Department of Nuclear Spectroscopy, Nuclear Physics Institute p.r.i.
Academy of Sciences of Czech Republic
Cross-section measurements
of neutron threshold reactions
using activation method
Ondřej Svoboda
Outline
• Motivation for cross-section measurements
• Requirements of cross-section measurements
• Irradiation facilities
• Evaluation process
• Background subtraction
• Example of results
• Conclusion
2
Project „Energy & Transmutation of Radioactive
Waste“
Ezhik
Gamma - 2
JINR Dubna, Russia
Gamma - 3
Energy + Transmutation
Kvinta
d
3
Activation detectors - (n,xn) reactions
E
Reaction
Half-life
[MeV]
8.1
6.183 d
197Au
(n,2n)
197Au
(n,3n) 195Au
14.8
186.1 d
197Au
(n,4n) 194Au
23.2
38.02 h
197Au
(n,5n) 193Au
30.2
17.65 h
197Au
(n,6n) 192Au
38.9
4.94 h
197Au
(n,7n) 191Au
45.7
3.18 h
Al
196Au
Threshold
Au
Bi
Co
In
Ta
4
Motivation – current (n,xn) cross-section knowledge
197
Au(n,2n)
2.0
2.0
Au
EXFOR
1.5
ENDF
1.0
0.5
0.0
197
Au(n,4n)
194
Au
1.5
EXFOR
1.0
ENDF
0.5
0.0
0
2.0
10
20
30
40
Neutron energy [MeV]
50
0
10
209
Cross-section [b]
Bi(n,xn)
1.5
(n,4n)206Bi
(n,5n)205Bi
1.0
0.5
0.0
Cross-section [b]
Cross-section [b]
2.5
196
Cross-section [b]
3.0
10
20
30
40
Neutron energy [MeV]
(n,6n)204Bi
(n,8n)202Bi
(n,10n)200Bi
(n,12n)198Bi
1
50
(n,7n)203Bi
(n,9n)201Bi
(n,11n)199Bi
0.1
209Bi(n,xn)
0.01
0
50
100
Neutron energy [MeV]
150
0
50
100
Neutron energy [MeV]
150
5
Requirements for cross-section measurements
Requirements for using activation method of measurement:
- high energy neutron source with good intensity
- (quasi)monoenergetic neutrons with well known spectrum
- pure monoisotopic samples
- good spectroscopic equipment:  and X-rays detectors
- knowledge about the corrections on beam fluctuation,
self-absorption, non-point like emitters…
Studied (mono)isotopic materials:
In all irradiations: Al, Au, Bi, I, In, Ta
In some irradiations: Co, Cu, Fe, Mg, Ni, Y, Zn
6
Irradiation facilities
7
TSL Uppsala Sweden
Cyclotron 15 – 180 MeV
Blue hall:
kvasi-monoenergetic neutron
source based on reaction
7Li(p,n)7Be
8
Cyclotron in NPI Řež
• Protons 18 – 37 MeV on 7Li target
• Fast
neutron generator (NRD - NPI)
• High intensities of neutrons: 108 cm-2 s-1
• Well equipped spectroscopic laboratory (NSD - NPI)
Beam-line
Graphite
stopper
Samples
Li-target
9
Evaluation procedure
Irradiation
HPGe
NYield
Spectra evaluation
1,01
1,00
0,99
0,98
0,97
2x2 cm emmiter
0,96
Ep
0,95
Et
0,94
0
2
4
6
8
10
12
14
16
Corrections
Cross-section
10
Uncertainty analysis – yield evaluation
11
- also possible to use
MCNPX to calculate
neutron spectra, but:
Both aproaches have its
problems… => It is
neccesary to study the
neutron sources further!!
- uncertainty in spectra
determination 10%
- uncertainty in neutron
intensity 5-10%
Neutron flux [n/MeV·sr·C]
Neutron spectra from p/Li at NPI Řež
Neutron energy [MeV]
12
Subtraction of background contribution
• cross-section comparisons between EXFOR and TALYS – mostly good agreement
• We believe the simulated s shape is OK, only the absolute value can be shifted
EXFOR data
197Au(n,2n)196Au
TALYS
•Following the neutron spectrum knowledge, we calculated ratio between production in
neutron peak and total production
• With this ratio we multiplied the yields to subtract background production
13
Cross-section versus neutron spectrum [-]
Subtraction of background contribution 2
1
beam 59 MeV (0.59)
beam 66.4 MeV (0.48)
beam 72.8 MeV (0.42)
beam 89.3 MeV (0.34)
Y-86 cross-section
0
0
20
40
60
Energy [MeV]
80
• unsensitive to absolute value of cross-section, but sensitive to changes in crosssection shape
• up to now, various conditions tested – different Talys versions, settings, manual
changes in cross-section shape – still more or less within 10% uncertainty
14
Various Talys 1.2 settings – level densities
2.5
196Au
in TALYS 1.2
ld1
Cross-section [barn]
2
ld2
ld3
1.5
ld4
ld5
1
0.5
0
0
10
20
30
40
50
60
70
Neutron energy [MeV]
80
90
100
15
Various Talys 1.2 settings – level densities
2.5
196Au
in TALYS 1.2
ld1
ld2
1.5
2.4
ld3
2.2
ld4ld1/ld2
2
Level density ratio [-]
Cross-section [barn]
2
1
0.5
0
0
10
ld1/ld3
ld5ld1/ld4
ld1/ld5
1.8
1.6
1.4
1.2
1
200.8
0.6
0
30
40
50
60
70
Neutron energy [MeV]
10
20
30
80
40
50
60
70
Neutron energy [MeV]
90
80
100
90
100
16
Impact on Au(n,xn) cross-section results
0.25
94 MeV - TSL data
Cross-section [barn]
0.20
0.15
TALYS 1.2 - ld1
0.10
TALYS 1.2 - ld2
TALYS 1.2 - ld3
TALYS 1.2 - ld4
0.05
TALYS 1.2 - ld5
TALYS 1.0 - ld1
0.00
196Au
194Au
193Au
192Au
Isotope
191Au
190Au
188Au
17
Background subtraction – other possibility
60 degrees
30 degrees
Y. Uwamino et al., NIM A389 (1997) 463
18
Yttrium studies
19
Yttrium cross-section measurement
Methodical measurement – neutron energy 32.5 MeV, May 2011, to prepare
systematic study of yttrium reactions by means of the NPI neutron source
Up to now very scare data about crosssections
No data about cross-sections of
isomeric state production
Special conditions of measurement:
- long irradiation
- intensive beam
- only limited number of samples – a
lot of measurement time for one
sample
Main aim - to study time evolution of
yttrium isomers
Reaction (n,3n) - production of
isomeric and ground state of 87Y
20
Production of two states of 87Y during irradiation
Stable beam intensity is assumed during whole irradiation – it is more or less true
for NPI cyclotron irradiations.
Unstable beam – correction on beam variations during irradiation.
N01= 6.05(8)109
NA1 = 10.01(16)109
NA1
N02
N01
N02= 7.12(21)109
NA2
NA2 = 3.63(17)109
21
Evolution of Yttrium isomers in short time
87mY
N1  N 01e
 1 t


1
1
 N 01 e 2 t 
 N 01e 1 t
87Y N 2   N 02 
1  2
1  2


1.6
1.4
1.2
1.0
0.8 N02
0.6 N01
0.4
0.2
0.0
22
Evolution of Yttrium isomers in long time
87mY
N1  N 01e
 1 t


1
1
 N 01 e 2 t 
 N 01e 1 t
87Y N 2   N 02 
1  2
1  2




1
N   N 02 
 N 01  e   2 t
1   2


Experimental
points
■ - 87Y
♦ - 87mY
N = N01exp(-λ1t)
23
Yttrium cross-section results
Higher threshold near to neutron energy → 93 % of radioactive nuclei is produced by peak, only 7 %
by background
Uncertainties of proton beam integral and neutron spectra description are about 10 - 15 % > gamma
spectroscopy uncertainties
σ(87mY) = 578(56) mbarn
σ(87gY) = 203(25) mbarn
Background production subtraction
89Y(n,3n)87Y
case
87Y
87mY
87gY
87Y
89Y(n,2n)88Y
case
squares - our data
lines - TALYS
- EXFOR
! Preliminary !
24
Summary of our cross-section measurements
25
Measured reactions
26
Results for the 197Au(n,2n)196Au reaction
3
EXFOR
NPI experiments
TSL experiments 2008
TSL experiments 2010
TALYS 1.0
Cross-section [barn]
2.5
2
1.5
197Au(n,2n)196Au
1
0.5
0
0
10
20
30
40
50
60
70
Neutron energy [MeV]
80
90
100
27
Summary
• measured some 39 reactions, 12 different energies, some of the reactions
measured for the first time
• measured also isomers (Y, In, Au)
• problems with background subtraction – solved by using the Talys code
• studies of all (minor) uncertainty sources under progress
• main uncertainty comes from determination of beam intensity and neutron
spectrum – need for further studies/developement/measurements of
neutron sources
• cross-section studies are the subject of Jitka Vrzalová PhD work
28
Acknowledgements
This work was financialy supported from following grants:
GA ASCR K2067107
GACR 202/03/H043
EFNUDAT
CTU0808214
F4E-2008-GRT-014.
Thank you for your attention...
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Back-up slides
30
Uncertainty analysis - cross-section calculation
Background subtraction
Neutron
spectrum
uncertainty
TALYS
uncertainty
TALYS
uncertainty
Yield
Neutron
spectrum
TALYS
version
TALYS
TALYS
setings
Cross-section
31
32