cross-section measurements of neutron threshold reactions using activation...
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Cross-section measurements of neutron threshold reactions
using activation method
Ondřej Svoboda
Department of Nuclear Spectroscopy, Nuclear Physics Institute p.r.i.Academy of Sciences of Czech Republic
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• Motivation for cross-section measurements
• Requirements of cross-section measurements
• Irradiation facilities
• Evaluation process
• Background subtraction
• Example of results
• Conclusion
Outline
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Project „Energy & Transmutation of Radioactive Waste“
JINR Dubna, Russia
d
Gamma - 2
Gamma - 3
Energy + Transmutation Kvinta
Ezhik
4
Activation detectors - (n,xn) reactions
Al Au Bi Co In Ta
ReactionE
Threshold[MeV]
Half-life
197Au (n,2n) 196Au 8.1 6.183 d197Au (n,3n) 195Au 14.8 186.1 d197Au (n,4n) 194Au 23.2 38.02 h197Au (n,5n) 193Au 30.2 17.65 h197Au (n,6n) 192Au 38.9 4.94 h197Au (n,7n) 191Au 45.7 3.18 h
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Motivation – current (n,xn) cross-section knowledge
0.01
0.1
1
10
0 50 100 150
Cro
ss-s
ectio
n [b
]
Neutron energy [MeV]
209Bi(n,xn)
(n,6n)204Bi (n,7n)203Bi(n,8n)202Bi (n,9n)201Bi(n,10n)200Bi (n,11n)199Bi(n,12n)198Bi
209Bi(n,xn)
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0.5
1.0
1.5
2.0
0 50 100 150Neutron energy [MeV]
Cro
ss-s
ectio
n [b
]
(n,4n)206Bi(n,5n)205Bi
197Au(n,2n)196Au
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0.5
1.0
1.5
2.0
2.5
3.0
0 10 20 30 40 50Neutron energy [MeV]
Cro
ss-s
ectio
n [b
]
EXFORENDF
197Au(n,4n)194Au
0.0
0.5
1.0
1.5
2.0
0 10 20 30 40 50Neutron energy [MeV]
Cro
ss-s
ectio
n [b
]
EXFORENDF
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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
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Irradiation facilities
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TSL Uppsala Sweden
Cyclotron 15 – 180 MeV
Blue hall:kvasi-monoenergetic neutron source based on reaction 7Li(p,n)7Be
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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)
BeamBeam--lineline
LiLi--targettarget
GraphiteGraphitestopperstopper
SamplesSamples
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Irradiation HPGe Spectra evaluation
Cross-section
Corrections0,94
0,95
0,96
0,97
0,98
0,99
1,00
1,01
0 2 4 6 8 10 12 14 16
Ep Et
2x2 cm emmiter
NYield
Evaluation procedure
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Uncertainty analysis – yield evaluation
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Neutron spectra from p/Li at NPI Řež
Neutron energy [MeV]
Neu
tron
flux
[n/M
eV·s
r·C
]
- 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%
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• 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
•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
197Au(n,2n)196AuEXFOR data
TALYS
Subtraction of background contribution
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Subtraction of background contribution 2
• unsensitive to absolute value of cross-section, but sensitive to changes in cross-section shape• up to now, various conditions tested – different Talys versions, settings, manual changes in cross-section shape – still more or less within 10% uncertainty
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1
0 20 40 60 80
Cro
ss-s
ectio
n ve
rsus
neu
tron
spec
trum
[-]
Energy [MeV]
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
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Various Talys 1.2 settings – level densities
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0.5
1
1.5
2
2.5
0 10 20 30 40 50 60 70 80 90 100
Cros
s-sec
tion
[bar
n]
Neutron energy [MeV]
ld1
ld2
ld3
ld4
ld5
196Au in TALYS 1.2
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0
0.5
1
1.5
2
2.5
0 10 20 30 40 50 60 70 80 90 100
Cros
s-sec
tion
[bar
n]
Neutron energy [MeV]
ld1
ld2
ld3
ld4
ld5
196Au in TALYS 1.2
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
2.4
0 10 20 30 40 50 60 70 80 90 100
Leve
l den
sity
ratio
[-]
Neutron energy [MeV]
ld1/ld2ld1/ld3ld1/ld4ld1/ld5
Various Talys 1.2 settings – level densities
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0.00
0.05
0.10
0.15
0.20
0.25
196Au 194Au 193Au 192Au 191Au 190Au 188Au
Cro
ss-s
ectio
n [b
arn]
Isotope
TALYS 1.2 - ld1TALYS 1.2 - ld2TALYS 1.2 - ld3TALYS 1.2 - ld4TALYS 1.2 - ld5TALYS 1.0 - ld1
94 MeV - TSL data
Impact on Au(n,xn) cross-section results
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Background subtraction – other possibility
30 degrees
60 degrees
Y. Uwamino et al., NIM A389 (1997) 463
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Yttrium studies
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Yttrium cross-section measurement
Reaction (n,3n) - production of isomeric and ground state of 87Y
Up to now very scare data about cross-sections
Methodical measurement – neutron energy 32.5 MeV, May 2011, to prepare systematic study of yttrium reactions by means of the NPI neutron source
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
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Production of two states of 87Y during irradiationStable 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.
NA1 = 10.01(16)109
NA2 = 3.63(17)109
N01= 6.05(8)109
N02= 7.12(21)109
N01
N02
NA1
NA2
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tt eNeNNN
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0121
101
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1022
teNN 1011
87mY 87Y
1.6
1.4
1.2
1.0
0.8
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0.0
N02
N01
Evolution of Yttrium isomers in short time
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teNNN
2
0121
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Experimental points
■ - 87Y♦ - 87mY
N = N01exp(-λ1t)
tt eNeNNN
12
0121
101
21
1022
teNN 1011
87mY 87Y
Evolution of Yttrium isomers in long time
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Higher threshold near to neutron energy → 93 % of radioactive nuclei is produced by peak, only 7 % by background
89Y(n,3n)87Y case
89Y(n,2n)88Y case
Uncertainties of proton beam integral and neutron spectra description are about 10 - 15 % > gamma spectroscopy uncertainties
Background production subtraction
σ(87mY) = 578(56) mbarn
σ(87gY) = 203(25) mbarn
Yttrium cross-section results
87mY
87Y
87gY
! Preliminary !
87Y - EXFOR
squares - our datalines - TALYS
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Summary of our cross-section measurements
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Measured reactions
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Results for the 197Au(n,2n)196Au reaction
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1
1.5
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0 10 20 30 40 50 60 70 80 90 100
Cro
ss-se
ctio
n [b
arn]
Neutron energy [MeV]
197Au(n,2n)196Au
EXFORNPI experimentsTSL experiments 2008TSL experiments 2010TALYS 1.0
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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
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Acknowledgements
This work was financialy supported from following grants:
GA ASCR K2067107GACR 202/03/H043
EFNUDATCTU0808214
F4E-2008-GRT-014.
Thank you for your attention...
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Back-up slides
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Yield Cross-section
Background subtraction
Neutron spectrum
Neutron spectrum
uncertainty
TALYS
TALYS uncertainty TALYS
version
TALYS setings
TALYS uncertainty
Uncertainty analysis - cross-section calculation
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