desired parameters of the gamma-
decay process. For excited levels below 2 MeV, 
their spectroscopic information in detail were 
known very well from investigations of (n, ), 
(n, e), (d, p)... reactions. However, for higher 
excited levels, the information is not enough 
because of low intensity of transitions and bad 
resolution of detectors [1]. 
The traditional gamma spectrometer 
allows getting more information about nuclear 
data and nuclear structure from their spectra. The 
background, however, is high due to Compton 
scattering. In order to reduce the background, it 
is necessary to develop advanced spectrometers 
such as Compton suppression, pair production, 
or coincidence systems. 
 In this work, the gamma two-step 
cascade (TSC) method has been developed to 
optimize solution and to reduce Compton 
scatter and pair-production phenomena in the 
gamma spectra of nuclei decay gamma 
cascades. This is allowed to determine 
precisely gamma cascade intensities and to find 
intermediate levels in an energy region near a 
binding energy. Since, the transition 
probabilities and quantum characteristics of 
intermediate levels are split. The characteristics 
allow comparing transition probabilities 
between theory and empirical results [2]. 
II. TSC METHOD 
The method is based on event-event 
coincidence measurements of two γ-rays from 
the cascade decay of a compound nucleus 
following thermal neutron capture. The total 
energies of the γ-rays and their time 
differences are measured by two germanium 
detectors. Coincidence events are selected 
which have a sum energy given by the energy 
©2014 Vietnam Atomic Energy Society and Vietnam Atomic Energy Institute 
THE GAMMA TWO STEP CASCADE METHOD AT DALAT RESEARCH REACTOR 
58 
difference between the capture state and the 
pre-selected low-lying state. The detected 
spectrum then contains information on two 
types of transitions. The 1
st
 type includes first 
transitions populated in the intermediate 
region of excited energy. Because of large 
number of levels in this region, no 
spectrometer is available for data acquisition. 
The 2
nd
 one includes transitions that the 
intermediate levels dominate low energy 
levels
[2, 3, 4]. In this case, the event-event 
coincidence spectroscopy can be used in 
advance for level densities determination. 
III. TSC GAMMA MEASUREMENT 
Neutron beam and detectors 
arrangement 
The experiment system has been 
installed at the tangential beam port of the 
DNRR. The thermal neutron beam was 
moderated by Si filter. The neutron flux, the 
cadmium ratio and the neutron beam diameter 
at the sample position were 2.4 105 n.cm-2.s-1, 
230 and 1.5 cm respectively. 
Two horizontal GMX35 detectors 
manufactured by ORTEC with the energy 
resolutions of 1.9 keV at 1332 keV (
60
Co) 
have been used in the - coincidence 
spectrometer. The detectors were shielded by 
lead blocks of 10 cm in thickness. The distance 
between the source and the detectors’ surfaces 
is 4 cm. In order to decrease the back scattered 
gamma rays and filter out X-ray, two lead 
plates of 2 mm in thickness were placed in 
front of the detectors and sample. The 
background count rate was less than 600 counts 
per second (cps) in 0.2 ÷ 8 MeV range [5]. 
Data acquisition system 
The electronics configuration used in 
those - coincidence experiments is shown in 
Figure 1. 
The detector signals are amplified with 
572 amplifier (AMP) modules with a shaping 
time of 3.0 µs and about 1 keV per channel. 
The output signals of the amplifiers are 
digitized by 7072 analog-to-digital converter 
(ADC) modules. The timing signals of both 
detectors are put through 474 timing filter 
amplifier (TFA) modules. 
The shaped and amplified timing 
signals by 474 TFA are plugged into 584 CFD 
modules, which are used in slow rise time 
rejection option (SRT) mode. The CFD output 
signal of the first channel is used as 556 time-
to-amplitude converter (TAC) start signal. 
Fig. 1. The - coincidence electronics. 
NGUYEN XUAN HAI et al. 
59 
The CFD output signal of the second 
channel is delayed 100 ns and served as a TAC 
stop signal. 
The full scale of TAC is set at 100 ns, 
and output signal is digitized in 8713 ADC 
with selection of 1024 channels for a 10 V 
input pulse. The TAC “Valid Convert” signal 
is used to gate 7072 ADCs, and the delay or 
synchronizing with AMP output signal is 
implemented by interface software. Recorded 
coincident events have three values, including 
coincidence gamma-ray energies from detector 
1, detector 2 and time interval between two γ-
rays in a pair event [5]. The resolving time for 
this configuration is about 10 ns with 
60
Co 
source measurement (see Figure 2). 
Coincidence Data Processing 
In the experiment, the data, which 
contains all pairs of - coincidence data from 
two HPGe-detectors, were stored in the 
memory of computer. Indeed, that is pairs of 
channel numbers associated with energies of 
- coincidence pairs. The coincidence 
spectrum of each detector can be created from 
the corresponding data file by the procedure 
that the count number of each channel of the 
spectrum is equal to times of that channel 
appearing in the corresponding coincidence 
data file. The coincidence spectrum of one 
detector with the chosen peak in another 
detector can be created by the same procedure. 
They are coincidence spectra between high-
energy primary and low-energy secondary 
transitions or among the low-energy secondary 
transitions as obtained in the work [3, 4]. 
Besides, the summation spectrum of amplitudes 
of coincidence pulses can be created by 
summation of pairs of coincidence data. Every 
full-peak in the summation spectrum is 
corresponding to the - cascade decays from 
the capture state to the determined low-lying 
excited level. The TSC spectrum of one 
detector associated with the defined energy (E) 
summation peak will be taken by choosing 
pairs of coincidence data having summation in 
the range of E ± E (with E/E ≤ 0.005) (see 
Figure 3). The TSC spectrum gives information 
on levels in the region between the capture state 
and the defined E low-lying level. From all 
obtained TSC spectra we can build up the decay 
scheme of the investigated nucleus on the base 
of methods and the criteria given in Ref. [5]. 
The measured values of gamma two-step 
cascade energies and intensities of 
35
Cl(nth, 
2γ)36Cl reaction were shown in Table 1. 
0 10 20 30 40
0
1000
2000
3000
4000
5000
C
o
u
n
ts
Resolving time (ns)
10ns
Fig. 2. The resolving timing spectrum 
2000 4000 6000 8000
0
100
200
300
400
500
E1+E2 = 8579 keV
7
8
8
.4
3
k
e
V
1
1
6
4
.8
7
k
e
V
1
6
0
1
.0
8
k
e
V
2
6
7
6
.3
0
k
e
V 2
8
6
3
.8
2
k
e
V
3
0
6
1
.8
6
 k
e
V
5
7
1
5
.1
9
k
e
V
5
5
1
7
.2
k
e
V
5
9
0
2
.7
k
e
V
1
9
5
9
.3
6
k
e
V
6
9
7
7
.8
5
k
e
V
6
6
2
7
.7
5
k
e
V
7
7
9
0
.3
2
k
e
V
C
o
u
n
ts
Energy keV
7
4
1
3
.9
5
k
e
V
Fig. 3. The TSC spectrum of 
36
Cl belongs to final 
level from 8579 keV. 
THE GAMMA TWO STEP CASCADE METHOD AT DALAT RESEARCH REACTOR 
60 
Table 1. The gamma two-step cascade energies and intensities of 
35
Cl(nth, 2γ)
36
Cl reaction. 
Measured values XCI 6/18/013 
I- Eγ 
 (keV) 
Up level 
(keV) 
Low level 
(keV) 
Eγ 
 (keV) 
Up level 
(keV) 
Low level 
(keV) 
787.03 1952.98 1164.01 786.30 1951.20 1164.89 10.520 
1164.01 1952.98 787.03 1162.78 1951.20 788.44 2.290 
1370.00 3331.99 1958.98 1372.86 3332.32 1959.41 0.384 
1958.98 1958.98 0.00 1959.36 1959.41 0.00 12.560 
1164.01 1164.01 0.00 1164.87 1164.89 0.00 27.20 
3723.00 4886.09 1164.01 3723.00 N/A 
517.05 517.05 0.00 517.08 2468.28 1951.20 24.300 
1950.98 2465.97 517.05 1951.14 1951.20 0.00 19.390 
789.03 789.03 0.00 788.43 788.44 0.00 16.320 
1164.60 1164.60 0.00 1164.87 1164.89 0.00 27.20 
1601.49 1601.49 0.00 1601.08 1601.12 0.00 3.484 
1958.48 1958.48 0.00 1959.36 1959.41 0.00 12.560 
2864.28 2864.28 0.00 2863.82 2863.96 0.00 5.770 
7413.06 8579.71 1165.01 7413.95 8579.70 1164.89 10.520 
6979.37 8579.71 1602.99 6977.85 8579.70 1601.12 2.290 
3062.98 8579.71 5518.16 3061.86 8579.70 5517.76 3.521 
5518.16 5518.16 0.00 5517.2 5517.76 0.00 1.689 
6621.31 8579.71 1957.98 6619.64 8579.70 1959.41 7.830 
5716.18 8579.71 2863.98 5715.19 8579.70 2863.96 5.310 
788.23 788.23 0.00 788.43 788.44 0.00 16.32 
1950.17 1950.17 0.00 1951.14 1951.20 0.00 19.39 
6629.20 8579.71 1950.17 6627.75 8579.70 1951.20 4.690 
7792.32 8579.71 788.23 7790.32 8579.70 788.44 8.310 
IV. RESULTS 
Within the framework of this research 
project, the obtained results are as follows: 
- Setting up successfully the event-
event coincidence spectrometer with for 
measuring nuclear structure data on thermal 
neutron beam. 
- Measuring and analyzing the 
gamma cascade transition data for nuclei of 
239
U, 
182
Ta, 
153
Sm, 
172
Yb, 
59
Ni, 
55
Fe and 
49
Ti. 
The experimental data are to evaluate 
excited states in the intermediate energy 
below the neutron binding energy. 
- Evaluating nuclear structure for 
those nuclei based on analyzed data and 
theoretical models. 
- Determining the lifetime level, width 
level and gamma transition strength from the 
experimental data of gamma intensity and 
electromagnetic transfer selection. 
- Providing methods and experimental 
facilities for basic researches, education and 
training. 
V. CONCLUSION 
The γ-γ coincidence spectrometer is a 
useful tool in research on nuclear spectroscopy in 
DNRR. Besides, the spectrometer can also be 
used in research on the lifetime of some excited 
states and γ-γ angular correlations that are 
completely new research fields. For some 
elements in the deformed nuclei region with high 
possibility of cascade transitions, this 
NGUYEN XUAN HAI et al. 
61 
spectrometer can be used for the neutron 
activation analysis because of very low 
gamma backgrounds. 
The research method and facilities 
for TSC measurements will play a 
significant role in carrying out R&D 
programs of nuclear technique applications 
so far, as well as in preparing human 
resources for the nuclear data program in 
Vietnam in the near future. 
ACKNOWLEDGMENTS 
The authors would like to express 
their sincere thanks to the researchers of 
DNRR for their cooperation concerning to 
neutron irradiations. This research is funded 
by Ministry of Science and Technology, 
Vietnam Atomic Energy Institute and 
Nuclear Research Institute. 
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on Neutron Capture Gamma Ray Spectroscopy 
and Related Topics, Grenoble, France, p.15-34 
(1981). 
[3] S.T. Boneva et al. Two-step cascades of neutron 
radiative capture: 1. The spectroscopy of excited 
states of complex nuclei in the range of the 
neutron binding energy, Physics of Elementary 
Particles and Atomic Nuclei, Vol.22, Part.2, 
p.479-511 (1991). 
[4] S.T. Boneva et al. Two-step cascades of neutron 
radiative capture: 2. Main parameters and 
peculiarities complex nuclei compound-states -
decay, Physics of Elementary Particles and 
Atomic Nuclei, Vol.22, Part.6, p.1431-1475 
(1991). 
[5] Vuong Huu Tan et al. Investigation of gamma 
cascade transition of 153Sm, 182Ta, 59Ni and 239U 
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(2005-2006).