jednorog

EFDA–JET–CP(09)06/03
S. Jednorog, M. Scholz, S. Popovichev, A. Murari
and JET EFDA contributors
Numerical Optimization of Activation
Samples for the Application of the
Activation Technique to Measure
Neutrons in Large Fusion Devices
like JET and ITER
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EFDA, Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK.”
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Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK.”
Numerical Optimization of Activation
Samples for the Application of the
Activation Technique to Measure
Neutrons in Large Fusion Devices
like JET and ITER
1
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S. Jednorog , M. Scholz , S. Popovichev , A. Murari
and JET EFDA contributors*
JET-EFDA, Culham Science Centre, OX14 3DB, Abingdon, UK
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Association EURATOM-IPPLM, IPPLM, Warsaw, Poland
EURATOM-UKAEA Fusion Association, Culham Science Centre, OX14 3DB, Abingdon, OXON, UK
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Associazione EURATOM-ENEA sulla Fusione, Consorzio RFX Padova, Italy
* See annex of F. Romanelli et al, “Overview of JET Results”,
nd
(Proc. 22 IAEA Fusion Energy Conference, Geneva, Switzerland (2008)).
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Preprint of Paper to be submitted for publication in Proceedings of the
36th EPS Conference on Plasma Physics, Sofia, Bulgaria.
(29th June 2009 - 3rd July 2009)
.
1. INTRODUCTION
The activation technique, based on the irradiation of suitable materials placed close to the plasma,
is used in large magnetic confinement devices for the measurement of the neutron yield and is
being developed to provide the neutron spectra for both DD and DT operation [1]. In previous
experiments at JET, stacks of separate elemental foils were used [2]. Nowadays High Purity
Germanium (HPGe) detectors, with high efficiency and resolution, can be numerically characterized
very well in terms of efficiency and energy calibration. This allows the application of samples with
mixed elemental compositions and shapes. The technique presented in this paper was implemented
on the JET neutron activation diagnostic.
2. SYSTEM FOR GAMMA RAY DETECTION AND ACCURACY OF NUMERICAL
EFFICIENCY CALIBRATION
The main components of system to detect the emission of the samples and discussed in this paper
are: a HPGe coaxial detector, a Numerical Characteristic (NCh) of the detector and Laboratory
Sourceless Object Calibration Software (LabSOCS). NCh allows estimating a sample activity without
having to perform an in-situ calibration. For the aim of this paper the efficiency of photon registration
always means Absolute Full Energy Peak Efficiency (AFEPE) [3]. The complete geometry of the
particular detector used, including its mounting and housing hardware, is included in a dedicated
MCNP model of LabSOCS. This software provides a calibration which comprises a set of energyefficiency curves for the relevant geometries. The calculated standard deviation is typically in the
interval 3%-7.1% [4],[5]. The efficiency calibration calculated by the LabSOCS and obtained as
the result of measurements with a real calibration source is presented on the Fig.1. It shows that in
this particular case, when the main characteristics of the real sample, i.e. its shape (geometry),
density and elemental composition, are precisely defined, the accuracy of LabSOC is very high.
3. THREE DIMENSIONS EFFICIENCY CALIBRATIONS FOR IDEAL AND REAL
SAMPLES
The efficiency calibration is the function of AFEPE versus photon energy. Usually it is a logarithmic
polynomial of order fifth or less. By plotting the calibration functions for different sample sizes, the
Three Dimensions (3D) curves of efficiency have been obtained. Figure 3 represents 3D AFEPE as
the function of energy and distance from the detector end cap for an ideal point source and as
function of the indium sample diameter. In case of a real sample, the self attenuation dominates,
whereas the geometrical factor represented by the inverse square low plays the main role for an
ideal sample.
3.1 OPTIMIZATION OF THE GEOMETRY FOR REAL ACTIVATION SAMPLES
The following relation (1) represents the dependence of AFEPE on the diameter of the used indium
sample as a result of Gaussian fitting of each particular cross section of the efficiency surface.
1
D (x, E = const) = y0 +
π .
2
A . exp -
2(x-xc)2
w2
w
(1)
X- diameter of the sample; xc, A, y0, and w are the parameters of the Gaussian fitting.
The AFEPE curve changes its shape below a particular inflection point, as shown in Fig.3 (left).
The sequence of inflection points represents the interval of optimal sample diameters and it is
presented in Fig. 3 (right). The sample optimization plays an important role particularly when the
number of neutrons activating the sample is very limited.
3.2 ABSOLUTE FULL ENERGY PEAK EFFICIENCY FOR A STACK OF SEPARATE
FOILS ACTIVATED DURING RECENT JET CAMPAIGNS, AND FOR A SAMPLE
MADE OF MIXTURE OF ELEMENTS
During recent JET campaigns a stack of different metal foils with constant diameter and different
thicknesses have been used. The processing of the sample efficiency calibration is expensive and
time consuming. This limits the number of metals which can be used during the activation
experiments. Actually the NCh of the HPGe detector practically gives unlimited access to a wide
number of activation materials with different geometry and density. The detector NCh allows
estimating the efficiency calibration for the mixture of nuclides as well as coping with a range of
sample geometries. Finally a set of samples consisting of up to five elements have been designed
for JET activation diagnostic. During 2008-2009 JET campaigns eleven mixed samples of Al, Se,
Y, Au, Fe powder with different composition have been used. This technique allows measurements
of a higher number of nuclear reactions in a single activation than previously.
CONCLUSIONS
- The performed analysis shows the applicability of the proposed mixed sample technique to
characterize the neutron yield in large fusion devices like JET and ITER [6].
- The relatively high AFEPE for mixed activation samples is the result of optimizing their
composition. In case of a stack of foils the lower sample density the higher its AFEPE.
- Optimization of sample size allows using the above technique in place with low neutron yields.
- The optimal sample diameter depends only slightly on photons energy.
ACKNOWLEDGEMENT
This work, supported by the European Communities under the contract of Association between
EURATOM/IPPLM, was carried out within the framework of the European Fusion development
Agreement. The views and opinions expressed herein do not necessarily reflect those of the European
Commission.
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REFERENCES
[1]. B. Esposito, L. Bertalot, M. Louglin, A. L. Roquemore, Neutron spectrum measurements in
DT discharges using activation technique, Rev. of Sci. Instrum., Vol 70 (1), 1130-1133,
[2]. L. Bertalot, L. Roquemore, M. Louglin, B. Esposito, Calibration of the JET neutron activation
system for DT operation, Rev. of Sci. Instrum, Vol 70 (1), 1137-1140,
[3]. Practical gamma-ray spectrometry, G. Klimor, J.D. Hemingway, pp 137-138, Chichestrer,
New York, Brisbane, Toronto, Singapore, 1995
[4]. F.L. Bronson, R. Venkataraman, Validation of the accuracy of the LabSOCS mathematical
Efficiency calibration for typical laboratory samples, 46th Annual Conference on Bioassay,
Analytical, and Environmental Radiochemistry, November 12-17, 2000, Seattle, Washington
DC
[5]. F.L. Bronson; R. Venkataraman, B. Young, Massemetric efficiency calibrations of HPGe
detectors for Laboratory applications, 44th Annual Conference on Bioassay, Analytical, and
Environmental Radiochemistry, November 15-19, 1998; Albuquerque NM
[6]. C. Barnes, M. Louglin, T Nishitani, Neutron activation for ITER, Rev. of Sci. Instrum., Vol
68 (1), 577-580
Calibration source
LabSOCS
Difference source
0.05
0.04
0.03
0.02
0.01
JG09.186-1c
Efficiency of photon registration
0.06
0
0
500
1000
1500
Photon energy (keV)
2000
2500
Figure 1: Efficiency calibration obtained with certificated calibration source (black squares) and LabSOCS (red
dotes) and differences between those data (green triangles).
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Absolute full energy peak efficiency for real indium disc
radiation source versus sample diameter & photon energy
Absolute full energy peak efficiency for ideal point radiation source
versus distance from detector end cap & photon energy
0.2
0.1
Efficiency
Efficiency
0.2
JG
09
.18
6-
0.1
0
2000
0
End cap 5
distance (cm)
500
10
JG
2c
1500
1000
Energy (keV)
5
Disc
diameter (cm)
1000
10
500
1500
09
.18
6-
3c
2000
Energy (keV)
Figure 2: The AFEPE of an ideal point radiation source (left figure) is the function of photon energy and sample
distance from detector end cup meanwhile the AFEPE of a thin real indium disk (right figure) is presented as the
function of photons energy and sample diameter.
LabSOCS
Gauss fit
Indium sample diameter (cm)
6.8
0.004
0.002
Convexity of the curve;
positive value
of second derivation
Infection point;
second derivation
equal to zero
0
2
4
6
8
Indium Sample diameter (cm)
6.7
6.6
6.5
6.4
10
6.3
JG09.186-5c
0.003
Concavity of
curve;
negative
value
of second
derivation
JG09.186-4c
Efficiency (photon energy = 45 keV)
0.005
0
500
1000
1500
Photon energy (keV)
2000
Figure 3: Typical cross section of the AFEPE for E=const (left figure) presents dependence of efficiency of a real
indium sample. Sequence of the inflection points (right figure) represents the interval of optimal diameters.
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0.18
0.2
Aluminum (ρ = 2.7)
Aluminum (ρ = 2.7)
MIX9 (ρ = 3.535)
Gold (ρ = 3.535)
Ytrium (ρ = 4.47)
Titan(ρ = 4.54)
Zirkon (ρ = 6.51)
Iron (ρ = 2.7)
Indium (ρ = 2.7)
Cobalt (ρ = 2.7)
Hafnium (ρ = 2.7)
Gold (ρ = 2.7)
0
10
100
Energy (keV)
1000
0.09
0
10
JG09.186-7c
0.1
Efficiency
Niciel (ρ = 2.7)
JG09.186-6c
Efficiency
Cadmium (ρ = 2.7)
100
Energy (keV)
1000
Figure 4: AFEPE for different activation samples (left figure) with diameter 18mm and thick 1mm and AFEPE for
real samples with 18mm diameter and 5,6mm thick (right figure). The efficiency of mixed sample is relatively high
nevertheless it consists of Al, Se, Y, Au, Fe. It is the result of careful choosing the sample composition.
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