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 Tan1, Pham Dinh Khang2, Nguyen Nhi Dien3, Nguyen Xuan Hai3,
Tran Tuan Anh3*, Ho Huu Thang3, Pham Ngoc Son3, Mangengo Lumengano4
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
(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
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].
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].
II. TSC METHOD
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.
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
difference between the capture state and the
pre-selected low-lying state. The detected
spectrum then contains information on two
types of transitions. The 1st 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 2nd 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.
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.
III. TSC GAMMA MEASUREMENT
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.
Neutron
arrangement
beam
and
detectors
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 2.4105 n.cm-2.s-1,
230 and 1.5 cm respectively.
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.
Two horizontal GMX35 detectors
manufactured by ORTEC with the energy
resolutions of 1.9 keV at 1332 keV (60Co)
have been used in the - coincidence
Fig. 1. The - coincidence electronics.
58
NGUYEN XUAN HAI et al.
The CFD output signal of the second
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 35Cl(nth,
2γ)36Cl reaction were shown in Table 1.
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 60Co
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
500
5000
4000
3000
E1+E2 = 8579 keV
400
300
200
100
0
10ns
2000
1000
0
0
10
20
30
40
2000
4000
6000
8000
Energy keV
Resolving time (ns)
Fig. 3. The TSC spectrum of 36Cl belongs to final
level from 8579 keV.
Fig. 2. The resolving timing spectrum
59
THE GAMMA TWO STEP CASCADE METHOD AT DALAT RESEARCH REACTOR
Table 1. The gamma two-step cascade energies and intensities of 35Cl(nth, 2γ)36Cl reaction.
Measured values
XCI 6/18/013
Up level
(keV)
Eγ
(keV)
Up level
(keV)
Low level
Eγ
(keV)
786.30
1162.78
1372.86
1959.36
1164.87
3723.00
517.08
Low level
(keV)
I-
(keV)
1164.01
787.03
1958.98
0.00
0.00
1164.01
0.00
787.03
1952.98
1952.98
3331.99
1958.98
1164.01
4886.09
517.05
1951.20
1951.20
3332.32
1959.41
1164.89
1164.89
788.44
1959.41
0.00
10.520
2.290
0.384
12.560
27.20
1164.01
1370.00
1958.98
1164.01
3723.00
517.05
0.00
N/A
2468.28
1951.20
788.44
1951.20
0.00
24.300
19.390
16.320
27.20
3.484
12.560
5.770
10.520
2.290
3.521
1.689
7.830
5.310
16.32
19.39
4.690
8.310
1950.98
789.03
2465.97
789.03
517.05
0.00
1951.14
788.43
0.00
0.00
0.00
0.00
1164.60
1601.49
1958.48
2864.28
7413.06
6979.37
3062.98
5518.16
6621.31
5716.18
788.23
1164.60
1601.49
1958.48
2864.28
8579.71
8579.71
8579.71
5518.16
8579.71
8579.71
788.23
0.00
0.00
0.00
0.00
1164.87
1601.08
1959.36
2863.82
7413.95
6977.85
3061.86
5517.2
6619.64
5715.19
788.43
1951.14
6627.75
7790.32
1164.89
1601.12
1959.41
2863.96
8579.70
8579.70
8579.70
5517.76
8579.70
8579.70
788.44
0.00
1165.01
1602.99
5518.16
0.00
1957.98
2863.98
0.00
0.00
1950.17
788.23
1164.89
1601.12
5517.76
0.00
1959.41
2863.96
0.00
0.00
1951.20
788.44
1950.17
6629.20
7792.32
1950.17
8579.71
8579.71
1951.20
8579.70
8579.70
IV. RESULTS
- Determining the lifetime level, width
level and gamma transition strength from the
experimental data of gamma intensity and
electromagnetic transfer selection.
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.
- Providing methods and experimental
facilities for basic researches, education and
training.
V. CONCLUSION
- Measuring and analyzing the
gamma cascade transition data for nuclei of
239U, 182Ta, 153Sm, 172Yb, 59Ni, 55Fe and 49Ti.
The experimental data are to evaluate
excited states in the intermediate energy
below the neutron binding energy.
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
- Evaluating nuclear structure for
those nuclei based on analyzed data and
theoretical models.
possibility
of
cascade
transitions,
this
60
NGUYEN XUAN HAI et al.
spectrometer can be used for the neutron
REFERENCES
activation analysis because of very low
gamma backgrounds.
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Data for Reactors. Helsinki 1970, IAEA, Vienna,
Vol.1, p.559 (1970).
The research method and facilities
for TSC measurements will play
a
[2] H.H. Bolotin. Thermal-neutron capture gamma-
gamma coincidence studies and techniques,
Proceedings of the 1981 International Symposium
on Neutron Capture Gamma Ray Spectroscopy
and Related Topics, Grenoble, France, p.15-34
(1981).
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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.
[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).
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.
[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
using the gamma two step cascade method, Final
report of the research project, Ministry of
Sciences and Technology, Code BO/05/01/05,
(2005-2006).
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