01. 06. 2011 EAF-2011 Prague
197Au(n,γ)198Au
cross-section in
Maxwellian-like neutron spectrum
A. Krása, A. Plompen, G. Georginis
European Commission – JRC – IRMM, Geel, Belgium
G. Feinberg, M. Friedman, A. Shor, Y. Eisen, D. Berkovits, M. Paul
Nuclear Research Centre Soreq, Yavne, Israel & Hebrew University, Jerusalem
Joint Research Centre (JRC)
IRMM - Institute for Reference Materials and Measurements
Geel - Belgium
http://irmm.jrc.ec.europa.eu/
http://www.jrc.ec.europa.eu/
Outline
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• Background and motivation
• Experimental setup
• Time of Flight
• Activation
• Summary
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7Li(p,n)7Be
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• Q = -1644.4 keV
• Ethres = 1880.8 keV
maximal neutron emission angle
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90°
7Li(p,n)7Be
80°
70°
60°
50°
40°
30°
20°
10°
0°
1880
1890
1900
1910
Proton energy [keV]
1920
7Li(p,n)7Be
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• for Ep = 1912 keV:
- neutron spatial distribution: cone with the emission angle 60°
- neutron energy distribution: similar to Maxwellian
Φn ~ En exp(- En / kT)
at kT = 25 keV (except En > 110 keV)
• this temperature is close to that of outer shells of
asymptotical giant branch stars,
where heavier elements are created via s-process
7Li(p,n)7Be
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• widely used to activate nuclides involved in s-process
⇒ neutron capture Maxwellian-averaged cross-section
(MACS) at kT = 25 keV
• Au – standard neutron fluence monitor
sample
gold foil
• Ratynski & Käppeler: σ (197Au(n,γ)) = (586 ±8) mb
7Li(p,n)7Be
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• to measure MACS for special cases, higher proton
beam (~ mA) intensities necessary
• possible with RF accelerators, but broader proton
energy distribution, which affects neutron spatial,
energy distributions and yields
⇒ new standard for neutron fluence monitoring required
Aim
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• investigate effects of proton beam with broad-energy
spread on emitted neutrons and σ (197Au(n,γ))
• two cases compared:
- 1912 keV proton beam with σ = 1.5 keV
- 1912 keV proton beam with σ = 15 keV
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Experimental setup
Neutron source
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• low-scatter target hall of VdG at IRMM Geel
• 2 mg/cm2 LiF targets evaporated in 1 mm Cu backing
LiF target
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LiF target
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• autoradiography
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Narrow-energy proton beam
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• scanning of narrow (< 0.2 keV) resonance
• measuring yield of 10.76 MeV γ-rays with NaI detector
Broad-energy proton beam
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• 2.06 μm Au degrager in front of LiF target:
2097±0.3 keV protons with σ = 1.5 keV →
1912±3 keV protons with σ = 15 keV
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Time of Flight
TOF setup
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TOF results
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TOF results
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TOF results
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Activation
Activation setup
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• two Au foils: 1 mm and 5 mm from LiF target
• fresh LiF target for each irradiation
• the same geometry for both
narrow-energy proton beam
broad-energy proton beam
Activation setup
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Au foil 1 mm from LiF target
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Au foil 5 mm from LiF target
γ-spectrometry
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• activities of
198Au (411.8 keV)
7Be (477.6 keV)
• HPGe detector
- 100% rel. eff.
• Pb+Cu shielding
- 10 cm Pb
- 1 mm Cu
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Radiography
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81 mm
• Ra226 source
88 mm
Data analysis
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• three steps:
a) number of 7Be = number of emitted neutrons
(also number of protons useful to provide n/p ratio)
b) number of 198Au
c) σ (197Au(n,γ) 198Au)
Correction factors
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• angular acceptance – neutrons within the angular
acceptance of Au foil
• scattering – neutrons scattered in Cu backing
• effective thickness - planar Au foil
gives bigger weight to neutrons
emitted at bigger angles
Simulations
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• correction factors simulated with SimLiT and Geant4
• SimLiT – simulation of 7Li(p,n)7Be ⇒ differential
energy-angular neutron distribution ⇒ input to Geant
• Geant4 – simulation of neutron scattering in
Cu backing and interactions in planar Au foils
• final correction factor is ratio of
198Au
nuclei using 4π spherical Au sample divided by
198Au nuclei using planar Au foils
Narrow beam
Au 1
1/1.25
Au 2
1/1.17
Broad beam
1/1.32
1/1.07
Results
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n/p
Narrow beam
Broad beam
Experiment
(1.10±0.02) x 10-6
(1.36±0.03) x 10-6
Simulated (SimLiT)
(1.13±0.02) x 10-6
(1.20±0.14) x 10-6
Simulated (PINO)
(0.90±0.02) x 10-6
(0.94±0.11) x 10-6
198Au
activity Narrow beam Broad beam
Au 1 (Bq)
6.92±0.14
10.63±0.21
Au 2 (Bq)
6.28±0.13
7.99±0.16
Results
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(197Au(n,γ))
Narrow
beam
Narrow
beam FZK
Broad
beam
Au 1 (mb)
596±12
586±8
652±24
Au 2 (mb)
582±11
Average (mb)
589±12
σ
607±24
586±8
630±32
Results
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σ ( E ) N ( E )dE
∫
σ=
∫ N ( E )dE
Spectrum-averaged
cross-section
Narrow-energy proton
beam (mb)
Broad-energy average
cross section(mb)
N(E) – experimental neutron
spectrum (TOF)
Maxwellian
Au(n,γ)
Au(n,γ)
Au(n,γ)
spectrum at
capture cross
capture cross capture cross
25keV folded
sections of
sections of
sections of
with capture
refs. 1+2
ref. 4 folded
ref. 3 folded
cross sections of
folded with
with N(E)
with N(E)
refs. 1+2
N(E)
618 ± 16
659 ± 18
640±12
595 ± 30
633 ± 30
616±30
1. ENDF/BVII
2. C. Lederer et al, Phys Rev C83(2011)034608
3. Kononov, Yad Fiz 26(1977)947
4. Yamamuro, Journal of NS&T 20(1983)797
595±16
Summary
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• σ (197Au(n,γ)) = (589±12) mb for narrow-energy proton
beam agrees with the standard value (586±8) mb
• broad-energy proton beam
– bigger angular acceptance (84°) results in 10% higher σ
– smaller angular acceptance (66°) the capture cross section is
similar to that obtained in narrow-energy proton beam
⇒ for experiments with RF accellerators it is preferable to
use a planar Au foil covering a smaller angle
• It is essential to incorporate Monte-Carlo simulations to
take into account the full geometry of the target and
neutron scattering in materials surrounding the target