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

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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

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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

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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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1

1.5

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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

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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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0.8

1

1.2

1.4

1.6

1.8

2

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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

12

0121

101

21

1022

teNN 1011

87mY 87Y

1.6

1.4

1.2

1.0

0.8

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0.4

0.2

0.0

N02

N01

Evolution of Yttrium isomers in short time

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teNNN

2

0121

102

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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Cro

ss-se

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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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