NATIONAL TECHNICAL UNIVERSITY OF ATHENS and
NATIONAL CENTER for SCIENTIFIC RESEARCH
“DEMOKRITOS”
Investigation of (n,2n) reactions using the high
energy neutron facility at NCSR “Demokritos”
R. Vlastou1, A. Kalamara1, M. Kokkoris1, M. Serris2, N. Patronis3
S. Harissopulos4, M. Axiotis4, A. Lagoyannis4
1
Department of Physics, National Technical University of Athens, 157 80 Athens, Greece,
2 Hellenic Army Academy, 16673 Vari, Athens, Greece
3 Department of Physics, University of Ioannina, 45110 Ioannina, Greece,
4 Institute of Nuclear Physics, NCSR "Demokritos", 153 10 Aghia Paraskevi, Greece
25th Symposium of the Hellenic Nuclear Physics Society, NCSR “Demokritos”, June 2016
The High Energy Neutron beam at “Demokritos”
is produced via the 3H(d,n)4He reaction
at energies ~15-21 MeV with deuteron beams ~2-4.5 MeV
The Ti-tritiated target of 373 GBq activity, consists of 2.1 mg/cm2 Ti-T layer on a
1mm thick Cu backing for good heat conduction. The flange with the tritium target
assembly is air cooled during the deuteron irradiation.
Neutron beam flux of the order of ~105-106 n/sec·cm2. To monitor the neutron flux
a BF3 detector is used, while the absolute flux of the beam is determined via the
27Al(n,α) reference reaction
Cross section measurements via the
activation technique
After the neutron irradiation : Off-line
measurements of γ-ray transitions from
the residual nuclei by using HPGe
detectors εr ≈ 80% and 56%
This new High Energy neutron facility has been characterized by
using NeuSDesc and MCNP simulations as well as the multiple foil
activation technique
It opens a vast field of research on a variety of neutron induced reactions at an
important energy range to test and improve nuclear models and investigate
reaction mechanisms.
The pre-equilibrium mechanism becomes important
in the de-excitation of the compound nucleus
Cross Section Measurements
of reactions already investigated with the low energy
neutron facility in the past using the activation technique
(n,2n) reactions on 191Ir, 193Ir, 174Hf, 176Hf, 197Au
recently measured at 15.3, 17.1 and 20.9 MeV
191Ir(n,2n)190Ir
reaction (N.Patronis et al. Phys.Rev.C75(2007)034607)
Isomeric cross sections are of fundamental interest since they are governed by the
spins of the levels involved in the compound nucleus evaporation process
Statistical model calculations with code STAPRE-F have been performed
by using the Generalized Superfluid Model (GSM) for the calculation of
nuclear level densities in the continuum to test it in the mass region ~190
and for isomeric cross section production.
The high spin isomeric state m2 is fed by a small part of the continuum
which depends on the spin cut-off parameter and consequently on the
moment of inertia. A reduced value of the rigid body moment of inertia is
needed for a better agreement with data.
The population of the isomeric state is also strongly dependent on the
details of the introduced level scheme.
Further measurements are needed above 15 MeV to resolve discrepancies in
experimental points and theoretical predictions
Cross section (mbarns)
2.5
191
Ir(n,2n)
190
g+m1, m2
Ir
Previous Data
ENDF
Present Data
2.0
1.5
1.0
0.5
0.0
8
10
12
14
16
18
20
22
24
26
Energy (MeV)
Further measurements are planned at 19 MeV to complete the energy range
which can be covered by the facility as well as theoretical calculations
using the code EMPIRE
197Au(n,2n)196Au
reaction (A.Tsinganis et al. Phys.Rev.C83(2010)024609)
Statistical model calculations were carried out with the use of three different
codes STAPRE-F, EMPIRE2.19 and TALYS1.2. which were also compared in
their implementation of the Generalized Superfluid Model (GSM) for the
calculation of nuclear level densities in the continuum. The σg+m1 cross section
was easily reproduced by the calculations, while for σm2, the theoretical results
could only reproduce the general trend of the experimental data, with the
distribution shifted at higher energies.
Several tests were performed to improve the theoretical predictions which
reveal the importance of the level scheme of the residual nucleus and the
limitations of the nuclear codes to embed high spin discrete states in the
continuum which would increase the feeding of the isomeric state.
Further measurements are needed above 15 MeV to resolve discrepancies in
experimental points and theoretical models
3.5
197
Cross section (barns)
3.0
Au(n,2n)
196
Au
g+m1, m2
Previous Data
ENDF
Present Data
2.5
2.0
1.5
1.0
0.5
0.0
0
5
10
15
20
25
30
35
40
45
50
Energy (MeV)
Further measurements are planned at 19 MeV to complete the energy range
we can cover as well as theoretical calculations using the code EMPIRE
174,176Hf
(n,2n) reactions
(M.Serris et al. Phys.Rev. C86(2012)034602).
Studies of neutron induced reactions on Hf are of considerable importance for practical applications in nuclear
technology: a) due to its high thermal neutron absorption cross section, Hf is used for reactor control rods in
nuclear submarines b) n-induced reactions on W and Ta in reactor materials could lead to long lived isomeric
states of Hf isotopes with rather harmful γ-ray production. Experimental data available only at ~14MeV
Natural Hf consists of 6 isotopes 174, 176, 177, 178, 179, 180 Hf and three of them produce long
lived residual nuclei and can thus be studied by using the neutron activation technique
Reaction
Q (MeV)
T1/2
Eγ (keV)
Iγ (%)
174Hf(n,2n)173Hf
-8.51
23.6h
123.7
83
176Hf(n,2n)175Hf
-8.16
70d
343.4
84
180Hf(n,n’γ)189Hf
5.5h
332.3
94
174Hf(n,γ)175Ηf
70d
343.4
84
The 176Hf(n,2n) reaction has an energy threshold of ~8.2MeV
Opens with En >~9MeV
While the contaminant 174Hf(n,γ) reaction has no threshold
Opens with parasitic low energy neutrons
However,
In the high energy region, the
176Hf(n,2n)175Hf reaction is also
contaminated by the 177Hf(n,3n)175Hf
reaction. Using the cross section from
ENDF, the expected number of counts
from the (n,3n) reaction has been
deduced to correct for this contribution
176
Hf(n,2n)
2015 Present Data
2012 Serris
2011 Semkova
2010 Zhu Chuan-Xin
2001 Kilary
Further measurements are
planned at 19 MeV to complete
the energy range which can be
covered by the facility
1999 Lu Hanlin
1998 Xianghzhong
1996 Meadows
1990 Patrick
1974 Lakshmana Das
1974 Quaim
1969 Hillman
1968 Dilg
ENDF/B-VII.1
0
2
4
6
8
10
12
14
16
18
20
22
Energy (MeV)
4.0
Theoretical
calculations will be
performed using the
code EMPIRE
174
Hf(n,2n)
2015 Present Data
2012 Serris
2011 Semkova
1998 Xiangzhong Kong
1990 Patrick
1981 Lakshmana Das
1974 Quaim
1969 Hillman
EDF/B-VII.1
3.5
Cross section (barns)
Cross section (barns)
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
3.0
2.5
2.0
1.5
1.0
0.5
0.0
8
10
12
14
16
Energy (MeV)
18
20
22